ABSTRACT
Vitamin A is involved in the biosynthesis of the visual pigment rhodopsin (1), in embryogenesis (2), as well as in cell growth and differentiation (3). It exists in interconvertible forms as retinol, its esters (the main storage form in the body) and retinaldehyde, whereas the latter is irreversibly converted to retinoic acid, the biologically active form of vitamin A that binds to nuclear receptors and modulate gene expression (Fig. 1) (4,5). Retinoic acid exists as three cis/trans isomers: all-trans-, 9-cis-and 13-cis-retinoic acid; all-trans-retinoic acid (retinoic acid, tretinoin) binds to the nuclear receptors such as retinoic acid receptor (RAR)-α, β, and γ with a high affinity, 9-cis-retinoic acid (alitretinoin) binds to RAR-α, β, γ and retinoid X receptor (RXR)-α, β, γ with a high affinity, whereas 13-cis-retinoic (isotretinoin) does not bind to nuclear retinoid receptors with significant affinity (6,7).
