Function
Retinoid dehydrogenases/reductases (oxidoreductases), including retinol dehydrogenase, catalyze the key oxidation-reduction reactions in the visual cycle, converting vitamin A to 11-cis retinal, which is the chromophore of the rod and cone photoreceptors. It is believed that RDHs at rod and cone are different, but related and can caatalyze the same reaction. RDH12 is the primary enzyme that reduces all-trans retinal released from bleached photopigments during recovery phase in the visual cycle. The RDH12 enzyme can use either cis or trans retinoid isomers as substrates and can also function as both dehydrogenase (i.e. retinol to retinal) and reductase (i.e. retinal to retinol).
The conversion of retinol to retinal is the rate-limiting step in the retinoic acid biosynthesis. In vertebrates, the retinoic acid is the ligand that controls nuclear receptor signaling pathway, which is responsible for growth and development as well as epithelial maintenance, therefore can be used for cancer and acne treatment. In human, ADH4 can exhibit at least 10 folds higher Vmax/Km than other ADH.
Some retinol dehydrogenases are in extra-ocular tissues, such as human retinol dehydrogenase-4 (RoDH-4), which converts retinol and 1-cis-retinol to different aldehydes in liver and skin. It was also found that 13-cis-retinoic acid (isotretinoin), 3,4-didehydroretinoic acid, and 3,4-didehydroretinol can act as competitive inhibitor of the 3α-hydroxysteroid dehydrogenase oxidative activity of the enzyme. This can potentially explain how isotretinoin, the active ingredient is Roaccutane (Accutane), can suppress sebaceous glands and be used for severe acne treatment.
Read more about this topic: Retinol Dehydrogenase
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