Interaction of 11-cis-retinol dehydrogenase with the chromophore of retinal G protein-coupled receptor opsin

Vertebrate opsins in both photoreceptors and the retinal pigment epithelium (RPE) have fundamental roles in the visual process. The visual pigments in photoreceptors are bound to 11-cis-retinal and are responsible for the ini tiation of visual excitation, Retinochrome-like opsins in the RPE are bound to all-trans-retinal and play an important role in chromophore metabolism. The retinal G protein-coupled receptor (RGR) of the RPE and Muller cells i s an abundant opsin that generates 11-cis-retinal by stereospecific photois omerization of its bound all-trans-retinal chromophore. We have analyzed a 32-kDa protein (p32) that co-purifies with bovine RGR from RPE microsomes. The co-purified p32 was identified by mass spectrometric analysis as 11-cis -retinol dehydrogenase (cRDH), and enzymatic assays have confirmed the isol ation of an active cRDH. The co-purified cRDH showed marked substrate prefe rence to 11-cis-retinal and preferred NADH rather than NADPH as the cofacto r in reduction reactions. cRDH did not react with endogenous all-trans-reti nal bound to RGR but reacted specifically with 11-cis-retinal that was gene rated by photoisomerization after irradiation of RGR. The reduction of 11-c is-retinal to 11-cis-retinol by cRDH enhanced the net photoisomerization of all-trans-retinal bound to RGR. These results indicate that cRDH is involv ed in the processing of 11-cis-retinal after irradiation of RGR opsin and s uggest that cRDH has a novel role in the visual cycle.