INTRODUCTION 423 



called the "fat-soluble accessory factor" (fat-soluble A). McCollum et 

 al. ^^ later demonstrated that this factor consisted of two distinct vitamins ; 

 the term, vitamin A, was retained for the growth-promoting, antixero- 

 phthalmic principle, while the term, vitamin D, was assigned to the anti- 

 rachitic substance, i.e., the compound which prevents rickets. 



The relationship of carotene to vitamin A was first suggested by Steen- 

 bock'^ on the basis of the fact that the intensity of color due to the yellow 

 lipochromes in butter, peas, and maize was roughly proportional to their 

 vitamin A biopotency. However, when it was later shown that extremely 

 potent vitamin A preparations were colorless (and that some highly-colored 

 yellow compounds were without vitamin A activity), it was necessary to 

 abandon the concept that vitamin A and carotene are identical. As a 

 result of the demonstration ])y von Euler and co-workers ^^ that highly puri- 

 fied iS-carotene, when administered in minimal amounts, could cure rats 

 suffering from vitamin A deficiency, it was evident that both a colorless 

 compound, having an absorption band at 328 mju, and the highly-colored /3- 

 carotene, which had no absorption band at 328 mju, had vitamin A activity. 

 The relationship of these two compounds was explained by Moore. ^^-^^ 

 He demonstrated that vitamin A appeared in the liver fat of the vitamin A- 

 deficient rat after the administration of either vitamin A or /3-carotene; 

 in both cases, the product had the characteristic absorption band at 328 m/x, 

 and it was also colorless. It was therefore suggested that colorless vitamin 

 A is the active principle; ;S-carotene was assumed to exhibit vitamin A 

 activity by virtue of its capacity to be converted to this vitamin in the ani- 

 mal body. /3-Carotene is therefore a precursor of vitamin A, or a "pro- 

 vitamin A." The relation of /S-carotene to vitamin A was further eluci- 

 dated by the demonstration of the structure of the two compounds. Where- 

 as /3-carotene was shown to have an empirical formula of C40H56, and to con- 

 tain two (S-ionone rings, vitamin A has one-half the number of carbons 

 (C22H29OH), and contains only one /3-ionone ring. Carotenes are essen- 

 tially of plant origin, and may serve as precursors of vitamin A, which is 

 found only in animals. There is no evidence that the animal can synthe- 

 size either carotene or vitamin A de novo.~^ The classic monographs which 



18 E. V. McCollum, N. Simmonds, J. E. Becker, and P. G. Shipley, /. Biol Chem., 

 53, 293-312 (1922). 



i« H. Steenbock, Science, 50, 352-353 (1919); with P. W. Boutwell, /. Biol. Chem., 

 41, 81-96 (1920). 



1^ H. von Euler, P. Karrer, and M. Rydbom, Ber., 62, 2445-2451 (1929). 



18 T. Moore, Biochem. J., 23, 803-811 (1929); 24, 692-702 (1930). 



19 T. Moore, Biochem. J., 25, 275-286 (1931). 



'^ T. W. Goodwan, The Chemical Pathology of Carotenoids, in R. T. Williams, The 

 Chemical Pathology of Animal Pigments, Biochem. Soc. Symposia No. 12, Cambridge 

 Univ. Press, 71-84 (1954). 



