74 VITAMINS A AND CAROTENES 



Rhodopsin 



alcohol dehydrogenase 

 cozymase ^ 



Vitamin A ^, — . Retinene 



hydroxy lamine 



Retinene oxime 



needed to make rhodopsin in this way, yet it does not visibly produce 

 rhodopsin. 



Careful extraction of whole frog retinas which had been bleached to 

 colorlessness and then incubated in the dark nevertheless showed that they 

 do form a little rhodopsin, perhaps 10 % as much as would be formed during 

 dark adaptations vivo. Retinal homogenates behave similarly. When retinal 

 homogenates were supplemented with cozymase, the yield of rhodopsin 

 was doubled. Kiihne had believed that the synthesis of rhodopsin from 

 colorless precursors — i.e., from vitamin A — demands the cooperation of 

 the pigment epithelium; and it was indeed found that the addition to 

 retinal homogenates of a homogenate of pigment epithelium doubles the 

 yield of rhodopsin again, bringing it to about 40%.''^ 



What the pigment epithelium contributes to this synthesis is not yet 

 wholly analyzed. One factor which it is known to contribute, however, is 

 vitamin A. A frog retinal homogenate can be shown to make rhodopsin 

 from vitamin A supplied by a pigment epithelium homogenate. Alterna- 

 tively, the addition of free vitamin A to a retinal homogenate increases 

 considerably the yield of rhodopsin. Another factor which helps is the 

 addition of respiratory enzymes — for example, the particles which form 

 the "succinoxidase" system of pig heart — which aid presumably by keeping 

 cozymase oxidized. When all these types of supplementation were combined, 

 yields of rhodopsin were obtained as high as 60 %.''^ 



It can be concluded that all factors which promote the oxidation of 

 vitamin A to retinene aid the synthesis of rhodopsin. The rhodopsin system 

 can therefore be formulated as in Fig. 19. What was originally thought to 

 be a special pathway for the synthesis of rhodopsin from vitamin A now 

 appears to consist in the special conditions required to drive the oxidation 

 of vitamin A to retinene. 



If this view of the rhodopsin system is correct, it should be possible to 

 assemble the entire system in solution by mixing four su])stances: vitamin 

 A, alcohol dehydrogenase, cozymase, and opsin. This is correct. We have 

 made such mixtures, using vitamin A from fish liver oils, cozymase from 



^8 R. Ilubbanl and G. Wald, Proc. Natl. Acad. Sci. U. S. 37, 69 (1951). 



