RETINAL PHOTOPIGMENTS 



125 



products of bleaching. In the isolated retina the orange 

 color fades and the tissue becomes colorless. The extracts of 

 completely faded retinas are also colorless. They contain 

 no retinene, but a large quantity of newly formed vitamin A, 

 the final product of bleaching. The same results are achieved 

 by prolonged illumination of the intact retina. In the living 

 animal placed back in darkness, vitamin A is resynthesized 

 to rhodopsin. The retinal cycle is thus represented by Wald 

 (1938, 1939a, 19396) as follows: 



/ 



(1) 



Vitamin A + protein -<— 



\ 



(328 m/t in chloroform) 

 (SbClg—^ 615-620 m/t) 



/ 



Rhodopsin 

 (500 m//) 



(3) 



Retinene + protein 



I 

 (387 m/t in chloroform) 

 (SbCl 3 ^ 664 m/x ) 



In isolated retinae the cycle is cut at (1) so that vitamin A 

 is the final product of bleaching. When rhodopsin is extracted 

 into aqueous solution, reactions (2) and (3) are also elimi- 

 nated, leaving only the succession of light and dark processes 

 which form retinene. 



Kiihne and Sewall (1880) noted that the dark-adapted 

 fish retinas which they studied were unlike those of all other 

 vertebrates in that they were purple-colored. Kottgen and 

 Abelsdorff (1896) confirmed this difference spectrophoto- 

 metrically. This purple photopigment was called porphy- 

 ropsin by Wald. Although it was thought for many years 

 that the fishes as a class possess the purple pigment por- 

 phyropsin, an examination of the retinas of a number of 

 marine fishes, by Wald (1936-1937), showed them to possess 

 typical rhodopsin cycles, and showed further that por- 



