title: The visual cycle operates via an isomerase acting on all-trans retinol in the pigment epithelium.
abstract: Thirty years have elapsed since Wald and his colleagues showed that 11-cis retinal was isomerized to all-trans when rhodopsin was bleached, yet little has been understood about the reverse process that generates 11-cis retinal for rhodopsin regeneration. It is not known whether the isomerization is enzyme-mediated, whether it occurs in the pigment epithelium or in the retina, or whether retinal, retinol, or a retinyl ester is the vitamin A compound that is isomerized. Radiolabeled all-trans retinol and high-performance liquid chromatography have now been used to demonstrate the existence of an eye-specific, membrane-bound enzyme (retinol isomerase) that converts all-trans to 11-cis retinol in the dark. Retinol isomerase is concentrated in the pigment epithelium; this localization clarifies the role of this tissue in rhodopsin regeneration and explains the need to transfer all-trans retinol from the rod outer segments to the pigment epithelium during the visual cycle.
mesh_terms: D000818:Animals; D001001:Anura; D002829:Choroid / Q000201:enzymology; D002851:Chromatography, High Pressure Liquid; D003623:Dark Adaptation; D006358:Hot Temperature; D066298:In Vitro Techniques; D007535:Isomerases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008027:Light; D008099:Liver / Q000201:enzymology; D010664:Phenylmethylsulfonyl Fluoride / Q000494:pharmacology; D010857:Pigment Epithelium of Eye / Q000201:enzymology*; D051381:Rats; D012160:Retina / Q000201:enzymology; D014357:Trypsin; D014801:Vitamin A / Q000378:metabolism*; D019745:cis-trans-Isomerases; D000099178:Retinoid Isomerohydrolase
chemical_list: D007535:Isomerases; D010664:Phenylmethylsulfonyl Fluoride; D014357:Trypsin; D014801:Vitamin A; D019745:cis-trans-Isomerases; D000099178:Retinoid Isomerohydrolase