Retinol dehydrogenase visual cycle

Since alcohol dehydrogenase is required for the conversion of retinol to retinal, excessive and prolonged ethanol ingestion can impair the physiological function of vitamin A. The decreased conversion of retinol to retinal results from competitive use of the enzyme by ethanol. Night blindness may result, since the visual cycle is a retinol-dependent physiological process. 

In the recovery of the visual apparatus, the G(a)-GTP-PDE complex is inactivated by spontaneous cleavage of the GTP. Activated rhodopsin is the substrate for a kinase which may deactivate it. The kinase is inhibited by cGMP, so that the amount of activated rhodopsin may be subject to feedback regulation. The trans-retinal may be isomerized directly to 11-cis-retinal, which recombines with opsin to form rhodopsin or it may first be reduced to retinol by NADH-dependent alcohol dehydrogenase, followed by isomerization of the retinol and reoxidation to 11-ci s-retinal (Fig.). Some of the retinal is continually lost from the cells of the retina, so that the continuation of the visual cycle depends on continual replacement from the blood. 

Driessen CA, Winkens HJ, Kuhlmann ED, Janssen AP, van Vugt AH, Deutman AF, Janssen JJ (1998) The visual cycle retinol dehydrogenase possible involvement in the 9-cis retinoic acid biosynthetic pathway. FEBS Lett 42S 135-140... 

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