Vitamins may be regarded as organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism. There are around 15 vitamins, essential for humans, classified as Fat Soluble (A,D,E,K) and Water Soluble (C, B-group).
It is a fat soluble vitamin, as such present only in foods of animal origin. However, Pro-vitamins carotenes are found in plants. Retinol, retinal and retinoic acid are regarded as vitamers of vitamin A. Vitamers are chemically similar substances that possess qualitatively similar vitamin activity.
Vitamin A is collectively used to represent many structurally related and biologically active molecules:
- Retinol (vitamin A alcohol): Its a primary alcohol containing ß-ionone ring and the side chain has two isprenoid units.
- Retinal (vitamin A aldehyde): This form is obtained by oxidation of retinol.
- Retinoic acid (vitamin A acid): Produced by oxidation of retinal.
- ß-carotene (provitamin A): This is found in plant foods. It is cleaved in intestine to produce two moles of retinal. In humans, this conversion is inefficient, hence ß-carotene possesses about one-sixth vitamin A activity compared to that of retinol.
Animal sources contain preformed Vit A, best sources are liver, kidney, egg, yolk, milk, cheese, butter. Fish liver oils are very rich in Vit A.
Vegetable sources contain provitamin A-carotenes, carrots, spinach, pumpkins, mango, papaya are some sources.
RECOMMENDED DIETARY ALLOWANCE
For adults- 5,000 (IU) for man and 4,000 (IU) for woman. The requirement increases in growing children, pregnant women and lactating mothers.
ABSORPTION, TRANSPORT AND MOBILIZATION
- Dietary retinyl esters are hydrolysed by intestinal hydrolases in the intestine, releasing retinol and free- fatty acids.
- Carotenes are hydrolysed by ß-carotene 15-15'- dioxygenase of intestinal cells to release 2 moles of retinal which is then reduced to retinol.
- In the intestinal mucosal cells, retinol is re-esterified to long chain fatty acids, incorporated into chylomicrons and transferred to lymph.
- The retinol esters of chylomicrons are taken up by the liver and stored.
- Retinol is transported in the circulation by plasma retinol binding protein. Many cells of the target tissues contain cellular retinol binding protein (RBP) that carries retinol to nucleus and binds to chromatin. It is here that retinol functions as a steroid hormone.
The biochemical function of vit A in vision was first given by George Wald and the events occur in a cyclic process known as Rhodopsin cycle or Wald's visual cycle. The sequence of events in the cycle are:
- The retina of the eye has two types of cells- Rods (10 million, involved in dim light vision), Cones (5 million, involved in bright light and colour vision)
- Rhodopsin is a conjugated protein present in rods. It contains 11-cis-retinal and the protein opsin.
- On exposure of light, Isomerization of 11-cis-retinal to all-trans-retinal takes place.
- This leads to coformational change in opsin which is responsible for the generation of nerve impulse.
- The all-trans-retinal then immediately is isomerized to 11-cis-retinal by retinal isomerase. This 11-cis-retinal then combines with opsin to regenrate rhodopsin.
- However, the conversion of all-trans-retinal to 11-cis-retinal is incomplete, and hence most of the all-trans retinal is transported to liver and converted to all-trans-retinol by alcohol dehydrogenase.
- The all-trans-retinol undergoes isomerization to 11-cis-retinol which is then oxidized to 11-cis-retinal to participate in the visual cycle.
- For more information click on the link https://youtu.be/8FV0FNy5iMg
FUNCTION AS STEROID HORMONES
Retinol and retinoic acid function almost like steroid hormones, regulate protein synthesis and thus are involved in the cell growth and differentiation.
FUNCTION AS ANTI-OXIDANTS
Carotenoids (most imp ß-carotene) function as anti-oxidants and reduce the risk of cancers initiated by free radicals. Also found to be beneficial to prevent heart attacks.
- Night Blindness: (Nyctalopia) one of the earliest symptoms of vit A deficiency. Difficulty to see in dim light.
- Xeropthalmia: Severe deficiency, characterized by dryness of conjunctiva and cornea, keratinization of epithelial cells. In certain areas of conjunctiva, white triangular plaques known as Bitot's spots are seen.
- Keratomalacia: persistent xerophthalmia causes corneal ulceration and degeneration leading to destruction of cornea and total blindness.
Symptoms include dermatitis, enlargement of liver, skeletal decalcification, tenderness of long bones, loss of weight, irritability, loss of hair, joint pain, etc.
Essentials of Biochemistry, Satyanarayana, 2nd edition