534 IX. CAROTENOIDS AND VITAMINS A 



that in the South African geelbek or yellow-backed cape salmon (Atrados- 

 cion (Otolithus) acquidens C. and V.), which appears to correspond to the 

 Atlantic squeteague, the total vitamin A in the liver decreased with the 

 onset of intensive feeding, and later gradually increased. The vitamin A 

 content of the visceral oil paralleled the total oil content of the viscera. 



7. The Metabolism of Carotenoids and of Vitamins A 



(7) The Transformation to Vitamins A 



a. kWr-trans Carotenoids as Provitamins A. The most important reaction 

 of the carotenoids concerning which information is available is their trans- 

 formation to vitamin A. The qualitative conversion of a compound to 

 vitamin A can be demonstrated by ascertaining whether or not the sub- 

 stance in question cures symptoms of hypovitaminosis A. A quantitative 

 evaluation of the extent of this conversion can be obtained by bioassay, 

 using vitamin A or pure all-/mw5-(S-carotene as a standard. The biopo- 

 tency of the unknown can be calculated in terms of the standard from the 

 log dose gain in body weight curve, by the method of Coward.® Only 

 those substances which contain one intact jS-ionone nucleus having a side- 

 chain patterned after vitamin A, and which is as long or longer than that 

 in vitamin A, can serve as precursors of this vitamin. There is some evi- 

 dence that dehydro-/3-carotene (isocarotene) can also act as a provitamin 

 A2, in spite of the fact that the /3-ionone rings have two double bonds rather 

 than one. For further information on this subject, the reader is referred 

 to the reviews of Karrer and co-workers,''-^ of Karrer and Jucker,-^ to 

 those of Zechmeister,^^'''-^'^-® and to that of Milas.''" Karrer''-^ also sum- 

 marized the biologic significance of the carotenoid epoxides. Table 3 

 lists the various carotenoids which have been found to exert vitamin A 

 activity. 



In addition to the compounds which have been listed in Table 3 as pro- 

 vitamins A, a number of carotenoids exist which are devoid of provitamin 

 A activity. The widely distributed carotenoid lycopene/'' in which both 

 of the (S-ionone rings are open, has been found to be inactive, as would be 

 expected ; the same situation obtains with the natural polym derivative of 



"« P. Karrer, K. Schopp, and R. Morf, Helv. Chim. Acta, 15, 1158-116,5 (1932). 



^25 L. Zechmeister, Bull. soc. chim. bioL, 31, 956-964 (1949). 



"6 L. Zechmeister, Harvey Lectures, 47, 221-241 (1951-1952). 



''^' N. A. Milas, Chemistry and Industrial Preparation, in W. H. Sebrell, Jr., and R. S. 

 Harris, The Vitamins, Chemistry, Physiology, Pathology, Vol. I, Academic Press, New York, 

 1954, pp. 4-58. 



728 p. Karrer, Fortschr. Chem. org. Naturstoffe, 5, 1-19 (1948). 



