572 



IX. CAROTENOIDS AND VITAMINS A 



favors reduction rather than oxidation. However, it has been found that 

 the dehydrogenase system can bring about an oxidation if the aldehyde so 

 formed is trapped, so that it is immediately removed from the sphere of 

 action. Cyanide, bisulfite, and semicarbazide have been employed for 

 this function, but Wald and Hubbard^^* used hydroxylamine successfully; 

 this behaves as shown below : 



C19H27CHO 

 Retinene 



+ H2NOH 



Ci9H27CH=NOH + H2O 

 Retinene oxime 



Hydroxvlamine 



The Method of "Trapping" Retinene by Condensation with Hydroxylamine to Form 



Retinene Oxime*^'^ 



Wald and Hubbard^^^ were able to effect a considerable oxidation of vitamin 

 A to retinene by the alcohol dehydrogenase system, in the presence of hy- 

 droxylamine. Since opsin is a good aldehyde de-trapping agent, it should 

 be able to bring about the formation of rhodopsin. 



Rhodopsin 



Vitamin Ai -|- Opsin 



letinene reductase 

 DPN — H2 



light 

 Lumi-rhodopsin 



>-20°C. 

 Meta-rhodopsin 

 HjO 



Retinenei + Opsin 



Vitamin Ai from 

 pigment epithelium 

 and circulation 



DPN 



oxidative^ 

 sjstems, 



O2 



DPN— H2 



Fig. 1. Diagram of the rhodopsin system. (The bulk of the system lies within the 

 outer segments of the retinal rods, but it must be supplemented with vitamin A, respira- 

 tory factors, and oxygen itself, from the pigment epithelium and from the circulatory 

 blood.)s" 



933 G. Wald and R. Hubbard, Proc. Nat. Acad. Sci., U.S., 36, 92-102 (1950). 

 ss" G. Wald, Science, 113, 287-291 (1951). 



