570 IX. CAROTENOIDS AND VITAMINS A 



protein is opsin, and it is identical Avith the protein moiety of rhodopsin. 

 Moreover, the enzymes which activate the porphyropsin system are iden- 

 tical with those which function in the rhodopsin system. 



(c) lodopsin. In 1937, Wald^-'^ was the first to report a light-sensitive 

 pigment (iodopsin) in the cones; this was identified in the retina of the 

 chicken. lodopsin was found to undergo the same series of reactions as 

 did rhodopsin and porphyropsin. The carotenoid prosthetic group was the 

 same as that in rhodopsin, namely retinenei, while the protein differed. "^ 

 The family of proteins associated with rod vision are called scotopsins, 

 while those concerned with cone vision are referred to as photopsins. 



b. Vitamin A Aldehyde, (a) Vitamin Ai Aldehyde (Retinenei). This 

 constitutes the only type of retinene originally recognized. It is the only 

 type of retinene formed when light acts upon rhodopsin. Wald and co- 

 workers^'^ listed lumirhodopsin (a highly unstable, orange-red product), 

 metarhodopsin, and then, in the presence of water, retinene, and protein, 

 as the intermediates in the rhodopsin -»- retinene + protein reaction. 

 Lumirhodopsin is converted to metarhodopsin in the dark. 



Retinene ( or retinenei) was found, by Ball, Goodwin, and Morton, -^^ to 

 be the aldehyde (C19H27CHO) of ordinary vitamin A alcohol (CjgHoy- 

 CH2OH). For a description of vitamin Ai aldehyde, including the chem- 

 ical and biochemical procedures for synthesizing it, the reader is referred 

 to an earlier section (see page 451). 



(6) Vitamin A2 Aldehyde {Retinene-^. Retinenco Avas first character- 

 ized by Wald"*'^-^ as a component of the visual systems of fresh-water 

 fishes; Wald^-^ also recognized its relationship to a vitamin A other than 

 vitamin Ai. In addition to Wald,^-^'^"^ a number of other investigators^"^ •^"' 

 recognized the fact that retinene2 is vitamin A2 aldehyde. Cama et al?^^ 

 demonstrated that vitamin A2 aldehyde (retinene2) was readily converted 

 to vitamin A2 in the rat. Retinene2 can be prepared from vitamin A2 by 

 oxidation with manganese dioxide.-''^ 



c. The Alcohol Dehydrogenase System. In order that the retinal pig- 

 ments may function in vision, it is necessary that provisions be made for 

 their continual regeneration. If the sensation of sight requires the forma- 

 tion of retinene and protein, with the consequent bleaching of the retina, 

 a new sensation cannot be recorded until the rhodopsin supply in the retina 

 has been restored. Although some regeneration can occur by a direct 

 combination of retinene and the appropriate protein, this is quite ineffective 



9" G. Wald, Nature, I4O, 545-546 (1937). 



s2« G. Wald, J. Durrell, and R. C. C. St. George, Science, 111, 179-181 (1950). 



