IMPORTANCE IN VISION 573 



However, it was impossible at first to bring about any appreciable syn- 

 thesis of rhodopsin when ordinary vitamin A and opsin were mixed, even 

 when the conditions outlined above were observed. Hubbard and Wald^^^ 

 found that the addition of succinoxidase in conjunction with retinal ho- 

 mogenate increased the yield of rhodopsin by 33 to 50%. The addition 

 of vitamin A and succinoxidase from beef heart^^^ increased the synthesis 

 of rhodopsin by 65%. 



Rhodopsin 



light 



Neo-6 retinene + opsin ' Al\-irans retinene + opsin 



(alcohol dehj'drogenase, cozymase) 

 Neo-6 vitamin A ' Al\-lrans vitamin A 



Fig. 2. Geometrical isomers of retinene and of vitamin A in the rhodopsin cycle."'' 



The succinoxidase functions by keeping the cozymase oxidized. Hub- 

 bard and Wald-^^ consider that the entire system consists of the following 

 components: vitamin A (fish liver oils), alcohol dehydrogenase (horse 

 liver), cozymase (yeast), and opsin (retina). It has been found that, with 

 this mixture, rhodopsin is synthesized in the dark. When returned to the 

 light, the system is again bleached. According to this concept, vitamin A 

 functions as a source of rhodopsin only after it is oxidized to retinene under 

 certain special conditions; the rest of the synthesis follows the same path- 

 way as that of retinene and of opsin. Figure 1 illustrates this modified 

 concept of the rhodopsin system. Presumably, the synthesis of porphyrop- 

 sin follows the same pattern as does that of rhodopsin. 



(a) Neo-b Retinene as a Required Precursor. Although Wald^^' was 

 able to demonstrate a functioning system when the vitamin A was a nat- 

 ural product, almost no rhodopsin was formed when a crystalline vitamin 

 A was employed in place of the natural product. It was concluded that 

 only certain stereoisomers of vitamin A are able to form the types of reti- 

 nene which are effective in combining with opsin to form rhodopsin. In 

 substantiation of this theory, Hubbard and Wald^^^ observed that, al- 

 though no rhodopsin resulted when all-^rans- vitamin A or neo-a vitamin A 



93* R. Hubbard and G. Wald, /. Gen. Physiol., 36, 269-315 (1952-1953). 



