METABOLISM OF CAllOTENOIDS AND VITAMINS A 545 



(3) The Transjormation to Products 

 Other Than Vitamin A 



Little is known about transformations of carotenoids other than their 

 conversion to vitamin A. Goodwin^^^ indicates that fishes are able to oxi- 

 dize alimentaiy carotenoids to highly oxygenated pigments. When these 

 pigments have the required structural characteristics, they may be con- 

 verted into vitamin A. 



It would seem probable that changes similar to those involved in the for- 

 mation of vitamin A from the carotenoids classed as provitamins A would 

 also occur in the non-provitamin A series of carotenoids, except that the 

 product derived from the latter group would be devoid of vitamin A ac- 

 tivity. An indication that such a change does occur in the case of this 

 latter group is given by the experiments of Painter and Glover,^^" who de- 

 tected traces of apo-15-lycopenal in the intestine but not in the livers of 

 rats which had previously received small doses of lycopene or of apo-12'- 

 lycopenal. Although lycopene could be demonstrated in the livers of rats 

 to the extent of 15% of the amount fed, no apo-15-lycopenal could be de- 

 tected in the livers of rats previously fed lycopene or apo-15-lycopenal. It 

 is also possible that the ionone derivatives, isolated by Prelog et alJ'''^ 

 from the urine of the pregnant mare, may be produced by the degradation 

 of alimentary xanthophylls. 



One interesting change, occurring in insects, is the transformation of ^- 

 carotene to a non-provitamin A. Thus, Goodwin^''- reported that, dur- 

 ing the development of the eggs of the African migratory locust (Locusta 

 migratoria migratorioides) and of the North African desert locust {Schisto- 

 cerca gregaria), ,S-carotene begins to disappear on the sixth to seventh day, 

 simultaneously with the appearance of astaxanthin. While /3-carotene is 

 the only pigment present in the newly fertilized eggs, astaxanthin comprises 

 70% of the total carotenoids present after their incubation. 



(3) Vitamin A Analogues and Isomers 



a. The Relative Biopotencies of Vitamin Ai and Vitamin A2. The first 

 correct characterization of vitamin A2 was made by Wald,'^^^ who proved 

 that it was a component of the visual system of freshwater fishes. Wald 

 was the first to demonstrate that it was a vitamin. He recognized it as 

 part of the porphyropsin system. 



'" V. Prelog, J. Fiihrer, R. Hagenbach, and R. Schneider, Helv. Chim. Acta, 31, 

 1799-1814 (1948). 



"8 G. Wald, Nature, 139, 1017-1018 (1937). 



