Retinoids

The term retinoid is used to include retinol and its derivatives and analogs, either naturally occurring or synthetic, with or without the biological activity of the vitamin. The main biologically active retinoids are shown in Figure 2.1; until the late 1990s, only retinol, retinaldehyde, all-frans-retinoic acid, and 9-cis-retinoic acid were known to be biologically active. However, a number of other retinoids are now also known or believed to have important functions, including 4-oxo-retinol, 4-oxo-retinoic acid, and a variety of retroretinoids. The term rexinoid has been introduced to include only those retinoids that bind to the retinoid X receptor (RXR) and not the retinoic acid receptor (RAR) (Section 2.3.2.1).

Free retinol is chemically unstable and does not occur to any significant extent in foods or tissues. Rather, it is present as a variety of esters, mainly retinyl palmitate. Retinyl acetate is generally used as an analytical standard and in pharmaceutical preparations. Dehydroretinol (vitamin A2) is found in freshwater fishes and amphibians; it has about half the biological activity of retinol.

Retinoic acid occurs in foods in only small amounts. In conventional biological assays, it has lower potency than retinol or retinyl esters because it is not stored, but is metabolized rapidly. Furthermore, because retinoic acid

Figure 2.1. Major physiologically active retinoids. Relative molecular masses (Mr): retinol, 286.5; retinaldehyde, 284.4; retinoic acid, 300.4; 13,14-dihydroxy-retinol, 318.5; 4-oxoretinol, 301.5; retroretinol, 286.5; 14-hydroxy-retroretinol, 302.5; anhydroretinol, 269.6; 4-oxo-retinoic acid, 315.4; 4-hydroxyretinoic acid, 316.4; dehydroretinol, 284.4; retinyl acetate, 328.5; and retinyl palmitate, 524.9.

Figure 2.1. Major physiologically active retinoids. Relative molecular masses (Mr): retinol, 286.5; retinaldehyde, 284.4; retinoic acid, 300.4; 13,14-dihydroxy-retinol, 318.5; 4-oxoretinol, 301.5; retroretinol, 286.5; 14-hydroxy-retroretinol, 302.5; anhydroretinol, 269.6; 4-oxo-retinoic acid, 315.4; 4-hydroxyretinoic acid, 316.4; dehydroretinol, 284.4; retinyl acetate, 328.5; and retinyl palmitate, 524.9.

cannot be reduced to retinaldehyde or retinol, it cannot support vision (or fertility) in deficient animals.

Anhydroretinol binds to plasma and intracellular RBPs, but not to the cellular retinoic acid binding proteins (CRABPs) or retinoid receptors. In experimental animals, it protects against the development of chemically induced tumors while showing none of the toxic effects of other retinoids.

4-Oxoretinol is synthesized from canthaxin (see Figure 2.2) and occurs in plasma. It binds to the retinoic acid nuclear receptor (but not the RXR) and is active in tissue differentiation. In the early embryo, the main biologically active retinoid is 4-oxoretinaldehyde, which both activates RARs and also acts as a precursor of oxo-retinoic acid and oxoretinol.

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