Comprehensive Biochemistry for Dentistry Textbook for Dental Students

430 15  Metabolism of Carbohydrates

• Glycogen primer
–– It is present in cytosol of the cell and it is also called as glycogenin.
–– It is a glycoprotein. Its protein (dimeric) component is made up of two similar
monomers. Each monomer is attached to oligosaccharide chain of seven glu-
cose residues.
–– Glycogen primer of glycogenin serves as a primer and an autocatalytic gly-
cosyltransferase enzyme (brings about transfer of carbohydrate moiety
from activated nucleotide sugar molecule to glycosyl acceptor molecule).

ATP ADP
Mg++
Glucose Glucose – 6 – Phosphate
Glucose – 6 – P Glucokinase Glucose – 1 – Phosphate
or

Hexokinase

Mg++

Phosphogluco mutase

Glucose – 1 – P UDP – Glucose pyrophosphorylase UDP –

(UDP – Glucose)

UDP – UTP PPi
Glycogen synthase Glycogenin

(UDP – Glucose)

OH

UDP

Glycogenin

OH
Glucose

1234
Glycogen primer

14 UDP – Glycogen
14 UDP Synthase

∝ – 1,4 Bonding

1 5 10 18
Fig. 15.8 Glycogenesis [Glycogen Chain] 15

15.10  Glycogenesis 431

∝ – 1, 4 – Bonding

9
13

15 10

Branching ∝ – 1, 6 – Bonding Main branch
by 18 Side Chain

Glucosyl
∝ – 4 – 6 – Transferase

Enzyme

Elongation [Glycogen synthase]

Branching
Continues

Glycogen

Fig. 15.8 (continued)

–– First glucose residue from UDP-G is transferred to nonreducing end of glyco-
gen primer and forms alpha-1,4-glycosidic bond. UDP is released for its
reuse.

Elongation of glycogen chain

• Glycogen chain elongates by successive addition of glucose residues at nonre-
ducing end of glycogen primer through formation of alpha-1,4-glycosidic
bonds.

• Glycogen synthase is the key regulatory enzyme that controls elongation of gly-
cogen chain as in Fig. 15.8a, b.

• Glycogen synthase
–– It is a glycosyltransferase enzyme.

432 15  Metabolism of Carbohydrates

–– It catalyzes transfer of glucose moiety from UDP-G to glycogen chain by
formation of alpha-1,4-glycosidic bond.

–– Glycogen synthase is unable to initiate formation of glycogen chain. It
requires a glycogen primer to accept glucose moieties from UDP-G and start
de novo synthesis of glycogen chain.

–– Glucose-6-P acts as an allosteric stimulator of glycogen synthase enzyme.

Branching of glycogen chain

• After the elongation of glycogen chain up to at least 11 glucose residues, branch-
ing of glycogen chain takes place by action of branching enzyme. It is called as
amylo-1,4 → 1,6-transglucosidase enzyme.

• Branching enzyme transfers six glucose residues from alpha-1,4 chain to another
chain by formation of alpha-1,6-glycosidic bond. This short chain of six glucose
residues serves as a branch on another chain. The growth of branch chain contin-
ues by addition of glucose residues by alpha-1,4 bonding.

• Glycogen synthase and branching enzymes act together in the synthesis of gly-
cogen molecule as in Fig. 15.8a, b.

Regulation of Glycogenesis
Glycogen synthase

• Glycogen synthase is the key regulatory enzyme in the synthesis of glycogen. It
exists in active glycogen synthase and inactive glycogen synthase in tissues. The
active GS promotes glycogenesis and inactive GS inhibits glycogenesis.

• These two forms are interconvertible through action of cyclic AMP.
• Active GS is changed into inactive GS by phosphorylation, while inactive GS is

converted into active GS by dephosphorylation.

Protein kinase

• This enzyme is found in cytosol of cells. Protein kinase activity is dependent on
cyclic AMP. The enzyme has two regulatory subunits (R2) and another two cata-
lytic subunits (C2) and is designated as (R2 C2).

• An increase in concentration of cyclic AMP in cytosol leads to detachment of
two regulatory subunits from catalytic subunits, and protein kinase is activated.

• Activated protein kinase catalyzes phosphorylation of glycogen synthase and con-
verts active form into inactive form of enzyme. This process inhibits glycogenesis.

Insulin

• Insulin promotes hydrolysis of cyclic AMP by phosphodiesterase enzyme and
converts it into AMP. Insulin decreases concentration of cAMP in cytosol.

15.11  Inherited Disorders of Glycogen Metabolism 433

• Insulin promotes conversion of inactive GS into active form and stimulates gly-
cogenesis in skeletal tissues and liver cells.

Glucose and glycogen concentration

• Increased concentration of glucose in cells promotes glycogenesis through posi-
tive feedback mechanism.

• Increased concentration of glycogen in cells inhibits glycogenesis by negative
feedback mechanism.

Glucocorticoid hormones

• Glucocorticoid hormones increase synthesis of glycogen synthase in the liver.
These hormones stimulate glycogenesis in liver cells.

15.11 I nherited Disorders of Glycogen Metabolism

Definition
A group of disorders related to glycogen metabolism which are inherited from
parents to offsprings are called as glycogen storage diseases.

Types of Glycogen Storage Diseases (GSD)
They are classified into six types as follows:

• Type I glycogen storage disease
–– This type of disorder is called as Von Gierke’s disease.
–– It is attributed to deficiency of glucose-6-phosphatase enzyme in hepato-
cytes and intestinal mucosa cells.
–– The disorder is an autosomal recessive trait.
Clinical manifestations
Hepatocytes, renal tubules, and intestinal mucosal cells contain large amount
of glycogen. It is not converted into glucose due to deficiency of glucose-6-­
phosphatase. It led to hypoglycemia.
Excessive oxidation of fats leads to ketosis and fatty liver.
Children suffer from stunted growth.
Prognosis is poor. Children die in younger age.

• Type II glycogen storage disease
–– It is called as Pompe’s disease.
–– Disorder is due to deficiency of maltase enzyme.
–– It is an autosomal recessive trait.
Clinical manifestations
Excessive amount of glycogen is stored in the liver, cardiac muscle fibers,
skeletal muscles, and smooth muscle fibers.

434 15  Metabolism of Carbohydrates

Enlargement of heart (cardiomyopathy).
Prognosis is poor. Affected children die in age of 9 months to 2 years due to
cardiac failure.
• Type III glycogen storage disease
–– This disorder is called as Forbes’ disease.
–– It is due to deficiency of debranching enzyme.
–– It is an autosomal recessive trait.
Clinical manifestations
Glycogen cannot be completely degraded due to deficiency of debranching
enzyme.
Excessive accumulation of limit dextran (altered structure of storage gly-
cogen) is found in the liver, heart, and skeletal muscles. Disorder is also called
as limit dextrinosis.
Enlargement of liver (hepatomegaly).
Weakness and wasting of muscles (myopathy).
Prognosis is average. Affected individuals can survive to adulthood.
• Type IV glycogen storage disease
–– Disorder is called as Andersen’s disease.
–– It is due to deficiency of branching enzyme.
Clinical manifestations
Excessive accumulation of unbranched glycogen chains (abnormal glycogen)
is found in the liver, heart, kidneys, and skeletal muscles. This abnormal gly-
cogen resembles amylopectin, and disorder is also called as
amylopectinosis.
Enlargement of the liver, spleen, and heart is commonly observed in affected
individuals.
Cirrhosis of the liver and heart failure are the commonest consequences in
sufferers.
Prognosis is poor. Affected children survive 3–4 years of life.
• Type V glycogen storage disease
–– Disorder is called as McArdle’s disease.
–– It is due to deficiency of phosphorylase enzyme in skeletal muscles.
–– It is an autosomal recessive trait.
Clinical manifestations
Excessive accumulation of glycogen is observed in skeletal muscles.
Weakness of muscles, muscle cramps.
Prognosis is average.
• Type VI glycogen storage disease
–– Disorder is called as Hers’ disease.
–– It is due to deficiency of phosphorylase enzyme in the liver.
Clinical manifestations
Excessive glycogen is stored in the liver.
Enlargement of liver.
Prognosis is average.

15.12  Disorders of Carbohydrate Metabolism 435

15.12 D isorders of Carbohydrate Metabolism

15.12.1  Diabetes Mellitus

Definition
Diabetes mellitus is an endocrine disorder associated with relative or absolute
deficiency of insulin.

It is a clinical condition characterized by increased blood glucose level. Today,
diabetes mellitus has emerged as the major cause of morbidity among young and
old-aged population alike. This condition is the main predisposing factor to diabetic
retinopathy (injury to retina), atherosclerosis (thickening of blood vessels), coro-
nary artery disease, cerebral stroke, and chronic renal failure.

15.12.2  T ypes of Diabetes Mellitus

I nsulin-Dependent Diabetes Mellitus (IDDM)
It is also called as type I diabetes mellitus or juvenile onset diabetes mellitus.

Occurrence
IDDM occurs early in life. It appears in adolescent age group. Both children and
adults can be affected by the disease. Patients need administration of insulin. These
individuals have high tendency to develop ketoacidosis.

Prevalence
IDDM affects 5–10% of diabetic young population.

Etiopathogenesis
IDDM is characterized by decreased synthesis of insulin by beta cells of islets of
Langerhans in the pancreas. Insulin plasma level is decreased.

• T-cell-mediated autoimmunity
–– Majority of the cases of IDDM are caused by autoimmunity against beta
cells. The T cells destroy beta cells of the pancreas. The affected person has
to rely upon insulin throughout their life owing to deficiency of insulin
production.
–– Heredity is also implicated in the pathogenesis of IDDM. Genetically prone
individuals have higher tendency to develop diabetes mellitus than normal
persons. Viral infection, obesity, and fat-rich diet become the predisposing
factors in the onset of diabetes mellitus in genetically susceptible persons.

• Idiopathic
–– Idiopathic type I diabetes mellitus has not ascribed any specific cause for its
pathogenesis. It occurs spontaneously in predisposed individuals.
–– Its prevalence is high among male African-American population. It has also
been observed in other ethnic population.

436 15  Metabolism of Carbohydrates

–– Patients with idiopathic type I diabetes develop ketoacidosis rapidly.
Individual characteristics appear similar to patients of type II diabetes.

–– Idiopathic type I diabetes patients do not show autoimmune markers in blood
circulation.

N on-insulin-Dependent Diabetes Mellitus (NIDDM)
It is also called as type II diabetes mellitus or maturity onset diabetes mellitus.

Occurrence
It appears late in life. It affects persons in middle age group and generally above
40 year of age. These individuals have less tendency to develop ketoacidosis.

Prevalence
NIDDM is prevalent in nearly 90% of diabetic population.

Etiopathogenesis

• NIDDM is characterized by nonresponsiveness of body tissues to insulin. It
is called as insulin resistance. As a compensatory mechanism, pancreas
secretes more amount of insulin to regulate blood glucose level. This con-
dition is called as hyperinsulinemia. It is associated with type II diabetes
mellitus.

• Insulin resistance is responsible for reduced glucose uptake and its utilization by
peripheral body tissues.

• There is manifestation of increase in blood glucose level (hyperglycemia).
• Predisposing factors include central obesity (↑ waist to hip ratio), alcohol con-

sumption, sedentary lifestyle, stress, and intake of high fat and carbohydrate
diet.

Gestational Diabetes Mellitus
Gestational diabetes mellitus appears during pregnancy. It affects nearly 5–10% of
pregnant women. It is associated with decreased insulin secretion in pregnancy.

The condition disappears after termination of pregnancy.

Clinical Manifestation of Diabetes Mellitus
Hyperglycemia

• Blood glucose level is increased. It is a characteristic manifestation of diabetes
mellitus.

Glycosuria

• It is the excretion of glucose in urine. Glucose is excreted by tubules when blood
glucose level is higher than renal threshold for glucose (160–180 mg/100 ml).

15.12  Disorders of Carbohydrate Metabolism 437

Polyuria

• It is the passage of large amount of urine by individual (>2 L/day). It is another

cardinal sign of diabetes mellitus.
• Frequency of micturition is also increased.

Polydipsia

• It is the excessive thirst in individuals who suffer from diabetes mellitus and
diabetes insipidus. It is followed by intake of large amount of water.

Polyphagia

• It is the increased appetite in individuals who suffer from diabetes mellitus. It is
associated with intake of large amount of foods repeatedly. Despite polyphagia,
there is weight loss in affected individuals. It is owed to impairment in glucose
utilization by body tissues.

Asthenia

• Physical weakness, lethargy, and inability to perform routine activities are com-
mon symptoms of diabetic patients.

Recurrent infection

• Owing to poor nutritional status of diabetic patients, immunity is compromised.
It leads to recurrent viral and bacterial infections in the body. Skin infections like
boils, diabetic foot, and infection of upper respiratory tract are common infec-
tions. These patients are predisposed to tuberculosis.

• Polyuria, polydipsia, and polyphagia are cardinal signs of diabetes mellitus

15.12.3  L aboratory Investigation of Diabetes Mellitus

Estimation of Blood Glucose Level
• Blood glucose estimation is an important test to confirm diagnosis of diabetes

mellitus.
• According to the WHO diabetes diagnostic criteria:
• Normal fasting blood glucose level should be <100 mg/dl.
• Normal post-prandial (2  h after food intake) blood glucose level should be

<140 mg/dl.
• Impaired fasting blood glucose level should be 110–125 mg/dl.
• Impaired post-prandial blood glucose level should be ≥140 mg/dl.

438 15  Metabolism of Carbohydrates

Glucose Tolerance Test
It determines the ability of body tissues to utilize carbohydrate after an intake of a
given amount of glucose.

Types of Glucose Tolerance Test

Oral Glucose Tolerance Test
Preparation of Patient

• Patient is advised to abstain from eating or drinking at least 8–12 h before test.
• Patient should be alert physically and mentally.

Procedure of Test

• A baseline fasting blood sample is collected.
• Patient is asked to drink a solution of 75 g of glucose (recommended by WHO

for adults) dissolved in 250 ml of water within 5 min.
• After every 30 min, five blood samples are collected.
• All six samples are estimated to determine blood glucose level. A graph is plot-

ted for six values of blood glucose concentration against time and it is called
glucose tolerance curve.

Interpretation
Normal Glucose Tolerance Curve

Characteristics

• Fasting blood glucose level should be < 110 mg/dl.
• At 1 h period, blood glucose level should be < 180 mg/dl. It is the highest peak

of blood glucose concentration. It should not exceed the renal threshold for
glucose.
• At 2.5 h period, fasting blood glucose level (<110 mg/dl) should be obtained.

Diabetic Glucose Tolerance Curve

• Fasting blood glucose level is elevated (>110  mg/dl). Fasting glucose level
between 110 and 125 mg/dl indicates borderline impaired glucose tolerance.

• At 1 h period, blood glucose level rises >180 mg/dl. The highest peak of blood
glucose concentration is obtained after 1 h.

• At 2.5 h period, fasting blood glucose level is not obtained. It confirms
hyperglycemia.

Intravenous Glucose Tolerance Test
It is indicated in condition of malabsorption of glucose from alimentary canal.

15.12  Disorders of Carbohydrate Metabolism 439

Indications

• Coeliac disease
• Environmental enteropathy
• Hypothyroidism

Procedure

• A dose of 3 g/kg of weight of glucose is administered intravenously in 50% solu-
tion in 5 min.

• Baseline blood sample and half hourly five blood samples are taken.

Interpretation of glucose tolerance curve is the same as in OGTT.

E stimation of Glycated Hemoglobin (HbA1C)
Glycated hemoglobin (HbA1C) offers the best test for monitoring long-term
blood glucose level in diabetic patients. It does not help to diagnose diabetes
mellitus. It provides a blood glucose level of previous 3 months.

Interpretation of Glycated Hemoglobin Test
WHO diabetes diagnostic criteria

• HbA1C value < 6% is indicative of very good blood glucose control.
• HbA1C value  <  7% is indicative of adequate blood glucose control, and

WHO has recommended a cutoff (6.5%) for diagnosis of diabetes mellitus.
• HbA1C value ≥ 8% is indicative of poor blood glucose control.

Glycation

• It is nonenzymatic attachment of glucose with protein.
• It occurs between free glucose moiety and polypeptide chain of hemoglobin in

blood circulation. Reducing end of glucose attaches covalently to N-terminal of
amino acid in chain.
• It is not physiological process. Glycated Hb predisposes to generation of higher
amount of free radicals within RBCs.
• It serves as a biomarker to determine level of blood glucose over an extended
period of 2–3 months in diabetic patients.

Glycosylation

• It is an enzymatic addition of carbohydrate moiety to a specific region of
polypeptide chain.

• It occurs as posttranslation modification.
• Glycosylation is necessary for rendering structural integrity and functional

patency to protein molecules.

440 15  Metabolism of Carbohydrates

Suggested Readings

Bender DA (2004) Introduction to nutrition and metabolism, 3rd edn. Taylor & Francis Group,
Philadelphia

Nelson DL, Cox MM (2004) Principles of biochemistry, 4th edn. W.H. Freeman and Company,
New York

Rosenthal MD, Glew RH (2009) Medical biochemistry – human metabolism in health and disease.
Wiley, Hoboken, NJ

Digestion and Absorption of Lipids 16

Lipids are important source of energy for living organisms. A healthy adult person
requires around 2800 calories per day. It is recommended that around 20–35% of
daily calories should be furnished by dietary lipids. Calorific value of fats is (9
calories/g). An adult person should consume around 60–90 g of fats per day. Intake
of calories from saturated fatty acids and trans fatty acids should be <10% and 2%
of total calories, respectively, per day.

16.1 D ietary Sources

Plant Sources of Lipids

• Vegetable oils like cottonseed oil, sunflower oils, mustard oil, rapeseed oil,
canola oil, and soya oil.

• Vegetable sources of lipids are rich in MUFA and PUFA.
• They are superior in quality to animal sources of lipids.

Animal Sources of Lipids

• Milk, butter, eggs, meat, cod liver oil, and pork.

16.1.1 Digestion of Triglycerides

Human diet contains a variety of lipids. Triglycerides constitute around 90–95% of
the total dietary lipids. They are the predominant lipids that are stored in body tis-
sues. Triglycerides serve as stored form of lipids.

© Springer Nature Singapore Pte Ltd. 2019 441
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_16

442 16  Digestion and Absorption of Lipids

Digestion in Oral Cavity
• Saliva contains lingual lipase. It is secreted by “Von Ebner’s glands” located

on dorsal surface of the tongue.
• The optimum pH for lingual lipase is between 4.0 and 4.5.
• Milk fat (butter) contains butyric acid (C4) fatty acids. Lingual lipase digests it

into glycerol and fatty acid.
• Lingual lipase hydrolyzes milk fats in the absence of bile salts.
• Enzymatic activity of lingual lipase continues in the stomach.
• It digests about 20–30% of dietary triglyceride (short chain fatty acids) into

monoglyceride, diglyceride, and fatty acid. Milk fat (butter) contains butyric acid
(C4). Lingual lipase favorably digests milk fat.

Lingual Lipase Diglyceride + Monoglyceride + Butyric acid
Milk fat (Butter)

Optimum pH 4.0

Digestion in the Stomach

• Gastric juice contains “gastric lipase.” It is secreted by chief cells of gastric
mucosa in fundus of the stomach.

• The optimum pH for gastric lipase is between 3.0 and 6.5. This is a weak lipo-
lytic enzyme. It is due to high pH (1.5–2.5) of the stomach.

• Gastric lipase hydrolyzes lipids containing short-chain fatty acids like milk fat
and egg yolk.

Lingual lipase and gastric lipase are acidic lipases which exhibit catalytic activ-
ity at acidic pH, in the absence of bile salts, and colipase.

• Fats retard the movement of food from the stomach to duodenum. This condition
is called as “gastroparesis” or “delayed gastric emptying.”

Digestion in the Small Intestine
• Intestinal juice contains bile salts, pancreatic lipase, and intestinal lipase.
• Bile salts are secreted by the liver.
• Role of Bile Salts

–– Bile salts are sodium taurocholate and sodium glycocholate secreted in
bile.

–– Lipids are insoluble in aqueous medium in the intestinal lumen. But alkaline lipase
is a water-soluble enzyme. This difficulty is solved by emulsification of fats.

–– Bile salts lower the interfacial tension. It helps in emulsification of fats in
duodenum.

–– “Emulsification of fats”  – Immiscible large fat drops are fragmented into
minute fat droplets in the duodenum. Fat droplets are suspended in aqueous
medium in duodenum. This process is called as emulsification.

–– Therefore, bile salts act as a “hydrotrope.”

16.1  Dietary Sources 443

Digestion of lipids

o–––– 1CH2–OHo
1CH2–O–Co– R1 –– R1
2CH–O–C– R2 ––
–
o Lipase 2CH–O–C– R2 +
3CH2–O–C– R3 H2O o
3CH2–O–C– R3 COOH
Triacylglycerol–

2,3–Diacylglycerol

Lipase Free fatty
acid

o H2O
CH2–O–C– R2
CH2–OHo
CHOH
CH2OH–– Lipase
CHOH ––

CH2OH –
––

–
–
Lipase CH2–O–C– R2 + R3
H2O

CH2OH CH2OH COOH

Glycerol 1–Monoacyl 2–Monoacyl Free fatty
glycerol glycerol acid (FFA)
+
R2 –CFOFOAH

Fig. 16.1  Hydrolysis of trialcylglycerol

–– Surface area of a fat drop is between 1 and 3 μm. After emulsification, it increases
around a thousand times. Lipase gets a larger surface area for its activity.

–– Bile salts help to attach a molecule of pancreatic lipase with two mole-
cules of colipase.

• Role of Pancreatic Lipase

–– Pancreatic lipase (steapsin) is secreted by the pancreas. It is the important

alkaline lipase for hydrolysis of triglycerides as in Fig. 16.1.

–– The optimum pH for pancreatic lipase is between 7.4 and 8.5.

–– This enzyme hydrolyzes ester bonds in triglycerides.
–– Cleavages ester bond at position-α – Initially, it cleavages ester bond at

position-α and liberates α-fatty acid. Triglyceride is converted into
α’,β-diglyceride.
–– Cleavages ester bond at position-α – Later on, it cleavages ester bond at
position-α. This action converts α’, β-diglyceride into β-monoglyceride.
–– Isomerization of β-monoglyceride into α-monoglyceride – The compound
β-monoglyceride offers resistance to hydrolysis by pancreatic lipase. It is
isomerized into α-monoglyceride by isomerase enzyme. It is digested into
glycerol and fatty acid.
• Role of Colipase

–– It is a coenzyme for lipase. It is secreted from the pancreas in a procolipase

form. It is activated by trypsin.

–– Colipase is required for optimum enzymatic activity of pancreatic lipase.

444 16  Digestion and Absorption of Lipids

• Role of Calcium Ions
–– Calcium ions precipitate free fatty acids into calcium soap. They increase
lipase activity.

16.1.2 Digestion of Cholesteryl Ester

• Cholesterol is present in diet in free state and bound state as cholesteryl ester
(10–15%). The liver also secretes free cholesterol in bile.

• Pancreatic and intestinal juices contain “cholesterol esterase.” It is a carboxyl
ester hydrolytic enzyme.

• Cholesterol esterase is a non-specific ester-splitting enzyme. It can cleavage ester
linkage at all three, namely, α, β, and α’ positions. It can digest triglycerides,
cholesteryl ester, and phospholipids.

• Its activity is enhanced by the presence of bile salts. The enzyme cleavages ester
bond in cholesteryl ester. It liberates cholesterol and free fatty acids.

16.1.3 Digestion of Phospholipids

• Pancreatic juice contains phospholipase A2.
• This enzyme exists in inactive form (zymogen) in pancreatic juice. It is acti-

vated by trypsin.
• Activated phospholipase A2 cleavages ester bond in phospholipids (lecithin).
• This enzyme converts phospholipid into free fatty acids and lysophospholipid

(lysolecithin).

Digestion End Products of Dietary Lipids

• About 70–75% diglycerides + monoglycerides
• About 20–25% glycerol + free fatty acids
• Free cholesterol
• Lysophospholipids
• Phospholipids

16.2 Absorption of Lipids

Absorption of digestion end products of lipids occurs in three stages. They are as
follows:

1. Luminal Stage
In this stage, digestion products of lipids exist in emulsified form in the lumen of
the intestine. They migrate toward brush border surface of enterocytes.

2. Cellular Stage
In this stage, digestion products of lipids pass through the membrane of enterocytes.

16.2  Absorption of Lipids 445

3. Transportation Stage
In this stage, lipids enter the lymph vessels and blood circulation.
Many theories have been proposed to explain the absorption of digestion end

products of lipids. Broadly, theories fall into two categories as complete hydrolysis
theory and partial hydrolysis theory of lipid absorption.

16.2.1 L ipolytic Theory

This theory was proposed by Pfluger, Verzer, and Mcdougall. Dietary lipids are
completely hydrolyzed into glycerol and free fatty acids. The free fatty acids are
precipitated into soaps. Water-soluble soaps can easily enter enterocytes.

16.2.2 Frazer’s Partition Theory

According to Frazer’s partition theory, dietary lipids are partially hydrolyzed into
monoglycerides, diglycerides, and free fatty acids. They form emulsion in the pres-
ence of bile salts. They can enter enterocytes.

16.2.3 Bergstrom Theory

It was proposed by Bergstrom in 1982. This latest theory explains the absorption of
digestion products of lipids. The theory has the following postulates:

1 . Unstirred Water Layer
• The brush border surface of intestinal epithelial cells (enterocytes) is covered
by a thick layer of water in the lumen of the intestine. This water layer is
called as “unstirred water layer” as in Fig. 16.4. This water layer is not
mixed with the fluid content of the rest of the intestinal lumen.
• Digestion products of lipids should cross this layer to reach brush border
surface of enterocytes.

2. Formation of Mixed Micelle
• “Hydrophilic Colloidal Aggregates” are called as micelle.
• Micelles are two types as “pure micelles” which are made up of bile salts only
and “mixed micelles” which contain bile salts and digestion end products as
in Figs. 16.2 and 16.3.
• Micelles have spherical shape with size between 3 and 6 nm. They are much
smaller than the size of fat drops which have the size between 1000 and
3000 nm.
• Formation of micelle starts when the concentration of bile salts in intestinal
lumen exceeds “critical micellar concentration” (CMC). It is the concentra-
tion of bile salts in lumen of the intestine above which formation of micelles
starts.

446 16  Digestion and Absorption of Lipids

Bile salts

Aquous medium

Hydrophilic head Phospholipid

Hydrophobic tail molecule

Fig. 16.2 Micelle Mixed micelle

Phospholipid Aquous
Monoglyceride medium

Long chain
Fatty acid
Bile salt

Cholesterol

Fig. 16.3  Mixed micelle

• Mixed micelle is composed of monoglycerides, diglycerides, cholesterol,
and long-chain fatty acids along with bile salts, calcium salts of fatty
acids, and bicarbonates as in Fig. 16.3.

16.2  Absorption of Lipids 447

• Mixed micelle has an external surface and internal core. External surface of mixed
micelle is made up of polar and hydrophilic heads of bile salts, and internal core
is made up of nonpolar and hydrophobic tails of bile salts. A mixed micelle con-
tains around 20–45 bile salt molecules. The external surface of micelle is soluble
in the fluid present in the lumen of the intestine. Its internal core is soluble in
monoglycerides, diglycerides, cholesterol, and long-chain fatty acids.

3 . Passive Diffusion of Digestion Products of Lipid
• Hydrophilic surfaces of micelles can easily pass through unstirred water
layer. They carry digested lipids to brush border surface of enterocytes. This
is the site of lipid absorption.
• Micelles align along the brush border surface of enterocytes. Digestion prod-
ucts of lipid are absorbed by passive diffusion through brush border surface of
enterocytes.
• Bile salts are dissociated from micelles. They are reabsorbed in the ileum
and enter hepatic portal vein to reach the liver. Again, bile salts are
secreted in bile and reach the duodenum. This is called enterohepatic
circulation as in Fig. 16.4.

Fat Drop

Bile sacts

Liver Unstirred Micelle
wate layer Lipase
Hepatic
portal Enterocyte
vein
SCFA
Smooth endoplasmic reticulum LCFA

Resynthesis Mono
T.G. glyceride

Glycerol

Blood Chylo Micelle
vessel microns

Lacteal Basolateral Nucleus Bile Apical
enterohepatic membrane salts membrane

circulation Reabsorbed Brush border
in ileum surface

SCFA - Short chain fatty acid - L.C.F.Acid
LCFA - Long chain fatty acid - Cholesterol
- Glycerol
- Bile salt

- Phospholipid

Fig. 16.4  Absorption of lipids in entreocyte

448 16  Digestion and Absorption of Lipids

4 . Resynthesis of Triglyceride, Cholesterol Ester, and Phospholipid Inside
Enterocytes
• Digestion products of lipid are transported to smooth endoplasmic reticulum
inside the enterocytes as in Fig. 16.4.
• Within smooth endoplasmic reticulum, long-chain fatty acids are activated
into fatty acyl CoA by acyl CoA synthetase enzyme. Two molecules of fatty
acyl CoA combine with monoglyceride and form triglyceride. This is called
as “monoacylglycerol pathway.”
• Lysophospholipid combines with long-chain fatty acid and is reconverted into
phospholipid by acyltransferase enzyme. Free cholesterol is again re-e­ sterified
into cholesterol ester by cholesterol acyltransferase enzyme.

5. Formation of Chylomicrons
• Chylomicrons are triglyceride-rich endogenous hydrophilic lipoprotein
particles. The word is derived from the Greek words “chylos” (juice or milky
fluid) and “micron” (very small particle).
• Triglycerides, cholesterol ester, and phospholipids aggregate together in the
presence of apolipoprotein in smooth endoplasmic reticulum to form
chylomicrons.
• Chylomicron has a central core and an external surface. The central core
contains nonpolar triglycerides and cholesterol ester molecules. They are cov-
ered by a single layer of polar phospholipid and cholesterol molecules. This
monolayer of phospholipids is associated with integral proteins called apoli-
poprotein B48.
• The size of the chylomicrons is between 30 and 60 nm. Chylomicron contains
around 88% triglycerides, 7% phospholipids, 3% cholesterol, and 2%
apo-B48.
• Chylomicrons are released into cisternae of Golgi bodies. They undergo mod-
ification by addition of apolipoprotein-A1 and glycosylation of apolipoporo-
tein-B­ 48. Chylomicrons are budded from Golgi bodies into cytosol of
enterocytes.

6. Transport of Chylomicrons
• From inside the enterocytes, chylomicrons are exported from the basolateral
surface of enterocytes by exocytosis. They move through interstitial spaces
enterocytes. Chylomicrons enter into lacteals present in intestinal villi.
• From lacteals, chylomicrons pass into lymph vessels.
• From lymph vessels, chylomicrons enter into the thoracic duct (also called as
left lymphatic duct, which arises at 12th thoracic vertebrae and drains lymph
and chyle (digested fats)).
• From the thoracic duct, chylomicrons pass into systemic circulation at the
level of the left subclavian vein. These chylomicrons are called as “nascent
chylomicrons.”

7 . Fate of Chylomicrons
• Nascent Chylomicrons. They enter the blood circulation and called as circu-
lating chylomicrons.

16.2  Absorption of Lipids 449

• Circulating Chylomicrons. They undergo delipidation in the blood circula-
tion. Lipoprotein lipase (LPL) brings about hydrolysis of triglycerides in chy-
lomicrons. The LPL is an enzyme located on the surface of endothelial cells
in capillaries. The released fatty acids are distributed to adipose tissues and
skeletal muscles where they are stored.

• Chylomicron Remnants. The remaining particles receive apolipoprotein
C-II and apolipoprotein E. The resultant particles are called as chylomicron
remnants.

• After a fat-rich diet, the concentration of chylomicrons in blood rises. The
plasma appears milky white due to chylomicrons.

• Chylomicron remnants are taken up by the liver from circulation. The triglyc-
erides of chylomicrons are hydrolyzed in the liver, and they are called as
“very low density lipoprotein (VLDL).” They are exported from the liver and
undergo delipidation by vascular lipoprotein lipase in circulation.

• VLDL are converted into LDL as in Fig. 16.4.

Absorption of SCF and MCF

• Length of fatty acid chain determines the absorption of fatty acids in the small
intestine.

• Short-chain fatty acids (<6C) and medium-chain fatty acids (6–12C) enter intes-
tinal epithelial cells (enterocytes) from lumen of the intestine.

• From the inside of the enterocytes, they directly enter the hepatic portal vein and
reach the liver.

• SCF and MCF are metabolized in liver cells. They are not stored in adipose
tissues.

• Absorption of SCF and MCF is independent of bile salts and micelles.

Absorption of Glycerol

• Glycerol is absorbed from the lumen of the small intestine. It passes into the
hepatic portal vein and enters the liver as in Fig. 16.4.

• Glycerol is converted into glycerol-3-phosphate in the liver. It can either enter
into glycolysis or is utilized in gluconeogenesis.

Absorption of Cholesterol

• Cholesterol in free form is absorbed from lumen of the intestine. It enters entero-
cytes and undergoes esterification to form cholesterol ester.

• It is incorporated into chylomicrons and enters lacteals.

450 16  Digestion and Absorption of Lipids

Absorption of Phospholipids

• Phospholipids are absorbed from lumen of the intestine as lysophospholipids.
They enter enterocytes and are restructured as phospholipids. They are incorpo-
rated into chylomicrons and are exported from enterocytes.

• Small amount of phospholipids are also absorbed from lumen of the intestine due
to their amphipathic nature.

16.3 C linical Significance

1 . Steatorrhoea
• It is a clinical condition characterized by excessive amount of fats in feces.
• Steatorrhoea is due to obstruction in the flow of bile, malignancy of pan-
creas, celiac disease, or tropical sprue (malabsorption syndrome in tropical
regions).
• Inadequate bile salts or pancreatic lipase results in impaired digestion of
dietary fats. They remain unabsorbed and are expelled from the intestine
along with stool.

2 . Chyluria
• It is a clinical condition characterized by excessive fats in the urine. It has a
milky appearance.
• The condition is due to an abnormal passage between the urinary tract and
lymphatic system of the intestine. This is called as “chylous fistula.”

Suggested Readings

Baron DN (1982) A short textbook of chemical pathology, 4th edn. Wiley, New York
Bender DA (2004) Introduction to nutrition and metabolism, 3rd edn. Taylor & Francis, Philadelphia
Ching K (2008) Fatty acids in foods and their health implication, 3rd edn. CRC Press, Boca Raton
Conn EE, Stump PK (1969) Outline of biochemistry, 2nd edn. Wiley, New Delhi
Hamosh M (1990) Lingual and gastric lipases. Nutrition 6(6):421–428 [Abstract]
Harper HA (1979) Review of physiological chemistry, 17th edn. Lange medical publisher,

New York
Iqbal J, Hussain MM (2009) Intestinal lipid absorption. Am J Physiol Endocrinol Metab

296:E1183–E1194
Kleiner IS, Orten JM (1966) Biochemistry, 7th edn. Mosby, St Louis
Murray RK, Granner DK, Mayes PA, Rodwell VW (1999) Harper’s biochemistry. Lange Medical

Publisher, New York
Murray RK, Granner DK, Mayes PA, Rodwell VW (2003) Harper’s illustrated biochemistry, 26th

edn. Lange Medical Books, New York
Nelson DL, Cox MM (2004) Principles of biochemistry, 4th edn. W.H. Freeman and Company,

New York
Oser BL (ed) (1965) Hawk’s: physiological chemistry, 14th edn. Mc-Graw Hill, New York
Rosenthal MD, Glew RH (2009) Medical biochemistry – human metabolism in health and disease.

Wiley, Hoboken, NJ

Metabolism of Lipids 17

17.1 I ntroduction

Lipids are stored in adipose tissues. Lipids are main reservoir of fuel for physiologi-
cal activities of human body. Stored lipids are always in a dynamic state. Lipids are
constantly decomposed and resynthesized depending on the calorie requirement,
age, metabolism, dietary intake, gender, and diseases. Tissue lipase hydrolyzes the
tissue triglycerides under the influence of hormones. Free fatty acids are released
into blood circulation. Under normal condition, serum free fatty acid concentration
varies between 6 and 15 mg/100 ml.

Sources of Free Fatty Acids in Plasma
Many sources liberate free fatty acids into blood circulation. These sources are as
follows:

Mobilization of Tissue Fat

• Tissue fat from adipose tissue constitutes the richest source of free fatty acids.
Tissue lipase degrades tissue fat and releases fatty acids.

Chylomicrons and Very Low Density Lipoprotein

• Chylomicrons and VLDL in blood circulation are decomposed by lipoprotein
lipase and release fatty acids.

Dietary Fats

• Triglycerides from diet are hydrolyzed by lipase in the lumen of the intestine.
Fatty acids are absorbed and enter blood circulation. Fatty acids in plasma are

© Springer Nature Singapore Pte Ltd. 2019 451
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_17

452 17  Metabolism of Lipids

distributed in bound form with albumin (albumin-FFA complex). Plasma has
nearly 30 mg/100 ml of free fatty acids in a postabsorptive state.

17.2 O xidation of Fatty Acids

Serum plasma concentration of fatty acids varies between 10 and 25 mg/100 ml in
postabsorptive state. Free fatty acids combine with albumin as albumin-FFA com-
plex. Fatty acids are distributed to body tissues in complexed state by blood
circulation.

Fatty acids have a high turnover rate. They have very short half-life of around
1–3 min. Their uptake by body tissues is rapid, and fatty acids are actively oxidized
within tissues. Hormones regulate plasma fatty acid concentration.

17.2.1 Types of Fatty Acids Oxidation

Alpha Oxidation
Definition

• Alpha oxidation of long-chain fatty acids is the oxidation at alpha carbon atom
of fatty acid that results in the removal of one carbon atom (carbon atom next to
carboxylic group called as alpha carbon) from fatty acid.

Site of Occurrence

• Alpha oxidation of long-chain fatty acids takes place in brain cells.

Mechanism

• It is the main mechanism for oxidation of phytanic acid. It is found in milk
and milk products and animal fat. Phytanic acid is also formed from plant-based
food sources. Chlorophyll in plants is hydrolyzed into phytol in human body.
Phytol is oxidized into phytanic acid.

• Long-chain fatty acid undergoes hydroxylation by alpha-hydroxylase enzyme to
form 2-hydroxy fatty acid. It undergoes oxidative decarboxylation to form long-­
chain fatty acid with one carbon atom shorter than parental fatty acid.

• Reactions occur in the presence of molecular oxygen and NADPH2.

Significance

• Alpha oxidation in brain tissues results in formation of hydroxy fatty acids like
cerebronic acid. It is structural component of brain cerebrosides.

• It serves to synthesize odd-chain fatty acids in brain tissues. They are essential
for synthesis of sphingomyelin.

17.2  Oxidation of Fatty Acids 453

Omega Oxidation
Definition

• It is the oxidation of medium-chain fatty acid at omega carbon atom (last carbon
atom of methyl group in hydrophobic side chain distant from COOH group).

Site of Occurrence

• Omega oxidation occurs in smooth endoplasmic reticulum in the liver and
kidneys.

• It also occurs in bacteria. It is an alternate metabolic pathway to beta oxidation.

Mechanism

• Omega oxidation of fatty acid was proposed by Verkade and colleagues.
• Medium-chain fatty acid undergoes hydroxylation at omega carbon atom to form

omega hydroxyl fatty acid. Reaction is catalyzed by omega hydroxylase enzyme.
• Oxidation of hydroxyl group at omega carbon atom takes place by alcohol dehy-

drogenase enzyme in the presence of NAD+ coenzyme which forms aldehyde
which again undergoes oxidation to form carboxylic group. Medium-chain fatty
acid is converted into dicarboxylic acid.
• Dicarboxylic acid can enter beta oxidation at both ends.

B eta Oxidation
Definition

Beta oxidation of fatty acid is the biochemical process involving oxidation at beta

carbon atom (second carbon atom from carboxylic group) of fatty acid chain that

results in cleavage of two carbon residue (acetyl CoA) from fatty acid chain.

Historical Aspect

• Beta oxidation of fatty acids was proposed by Franz Knoop in 1904.
• Knoop attached phenyl residue to omega carbon of fatty acids. Dogs were fed

diet rich in such fatty acids. He observed phenyl derivative of fatty acids in the
urine of dogs.
• Knoop fed diet rich in phenyl propionate (odd carbon atoms) to dogs and detected
excretion of hippuric acid (benzoic acid conjugation with glycine) in urine.
While, he fed diet rich in phenylbutyrate (even carbon atoms) to dogs and
detected excretion of phenylacetate in urine.
• On the basis of above observations, Knoop proposed beta oxidation of fatty
acids.

454 17  Metabolism of Lipids

Site of Occurrence

• Fatty acids undergo beta oxidation in the liver, kidneys, cardiac muscles, adipose
tissues, lungs, and gonads.

• Brain tissues, RBCs, and adrenal medulla cannot oxidize fatty acids.

Enzymes

• Initial step in beta oxidation occurs in cytosol as the enzymes are located in
cytosol.

• Beta oxidation proper occurs in mitochondrial matrix as enzymes are located in
mitochondrial matrix.

Steps in β-Oxidation of Fatty Acids
Beta oxidation of fatty acids is carried in three distinct steps:

1 . Activation of Fatty Acids (Cytosol)
2 . Transport of Activated Fatty Acids into Mitochondria
3 . Beta Oxidation Proper (Mitochondria)

Activation of Fatty Acids
Activation of fatty acids involves two biochemical reactions in stages. Process
requires a molecule of ATP, coenzyme A, and Mg++ ions as in Fig. 17.1.

• In the first stage, fatty acid interacts with ATP to form acyladenylate.
• In the second stage, acyladenylate combines with coenzyme A and forms Acyl CoA.
• Reaction is catalyzed by Acyl CoA synthetase (thiokinase).

Activation process consumes two high-energy phosphate bonds. ATP is con-
verted into pyrophosphate which is hydrolyzed by inorganic pyrophosphatase to
form phosphate.

R–CH2–CH2–CH2–CH2– COOH Long chain fatty acid

ATP CoA∼SH
Acyl CoA synthetase
2Pi Pyrophosphatase PPI + AMP Mg++
H2O

R–CH2–CH2–CH2–CH2– CO∼S. CoA
Acyl CoA

Fig. 17.1  Activation of fatty acid

17.2  Oxidation of Fatty Acids 455

• Activation of fatty acids occurs in cytosol.
• Activated fatty acid is called Acyl CoA.

Types of Acyl CoA Synthetases
1. Acetyl CoA Synthetase

• It is a short-chain fatty acid activation-catalyzing enzyme.
• It catalyzes (C2) acetic acid and butyric acid (C4).
2 . Medium-Chain Synthetase
• It is a medium-chain fatty acid activation-catalyzing enzyme.
• It catalyzes C4–C12 fatty acids.
3 . Long-Chain Synthetase
• It is a long-chain fatty acid activation-catalyzing enzyme.
• It catalyzes C12–C22 fatty acids.

Transport of Activated Fatty Acids into Mitochondria
Activated fatty acids, Acyl CoA, require transportation across mitochondrial mem-
branes for beta oxidation proper as in Fig. 17.2.

Inner membrane of mitochondria is impermeable to Acyl CoA. Its transport is
carried by a carrier molecule. It is called as carnitine.

Carnitine

• Chemically, it is β-hydroxy-γ-trimethyl aminobutyrate.
• Carnitine is found in the milk, meat, liver, skeletal muscles, and yeast.
• Carnitine is made up of two amino acids, namely, lysine and methionine.
• It is synthesized in the liver and kidneys.

Mechanism of Transportation

• Acyl CoA reacts with carnitine on the outer surface of the inner mitochondrial
membrane and forms acyl-carnitine. Reaction is catalyzed by carnitine acyltrans-
ferase I (CAT I). CAT-I is located on the outer surface of the inner mitochondrial
membrane.

• Acyl-carnitine is translocated across the inner mitochondrial membrane. It enters
the mitochondrial matrix.

• Inner surface of inner mitochondrial membrane contains carnitine acyltransfer-
ase II (CAT-II). This enzyme catalyzes acyl-carnitine into Acyl CoA and carni-
tine. This step requires coenzyme A.

• Carnitine is released which enters cytosol for reuse as a carrier.
• Two pools of coenzyme A in cells

456 17  Metabolism of Lipids

Cytoplasm

Long chain fatty acid

ACYL-CoA CoA∼SH
Syntetase
ATP

Mg+A+ MP+PPI

ACYL-CoA R–CH2–CH2–CH2–CH2–Co∼S.CoA

Outermmeitmocbhraonnedrial– ACYL carnitine

CH2+ γ β α
H3C–N–CH2–CH–CH2–COOH

CH3 OH

Carnitine + ACYL-CoA

Carnitine acy transferase I

Cytosolic size of CAT I
inner membrane

Translocase

CAT II

Matrix Carnitine
Acyl
size of transferase
membrane Carnitine Acyl Carnitine
II CAT II

Acyl CoA CoA∼SH

Mitochondria
Matrix

β−oxidation of
acyl CoA

Fig. 17.2  Transport of Acyl CoA with carnitine

17.2  Oxidation of Fatty Acids 457

Cytosol coenzyme A
–– This coenzyme is helpful in activation of fatty acids in cytosol. It is a prepara-

tory phase before transport into mitochondria.
Mitochondrial coenzyme A
–– This coenzyme is helpful in activation of fatty acids in mitochondrial matrix.

It is a preparatory phase before beta oxidation proper.

β-Oxidation Proper
It occurs in sequential cycles. Each cycle of oxidation liberates two carbon residues
from fatty acids (acetyl CoA). This phase is described in the following four steps:

1 . Oxidation
Acyl CoA is oxidized into α,β-unsaturated acyl CoA. It has double bond between
α and β carbon atoms. Reaction is catalyzed by acyl CoA dehydrogenase enzyme.
Reaction requires coenzyme FAD which accepts hydrogen atoms and is reduced
into FADH2. It is transported across ETS chain to produce 2 ATP molecules.

2. Hydration
α,β-unsaturated acyl CoA undergoes hydration to form β-hydroxyacyl
CoA. Reaction is catalyzed by enoyl CoA hydratase.

3. Oxidation
Enoyl CoA hydratase undergoes oxidation to form β-keto acyl CoA. Reaction is
catalyzed by β-hydroxyacyl CoA dehydrogenase enzyme.

4. Cleavage
β-keto acyl CoA undergoes splitting to form acetyl CoA with release of new acyl
CoA which is two carbon atoms shorter than parental acyl CoA.  Reaction is
catalyzed by β-keto acyl CoA thiolase.
The new acyl CoA reenters beta oxidation cycle to produce another acetyl CoA,
and the process continues till complete oxidation of participating fatty acid as in
Fig. 17.3.

Energetic in Beta Oxidation
Oxidation of Even Carbon Chain Fatty Acids

Even chain fatty acid like palmitic acid (C15H31COOH) undergoes beta oxidation
and yields the following important facts:

• Seven cycles of beta oxidation for palmitic acid

–– In each cycle, five ATP molecules are formed from oxidation of NADH2 and
FPH2 in ETC.

–– Total production is 35 ATP molecules in 7 cycles (5 × 7 cycles).
• Production of eight Acetyl CoA residues in beta oxidation of palmitic acid

–– One Acetyl CoA residue enters TCA cycle and produces 12 ATP.
–– A total of 96 ATP molecules are produced (12 × 8 Acetyl CoA residues).

458 17  Metabolism of Lipids

γ β αO
R–CH2–CH2–CH2–CH2 C~S–CoA

Acyl – CoA

FP Acyl CoA
FP.H2 Dehydrogenase

β αO
R–CH2–CH2–CH–CH–C~S–CoA

α, β – Unsaturated
Acyl CoA

Enoyl H2O
CoA
hydratase

OH O

R–CH2–CH2–CH–CH2–C~S–CoA
βα

β – Hydroxy acyl CoA

β – OH – Acy NAD+
CoA NADH+ H+

Dehydrogenase

OO
R–CH2–CH2–C– CH2–C~S–CoA

βα
β – Keto acyl CoA

Thiolase CoA~SH

TCA O O
CYCLE CH3–C~SO–CoA R–CH2–CH2–CH2–C~S–CoA

Acetyl CoA Acyl CoA

[Shorter by 2c]

CO2 H2O ATP
Fig. 17.3  β Oxidation of fatty acids

Aggregate production is 131 ATP molecules (35 + 96 ATP).

Consumption of ATP in activation of fatty acid = 2 ATP.
Net Gain = 129 ATP (131 − 2 ATP) through beta oxidation of palmitic acid.

17.3  Biosynthesis of Cholesterol 459

Oxidation of Odd Carbon Chain Fatty Acids

• Beta oxidation of odd carbon chain fatty acid occurs through similar steps as

even carbon chain fatty acids.

• In the last cycle of beta oxidation, a three-carbon residue is produced unlike a
two-carbon residue in even carbon chain fatty acids. The three-carbon residue
is called as propionyl CoA.

Fate of Propionyl CoA

• Propionyl CoA undergoes carboxylation to form D-methyl malonyl
CoA. Reaction is catalyzed by propionyl CoA carboxylase in the presence of a
molecule of CO2, ATP, and biotin as coenzyme.

• D-methyl malonyl CoA is changed into L-methyl malonyl CoA by enzyme
methyl malonyl CoA epimerase.

• L-methyl malonyl CoA is converted into succinyl CoA by enzyme methyl malo-
nyl CoA mutase in the presence of vitamin B12. In the deficiency of vitamin
B12, L-methyl malonyl CoA is converted into methyl malonic acid which accu-
mulates in body, and it is excreted in urine. Its detection in urine has a diagnostic
value in the screening of deficiency of vitamin B12.

• Succinyl CoA enters TCA cycle.

Metabolism of Cholesterol
Cholesterol is a steroid and is present in animals. It is called as animal sterol. A
healthy person has nearly 2 g of cholesterol per kg of body weight (140 g in 70 kg
weight).

17.3 B iosynthesis of Cholesterol

Site of Occurrence

• Cholesterol is synthesized in most of the body tissues.
• The liver, adrenal cortex, gonads, and skin are actively involved in the synthesis

of cholesterol.
• Brain tissues of infants synthesize cholesterol, while, in adults, synthesis of cho-

lesterol is absent in brain tissues.

Enzymes

• Enzymes for cholesterol biosynthesis are located in cytosol and microsomes.

460 17  Metabolism of Lipids

17.3.1 Steps in Biosynthesis

Cholesterol biosynthesis can be explained in the following steps:

1 . Formation of HMG CoA
• Acetyl CoA (CH3COS.CoA) is the precursor of cholesterol. The Acetate
(CH3COO) from acetyl COA constitute carbon skeleton of cholesterol (27C).
• Two molecules of acetyl CoA (2C) condense together to form acetoacetate
(4C). It further condenses with one molecule of acetyl CoA to form β-hydroxy
β-methylglutaryl CoA (6C).
• Reaction is catalyzed by HMG CoA synthase (cytosolic enzyme) as in
Fig. 17.4.

2 . Formation of Mevalonate
• β-Hydroxy β-methylglutaryl CoA (6C) is converted into mevalonate (6C).
• Reaction is catalyzed by HMG CoA Reductase. It is a regulatory enzyme
and it is a rate-limiting step in cholesterol biosynthesis. Enzyme is present
in endoplasmic reticulum.
• NADPH (reducing equivalents) provides hydrogen atom for reduction and it
is oxidized into NADP.

3. Formation of Isoprenoid (5C) Units
• Mevalonate undergoes phosphorylation to form 3-phospho
5-pyrophosphomevalonate.
• Reaction is catalyzed by kinase enzyme in the presence of ATP.
• 3-Phospho 5-pyrophosphomevalonate (6C) is unstable and undergoes decar-
boxylation to form isopentenyl pyrophosphate (5C).
• Isopentenyl pyrophosphate (IPP) isomerizes into dimethyl allyl pyrophos-
phate (6C) (DPP) as in Fig. 17.4.

4 . Formation of Squalene
• One molecule of isopentenyl pyrophosphate condenses with one molecule of
dimethyl allyl pyrophosphate to form geranyl pyrophosphate (10C). Reaction
is catalyzed by geranyl pyrophosphate synthase.
• One molecule of isopentenyl pyrophosphate condenses with geranyl pyro-
phosphate to form farnesyl pyrophosphate (15C). Reaction is catalyzed by
farnesyl pyrophosphate synthetase.
• Two molecules of farnesyl pyrophosphate condense together to form squa-
lene (30C). Reaction is catalyzed by squalene synthetase as in Fig. 17.4.

5 . Conversion of Squalene into Cholesterol
Squalene is converted into cholesterol in two separate reactions which are
described as follows:
• Squalene conversion into lanosterol
–– Squalene undergoes hydroxylation to form squalene-2,3-epoxide. Reaction
is catalyzed by squalene monoxygenase. Process requires NADPH and
oxygen molecule.
–– Squalene-2,3-epoxide undergoes cyclization into lanosterol. Reaction is
catalyzed by cyclase enzyme.

17.3  Biosynthesis of Cholesterol 461

O O
CH3–C∼S–CoA + CH3–C∼S–CoA
Acetyl CoA (2c) Acetyl CoA (2c)

Thiolase CoA∼SH

OO
CH3–C–CH2–C∼S–CoA

Acetoacetyl-CoA (4c)

H2O HMG-CoA
Acetyl-CoA Synthase

β - Hydroxy - β - Methyl glutaryl CoA
HMG - CoA (6c)

2 NADPH2 HMG - CoA
2 NADP+ Reductase

CoA∼SH

OH H

CH3–C–C–CH2OH
H

H2–C–COOH

Mevalonate (6c)

ATP Kinase
Mg++

ADP

Mevalonate-5-phosphate
ATP Phosphomevalonate
kinase

ADP

Mevalonate-5-pyrophosphate

ATP Kinase
Mg++

CO2 H2O Pi ADP

ISO-pentenyl Mevalonate-3-phospho-5pyrophosphate
pyrophosphate Decarboxylase

(5c)

Fig. 17.4  Biosynthesis of Cholesterol

462 17  Metabolism of Lipids
Isopentenyl 5c Pyrophosphate
Geranyl-Pyrophosphate
Synthetase Isomerase
3,3 Dimethyl allyl-Pyrophosphate (5c)

PPI

Geranyl-Pyrophosphate (10c)

Isopentenyl Farnesyl-Pyrophosphate Synthetase
Pyrophosphate (5c)

PPI

Farnesyl-Pyrophosphate (15c)

Condensation Squalene Synthetase NADPH2
2PPI Mg++
Farnesy
Pyrophosphate (15c) NADP+

Squalene (30 C)
NADPH2

Squalene monoxygenase

NADP+ O2

Squalene – 2,3 – epoxide
Cyclase

Lanosterol
(C14) Methyl group

14 – Desmethyl lanosterol
2 CH3 groups (C4)

Fig. 17.4 (continued) 

17.3  Biosynthesis of Cholesterol 463
Mosterol

Cholestadienol

NADPH2

O2

NADP+

Desmosterol (24-Dehydrocholesterol)

NADPH2 Reductase

NADP+
Cholesterol

Fig. 17.4 (continued) 

• Lanosterol conversion into cholesterol
This conversion follows enzyme-controlled series of biochemical reactions.
Important reactions have been mentioned as follows:

–– Demethylation of three methyl groups from lanosterol. This step decreases
carbon atoms from 30C to 27C.

–– Double bond between C8 and C9 position is shifted to new position between
C5 and C6.

–– Double bond between C24 and C25 is saturated.

These reactions are catalyzed by enzymes which are located in endoplasmic
reticulum of cells as in Fig. 17.4.

17.3.2 R egulation of Cholesterol Biosynthesis

HMG CoA Reductase Enzyme

• HMG CoA reductase is the key regulatory enzyme for synthesis of cholesterol in
body tissues. Activity of HMG CoA reductase in turn is controlled at the fol-
lowing stages:
–– Gene expression
Cholesterol controls gene expression of HMG CoA reductase. Normal serum
cholesterol concentration is between 140 and 200  mg/dl. In condition of
hypercholesterolemia, cholesterol itself inhibits gene expression and synthe-
sis of mRNA. Therefore, synthesis of HMG CoA reductase is reduced, and
cholesterol synthesis is minimized.
–– Cholesterol feedback inhibition
↑ cholesterol concentration in serum serves as a negative feedback and inhib-
its HMG CoA reductase.

464 17  Metabolism of Lipids

–– Proteolysis of enzyme
HMG CoA reductase contains sterol-sensing domain (a segment of 180
amino acid residues that bind with sterol group). Cholesterol promotes
proteolysis of HMG CoA reductase enzyme.

–– Phosphorylation-dephosphorylation mechanism
Cyclic AMP-activated protein kinase catalyzes phosphorylation of HMG
CoA reductase and its enzymatic activity is reduced, while dephosphorylation
of HMG CoA reductase increases its activity.

Hormones

• Insulin and thyroxine hormones increase cholesterol synthesis.
• Glucagon and cortisol decrease cholesterol synthesis.

Role of Diet

• Diet rich in saturated fatty acids promotes synthesis of cholesterol.
• Diet rich in PUFA decreases cholesterol synthesis and serum cholesterol level.

PUFA possibly stimulates cholesterol oxidation into bile acids and their excre-
tion through the intestine.
• Fiber-rich diet decreases cholesterol synthesis and controls serum cholesterol level.
• Diet rich in glucose, sucrose, and fructose stimulates cholesterol synthesis.

Heredity

• Heredity is a predisposing factor in hypercholesterolemia and hyperlipidemia.

17.4 B iodegradation of Cholesterol

Cholesterol is converted into bile acids, steroidal hormones, and calcitriol. It cannot
be decomposed into CO2 and water. Biodegradation of cholesterol occurs in the fol-
lowing ways:

Formation of Bile Acids
Bile acids are formed in the liver. They serve as emulsifying agents and help in
digestion of dietary triglycerides. Bile acids contain polar and non-polar groups
(amphipathic). Bile acids are of two types as follows:

Steps in Bile Acid Synthesis

• Cholesterol undergoes hydroxylation in the liver to form 7-hydroxycholesterol.
• Reaction is catalyzed by 7-alpha-hydroxylase enzyme in the presence of

NADPH2 and molecular oxygen.
• The 7-hydroxycholesterol undergoes series of enzymatic reactions to form cho-

lic acid and chenodeoxycholic acid. They are primary bile acids which undergo
the following changes as described below:

17.5  Ketogenesis 465

Primary Bile Acids

• Cholic acid
• Chenodeoxycholic acid

Cholic acid predominates in bile. It undergoes conjugation with glycine and tau-
rine amino acids to form glycocholic acid and taurocholic acid. Conjugated primary
bile acids have better emulsifying activity. These conjugated acids exist in bile in
the form of salts of sodium or potassium as:

Sodium glycocholate and sodium taurocholate are found in bile acids.
Secondary bile acids are formed from deconjugation of primary bile acids by
intestinal bacteria.
Secondary bile acids

• Deoxycholic acid
• Lithocholic acid

Formation of Steroidal Hormones
Cholesterol is a precursor molecule for the synthesis of mineralocorticoids, gluco-
corticoids, androgens, progesterone and estrogens.

Formation of Cholecalciferol and Calcitriol
7-Dehydro-cholesterol is a precursor for synthesis of cholecalciferol which is
hydroxyalted in the liver and kidneys to form calcitriol.

17.5 K etogenesis

Definition
Ketogenesis is a metabolic process in which a group of organic compounds are
synthesized in body.

Types of Ketone Bodies
Ketone bodies are of three types:

• Acetone
• Acetoacetate
• Β-hydroxy butyrate

Site of Occurrence and Enzymes

• Ketogenesis occurs in the liver.
• Enzymes catalyzing reactions are located in mitochondrial matrix of hepatocytes.

466 17  Metabolism of Lipids

17.5.1 Steps in Ketogenesis

Process occurs through enzyme-controlled biochemical reactions. These are
explained as follows:

• Formation of Acetoacetyl CoA
–– Two molecules of acetyl CoA undergo condensation to form acetoacetyl CoA
–– Reaction is catalyzed by thiolase enzyme. A molecule of CoA-SH is released
in reaction as in Fig. 17.5.

OO
CH3 – C – CH2 – C – S.CoA

Acetoacetyl-CoA

CoA–SH H2O
O
CH3 – C ∼ S.CoA HMG-CoA
Acetyl-CoA Synthase

OH H O

HO –C –CH2 – C – C ∼C ∼ S – CoA
O CH3 H

β - Hydroxy - β - Methyl glutaryl CoA
[HMG - CoA]

HMG - CoA
Lyase

ACETYL-CoA

β-hydroxy butyrate Acetoacetate Ketone body
dehydrogenase H Ketone body

NAD+ NADH+H+ CH3–C–C C O
O H OH
OH Ketone body Spontaneous
CH3–C–CH2–COOH decarboxylation
CO2
H
CH3–C–CH3
β-hydroxy butyric O
acid
Acetone

Fig. 17.5  Ketogenesis

17.5  Ketogenesis 467

• Formation of β-Hydroxy β-Methylglutaryl CoA
–– Acetoacetyl CoA condenses with a molecule of acetyl CoA to form β-hydroxy
β-methylglutaryl CoA.
–– Reaction is catalyzed by HMG CoA synthase (mitochondrial enzyme). A

molecule of CoA-SH is released.

–– HMG CoA synthase is key regulatory enzyme.
• Formation of Acetoacetate

–– β-Hydroxy β-methylglutaryl CoA undergoes cleavage to form a molecule of
acetoacetate with the release of a molecule of acetyl CoA.

–– Reaction is catalyzed by HMG CoA lyase.
• Formation of Acetone

–– Acetoacetate can undergo nonenzymatic spontaneous decarboxylation to

form acetone as in Fig. 17.5.
• Formation of β-Hydroxy Butyric Acid

–– Acetoacetate is converted into β-hydroxy butyric acid by β-hydroxy butyrate
dehydrogenase. Reaction occurs in the presence of NADH2 which supplies
hydrogen atom. It is oxidized into NAD+.

–– β-Hydroxy butyric acid is the dominant ketone body in plasma and urine
during ketosis.

17.5.2 B iological Significance of Ketone Bodies

Source of Energy for Extrahepatic Tissues

• Ketone bodies are transported from the liver to the body tissues by blood circula-
tion. Acetoacetate and β-hydroxy butyric acid are chiefly utilized by heart mus-
cles, skeletal muscles, and renal cortex to generate energy.

• In adverse conditions like starvation, ketone bodies are the source of energy for
extrahepatic tissues due to deficient supply of glucose.

• In diabetes mellitus, glucose uptake by peripheral tissues is reduced due to defi-
ciency of insulin. Peripheral tissues utilize ketone bodies.

• In starvation, brain tissues are primarily dependent on ketone bodies for supply
of energy.

Steps for Ketolysis
Ketone bodies decomposed and are utilized by extrahepatic tissues through the
following reactions:

• β-Hydroxy butyric acid
β-Hydroxy butyric acid undergoes oxidation to form acetoacetate by
β-hydroxy butyrate dehydrogenase. Reaction requires NAD+.

• Acetoacetate
Acetoacetate interacts with succinyl CoA in the presence of enzyme CoA trans-
ferase (thiophorase) to form acetoacetyl CoA and succinic acid.

468 17  Metabolism of Lipids

• Acetone
Acetone is not oxidized in living tissues for energy. Excessive accumulation
of acetone produces a fruity smell in breath and urine.
The liver lacks CoA transferase enzyme. Therefore, the liver cannot utilize
ketone bodies.

17.6 Ketosis

Definition
It is a clinical condition characterized by excessive synthesis and accumulation
of ketone bodies in blood circulation.

Under normal health condition, a state of equilibrium exists between the
synthesis of ketone bodies in the liver and their utilization in extrahepatic tis-
sues of body.

Normal plasma ketone bodies level is nearly 1 mg/100 ml. Ketone bodies are
present in urine in undetectable concentration.

Predisposing Factors for Ketosis
Diabetes Mellitus

• It is an endocrine disorder characterized by decreased synthesis of insulin and
insulin resistance among peripheral tissues of body.

• Insulin regulates the uptake and utilization of glucose by extrahepatic tissues.
In the presence of insulin resistance and insulin deficiency, glucose is not
utilized by body tissues for energy production. Glucose becomes surplus in
blood circulation. Alternately, tissue lipids are mobilized to provide energy to
body. It leads to excessive accumulation of free fatty acids and acetyl CoA. The
vicious cycle terminates into excessive production of ketone bodies
(ketosis).

Starvation

• It is a condition in which an individual is severely deprived from intake of food
for a prolonged period. There is an acute deficiency of calories in the body for the
maintenance of basal metabolic rate. Starvation is an acute and a severe form of
malnutrition. In its initial stage, glycogen store of the body is utilized to provide
energy to the body. In the next stage, tissue lipids from adipose tissues are
decomposed to release fatty acids.

• There is excessive concentration of free fatty acids in circulation. Free fatty acids
are oxidized to provide energy to body, and it leads to the accumulation of acetyl
CoA in blood circulation.

• Excessive acetyl CoA cannot be utilized in citric acid cycle. They are converted
into surplus amount of ketone bodies (ketosis).

17.7  Lipoproteins 469

Pathophysiology

Certain conditions predispose to excessive synthesis of ketone bodies in the liver.

Their utilization does not increase proportionately in extrahepatic tissues.
It results in excessive accumulation of ketone bodies in plasma and it is called

as ketonemia. There is an increase in excretion of ketone bodies by kidneys in
urine and is called as ketonuria.

Ketoacidosis is a metabolic disorder characterized by excessive accumulation of

keto acids in blood circulation owing to excessive synthesis of ketone bodies.

Due to uncontrolled production of ketone bodies and their limited utilization,

there is excessive production of keto acids in body. The buffer systems of body are

unable to neutralize keto acids. There is rise in plasma concentration of keto acids

and it leads to fall in pH of blood. It is called as ketoacidosis.

Ketoacidosis is common consequence of uncontrolled diabetes mellitus and is

labeled as diabetic ketoacidosis. It is manifested as a fruity smell (acetone) from the

mouth of patient.
Ketosis and ketoacidosis are clinical conditions owing to excessive produc-

tion of ketone bodies. However, ketosis leads to ketoacidosis which is a life-­
threatening condition that requires immediate medical intervention to save the
life of patient.

17.7 L ipoproteins

Definition
Lipoproteins are conjugated proteins that are composed of protein molecules
complexed with lipid moieties.

Lipoprotein molecules are carriers of lipid molecules. They serve as transport
vehicles and carry lipid molecules from the intestine to the liver and distribute lipids
to body tissues through blood circulation.

17.7.1 Classification of Lipoproteins

Lipoproteins are divided into five categories based on electrophoresis property as
follows:

C hylomicrons
Characteristics

• Chylomicrons are synthesized inside enterocytes.
• They have a density of <0.96.
• They have a diameter of 100–1000 nm.

470 17  Metabolism of Lipids

Composition

• Chylomicrons contain about 2% of proteins.
• Chylomicrons contain approximately 98% of total lipids.

–– Triglycerides constitute 85–88% of total lipids of chylomicron.
–– Free cholesterol and cholesterol ester constitute 4% of the total lipids of

chylomicrons.
–– Phospholipids represent about 7–8% of the total lipids in chylomicrons.
–– Free fatty acids are absent in chylomicrons.
–– Chylomicrons contain least amount of proteins (2%) and the maximum

amount of total lipids (98%).
• Chylomicrons contain B48 apoprotein molecule.

Functions

• Chylomicrons are the lightest in weight (least density) and have the largest size
among lipoproteins.

• Chylomicrons serve to transport exogenous triglycerides (dietary) from the small
intestine to the liver and body tissues.

V ery Low Density Lipoproteins (VLDL)
Characteristics

• Very low density lipoproteins VLDL are synthesized in the liver.
• They have a density of 0.96–1.006.
• They have a diameter of 30–90 nm.
• Normal serum VLDL concentration should be 2–25 mg/100 ml.

Composition

• Proteins constitute 10% of the total weight of VLDL.
• VLDL contain 90% of the total lipids.

–– Triglycerides constitute about 55% of the total lipids in VLDL.
–– Free cholesterol and cholesterol ester constitute 20–24% of the total lipids in

VLDL.
–– Phospholipids represent about 20% of the total lipids in VLDL.
–– Free fatty acids are present in negligible amount (1%) in VLDL.
• VLDL contain B100 apoprotein molecule.

Functions

• Very low density lipoproteins serve to transport endogenous triglycerides from
the liver to body tissues.

17.7  Lipoproteins 471

L ow Density Lipoproteins
Characteristics

• Low density lipoproteins are synthesized from VLDL in the blood circulation.

• They have a density of 1.006–1.063.

• They have a diameter of 20 nm.
• Normal serum LDL should be <100 mg/100 ml.
• LDL are considered as bad cholesterol. It is implicated in pathogenesis and

progression of atherosclerosis and coronary artery disease.

Composition

• Proteins constitute about 20% of the weight of LDL.
• LDL contain 80% of total lipids.

–– Triglycerides represent about 10–12% of the total lipids in LDL.
–– Free cholesterol and cholesterol ester constitute 60% of the total lipids in

LDL.
–– Phospholipids represent 25–30% of the total lipids in LDL.
–– Proportion of free fatty acids is negligible (1%) in LDL.
• LDL contain B100 apoprotein molecule.

Functions

• Low density lipoproteins serve to transport cholesterol from the liver to body
tissues.

H igh Density Lipoproteins
Characteristics

• High density lipoproteins are synthesized in the liver.

• They have a density between 1.063 and 1.20.

• They have a diameter of 10–20 nm.
• Normal serum HDL concentration is between 40 and 70 mg/100 ml. HDL is

considered as a good cholesterol.

Composition

• Proteins constitute about 40% of the HDL.
• HDL contain about 60% of the total lipids in HDL.

–– Triglycerides constitute 10–12% of the total lipids in HDL.

–– Free cholesterol and cholesterol ester constitute 40% of the total lipids.

–– Phospholipids represent about 45% of the total lipids in HDL.

–– Free fatty acids are present in negligible amount (1%) in HDL.
–– HDL contain the maximum amount of proteins (40%) and least amount

of total lipids (60%).

472 17  Metabolism of Lipids

Functions

• HDL serve to transport endogenous cholesterol from peripheral tissues to the
liver.

Structure of Lipoprotein

• It is composed of a central core of triglycerides and cholesterol ester.
• Central core is surrounded by a layer of apoproteins molecules, cholesterol, and

phospholipid molecules.
• Phospholipids and cholesterol are amphiphilic molecules. Their polar heads are

directed outward toward aqueous medium. Their nonpolar tails are oriented
toward the interior of the lipoprotein molecule.
• Lipoprotein molecules are soluble in aqueous medium in blood circulation and
body tissues.

Suggested Readings

Alberti KGMN (ed) (1978) Recent advances in clinical biochemistry. Churchil Livingstone,
London

Baron DN (1982) A short textbook of chemical pathology, 4th edn. Wiley, New York
Conn EE, Stump PK (1969) Outline of biochemistry, 2nd edn. Wiley, New Delhi
Harper HA (1979) Review of physiological chemistry, 17th edn. Lange Medical Publisher,

New York
Kleiner IS, Orten JM (1966) Biochemistry, 7th edn. Mosby, St Louis
Latner AL (1975) Cantarow and Trumper. Clinical biochemistry, 7th edn. Saunders, Philadelphia
Murray RK, Granner DK, Mayes PA, Rodwell VW (1999) Harper’s biochemistry. Lange Medical

Publisher, New York
Murray RK, Granner DK, Mayes PA, Rodwell VW (2003) Harper’s illustrated biochemistry, 26th

edn. Lange Medical Books, New York
Oser BL (ed) (1965) Hawk’s: physiological chemistry, 14th edn. Mc-Graw Hill, New York

Metabolism of Minerals 18

18.1 C alcium

Calcium is the most significant mineral of hard tissues of the body. It is distributed
in various foods. The human body requires calcium for strength of bones and teeth.
It is also essential for many physiological functions. Calcium is necessary for con-
tractility of muscles and nerve conduction. It is essential cofactor in activity of
enzymes and hormones.

Distribution of Calcium in the Body
An adult person of 70 kg body weight contains around 1000 g calcium. It represents
around 1.5% of the total weight of the body. Endoskeleton of the body contains
around 990 g calcium. It amounts to nearly 99% of the total calcium content of the
body. Calcium in bones exists as complexes of phosphate and carbonate with cal-
cium. Skeletal calcium serves two functions. It is a labile pool of calcium to main-
tain intracellular and extracellular levels of calcium. Skeletal calcium provides
bone strength.

Non-skeletal tissues contain approximately 10g calcium which represents
nearly 1% of the total calcium content of the body. Soft tissues possess 0.5% cal-
cium, and extracellular compartment has 0.1% calcium. Normal plasma calcium
level is between 9 and 11 mg/100 ml.

Dietary Sources
Animal Sources
• A rich source is yogurt (dairy edible obtained by bacterial fermentation of

milk).
• Good sources are milk, cheese, sardine, salmon, and egg yolk.

Plant Sources
• Good sources are cereals, lentils, nuts, turnip, and cabbage.

© Springer Nature Singapore Pte Ltd. 2019 473
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_18

474 18  Metabolism of Minerals

Recommended Dietary Allowance
Adults
• RDA for adults is about 800 mg per day.

Children
• RDA for children is 1.2 g per day.

In Pregnancy and Lactation
• RDA in pregnancy and lactation is 1.5 g per day.

Forms of Calcium in Plasma
Calcium in plasma exists in three forms as:
• Ionized Calcium

–– Ionized calcium is about 51% of total non-skeletal calcium (>5 g).
–– Ionized calcium is metabolically active, and it is involved in physiological

and metabolic functions.
• Protein-Bound Calcium

–– Protein-bound calcium is nearly 40% of total non-skeletal calcium
(~ 4 g).

–– Important plasma proteins that bind to calcium are albumin and globulin.
–– Calmodulin (calcium-modulated protein) is present in cytoplasm of cells.
• Calcium Complexes
–– Calcium complexes are nearly 9% of total non-skeletal calcium (~1 g).
–– Important complexes of calcium are calcium phosphate, calcium oxalate, and

calcium carbonate.

Non-skeletal calcium performs multiple functions.

• It is necessary for cell signaling.
• It is necessary for muscle contraction.
• It is responsible for nerve impulse transmission.
• It is important in blood clotting.

Absorption of Calcium
• Dietary calcium is found in complexes like calcium carbonate, calcium

phosphate, and calcium tartrate.
• Calcium absorption occurs by mucosa of the small intestine.
• At low intraluminal calcium level

–– Calcium is absorbed by passive diffusion
–– It occurs in favor of concentration gradient and is the slow process of

absorption.
• At high intraluminal level

–– Calcium absorption occurs by active transport.
–– It is an ATP-dependent process.
–– Active transport is regulated by 1,25-dihydroxy cholecalciferol (calcitriol).

18.1 Calcium 475

–– Calcitriol controls synthesis of calbindin (calcium-binding protein located in
brush borders of duodenum).

–– Calbindin transports calcium across cell membranes of enterocytes.

Factors Influencing Absorption of Calcium
Absorption Promoters
• Calcitriol

–– Calcitriol is a hormone.

–– It governs synthesis of calcium-binding protein on the surface of

enterocytes.
–– It enhances absorption of calcium
• Parathyroid Hormone

–– Parathyroid hormone activates hydroxylase enzyme in kidneys.

–– Synthesis of calcitriol is increased.
–– Parathyroid hormone indirectly enhances calcium absorption.
• Dietary Proteins
–– High proteins in diet enhance calcium absorption.
–– Amino acids like arginine and lysine enhance calcium absorption.
• Dietary Carbohydrates
–– Dietary carbohydrates like lactose enhance calcium absorption.

–– Dietary sugars are fermented by colonic bacteria into organic acids.
–– Organic acids like citric acid enhance calcium absorption.
• Low pH of Intestinal Lumen
–– Calcium complexes like calcium carbonate and calcium phosphate are

readily soluble at low intraluminal pH.
–– Low pH of intestinal lumen enhances calcium absorption.

Absorption Inhibitors
• Dietary Fatty Acids

–– Free fatty acids combine with calcium salts and form insoluble calcium soap.

–– It is excreted in stools.
–– Dietary fatty acids inhibit calcium absorption.
• Dietary Phytic Acids
–– Phytic acids like inositol hexaphosphate are found in cereals.
–– They complex with calcium to form insoluble salts of calcium.
–– Phytic acids inhibit calcium absorption.
• Dietary Oxalates

–– Vegetables like spinach and cabbage possess oxalates.
–– Oxalates complex with calcium to form calcium oxalate.

–– It is poorly absorbed and excreted in stools.
–– Dietary oxalates inhibit calcium absorption.
• Dietary Fibers
–– Dietary fibers inhibit calcium absorption.
• High pH of Intestinal Lumen
–– Alkaline medium of the intestine inhibits calcium absorption.

476 18  Metabolism of Minerals

• Dietary Minerals (Phosphate, Iron, and Magnesium)
–– High level of phosphates in food form calcium phosphate which is
insoluble.
–– A ratio of (1:1) of calcium and phosphate in food is favorable for calcium
absorption.
–– High level of magnesium in food interferes in calcium absorption.
–– High iron in diet interferes in calcium absorption.

• Glucocorticoids
–– Glucocorticoids are steroids synthesized by the adrenal cortex.
–– Glucocorticoids diminish calcium absorption.

18.1.1 Functions of Calcium

Mineralization of Bones and Teeth
• Calcium and phosphate are necessary for mineralization of bones and teeth.
• Calcium is deposited in the form of hydroxyapatite crystals.
• Bones serve as a dynamic store of calcium in the body.

Clotting of Blood
• Ionized calcium represents a blood clotting factor IV.
• Calcium is necessary for formation of extrinsic and intrinsic prothrombin

activator.
• Calcium is helpful in cascade of blood clotting.

Muscle Contraction
• Calcium is necessary for formation of actin-myosin complex in the skeletal

muscle.
• Calcium plays a role in excitation-contraction coupling.
• Calcium is helpful in skeletal muscle contraction.

Myocardial Excitability
• Myocardial contractibility and excitability is dependent on calcium.
• Calcium is necessary for normal systole and diastole.

Conduction of Nerve Impulse
• Influx of calcium at a synapse is helpful in release of a neurotransmitter across

synapse.
• Thus calcium is necessary for nerve impulse conduction.

Enzyme Activity
• Activity of lipase is dependent on calcium as cofactor.
• Activity of neuronal nitric oxide synthase (secreted by specific neurons in the

brain) is dependent on calcium as cofactor.

18.1 Calcium 477

• Activity of calpain (proteolytic enzyme; cysteine protease) is dependent on cal-

cium as cofactor.
• Activity of calcineurin A, B, and B2 (protein serine/threonine phosphatase; role

in T-cell activation) is dependent on calcium as cofactor.

Activity of Calmodulin
• Calmodulin is calcium-modulated protein. It is a cytosolic protein.
• It can attach with four calcium ions and can form calcium-calmodulin complex.
• The complex activates adenylate cyclase and it regulates cell functions.

Intracellular Messenger
• Calcium (intracellular) acts as second messenger (intracellular molecule for cell

signaling).
• Calcium as second messenger is responsible for cell functions like muscle con-

traction, nerve impulse transmission, cell growth, and apoptosis.
• Other second messengers are cAMP, cGMP, inositol triphosphate, and

diglyceride.
• First messengers are peptide hormones like TSH, ACTH, prolactin, and catechol-

amines like adrenaline.
• Calcium (extracellular) acts as tertiary messenger for cell function. In gastric

mucosa, extracellular calcium acts as tertiary messenger for secretion of pepsinogen.

Permeability in Gap Junctions
• Gap junctions are found in almost all tissues of human body except erythrocytes

and sperms.
• It is an intercellular communication. Gap junction has an intercellular space of

2 nm, and it is filled with extracellular fluid.
• Calcium concentration in gap junction is helpful in transmission of electri-

cal impulse, movement of ions, and movement of metabolites between two
cells.

Activity of Neuromuscular Junction
• Transmission of nerve impulse from a neuron to skeletal muscle is regulated by

calcium.
• Influx of calcium at neuromuscular junction brings about release of

acetylcholine.
• This neurotransmitter is responsible for the activity of neuromuscular junction.

18.1.2 R egulation of Plasma Calcium Level

Normal plasma calcium level is between 9 and 11 mg/100 ml. Plasma contains cal-
cium in three forms. Ionized calcium in plasma is a metabolically active form. It
performs various functions.

Calcium homeostasis is regulated by the following factors:

478 18  Metabolism of Minerals

Role of Calcitriol
• Effect on Intestine

–– Calcitriol is 1,25-dihydroxy cholecalciferol. It is an active Vitamin D3.
–– It acts as hormone and induces the synthesis of calbindin from enterocytes.
–– Calbindin (calcium-binding protein) is located on brush border surface (api-

cal surface) of enterocytes.
–– Presence of calbindin increases absorption of dietary calcium from lumen of

gut.

Calcitriol increases plasma calcium level.

• Effect on Bones
–– Calcitriol stimulates osteoblastic activity in bones. Osteoblasts express alka-
line phosphatase enzyme in cell membrane.
–– Alkaline phosphatase increases level of phosphorous in developing
bones.
–– It results in increased mineralization of bones.

Calcitriol enhances mineralization of bones.

Role of Parathyroid Hormone (PTH)
Parathyroid hormone is a peptide hormone. It is secreted by chief cells of two pairs
of parathyroid glands. It binds with receptors located on target tissues. It acts on the
following tissues:

• Effect on Bones
–– Bones are dynamic store house of calcium.
–– PTH enhances osteoclastic activity in bones. The number of osteoclasts is
increased in the bone.
–– Osteoclasts secrete lactic acid and collagenase enzyme.
–– It results in demineralization of the bone.

PTH enhances demineralization of the bones.

• Effect on Kidneys
–– PTH stimulates reabsorption of calcium ions through distal convoluted tubules
and collecting tubules (daily excretion of calcium is 5 mmol/day).
–– It diminishes excretion of calcium ions by renal tubules.
–– PTH decreases reabsorption of phosphates by proximal renal tubules.
–– PTH stimulates hydroxylation of 25-hydroxy cholecalciferol into
1,25-d­ ihydroxy cholecalciferol.

PTH decreases plasma phosphate level.
PTH increases plasma calcium level.

18.1 Calcium 479

• Effect on Intestine
–– PTH enhances absorption of calcium indirectly.

Role of Calcitonin
Calcitonin is a peptide hormone. It is secreted by parafollicular cells of the thyroid
gland.

• Calcitonin is antagonistic to PTH.

• Calcitonin inhibits osteoclastic activity in bones.
• Calcitonin increases mineralization of bones.
• Calcitonin diminishes plasma calcium level.

• Calcitonin increases excretion of phosphates by kidneys.

Role of Kidneys
• Kidneys bring about hydroxylation of 25-hydroxy cholecalciferol.
• Kidneys reabsorb calcium.
• Kidneys excrete phosphate.

Role of Intestine
• The small intestine absorbs calcium and phosphates.

Role of Bones
• Bones are reservoir of calcium.
• Bones undergo mineralization and demineralization (bone remodeling) under the

control of hormones.

18.1.3 D isorders of Calcium Metabolism

Hypercalcemia
It is a clinical condition characterized by an increase in plasma calcium level
more than reference value (>11 mg/100 ml).

Etiology
• Hyperparathyroidism
• It may be caused by any one of the following factors:

–– Parathyroid adenoma (benign tumor)
–– Parathyroid malignancy
• Iatrogenic (drug-induced)
–– Loop diuretics like thiazides
• Granulomatous diseases
–– Tuberculosis
–– Sarcoidosis