508 VI. CAKOTENOIDS AND RELATED COMPOUNDS 



some fats, which they termed "fat-soluble accessory factor." Drummond 

 later suggested that this somewhat cumbersome term be replaced by the 

 expression vitamin A. This terminology has been employed almost 

 exclusively since that time. In 1922, it was demonstrated that the fat- 

 soluble A of Osborne and Mendel was in fact not a single vitamin but at 

 least two different factors.^- The designation vitamin A was retained for 

 the growth-promoting, antixerophthalmic factor, while the name vitamin 

 D was assigned to the antirachitic factor, i.e., the compound which pre- 

 vents rickets. 



Because the early work on the qualitative identification of vitamin A and 

 on its quantitative estimation was based largely upon the biological re- 

 sponse, considerable confusion has existed as to its chemical nature, due to 

 the fact that several widely varying substances appeared to have vitamin 

 A potency. For example, Steenbock^^ alone, and in collaboration with co- 

 workers,^^"" postulated as early as 1919 that vitamin A was related to the 

 yellow carotenoid pigments. Thus, butter, peas, and maize had a vita- 

 min A potency which was to some extent proportional to their content of 

 the yellow lipochromes. 



However, a number of facts soon accumulated which argued against the 

 identity of the carotenoid with vitamin A. Preparations which were color- 

 less were shown in some cases to be extremely potent in vitamin A. Second, 

 when vitamin A is treated with antimony trichloride, an intense blue color 

 obtains which has an absorption maximum at 610-630 m/i. On the other 

 hand, the reaction between carotene and antimony trichloride gives rise 

 to a greenish blue color where the absorption band occurs, with a maximum 

 at 590 m/i. Another marked difference is that vitamin A has a charac- 

 teristic absorption band in the ultraviolet region of the spectrum with a 

 maximum at 328 m^t, while carotene does not exhibit specific absorption in 

 this region. 



The reason for the interrelationship in biological activity between caro- 

 tene and vitamin A became evident shortly after Euler and associates'^ 

 demonstrated the effectiveness of pure crystalline carotene prepared from 

 carrots in curing rats suffering from vitamin A deficiency. Quantities of 

 lO/xg. of carotene were found to be sufficient to alleviate the symptoms of 

 avitaminosis and to restore growth promptly. Several years after von 



12 E. V. McCoUum, N. Simmonds, J. E. Becker, and P. G. Shipley, J. Biol. Chem., 53 

 293-312 (1922). 



13 H. Steenbock, Science, 50, 352-353 (1919). 



" H. Steenbock and P. W. Boutwell, J. Biol. Chem., J,l, 81-96 (1920). 



IB H. Steenbock, M. T. Sell, and M. V. Buell, J. Biol. Chem., 47, 89-101 (1921). 



" H. Steenbock, M. T. Sell, and P. W. Boutwell, J. Biol. Chem., 47, 303-308 (1921). 



" H. Steenbock and M. T. Sell, J. Biol. Chem., 51, 63-76 (1922). 



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



