THE VISUAL PIGMENTS 

 AN APPRAISAL OF THE RHODOPSIN-PORPHYROPSIN THEORY 



Recent work appears to cast some doubt on the rhodopsin- 

 porphyropsin theory, as epitomized in the last two quotations from 

 WALD. Some of this doubt stems from recent researches by wald 

 and his colleagues and some from work by the author. 



We have seen that wald's conclusions regarding the distribution 

 of the visual pigments had been reached primarily as a result of 

 studies on the decomposition products of visual pigments in bleached 

 retinae. These products were found to be either vitamin A^ or Ag or 

 a mixture of both. The density spectra of the visual pigment solu- 

 tions which could be prepared from the unbleached retinae were only 

 summarily examined, and were placed in one of three groups 

 accordingly as the maximum occurred at 500 mju (rhodopsin or 

 vitamin Aj group) or 520 m^u (porphyropsin or vitamin Ag group) or 

 between the two (mixed group). No homogeneity tests (Chap. 6) 

 were carried out. 



The main evidence for the rhodopsin-porphyropsin theory thus 

 rests on the vitamin Ai and Ag tests. 'Retinal vitamins A . . . 

 possess a special significance ; they are diagnostic of particular types 

 of visual purple system. The retinal distribution of vitamins A 

 ordinarily is paralleled precisely by distributions of the other com- 

 ponents of the visual cycles, the retinenes, and rhodopsin and 

 porphyropsin.' (wald, 1939a.) 



But it is now known that retinenes and retinencg, and vitamins A^ 

 and Ag, can exist in a number of different cis-trans forms, some of 

 which combine with suitable proteins to yield photosensitive pig- 

 ments of quite distinct properties (see Chapter 5). Nevertheless all 

 five known isomers of retinene^ give an identical density spectrum 

 when tested with the Carr-Price reagent. 'If one wishes to have a 

 measure of retinene concentration — and presumably also of vitamin 

 A — that is independent of cis-trans configuration, this is the way to 



do it' (HUBBARD, GREGERMAN and WALD (1953)). 



Thus the Carr-Price reaction can only be used for deciding 

 whether a visual pigment is derived from vitamin A^ or vitamin Ag; 

 it is useless for deciding which particular isomers of the vitamins are 

 involved. 



We are left, therefore, with the density spectra of the visual 

 pigments themselves as the only guide to their identity or otherwise. 

 The presence of varying amounts of yellow impurities in the extracts, 



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