2. zContents
2
Vitamin classification
Structure of vitamin A
Sources of vitamin A
Absorption and transport
Mechanism of action
Functions & body requirement
3. zVitamin classification
3
Vitamins
Water soluble Fat soluble
Non-B complex B complex
Energy - releasing Hamatopoietic Other
• Vitamin A • Vitamin D
• Vitamin E • Vitamin K
• Vitamin C
• Thiamin (B1)
• Riboflavin (B2)
• Niacin (B3)
• Folic acid
• Vitamin B12
• Pyridoxine (Vitamin B6)• Biotin
• Pantothenic
acid
5. zStructure of Vitamin A
5
Vitamin A – fat soluble vitamin
Retinoids includes
natural & synthetic forms of vitamin A
that may or may not show vitamin A activity
Structure
Retinol Retinal Retinoic acid β-Carotene
6. zStructure of Vitamin A
6
Retinol
Found in animal tissue, as retinyl ester
Primary alcohol,
containing a β-ionone ring,
with an unsaturated side chain
β-carotene
Found in plant food
Oxidatively cleaved in the intestine to yield two molecules of retinal
In human this conversion is inefficient,
Vit A activity of β-carotene is only about 1/12 that of retinol
7. zStructure of Vitamin A
7
Retinal
Aldehyde derived from the oxidation of retinol
Retinal and retinol can readily be interconverted
Retinoic acid
Acid derived from the oxidation of retinal
Cannot be reduced in the body
Retinol Retinal Retinoic acid
Oxidation
Aldehyde Acid
Oxidation
8. zSources of Vitamin A
8
Preformed vitamin A
Liver, kidney, cream, butter and egg yolk
Beta carotene – precursor of vitamin A
Yellow & dark green vegetables & fruits
9. zAbsorption and transport of vitamin A
9
Retinyl esters
β-Carotene
Fatty
acids
Retinol
β carotene
Retinal
Retinol
Retinyl
esters
Hydrolysed
Enterocyte
Fatty acyl
CoA
Re-
esterification
to long chain
fatty acid
Secreted as a
component of
chylomicrons lymph
Chylomicron
remnants
10. zAbsorption and transport of vitamin A …cnt
10
RBP
Retinyl
pulmitate
All-trans-Retinol
Retinol- RBP
complex
Retinol Retinol
Chylomicron
remnants
RBP
11. zMechanism of action of vitamin A
11
Eg: retinoids control
the expression of
the gene of keratin
in most epithelial
tissues of the body
Retinoic acidRetinol
oxidation
Specific
receptor proteins
Epithelial cell
Activated
retinoic acid-
receptor complex
Interact with
nuclear chromatin
To regulate retinoid-
specific RNA
synthesis
Control the
production of
specific proteins
12. z
Mechanism of action of vitamin A12
Retinoic acid
Retinol
oxidation
Epithelial cell
Plasma retinol-RBPRBP
Retinoic acid
Inactivate
receptor
Activated
receptor complex
gene
mRNA
mRNA Specific proteins
Cellular
differentiation
cytasol
nucleus
14. zFunctions of Vitamin A
14
Retinol- RBP
complex
All- trans-Retinol
RBP
Retinal rod/cone
All- trans-retinyl
esters
11-cis Retinol
11-cis Retinal
Rhodopsin
All- trans-Retinal
Opsin
Opsin
When exposed
to light
Bleaching
reduced
esterified
isomerized
oxidized
15. zRequirement of vitamin A
15
Measured with – retinol activity equivalents (RAE)
900 RAE daily adult male
700 RAE daily adult female
1 RAE equals to
1 mg of retinol
12 mg of β carotene
24mg of other carotinoids
16. zIn Summary
16
Vitamin A is a fat soluble vitamin
There are 4 forms of vitamin A
Retinol, Retinal, Retinoic acid & β- carotene
They act by controlling the production of specific proteins
It is important in maintaining vision, growth, reproduction &
epithelial differentiation
It has a therapeutic importance in the treatment of psoriasis,
promyelocytic leukemia and severe acne