Vitamin A: recent advances in the biotransformation, transport, and metabolism of retinoids

Advances in vitamin A research in 1999 and 2000 have improved the understanding the molecular processes through which beta-carotene and other provitamin A carotenoids are converted to vitamin A, the roles of cellular retinoid-binding proteins that serve as retinoid chaperones during metabolism, the regulation of retinoid transport, and the nature and regulation of several enzymes required for the absorption, storage, activation, and inactivation or degradation of retinoids. Not only has a clearer picture emerged of specific molecular processes, but it is also becoming evident that whole-body retinoid homeostasis is facilitated by close communication among organs due to the rapid interorgan recirculation of retinoids, and by the "autoregulation" by retinoic acid of several enzymes and retinoid-binding proteins that mediate retinoid homeostasis.