The Action of Light on Rhodopsin

Rhodopsin, a red visual pigment of vertebrate rods, is composed of the yellow carotenoid derivative, retinene, joined to the colorless protein, opsin.It is syn- thesized by the combination of opsin with a specific hindered cis isomer of retinene, neo-b (11-cis) and bleaches in the light to a mixture of opsin and all-trans retinene (cf.Fig. 1).Another cis isomer of retinene-iso-a (9-cis)-combines with opsin to form a second photosensitive pigment, isorhodopsin.Isorhodopsin differs spec- troscopically from rhodopsin but bleaches in the light to the identical mixture of all- trans retinene and opsin.The synthesis and bleaching of these pigments therefore involve cycles of stereoisomerization of retinene.' This seems to be a general situ- ation that involves all the known visual pigments-rhodopsin, iodopsin, porphyropsin, and cyanopsin-and the corresponding isopigments.2'3If rhodopsin or isorhodopsin is bleached with heat instead of light, the retinene is released in its original cis configuration.4Stereoisomerization to the all-trans con- figuration is therefore associated not with all bleaching but specifically with bleach- ing by light.,It has been known for some years that, in the light, rhodopsin bleaches in stages, over intermediates which are short-lived at room temperature, yet stable at low temperatures or in the absence of water.6'7Light alone does not bleach (i.e., decolorize) rhodopsin but converts it to the red photoproduct, lumi-rhodopsin.Lumi- rhodopsin is stable below about -45°C.Above this temperature, in light or darkness, it goes over to a second orange-red compound, called meta-rhodopsin.In lumiand meta-rhodopsin the chromophore is still attached to opsin.The major color shift-i.e., bleaching-accompanies the hydrolysis of meta-rhodopsin to retinene and opsin.In light or darkness, vertebrate meta-rhodopsins hydrolyze above about -150 C.,7 squid and lobster meta-rhodopsins above about +20°C.8, 9The slow course of this latter reaction strongly suggests that visual excitation pre- cedes the release of retinene and depends upon the light reaction itself or, at most, on the conversion of lumi-rhodopsin to meta-rhodopsin. 10n warming meta-rhodopsin in the dark, one obtains a mixture containing roughly one part of retinene and opsin and one part of rhodopsin and isorhodop- sin.7' 11 It was assumed earlier that all these products were formed in the dark, i.e., that meta-rhodopsin in the dark was converted back to rhodopsin and isorho- dopsin, in addition to being degraded to retinene and opsin.7' 1' Recent experiments, however, have clarified the mechanism of the light reaction and necessitate a reinterpretation of these observations.Squid rhodopsin.-Squidrhodopsin does not usually bleach in the light, owing to the relative stability of squid meta-rhodopsin (see above).This enables one to examine squid meta-rhodopsin under more favorable conditions than vertebrate meta-rhodopsins.Thus we have found that, whereas the chromophore of squid rhodopsin, like vertebrate rhodopsin, has the neo-b (1 -cis) configuration, that of meta-rhodopsin is all-trans.Thermal bleaching, which does not isomerize the