Chemistry of Vitamin-A
Vitamin-A is a fat soluble vitamin. Its active form is present only in animal tissues. The pro-vitamin, beta-carotene is present in plant tissues. Beta-Carotene has two beta ionone rings connected by a polyprenoid chain. One molecule of beta-carotene can theoretically give rise to two molecules of Vitamin-A; but it may produce only one in biological systems. All compounds with Vitamin-A activity are referred to as retinoids. They are poly-isoprenoid compounds having a beta-ionone ring system. Three different compounds with Vitamin-A activity are:
- Retinol (Vitamin-A Alcohol)
- Retinal (Vitamin-A Aldehyde) &
- Retinoic Acid (Vitamin-A Acid)
Retinal may be reduced to Retinol by Retinal Reductase. This reaction is readily irreversible. Retinal is oxidized into Retinoic Acid, which cannot be converted to other forms. The side chain contains alternate double bounds, and hence many isomers are possible. The all-trans variety of Retinal, also called as Vitamin-A1 is the most common. Vitamin-A2 is found in Fish oils and has an extra double bond in the ring. Biologically important compound is 11-cis-retinal.
Absorption of Vitamin-A
Beta-Carotene is cleaved by a di-oxygenase to form Retinal. The Retinal is reduced to Retinol by an NADH or NADPH dependent retinal reductase present in the intestinal mucosa. Intestine is the major site of absorption. The absorption is along with other bile salts. In biliary tract obstruction and steatorrhoea, Vitamin-A absorption is reduced. Within the mucosal cell, the retinol is re-esterified with fatty acids, incorporated into chylomicrons and transported to liver. In the liver stellate cells, vitamin is stored as Retinol Palmitate.
Transport from Liver to Tissues
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