ABSTRACT
Vitamin A plays a major role for the regular formation of epithelial tissues and its maintenance by the availability and activity of its metabolites-in particular retinoic acid (RA). Therefore, interconversion, storage, uptake, metabolism, and efflux are factors that determine retinoid effects on individual skin cells as well as the body as a whole. The general metabolic pathways of retinoids, which have been reviewed in Chapter 1 and elsewhere (1-3). The metabolic pathways include inter alia: (i) retinyl ester storage in the liver; (ii) transport from the liver to the target organ after binding to plasma retinol-binding protein; (iii) conversion of retinol to retinal and RA respectively by alcohol dehydrogenase (ADH) and retinaldehyde dehydrogenase (RALDH); (iv) binding to the cellular retinol binding protein (CRBP) or cellular retinoic acid binding protein (CRABP) as well as to retinoic receptors (RARα, β, and γ ) or retinoic X receptors (RXRα, β, and γ ) (2).
