in. lUOCHEMICAL SYSTEMS 



75 



yeast, and alcohol tlehytlrog(>uaso from liorsc livers. Only ojxsin needs to 

 be obtained from the retina. When such a mixture is placed in the dark, it 

 synthesizes rhodopsin (Fig. 20). liioughl into the light, it bleaches. Re- 

 placed in the dark, it makes more rhodopsin. This mixture of four sui)- 

 stances carries out in solution all the reactions of the rhodopsin system. ''s 



F. cis-trans ISOMERS OF VITAMIN A AND RETINENE 



The four-component system just described was assembled originally 

 usins: a fish liver oil concentrate as the source of vitamin A. When this 



Rhodopsin 



alcohol delivdmgcnasc 

 DPN-H, 



Vitamin Ai -t- Opsin -^ 



Vitamin Ai from 



pigment epithelium 



and circulation 



DPN 



oxidative 



systems, 



O, 



light 

 Lumi-rhodopsin 



>-20°C. 



-Mcta-ihodopsin 

 H,0 



r 



Retinenei + Opsin 



DPX-H, 



Fig. 19. Diagram of the rhodopsin system. The bulk of the system lies within the 

 outer segments of the retinal rods, hut it is supplemented with vitamin A, respiratory 

 factors, and oxygen itself from the pigment epithelium and the blood circulation. 

 (From G. Wald.«^) 



experiment was repeated using (Mystalline vitamin A, almost no rhodopsin 

 was formed. 



Such difference in behavior in two preparations of vitamin A could have 

 only one explanation. \'itamin A, like other carotenoids, is known to exist 

 in several different forms, geometrical or cis-trans isomers of one another. 

 Most of what is known of cis-trans isomerization among carotenoids is due 

 to the work of Zechmeister and his colleagues; in close association with 

 Zechmeister's experiments, Pauling has developed the theory of the sub- 

 ject.''^ 



According to Pauling, in such structures as the carotenoids, cis-trans 



«« G. Wald, Science 113, 287 (1951). 



^» L. Zechmeister, Chetn. Kevs. 34, 267 (1944). 



