536 IX. CAROTENOIDS AND VITAMINS A 



Although kitol has been reported in the hver oils of some terrestrial animals 

 such as sheep/'*'' it is not formed by the rat, even after excessive doses of 

 vitamin A7^^ For a further description of the occurrence and properties 

 of kitol, the reader is referred to The Lipids, Vol. I, pages 710-713. 



Only the carotenoids which have at least one intact ;S-ionone ring can 

 serve as provitamins A. /3-Carotene has repeatedly been found to possess 

 the highest provitamin A activity of any of the carotenoids; its biopotency 

 is approximately twice or more that of the other provitamins A. Euler, 

 Karrer, and co-workers^^'^^'''^^ made a study of the biopotencies of jS-caro- 

 tene and of other carotenoids. 



The biologic activity of a-carotene was found to be 53% of that of /3- 

 carotene when the evaluation was made on the basis of gain-in-weight over 

 twenty-eight days." Kuhn and co-workers^'*" had likewise arrived at ap- 

 proximately the same value in bioassays, also based upon weight gains. 

 However, Johnson and Baumann^^^ reported, on the basis of vitamin A 

 storage in the liver, that all-^mns-a-carotene had a biopotency of only 25% 

 of that of all-/rans-/3-carotene. In later studies of Johnson, Swick, and 

 Baumann,'''** a-carotene was found to have about one-half the effectiveness 

 of /3-carotene in the chick when the comparative biopotencies were based 

 upon growth and prevention of vitamin A-deficiency symptoms; on the 

 other hand, a-carotene was considerably less than one-half as potent as 

 j8-carotene, when the accumulation of vitamin A in the liver after the ad- 

 ministration of fixed dosages of the test substances was the basis of com- 

 parison. In rats, a-carotene yielded lower stores of vitamin A than its 

 growth-promoting power would lead one to expect.®^^ The levels of bio- 

 activity of the stereoisomeric forms of all-^r ans- a-carotene (see Table 4) 

 would also indicate that the higher figure (53%) represents the biologic 

 value of all-fmws-a-carotene in rats." The biopotency of echinenone^^^ 

 was found to be similar to the higher value for a-carotene, i.e., 54%. 



On the other hand, the all-^raws-7-carotenes obtained from several sources 

 showed wide variations in calculated biologic^^^ activity. Although the 

 biopotency of 7-carotene prepared from the Moluccan plant (Gonocaryum 

 pyriforme), described by Winterstein,^'** was reported as approximately 



'^8 E. M. Shantz, personal communication, Sept. 29, 1948. 



"9 B. V. Euler, H. v. Euler, and P. Karrer, Helv. Chim. Acta, 12, 278-285 (1929). 



'« R. Kuhn, H. Brockmann. A. Scheunert, and M. Schieblich, Z. physiol. Chem., 221, 

 129-136 (1933). 



'41 R. M. Johnson, R. W. Swick, and C. A. Baumann, Arch. Biochem., 22, 122-131 

 (1949). 



'" J. Ganguly, N. I. Krinsky, J. H. Pinckard, and H. J. Deuel, Jr., Z. physiol. Chem., 

 296, 61-66 (1953). 



'" H. Brockmann and M. L. Tecklenburg, Z. physiol. Chem., 221, 117-128 (1933). 



'" A. Winterstein, Z. physiol. Chem., 219, 249-252 (1933). 



