92 VITAMINS A AND CAROTENES 



more important for special cases is the method of Demarest/^ which is 

 based on selective destruction of vitamin A by means of ultraviolet irradia- 

 tion. 



A paper-chromatographic method for the determination of vitamin A is 

 given by Datta and Overell/^ The separation of vitamin A, vitamin A 

 esters, and carotene by chromatography on aluminium oxide activated to 

 different degrees was carried out by Miiller;'*^ this method is generally 

 applicable. Ritsert*^ discussed thoroughly the separation of vitamin A from 

 vitamin A esters by shaking the extract with methanol. In order to isolate 

 neovitamin A one makes use of its slow reaction with maleic acid anhydride, 

 with which vitamin A condenses rapidly.^" 



The amounts of vitamin Ao occurring in commercial liver oils are so 

 small that one can dispense with their determination. Otherwise, this may 

 be done only spectrophotometrically after separation by chromatography;^^ 

 the same applies to kitols.^^ 



3. Determination of Provitamins A 



Although the determination of vitamin A seems to be fully worked out, 

 that of the provitamins A is far from satisfactory. A determination of 

 provitamins A means, in practice, the measurement of carotenes. Hitherto, 

 the basic method of carotene determination, as documented by Kuhn and 

 Brockmann's classical procedure, has not undergone important changes, 

 but many new findings in recent years have led to refinements. (3ne serious 

 problem is the quantitative extraction of the carotenes from vegetable 

 material. Kuhn and Brockmann'* determined the total content of carotene 

 without separation of the isomers. But lately such a separation has become 

 a rule, for it is realized that a-carotene shows only 50 %, and 7-carotene 

 only 25%, of the biological activity of /3-carotene.^--^* Recently, this prob- 

 lem became more complicated by the results of Fraps, Zechmeister, Zscheile, 

 and their collaborators,^* who found the stereoisomers of carotene in various 



46 B. Demarest, Z. Vitajninforsch. 9, 20 (1939); A. Chevallier, ibid. 7, 10 (1938). 

 " S. P. Datta and B. G. Overell, Biocheyn. J. 44, XLIII (1949); H. P. Kaufmanii, 



Fetle u. Seifen 52, 331 (1950). 

 « P. B. Miiller, Helv. Chim. Acta 26, 1945 (1943); 30, 1172 (1947). 

 4' K. Ritsert, Vitamine u. Hormone 3, 57 (1943). 



^0 C. D. Robeson and J. G. Baxter, /. Am. Chem. Soc. 69, 136 (1947). 

 " R. A. Morton and A. Stubbs, Biochem. J. 40, LVIII (1946); R. K. Barua and R. A. 



Morton, Biochem. J. 45, 308 (1948). 

 ^2 H. J. Deuel, Jr., E. Sumner, C. Johnston, A. PolgAr, and L. Zechmeister, Arch. 



Biochem. 6, 157 (1945). 

 " H. J. Deuel, Jr., C. Johnston, E. Sumner, A. Polgdr, W. A. Schroeder, and L. 



Zechmeister, Arch. Biochem. 5, 365 (1944). 

 " A. R. Kemmerer and G. S. Fraps, J. Biol. Chem. 161, 305 (1945). 

 " B. W. Beadle and F. P. Zscheile, J. Biol. Chem. 144, 21 (1942). 



