THE ACTION OF LIGHT ON RHODOPSIN

Publisher Website
Google Scholar
Abstract
Experiments with rhodopsin obtained from the squid have yielded the following information. Rhodopsin is composed of the colorless protein, opsin, combined with the hindered cis isomer of retinene, neo-b(ll-cis). Light isomerizes the neo-b chromophore, while still attached to opsin, to the all-trans configuration, thereby converting rhodopsin to all-trans lumi- and meta-rhodopsin. This is probably the reaction responsible for visual excitation. Having converted rhodopsin to all-trans lumirhodopsin, the continued presence of light can isomerize the all-trans chromophores of lumi- or meta-rhodopsin to steady-state mixtures of stereoisomeric chromophores, all still attached to opsin. The neo-b fraction of this mixture constitutes rhodopsin, the iso-a fraction isorhodopsin. The other isomers form stereoisomeric sets of lumi- and meta-rhodopsins, all of which hydrolyze in the dark to retinene and opsin. Since light induces not only the bleaching of rhodopsin but also its regeneration, the quantum efficiency of bleaching should decrease under all conditions which favor the photoregeneration of rhodopsin from lumi-or meta-rhodopsin. The quantum efficiency of vision, however, probably, depends only on the initial photoisomerization of rhodopsin to all-trans lumi-rhodopsin and should not be affected by the photochemical back-reactions.