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Vitamins
5th lecture
2 nd class
Biochemistry
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Vitamins are defined as small organic molecules present in diet which
are required in small amounts. Most of the vitamins are not synthesized in
the body and hence they must be supplied in the diet. However few
vitamins are synthesized in the body. Though most of them are present in
diet as such some are present as precursors. The precursor forms of
vitamins are called as provitamins. In the body these provitamins are
converted to vitamins.
Types of vitamins:
1-Fat Soluble Vitamins
They are vitamins A, D, E and K. They have some common properties.
They are: 1. Fat soluble. 2. Require bile salts for absorption. 3. Stored in
liver. 4. Stable to normal cooking conditions. 5. Excreted in feces.
2-Water Soluble Vitamins
They are members of vitamin B complex and Vitamin C. Their common
properties are 1. Water solubility. 2. Except Vitamin B12 others are not
stored. 3. Unstable to normal cooking conditions. 4. Excreted in urine.
MEDICAL AND BIOLOGICAL IMPORTANCE
1. Vitamins are essential for growth, maintenance and reproduction.
However, they are not used for energy production.
2. Fat soluble vitamins are required for normal and color vision, blood
clotting, bone formation and maintenance of membrane structure.
3. Most of the water soluble vitamins function as coenzymes or prosthetic
groups of several enzymes involved in carbohydrate, lipid and amino acid
metabolism etc.
4. Vitamins A and D act as steroid hormones.
5. Pregnant and lactating women require higher amounts of vitamins.
During post operative recovery also vitamin requirement is more.
6. Lack of vitamin in the diet produce characteristic deficiency
symptoms. Since intestinal flora synthesizes some vitamins prolonged use
of antibiotics also produce vitamin deficiency.
7. Deficiency of fat soluble vitamins produce night blindness, skeletal
deformation, haemorrhages and hemolysis.
8. Deficiency of water soluble vitamins produce beriberi, glossitis,
pellagra, microcytic anaemia, megaloblastic anaemia and scurvy.
FAT SOLUBLE VITAMINS
VITAMIN A
Chemistry
The word Vitamin A refers to group of compounds which exhibit Vitamin
A activity. They are retinol (Vitamin A alcohol), retinal (Vitamin A
Vitamins
5th lecture
2 nd class
Biochemistry
-----------------------------------------------------------------------------------------------aldehyde) and retinoic acid (Vitamin A acid). They are also referred as
retinoids. Retinal and retinoic acid are formed from retinol.
Further retinal and retinol are inter convertible. But retinoic acid cannot
be converted to either retinal or retinol.
Vitamin A metabolism
In the intestine pancreatic esterase hydrolyzes retinol esters present in the
diet to retinol and free fatty acid in presence of bile salts. Retinol is
absorbed by mucosal cells. β-carotenes are also absorbed by mucosal
cells.
Vitamins
5th lecture
2 nd class
Biochemistry
-----------------------------------------------------------------------------------------------Functions of vitamin A
The three major retinoids retinal, retinol and retinoic acid have unique
functions.
1. Retinal is required for normal and color vision.
2. Retinol is required for reproduction and growth. Retinol supports
spermatogenesis, oogenesis and placental development.
3. Retinol is required for differentiation and function as steroid hormone.
4. Retinoic acid is required for the synthesis of glycoproteins or
mucopolysaccharides. Retinoyl phosphate act as glycosyl carrier.
5. Retinoic acid also act as steroid hormone. It also promote growth and
differentiation but only to some extent.
6. Retinol and retinoic acid are involved in regulation of gene expression.
Fig: visual cycle
Vitamins
5th lecture
2 nd class
Biochemistry
-----------------------------------------------------------------------------------------------Deficiency manifestations of Vitamin A
1. Night Blindness or Nyctalopia
Visual acuity is diminished in dim light. The patient cannot read or drive
a car in poor light. The dark adaptation time is increased.
2. Xerophthalmia
The conjunctiva becomes dry, thick and wrinkled. The conjunctiva gets
keratinized and loses its normal transparency. Dryness spreads to cornea.
It becomes glazy and lusterless due to keratinization of corneal
epithelium.
3. Bitot's Spots
These are seen as greyish-white triangular plaques firmly adherent to the
conjunctiva.
Vitamin D
The term vitamin D refers to group of two compounds that exhibit
vitamin D activity. They are vitamin D2 also called as ergo calciferol and
vitamin D3 also called as cholecalciferol. These active forms of vitamin
D2 and vitamin D3 are formed from provitamins which are sterols. The
provitamin of vitamin D2 is ergosterol which is found in ergot and yeast.
It is a derivative of cholesterol. The provitamin of vitamin D3 is 7dehydrocholesterol which is found in animals. It is also a derivative of
cholesterol.
Synthesis of cholecalciferol or vitamin D3
Absorption, transport and storage
Dietary vitamin D2 and vitamin D3 are absorbed in the small intestine in
presence of bile salts. In the intestinal mucosal cells absorbed Vit D is
incorporated into chylomicrons and enters circulation via lymph. In the
circulation vitamin D dissociates from chylomicrons andbinds to specific
vitamin D binding protein (DBP) which has higher affinity for vitamin
Vitamins
5th lecture
2 nd class
Biochemistry
-----------------------------------------------------------------------------------------------D3. So a binary complex containing vitamin D and DBP is found in
plasma. Further, vitamin D3 formed in the skin also combines with
vitamin D binding protein and forms a binary complex. Different tissues
take up vitamin D from DBP and vitamin D complex. Vitamin D is stored
in liver and adipose tissue. Vitamin D binding protein can combine with
different forms of Vitamin D.
A. Vitamin D and Absorption of Calcium
Calcitriol promotes the absorption of calcium and phosphorus from the
intestine. In the brush-border surface, calcium is absorbed passively.
From the intestinal cell to blood, absorption of calcium needs energy. It is
either by the sodium-calcium exchange mechanism or by pumping out the
calcium-calbindin complex. Calcitriol acts like a steroid hormone. It
enters the target cell and binds to a cytoplasmic receptor. The hormonereceptor complex interacts with DNA and causes derepression and
consequent transcription of specific genes that code for Calbindin (Fig.).
Due to the increased availability of calcium binding protein, the
absorption of calcium is increased.
B. Effect of Vitamin D in Bone
Mineralization of the bone is increased by increasing the activity of
osteoblasts. Calcitriol coordinates the remodelling action of osteoclasts
and osteoblasts. Calcitriol stimulates osteoblasts which secrete alkaline
phosphatase. Due to this enzyme, the local concentration of phosphate is
increased. The ionic product of calcium and phosphorus increases,
leading to mineralization.
Vitamins
5th lecture
2 nd class
Biochemistry
-----------------------------------------------------------------------------------------------C. Effect of Vitamin D in Renal Tubules
Calcitriol increases the reabsorption of calcium and phosphorus by renal
tubules, therefore both minerals are conserved (PTH conserves only
calcium).
Note:
Calcitriol is the physiological active form of vitamin D. It increases the
blood calcium level.
Calcitonin is the peptide hormone released from thyroid gland. It
decreases the blood calcium.
Deficiency of Vitamin D
The deficiency diseases are rickets in children and osteomalacia in
adults. Hence vitamin D is known as antirachitic vitamin.
WATER SOLUBLE VITAMINS
VITAMIN B COMPLEX
Members of vitamin B complex are (1) Thiamin (Vitamin B1) (2)
Riboflavin (Vitamin B2) (3) Niacin (4) Pyridoxine (Vitamin B6) (5)
Biotin (6) Folic acid (7) Cyanocobalamin (Vitamin B12) and (8)
Pantothenic acid.
Vitamins
5th lecture
2 nd class
Biochemistry
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VITAMIN B12
Chemistry
Vitamin B12 is water soluble, heat stable and red in color. It contains
4.35% cobalt by weight. It contains one cobalt atom. Four pyrrole rings
co-ordinated with a cobalt atom is called a Corrin ring.
a. Cyanocobalamin
When cyanide is added at the R position, the molecule is called
cyanocobalamin. During the isolation procedure, cyanide is added to get
stable crystals. The CN group has no physiological function.
b. Hydroxy cobalamin
When hydroxyl group is attached at the R position, it is called hydroxy
cobalamin or vitamin B12a. Injectable preparations are in this form.
c. Adenosyl cobalamin (Ado-B12)
When taken up by the cells, these groups are removed and deoxy
adenosyl cobalamin or Ado-B12 is formed (Fig.). This is the major
storage form, seen in liver.
d. Methyl cobalamin
When the methyl group replaces adenosyl group, it is known as methyl
cobalamin. This is the major form seen in blood circulation as well as in
cytoplasm of cells. The Ado-B12 and methyl B12 are the functional coenzymes in the body.
Fig: structure of vit. B12
Vitamins
5th lecture
2 nd class
Biochemistry
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Fig: Storage form of vit. B12
Deficiency Manifestations
i. Folate trap: Vitamin B12 deficiency causes simultaneous folate
deficiency due to the folate trap. Therefore all the manifestations of folate
deficiency are also seen (for details see under folic acid).
ii. Megaloblastic anemia: In the peripheral blood, megaloblasts and
immature RBCs are observed.
iii. Abnormal homocysteine level: In vitamin
B12 deficiency, so that homocysteine is accumulated, leading to
homocystinuria. Homocysteine level in blood has a positive correlation
with myocardial infarction. So, B12 and folic acid are protective against
ischemic heart disease.