20 VITAMINS A AND CAROTENES 



biologically active provitamin A products by chemical treatment. Both 

 xanthophyll and zeaxanthin are biologically inactive, but when treated with 

 phosphorus tribromide, biologically active products are obtained.^" 



TABLE VII 

 Biologically Active Derivatives of Provitamin A Carotenoids 



Kitol,^^"^^ which belongs to the last group, is not only devoid of vitamin 



" H. von Euler, P. Karrer, and O. Walker, Helv. Chini. Acta 15, 1507 (1932). 

 '* R. Kuhn and H. Brockmann, Ber. 67, 1408 (1934); Ann. 516, 99 (1935). 



45 R. Kuhn and H. Brockmann, Ber. 66, 1319 (1933). 



46 R. Kuhn and H. Brockmann, Ann. 516, 113, 122 (1935). 



« P. Karrer and H. Solemssen, Helv. Chini. Acta 20, 682 (1937). 



« H. von Euler, P. Karrer, and H. Solemssen, Helv. Chirn. Acta 21, 211 (1938). 



49 P. Karrer, H. von Euler, H. Solemssen, and 0. Walker, Helv. Chim. Acta 17, 417, 



1169 (1934). 

 60 H. von Euler, P. Karrer, and A. Zubrys, Helv. Chim. Acta 17, 24 (1934). 



51 H. Pritchard, H. Wilkinson, J. R. Edisbury, and R. A. Morton, Biochem. J . 31, 

 258 (1937). 



52 H. Willstaedt and PI. B. Jensen, Nature 143, 474 (1939). 



53 H. Kringstad and J. Lee, Tidskr. Kjemi Bergvesen Met. 1, 82 (1941). 



54 N. D. Embree and E. M. Shantz, J. Am. Chem. Soc. 65, 910 (1943). 



55 F. B. Clough, II. M. Kaschcr, C. D. Robeson, and J. G. Ba.xter, Science 105, 436 

 (1947). 



