THE VISUAL PIGMENTS 



when the retinae of dark-adapted white perch {Perca flavescens) 

 were extracted with benzine, the visual pigment (porphyropsin) was 

 not affected. The extract, which was only faintly yellow gave a very 

 pale blue colour (Amax = 696 m/0 with the Carr-Price reagent. 

 When the retinae were bleached and immediately extracted, however, 

 the extract was deep orange in colour and gave an intense blue 

 colour (Amax = 703) with antimony trichloride. Bleached and faded 

 retinae yielded a light yellow extract giving an intense blue reaction 

 (Amax = 696 m[j). 



In an examination of fish liver oils, edisbury, morton and 

 SIMPKINS (1937) reported that they had repeatedly found, on testing 

 with the Carr-Price reagent, a band at 693 m// in addition to that 

 at 620 TCifi (due to vitamin A). They stated 'wald's discovery that 

 a substance apparently identical with the 693 m^ chromogen can 

 replace the vitamin A of rhodopsin without loss of physiological 

 function runs parallel with the similar replacement in the viscera and 

 liver of the brown trout. It therefore seems desirable provisionally to 

 designate as 'vitamin Ag' the 693 m/^ chromogen with its characteris- 

 tic ultra-violet absorption.' 



Because of this suggestion, wald (1939b) proposed the name 

 retinencg for the orange substance (yielding the 703 m/^ band with 

 antimony trichloride) extracted from freshly bleached fish retinae. 



Thus WALD proposed a porphyropsin visual cycle with vitamin 

 A2 and retinencg, analogous to that of rhodopsin : 



Porphyropsin 

 (522 m/0 



Vitamin Ag + protein < Retinencg + protein 



(355 m^ in chloroform) (405 m/f in chloroform) 



(SbClg -> 696 mfx) (SbClg -^ 703 m//) 



The density spectra of rhodopsin and porphyropsin, and of their 

 related retinenes and vitamins A, are shown in Fig. 2.2. 



the rhodopsin-porphyropsin theory 



The early investigations of kottgen and abelsdorff (p. 29) 

 had indicated that there were two kinds of visual pigment. One with 



32 



