ISOMERISM AND THE VISUAL PIGMENTS 



Species is premature, particularly since the 'porphyropsin* band is 

 centred at a wavelength which, in different fresh-water fish, varies at 

 least from 520 to 533 m/u (see Chap. 2). Again, iodopsin, a presump- 

 tive cone pigment which, to date, has been found only in chicken 

 retinae (wald, 1937, bliss, 1941) is, in the table, so related to 

 rhodopsin as to imply that all retinae containing it, contain iodopsin 

 also — provided that retinal cones (and hence 'photopsin') are present. 

 Similarly with cyanopsin and porphyropsin. 



In effect the table combines the results of the experiments on cis- 

 trans isomerism with wald's earlier conclusion (Chap. 2) that fhere 

 are only two rod pigments — rhodopsin and porphyropsin, based 

 respectively on 'retinenci' and 'retineneg.' Now that it has been 

 shown that retinenej (for example) can exist in several cis-trans forms 

 — all of which give the same Carr-Price reaction the validity of 

 the earlier conclusion is questionable (cf. Chap. 2, p. 38 et seq). 

 Nevertheless even in 1953b wald wrote 'in the rods of vertebrates 

 there is substantial evidence for the existence of only two visual 

 pigments, rhodopsin and porphyropsin. The distribution of these 

 pigments has been explored to the point at which it begins to seem 

 probable that there are no others. On the other hand, iodopsin 

 marks only a beginning with the visual pigments of vertebrate cones. 

 More pigments than this are needed, if only to provide a basis for 

 colour vision in animals.' 



But if more cone pigments than iodopsin (and cyanopsin) are 

 necessary, how are we to envisage their composition? If, for ex- 

 ample, a new cone pigment based on a retinenci isomer were dis- 

 covered, how would it modify the table ? One would have to suppose 

 either that the pigment contained a protein different from 'photopsin,' 

 or a retinenci isomer different from neo-b or iso-a. In either case a 

 difficulty arises. For the one supposition destroys the scotopsin- 

 photopsin conception and the other endangers the rhodopsin- 

 porphyropsin theory (because the new retinenci isomer would, 

 presumably, combine with a scotopsin to form a new rod pigment). 



In the next chapter we review the results of some homogeneity 

 tests which give illustrative point to these remarks. 



REFERENCES 



Ball, S., Goodwin, T. W. and Morton, R. A. (1948). Studies on vita- 

 min A. 5 : The preparation of retinenej-vitamin A aldehyde. Biochem, 

 /., 42, 516-523. 



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