Vitamin A is a fat-soluble substance essential to the health, survival, and reproduction of all vertebrates. As with all vitamins, it is needed in only small amounts in the human diet, about 1 to 1.5 milligrams a day. Vit-amin A does not occur in the plant kingdom, but plants supply animals with precursors (or provitamins), such as beta-carotene and other carotene-related compounds (carotenoids), that are converted to vitamin A in the intestinal mucosa of animals and humans. Beta-carotene (and other carotenoids) are abundant in all photosynthesizing parts of plants (green leaves), as well as in yellow and red vegetables. Vitamin A, also known as “retinol,” is itself a precursor of several substances active in the vertebrate organism; these are collectively termed “retinoids.” One retinoid is retinoic acid, an oxidation product of retinol, formed in the liver and other organs and existing in different chemical isomers, such as all-transretinoic acid and 9-cis-retinoic acid, with different functions. Other retinoids are all-trans-retinaldehyde and its 11-cis-isomer. The latter is active in the retina of the eye, forming the light-sensitive pigment rhodopsin by combination with the protein opsin. In the liver, retinol is stored in the form of its ester (retinyl palmitate).
Retinoids in the animal organism are generally not found in the free state but are bound to specific pro-teins. Thus, in the blood, retinol is carried by a retinolbinding protein and, within cells, by an intracellular retinol-binding protein. Retinoic acid and retinalde-hyde are carried by specific intracellular binding proteins. When carrying out its hormonal function, retinoic acid combines with another set of proteins, called retinoic acid receptors, located in the cell nucleus. The retinoic acid-receptor complex can then interact with specific genes at sites known as retinoic acid response elements, thereby activating these genes and causing them to stimulate (or repress) the expression of specific proteins or enzymes involved in embryonic development, cell differentiation, metabolism, or growth.
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