3 FUNCTION IN THE VISUAL PROCESS 17 



retinene for this yellow pigment. Retinene is subsequently converted into 

 vitamin A and the latter is converted back into rhodopsin in the dark. To 

 some extent retinene can also be directly transformed into rhodopsin. Both 

 retinene and vitamin A always occur in the eye combined with protein^^^ j^g 

 transformations described can be summarised schematically as follows: 



Rhodopsin 

 .71 



Vitamin A^ + Protein < Retinene + Protein 



According to Wald^^, retinene can also be obtained directly without 

 illumination from eyes adapted to darkness by the extraction of visual purple 

 with chloroform. This fact, as well as certain other considerations, led Wald 

 to believe that rhodopsin is a carotenoid (retinene) -protein combination which 

 can be destroyed by illumination, heat, or suiviable solvents such as chloroform, 

 thus liberating the protein-bound carotenoid (retinene). It should be mentioned 

 that neither retinene nor rhodopsin have ever been obtained in the crystalline 

 state or analysed. Nevertheless, it cannot be doubted that the chromophoric 

 system of the carotenoids is utilised in the act of vision. Rhodopsin, ret^ene 

 and vitamin A have also been found in the eyes of mammals and salt water 

 fish°^ e.g. Prionotus carolinus, Centropristes stiratus, Stenotomus chrysops, and 

 the cycle between these substances again appears to be the same. 



Many fresh water fish contain a different light-sensitive pigment, por- 

 phyropsin^^, in place of rhodopsin. The reactions involved in the visual process 

 of such fresh water fish, e.g. Morone americana, Perca flavescens, and Esox 

 rcticulatus were examined by Wald^^ who found that they were similar in 

 character to those occurring with rhodopsin. During the illumination of por- 

 phyropsin, retinenCg is formed, which is in turn converted into vitamin Ag. 



Morton and his collaborators have recently shown^^ that retinene^ is 

 identical with vitamin A^ aldehyde and retinenCg with vitamin A^ aldehyde. 



REFERENCES 



1. P. Karrer, a. Helfenstein, H. Wehrli and A. Wettstein, Helv. Chim. Acta 

 13 (1930) 1084. 



2. R. KuHN and Ch. Grundmann, Ber. 65 (1932) 898, 1880. — R. Kuhn and A. Winter- 

 stein, Naturwissenschaften 21 (1933) 527; Ber. 6j (1934) 344; (>5 (1932) 646. — 

 R. Kuhn, Forsch. u. Fortschr. 9 (1933) 426. — R. Kuhn and A. Deutsch, Ber. 66 (i933) 

 883. 



P. Karrer and T. Takahashi, Helv. Chim. Acta 16 (1933) 287. 



3. A very detailed description of these processes will be found in L. Zechmeister's 

 Monograph Die Carotinoide, BerUn, 1934, Springer- Verlag. 



Carotenoids 2 



