Interconversions and active forms
Retinol and retinal are easilv interconvertible in the body, and both can be supplied by the retinyl esters from the diet or liver reserves. Retinoic acid is formed in the tissues by oxidation of retinal, but this reaction is not reversible. Neither retinal nor retinoic acid is stored in the body, and their circulating levels are very low compared to retinol. Retinal (11-cis) is known to be the active form in the visual process, but the active form or forms in the other functions of vitamin A are not known. The fact that some cells have both CRBP and CRABP suggests that both may be capable of performing the same functions in these cells, or that perhaps both are needed. It is also possible that they undergo further activation; retinoic acid is known to be converted to 5,6-epoxyretinoic acid in some tissues, but whether this represents further activation or catabolism remains to be established. Retinol can provide the active forms for all tissues, whereas retinoic acid for only some; it cannot supply retinal for the visual pigments, and it has only partial activity in the reproductive tissues, which also appear to have sex and species differences. However, the acid can support growth and differentiation of the epithelial tissues; it is actually more active than retinol in these functions and, therefore, may be the active form.
