THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



formulations for the alkaline and acid forms of the indicator yellow 

 analogues prepared by the Liverpool group are : 



I ^ 



a r 



R ^ OH© \^/\ R 



Amax = 360-370 m^ Amax = 435^60 myu 



Analogue of alkaline indicator yellow Analogue of acid indicator yellow 



where R is protein in the case of true indicator yellow. 



IS THE STRUCTURE OF INDICATOR YELLOW A 

 CLUE TO THE STRUCTURE OF VISUAL PURPLE? 



According to wald (1938) lythgoe's pH-sensitive indicator 

 yellow is 'a mixture of retinene^ and protein, about two-thirds still 

 loosely attached to each other.' wald states (1949) that when visual 

 purple is bleached in the retina 'the carotenoid is cleaved from 

 protein.' He regards indicator yellow as an unimportant pheno- 

 menon of visual purple solutions and certainly not a part of the visual 

 cycle in vivo. In their 1948 paper, ball, colli ns, morton and 

 STUBBS were apparently of the same opinion. Struck by the fact that 

 indicator yellow analogues could be prepared by the interaction of 

 retinenci and many amino compounds (see table, p. 106) they con- 

 sidered that 'the conclusion to be drawn concerning "indicator 

 yellow" is that free or loosely bound retinenci (the final product of 

 bleaching rhodopsin) may combine with any suitable protein or 

 amino-acid which may be available. In that case "indicator yellow" 

 would be a fortuitous artefact having no direct relevance to visual 

 chemistry, and wald's cycle connecting vitamin A retinene and 

 rhodopsin need not be compHcated by trying to fit in indicator 

 yellow.' 



This view was questioned by dartnall (1948) who referred to 

 lythgoe's (1937) inability to extract retinene from bleached visual 

 purple solutions using the bland petroleum ether alone as solvent 

 and to the statement by hecht (1942) that in order to extract the 

 retinene the active agent alcohol must be added to petroleum ether. 

 HECHT had also observed that if cold acetone is added to a bleached 

 visual purple solution the precipitate 'contains not only the protein 

 but most of the yellow colour. The retinene must therefore still be 



109 



