A Newly Discovered Visual Cycle Necessary for Vision during Continuous Illumination

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Abstract
Rhodopsin, the light-sensitive visual pigment of the retina, is activated through photoisomerization of its prosthetic group, 11-cis-retinal, to all-trans-retinal. A protein found in the retinal pigment epithelium named retinal G protein- coupled receptor (RGR) reacts to light in an opposite but parallel way: its prosthetic group, all-trans-retinal, is photoisomerized to 11-cis-retinal. The latter is reduced to 11-cis-retinol by a cis-retinol dehydrogenase that co-purifies with the RGR. The resulting 11-cis-retinol feeds into the visual cycle to be oxidized to 11-cis-retinal, thus replenishing the 11-cis-retinal of the rhodopsin. During continuous intense illumination, RGR can supplement the 11-cis-retinal required to regenerate rhodopsin.

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