THE PHOrORECEPTOR PROCESS IN VISION 



677 



CH, 



H,C 



HC 



\ 



C 

 H 



CH3 



/ 



CH, CH;, 



^ H H I H H H 1 H H 

 C— C=C— C=C— C=C— c:=c— c=o 



II 

 C-CH3 



Retinene., 



The properties of the porphyropsin system arc in 

 general precisely parallel with those of the rhodopsin 

 system. Alcohol dehydrogenase and DP\ catalyze 

 the equilibrium between retinenes and vitamin Ao 



just as between retinenei and vitamin Ai (64). The 

 bleaching of porphyropsin yields an inactive form of 

 retineneo, apparently the dW-lrans isomer. 



The geometrical isomers of retinenez have not been 

 investigated as thoroughly as those of retinenei. 

 Nii-trans retinene2 has been crystallized. Two cis 

 forms of retinene2 have been partially purified, though 

 not crystallized. These resemble in their spectro- 

 scopic properties respectively the neo-i and iso-o 

 isomers of retinenei. Neo-fe retinene^, when incubated 

 in the dark with opsin, yields porphyropsin, in- 

 distinguishable from that extracted from a dark- 

 adapted fresh-water fish retina; whereas iso-a retinene 

 treated similarly yields a comparable pigment, iso- 



3.0 



2.5 - 



2.0 Y 



C 

 .0 







K 



1.0 



OS 



- 



1 \ — r 



"Porphyropsin - yo//ow/ perch 

 • unbleached 

 o bleached 



J L 



J 1 1 1 ^_ 



\ L 



300 



400 



£00 



600 



FIG. 8. Absorption spectra of porphyropsin and of the product of its bleaching (pH 7.0) from 

 the fresh-water yellow perch, Perca flavescens. This preparation was extracted with 2 per cent digi- 

 tonin from a suspension of rod outer segments, which had been previously hardened with alum, 

 and pre-e.\tracted with water and with petroleum ether. Porphyropsin, like rhodopsin, possesses 

 three absorption bands: the a-band about 522 m^ at, the /3-band at about 377 m/x, and the 7-band 

 (opsin) at about 280 m/i. On bleaching, the a- and /J-bands are replaced by the absorption band 

 of retineneo , at about 400 m^i. [From Wald, G., P K. Brown & P. .S. Brown, unpublished ob- 

 servations, i 



