ABSTRACT
Retinoic acid (RA) can support the systemic functions of vitamin A in vivo, but unlike retinol, orally or peritoneally dosed RA cannot support the visual or the reproductive functions of vitamin A in mammals. As expected from its activity in vivo, RA induces the differentiation of a broad variety of cells and tissues in culture; generally, RA is a more potent promoter than retinol of differentiation in vitro. This chapter discusses the evidence that RA is an endogenous retinoid in the pathway of retinol metabolism under physiological conditions. The observation that beta-carotene serves as a substrate for RA synthesis in cytosol prepared from rat liver, intestine, lung, kidney and testes adds another dimension to RA biosynthesis. Retinoids containing photolabile azide groups should be useful photoaffinity probes for studying the mechanism of retinoid action, particularly in the identification of nuclear retinoid receptors.
