Vitamin A is a generic term that is used to describe compounds that have the biological activity of retinol, such as all-trans retinol, retinyl palmitate, retinyl oleate, and retinyl stearate, and provitamin A carotenoids such as a-carotene, P-carotene, and P-cryptoxan-thin (Fig. 4). The term "retinoid" refers to both naturally occurring forms of vitamin A and synthetic analogs of retinol, with or without biological activity. All-trans retinol has a molecular weight of 286 and consists of a substituted cyclohexenyl (P-ionone) ring, a tetraene side chain, and a primary hydroxyl group at position C-15. The hydroxyl group can be esterified with long-chain fatty acids to form compounds such as retinyl palmitate and retinyl stearate, which are common forms of vitamin A in animal tissues and products. Active metabolites of retinol include all-trans retinoic acid and 9-cis retinoic acid, probably found in all nucleated cells, and 11-cis retinal is found in the retina, where it plays a role in the visual cycle.
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