THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



benzine or carbon disulphide, the visual purple was not affected and 

 nothing, save a trace of vitamin A, was extracted. When the retinae 

 were treated with chloroform, however, the visual purple was de- 

 colourized, and the extract contained a yellow pigment (^^max = 

 385 mju) which wald named 'retinene.' With the Carr-Price 

 reagent, retinene yielded a blue colour (Amax = 664 mju). 



WALD found that retinene was freely soluble in carbon disulphide 

 or benzine, even though these homopolar solvents would not extract 

 it from dark-adapted retinae. He concluded therefore that the 

 retinene was originally bound to a non-lipoidal complex. 



When the retinae of frogs which had been Hght-adapted were 

 extracted with benzine or chloroform, colourless solutions were 

 obtained, containing appreciable amounts of vitamin A, but no 

 retinene. 



The relationship between visual purple, retinene and vitamin A was 

 revealed by further experiments with isolated retinae. On exposing 

 the retinae to bright daylight they changed from a deep red colour to 

 orange. They then slowly faded until, in about an hour they were 

 colourless, wald found that when retinae were extracted immedi- 

 ately after bleaching, the extracts contained large quantities of 

 retinene, but only a trace of vitamin A. After fading had started, 

 however, the extracts contained less retinene and appreciable 

 amounts of vitamin A. As fading proceeded, the amount of retinene 

 became less and that of the vitamin A, more. 



From these and other experiments wald concluded that vitamin 



A, retinene and visual purple (rhodopsin) were related in a visual 



cycle as follows : 



Rhodopsin> 



(500 mju) 



Vitamin A + protein 

 (328 m// in chloroform) 

 (SbClg-^ 615-620 m^) 



Retinene + protein 

 (385 m/j, in chloroform) 

 (SbCls -> 664 m/x) 



THE PORPHYROPSIN (VISUAL VIOLET) CYCLE 



Similar experiments were subsequently carried out with fresh- 

 water fish— and with parallel results. Thus wald (1937) found that 



31 



