Cis-trans Isomers of Vitamin

We have recently described the synthesis of rhodopsin in a solution containing four components: vitamin A, the precursor of the rhodopsin chromophore; opsin, the protein of rhodopsin; and liver alcohol dehydrogenase and cozymase, the enzyme and coenzyme which oxidize vitamin A to retinene (Hubbard and Wald, 1951). 1 This experiment was first performed with a fish liver 0il concentrate as the source of vitamin A. When later we attempted to repeat it using crystalline vitamin A, almost no rhodopsin was formed. Such differences in behavior in two samples of what is ordinarily thought of as the same substance could have only one explanation. The vitamin A molecule exists in several different shapes, as different geometrical or cis-trans isomers. Liver oils are known to contain a mixture of such isomers, while ordinary crystalline vitamin A is a single isomer. Carotenoids in general are converted from any single cis-trans configuration to an equilibrium mixture of stereoisomers by irradiation with light in the presence of a trace of iodine. When crystalline vitamin A was treated in this way, it became as effective a precursor of rhodopsin as the liver oil concentrate. These experiments made it plain that the geometrical configuration of vitamin A is a dominant factor in the rhodopsin system. The present paper is concerned with the analysis of this relation.