Retinaldehyde biological activity

Q s-retinol has 75% of the biological activity of aU-trans-retinol, and reti-naldehyde has 90%. Food composition tables give total preformed vitamin A as the sum of aU-trans-retinol -i- 0.75 x 13-c/s-retinol - - 0.9 x retinaldehyde (Holland et al., 1991). 

A-2 Vitamin A consists of three biologically active molecules, retinol, retinal (retinaldehyde) and retinoic acid. Each of these compounds are derived from a group of molecules know as carotenoids also referred to as the provitamin A. Beta-carotene, which consists of two molecules of retinal linked at their aldehyde ends. 

Fig. 1. The structures of key retinoids and their precursors. Fish convert retinyl esters (e.g. retinyl palmitate (RP)) and carotenoids (e.g. /3-carotene) to retinol in the gut lumen prior to intestinal absorption. Retinyl esters (e.g. RP) stored in the liver are synthesized from retinol by lecithin retinol acyltransferase (LRAT) and acyl CoAiretinol acyltransferase (ARAT). The retinyl esters are mobilized through their conversion to retinol by retinyl ester hydrolase (REH), which is then transported in the circulation to various sites in the body. Retinol is further metabolized within specific tissues to retinal by alcohol dehydrogenases (ADH) or short-chain dehydrogenase/reductase. Retinal is converted to the two major biologically active forms of retinoic acid (RA) (all-trans and 9-cis RA). Retinaldehyde dehydrogenase 2 (Raldh2) synthesizes all-trans RA from all-trans precursors and 9-cis RA form 9-cis precursors.

Retinol is found only in foods of animal origin and a small number of bacteria, mainly as the ester retinyl palmitate. Retinoic acid is a metabolite of retinol and has important biological activities in its own right. The oxidation of retinaldehyde to retinoic acid is irreversible, and retinoic acid cannot be converted to retinol in vivo, and does not support either vision or fertility in deficient animals. 

Central oxidative cleavage of 3-carotene, as shown in Figure 11.3, should give rise to two molecules of retinaldehyde, which can be reduced to retinol. However, the biological activity of P-carotene, on a molar basis, is considerably lower than that of retinol, not twofold higher as might be expected. In addition to the relatively poor absorption of carotene from the diet, three factors may account for this ... 

Retinaldehyde is biotransformed into all-frani -retinoic acid and induces biologic effects including comedolytic activity similar to those of topical tretinoin when administered at comparatively lower concentrations. ... 

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