680 



VII. 



VITAMINS A 



TABLE 3 

 Distribution of Vitamin A2 in the Pike Perch {Lucioperca sandra) 



Adapted from E. Lederer and F. H. Rathmann, Biochem. J., 32, 1252-1261 (1938); 

 by R. A. Morton, The Application of Absorption Spectra to the Study of Vitamins, Hor- 

 mones, and Coenzymes, 2nd ed., Jarrell, Ash, Boston, 1942, p. 79. 



{4) Vitamin- A-Like Compounds in the Retina 



The retina has long been recognized as a concentrated source of a pig- 

 ment resembling vitamin A. This pigment has been known as visual 

 purple or rhodopsin; apparently it is a conjugated protein in which the 

 vitamin- A-like product serves as the prosthetic group. Rhodopsin has 

 frequently been referred to in the literature as a "carotenoid" protein. 

 Inasmuch as recent findings have indicated that the prosthetic group is 

 vitamin A aldehyde,''^"^^ rather than one of the C40 polyenes, this terminol- 

 ogy would appear to be somewhat misleading. 



On being bleached by light, the retinas are changed from the pinkish 

 color of rhodopsin to a light yellow color. This occurs as a result of a ser- 

 ies of photochemical and thermal reactions. Wald^^ was able to prepare 

 the vitamin-A-like product so formed by a petroleum ether extract of il- 

 luminated retinas. He called it retinene (now subdivided into retinenei 

 and retinene2). Wald reported that this product was not identical with 

 vitamin A, since it had an absorption maximum at 365 m^u in petroleum 

 ether (instead of 325 mju), and the absorption maximum in chloroform was 

 displaced to 385 m/x. When treated with antimony trichloride, the retinene 

 preparation displayed a bluish color similar to that exhibited by a number 

 of carotenoids, but the absorption maximum was at 664 m^i instead of at 

 615-620 mix, as is the case with vitamin A. Although retinene and the 

 specific protein revert to rhodopsin in the dark, in the intact retina, retinene 

 is to some extent converted to vitamin A. In the case of retinenei, vita- 

 min Ai can be identified by its typical absorption band in chloroform (328 



78 R. A. Morton, Nature, 153, 69-71 (1944). 



" R. A. Morton and T. W. Goodwin, Nature, 153, 405-406 (1944). 



80 E. G. E. Hawkins and R. F. Hunter, /. Chem. Soc, 19U, 411. 



81 S. Ball, T. W. Goodwin, and R. A. Morton, Biochem. J., 42, 516-523 (1948). 



82 G. Wald, J. Gen. Physiol, 19, 351-371, 781-795 (1935-1936). 



