Overview of retinoid metabolism.

Substantial and exciting progress has been made in understanding the cytoplasmic metabolism and the nuclear functions of vitamin A. Keys to this understanding have come from studies of the plasma and cytoplasmic proteins that bind retinol and retinoic acid, thereby regulating retinoid concentration and specifying the enzymatic reactions through which retinol is absorbed, stored as esterified retinol or oxidized to retinoic acid. In the nucleus, the hormone-like activity of retinoic acid is transduced by a family of retinoic acid receptors that regulate a large number of retinoic acid-responsive genes. The coordination of cytoplasmic retinoid metabolism and the nuclear effects of retinoic acid or other retinoid metabolites is now recognized as critical to embryonic development and the control of cellular differentiation throughout life.