ISOMERISM AND THE VISUAL PIGMENTS 



The only other isomer which combines with cattle opsin is iso- 

 retinenei a. But in this case, a new photosensitive pigment, having 

 -^max at 487 m^ is formed. This new pigment, zjo-rhodopsin, has not 

 been identified with any naturally-occurring visual pigment, wald 

 regards it, for the present, as an artifact. 



Recently, wald and his colleagues have briefly reported the 

 results of parallel experiments with other visual pigments. The 

 experimental details of these researches have not yet been pubhshed 

 in full. 



PORPHYROPSIN 



The first of these researches concerns porphyropsin the 'purple 

 light-sensitive pigment, associated characteristically with the rods of 

 vertebrates which originate in fresh water — freshwater fishes, lam- 

 preys, and certain larval and adult amphibia' (wald 1953a). Refer- 

 ring to unpubHshed work in collaboration with brown and smith, 

 WALD (1953a) writes, 'the bleaching of porphyropsin yields what is 

 apparently the all-trans isomer of retinencg. This is inactive in re- 

 synthesizing porphyropsin. From a mixture of retinencg isomers, 

 however, we have isolated two active fractions, which appear to 

 contain two cis isomers of retinencg, which can for the present be 

 called cisi and cis 2. Neither has been completely purified or crystal- 

 lized. The cisi fraction, incubated in the dark with a solution of 

 opsin from a fresh- water fish, yields porphyropsin (Amax 523 m/x). 

 The cis2 fraction, treated similarly, yields a comparable pigment with 

 -^max 507 mju; this is /50-porphyropsin.' wald, brown and smith 

 (cited by wald, 1953a) found that the active retinencg isomers would 

 also combine with cattle opsin to form photosensitive pigments. 

 With cattle opsin, however, the spectra of the synthesized pigments 

 were displaced to shorter wavelengths than in the case of fish opsin, 

 viz. maxima at 517 m/t (ciSi isomer) and 501 m/< (c/^g isomer). Since 

 the absorption spectrum of cattle rhodopsin (Amax 498 m/u) is simi- 

 larly displaced from frog rhodopsin (Amax 502 mju) wald (1953a) 

 considers these experiments show that 'the opsins are so closely 

 related in the rhodopsin and porphyropsin systems that it seems 

 proper to regard them as belonging to the same family, the scotop- 

 sins. The rhodopsin and porphyropsin systems therefore share 

 throughout the same proteins; it is only their carotenoids which 

 differ' (wald, 1953a). 



151 



