Fig. 4.6 Some important metabolic transformations of vitamin A. The parent compound of the vitamin A group is all-frans-retinol (A). It can be obtained directly from the diet or formed from the breakdown of dietary P-carotene (provitamin A) from plants (B). The active form of vitamin A in the visual process is 11-cis-retinal (C). Retinyl esters (mainly palmitate) provide a storage form of the vitamin in tissues, especially the visual pigment at the back of the eye (D). All-frans-retinal can be converted into retinoic acid (E), which has no visual function, but is involved in processes of cellular differentiation. Vitamin A can be excreted via the formation of glucuronides from retinoic acid or from vitamin A itself. Reactions 1-10 are described in further detail in the text.
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