Principles of Biochemistry

10.2.1. Vitamin A

 Vitamin A, or 3 hydroxy retinol, is a polyisoprenoid compound containing a cyclohexenyl ring . Vitamin A is a generic term referring to all compounds from animal sources that exhibit the biologic activity of vitamin A. In the body, the main functions of vitamin A are carried out by retinol, an alcoholic form of vitamin A, and its two derivatives retinal (an aldehyde form) and retinoic acid (an acid form). The term retinoid has been used to describe both the natural forms and the synthetic analogs of retinol.

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In vegetables, vitamin A exists as a provitamin or precursor, in the form of the yellow pigment, b-carotene. It consists of two molecules of retinal joined at the aldehyde end of their carbon chains. However, b-carotene is not efficiently metabolized to vitamin A and it is only about one-sixth as effective a source of vitamin A as retinol, weight for weight. b-Carotene-like compounds are known as carotenoids.

Functions

1. Role in vision: Rhodopsin occurs in the rod cells of the retina, which are responsible for vision in poor light. 11 cis- Retinal, and isomer of all-trans-retinal, is specifically bound to the visual protein opsin to form rhodopsin. When rhodopsin is exposed to light, it dissociates as it bleaches and forms all trans-retinal and opsin. This reaction is accompanied by a conformational change that induces a calcium ion channel in the membrane of the rod cell. The rapid influx of calcium ions triggers a nerve impulse, allowing light to be perceived by the brain. All trans retinal is inactive in the synthesis of rhodopsin and it has to be converted to 11-cis retinal by retinal isomerase.

2. Vitamin A helps in maintaining the integrity of epithelial tissues.

3. It helps in the preservation of the structural integrity and the normal permeability of the cell membrane. It accelerates the normal formation of boned and teeth.

4. Vitamin A and Immune function: Vitamin A also supports the generation of T lymphocytes maintaining the body’s ability to mount an immune response against invading infecting agents.

5. It is needed for the formation of chondroitin sulphate.

6. Retinoic acid participates in glycoprotein Synthesis

7. In cell differentiation, large part of vitamin A in the form of retinoic acid takes part in promoting growth and differentiation of tissues. It has been proposed that retinoyl phosphate functions as a carrier of oligosaccharides across the lipid bilayer of the cell, by way of an enzymatic trans-cis isomerization analogous to that described above in the trans-cis isomerization of rhodopsin.

8. Both retinoids and carotenoids have anticancer activity. b-Carotene is an antioxidant and may play a role in trapping peroxy free radicals in tissues at low partial pressures of oxygen.

Deficiency

Vitamin A deficiency causes symptoms due to malfunction of the various cellular mechanisms in which retinoids participate.

1. Night blindness- Nyctalopia: It is defective night vision at night, which occurs when liver stores are nearly exhausted.

2. Drying of conjunctiva: Further depletion leads to keratinization of epithelial tissues of the eye “ Bitot spots”
3. Keratomalacia: Softening of cornea, in advanced cases with ulceration and necrosis.
4. Keratinising metaplacia: Of the epithelium of lungs, gastrointestinal, and genitourinary tracts, coupled with reduction in mucous secretion.
5. Xerophthalmia: Deterioration in the tissues of the eye, leads to total blindness,

RDA

  Infants  :1500IU

 Children  :2000-3000IU

 Adults  :5000IU

 Pregnant& Lactating women :6000-8000IU

The amounts of vitamin A or retinol are often quoted in international units (I.U.). One I.U. of vitamin A is equal to 0.3 µg retinol or 0.6 µg beta carotene.

Sources

Plant Source: All yellow fruits and vegetables-Sweet potatoes, carrots, pumpkin, papayas, tomatoes and green leafy vegetables

Animal sources: Liver, milk, butter, eggs, kidney, fish liver oil, animal fats apple, banana, jackfruit etc.

Hypervitaminosis: Hypervitaminosis A (Vitamin A toxicity) occurs when the intake exceeds the body’s storage capacity and the cells are exposed to unbound retinol. This can occur with excessive use of vitamin A supplements. Acute symptoms include drowsiness, sluggishness, severe headache, vomiting peeling of the skin about the mouth and increased intracranial pressure. Chronic high doses can cause liver damage and hyperostosis.