The Role of Sulfhydryl Groups in the Bleaching and Synthesis of Rhodopsin by George Wald And

A mixture of the carotenoid retinenel and the protein Qpsin, placed in the dark, reacts spontaneously to form the light-sensitive pigment of rod vision, rhodopsin (Wald and Brown, 1950). The nature of the binding between retinenel and opsin governs the conditions under which this synthesis occurs. Conversely the opening of the linkage between opsin and its chromophore as rhodopsin bleaches may have some fundamental connection with the excitation process in rod vision. The demonstration by Morton and his colleagues that retinenex is vitamin A1 aldehyde aroused interest in the general array of aldehyde reactions in which retinenex takes part. Attention centered originally on the condensation of retinenel with amino groups in a variety of organic amines, amino acids, and proteins. Such complexes vary in spectrum with pH, suggesting a relationship to the product of bleaching rhodopsin ("indicator yellow"). Some of them also are brightly colored in acid solution (0.1 N HC1), suggesting some connection with the rhodopsin chromophore (Ball, Collins, Morton, and Stubbs, 1948; Ball, Collins, Dalvi, and Morton, 1949; Collins and Morton, 1950). We shall present evidence here, however, that sulfhydryl groups of opsin play a central part in the synthesis and bleaching of rhodopsin. The optimal conditions for rhodopsin synthesis favor the condensation of retinenel with sulfhydryl rather than with amino groups. The synthesis is blocked completely by low concentrations of the sulfhydryl reagent, p-chloromercuribenzoate; and this inhibition is reversed by the addition of glutathione. Furthermore the bleaching of rhodopsin exposes new sulfhydryl groups. In a subsequent paper we will show that retinene~ condenses spontaneously in solution with the sulfhydryl groups of cysteine and glutathione (Wald and Brown, 1952-53). Such complexes arise under the same conditions as are optimal for the synthesis of rhodopsin; and like rhodopsin, they do not vary in spectrum with pH.