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 synthesized 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 spectroscopically from rhodopsin but bleaches in the light to the identical mixture of alltrans retinene and opsin. The synthesis and bleaching of these pigments therefore involve cycles of stereoisomerization of retinene. ' This seems to be a general situation that involves all the known visual pigments-rhodopsin, iodopsin, porphyropsin, and cyanopsin-and the corresponding isopigments.2' 3 If rhodopsin or isorhodopsin is bleached with heat instead of light, the retinene is released in its original cis configuration.4 Stereoisomerization to the all-trans configuration is therefore associated not with all bleaching but specifically with bleaching 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.6'7 Light alone does not bleach (i.e., decolorize) rhodopsin but converts it to the red photoproduct, lumi-rhodopsin. Lumirhodopsin 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, 9 The slow course of this latter reaction strongly suggests that visual excitation precedes the release of retinene and depends upon the light reaction itself or, at most, on the conversion of lumi-rhodopsin to meta-rhodopsin. 10 On warming meta-rhodopsin in the dark, one obtains a mixture containing roughly one part of retinene and opsin and one part of rhodopsin and isorhodopsin.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 isorhodopsin, 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. -Squid rhodopsin 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