Involvement of Alcohol Dehydrogenase, Short-Chain Dehydrogenase/Reductase, Aldehyde Dehydrogenase, and Cytochrome P450 in the Control of Retinoid Signaling by Activation of Retinoic Acid Synthesis

The effects of vitamin A (retinol) on growth and development are mediated by the active metabolite retinoic acid which controls a nuclear receptor signaling pathway. While elegant work on the retinoic acid receptor family has focused attention upon how the receptor controls this pathway, there now exists a relatively large gap in our understanding of how retinol is activated to form the ligand. During vertebrate embryogenesis and in adult organs retinoic acid is detected in a distinct spatiotemporal pattern, suggesting that it is produced from retinol in a regulated fashion. Enzymes involved in retinol and retinal metabolism are likely candidates for regulators of tissue retinoic acid levels. Members of the alcohol dehydrogenase and short-chain dehydrogenase/reductase enzyme families catalyze the reversible interconversion of retinol and retinal, the rate-limiting step, whereas members of the aldehyde dehydrogenase and cytochrome P450 enzyme families catalyze the irreversible oxidation of retinal to retinoic acid. The identification of enzymes likely to catalyze retinol oxidation in vivo has been particularly controversial, and this is made even more difficult by the reversible nature of this reaction. Taking into account enzymatic properties and coenzyme preferences, a case can be made that class IV alcohol dehydrogenase catalyzes retinol oxidation to provide retinal for retinoic acid synthesis, whereas microsomal retinol dehydrogenase (a short-chain dehydrogenase/reductase) catalyzes the reduction of retinal to retinol to promote retinoid storage. Further studies on these enzyme families will allow this layer of control in the retinoid signaling pathway to be understood.