546 IX. CAROTENOIDS AND VITAMINS A 



Lederer et al?''^ recognized a "second vitamin A," and Edisbiiry and co- 

 workers^^^ introduced the term vitamin A2, based upon the earlier observa- 

 tions of Wald."^ Vitamin A2 was found to have a wide distribution in the 

 livers of fresh-water fishes (see page 498) . There has been considerable dif- 

 ference of opinion as to its biologic activity. Although Gillam and asso- 

 ciates^^" first reported that vitamin A2 possessed growth-promoting powers, 

 this was denied by other investigators/^^ and was then later reaffirmed. '^^^''^^^ 



Vitamin A2 has been synthesized as 3'-dehydrovitamin A^" (the Milas 

 numbering system/^* see The Lipids, Vol. I, p. 708). An indication of the 

 fact that the synthetic compound is vitamin A2 consists in the absorption 

 spectrum and antimony trichloride reaction, which correspond closely 

 with those reported by Shantz^^^ for natural vitamin A2. Moreover, the bio- 

 potencies of the natural and synthetic vitamin A2 are in the same range. 

 Shantz and Brinkman^^^ reported that the biologic activity of pure vitamin 

 A2 obtained from natural sources is equivalent to 1,300,000 I.U./g. as 

 compared with a value of 3,300,000 I.U./g. for crystalline vitamin Ai, 

 based upon the new U.S. P. standard (No. 3).^"-^*^ Thus, natural vitamin 

 A2 has a biopotency of 40% of that of vitamin Ai,^^^ while synthetic vita- 

 min A2 has been found to have a bioactivity equivalent to only 30% of vita- 

 min Ai.-" These facts would indicate that vitamin A2 is 3'-dehydro- 

 vitamin Ai.''^^ This opinion is shared by Cama et al.p° although it is con- 

 trary to the earlier views of Karrer, Geiger, and Bretscher'^^^ and of 

 Shantz. ^85 



When vitamin A2 is given to mammals which ordinarily store vitamin Ai, 

 both forms appear in the tissues. Thus, Shantz and co-workers^^^ found 



'" E. Lederer, V. A. Rosanova, A. E. Gillam, and I. M. Heilbron, Nature, I40, 233 

 (1937). 



^8" A. E. Gillam, I. M. Heilbron, W. E. Jones, E. Lederer, and V. Rosanova, Biochem. 

 J., 32, 405-41G (1938). 



'81 P. Karrer, A. Geiger, and E. Bretscher, Helv. Chim. Acta, 24, 161 E-172 E (1941). 



'82 J. L. Jensen, E. M. Shantz, N. D. Embree, J. D. Cawley, and P. L. Harris, J. 

 Biol. Chem., 149, 473-477 (1943). 



"3 p. Karrer and E. Bretscher, Helv. Chim. Acta, 26, 1758-1778 (1943). 



'8^ N. A. Milas, Vitamins and Hormones, 5, 1-38 (1947). 



'86 E. M. Shantz, Science, 108, 417-419 (1948). 



'86 E. M. Shantz and J. H. Brinkman, J. Biol. Chem., 183, 467-471 (1950). 



'8' J. G. Baxter and C. D. Robeson, J. Am. Chem. Soc, 64, 2411-2416 (1942). 



'88 J. D. Cawley, C. D. Robeson, L. Weisler, E. M. Shantz, N. D. Embree, and J. G. 

 Ba.\ter, Science, 107, 346 (1948). 



'89 K. R. Farrar, J. C. Hamlet, H. B. Henbest, and E. R. H. Jones, Chemistry & 

 Industry, 1951, 49-50. 



"0 H. R. Cama, P. D. Dalvi, R. A. Morton, and M. K. Salah, Biochem. J., 52, 542- 

 547 (1952). 



'91 E. M. Shantz, N. D. Embree, H. C. Hodge, and J. H. Wills, Jr., J. Biol. Chem., 163, 

 455-464 (1946). 



