60 



VITAMINS A AND CAROTENES 



Rhodopsin 

 / \\light 



Vitamin A + protein 



Retinene + protein 



Rhodopsin bleaches in the Hght over orange intermediates to a mixture 

 of the yellow carotenoid, retinene, and a protein which we now call opsin. 



zoo 



400 SCO 



Wa vs length- m^ 



Fig. 12. Spectra of rhodopsin and of the product of its bleaching in aqueous solu- 

 tion. Bullfrog rhodopsin in solution in 2% aqueous digitonin at pH 5.55. Rhodopsin 

 possesses three absorption maxima: the a band, which is mainly responsible for the 

 spectral sensitivity of rod vision; the /? band which together with the a band belongs 

 to the carotenoid prosthetic group; and the high y band, which is due to the protein, 

 opsin. On bleaching in solution the a and /3 bands are replaced by the absorption 

 spectrum of retinene; the protein band remains unchanged. (From G. Wald.'*^) 



In the usual solutions of rhodopsin these are the final products of bleaching 

 (Fig. 12). In the retina, however, and in the enzyme systems to be de- 

 scribed below, retinene is converted to the colorless vitamin A. Both vita- 

 min A and retinene recombine with opsin to regenerate the visual pigment 

 (Fig. 13). 



In the rods of fresh-water fishes, lampreys, and certain amphibians, 

 another light-sensitive pigment is found. This, because of its purple color, 



