The photochemical system upon which the excitation of rod vision depends has been analyzed, and all its component processes brought into free solution. The only action of light in this system is to convert rhodopsin to the highly unstable lumi-rhodopsin. This bleaches in the dark via the intermediate meta-rhodopsin to a mixture of the carotenoid, retinene1, and the colorless protein, opsin. Retinene1 is reduced to vitamin A1 by the coenzyme, reduced cozymase, acting in concert with the enzyme retinene reductase or alcohol dehydrogenase. These are the degradative processes in the rhodopsin system.
The resynthesis of rhodopsin from these products is the basis of dark adaptation. A mixture of opsin and retinene1, forms rhodopsin spontaneously in the dark. The retinene reductase system, left to itself, reduces retinene1 to vitamin A1. In the presence of opsin, however, it oxidizes vitamin A1 to retinene1 as rapidly as retinene, condenses with opsin to form rhodopsin.
A mixture of four substances in solution performs all the reactions of the rhodopsin system: vitamin A1, cozymase, alcohol dehydrogenase, and opsin. Opsin is the only one of these specific to the retina.
The synthesis of rhodopsin requires the presence of free sulfhydryl (-SH) groups in opsin. Conversely when rhodopsin bleaches, two such groups are liberated for each retinene1 formed. Based upon these changes, an artificial system has been devised in which the bleaching of rhodopsin results in an electrical variation. This may provide a model for the excitation process in rod vision.
GEORGE WALD, "The Photochemical Basis of Rod Vision," J. Opt. Soc. Am. 41, 949-955 (1951)