METABOLISM OF CAROTENOIDS AND VITAMINS A 535 



this hydrocarbon, prolycopeneJ-^ Although Lund"° reported that zeaxan- 

 thin, which has a hydroxyl group on each of the two /3-ionone rings, is a 

 vitamin A precursor, the weight of evidence is contrary to this finding.''" •'^^^ 

 Braude et al?^^ observed that zeaxanthin is biologically inactive in the case 

 of both rats and pigs. Lutein is also inactive as a provitamin A.''^- The 

 results of Kemmerer et al.,^^ Kelley and Day,^- and of Callison and co- 

 workers,^^ which involved the question of whether or not lutein in large 

 doses decreases the provitamin A activity of ;S-carotene, are uniform in sup- 

 porting the fact that lutein is biologically inactive as a provitamin A. 

 Euler et al?^- did report that both zeaxanthin and lutein are converted to 

 products with provitamin A activity when they are treated with phos- 

 phorus tribromide (PBrs). ei-Carotene, C40H56, which contains two a- 

 ionone rings, was synthesized by Karrer and Eugster.''^^ As was to be ex- 

 pected, it is completely inactive as a precursor of vitamin A.- 



Kitol (C4oH58(OH)2) which is a dihydric alcohol of approximately twice 

 the size of vitamin A, does not possess any vitamm A activity in the 

 animal, ""■"''"''^^ although it may be converted to vitamin A by heat^-*"'^'*^ 

 and by the use of radon alpha particles.'^" Embree and Shantz^^" calcu- 

 lated that one molecule of vitamin A was formed for each molecule of kitol 

 destroyed, but Clough et al}'^^ recorded a figure of only 0.67 to 0.75 mole- 

 cule of \'itamin A as arising for each molecule of kitol which disappears. 

 Two types of kitol have been reported, one of which yields ordinary vita- 

 min A (Ai) while the second type, found in fresh-water fishes such as the 

 northern or Great Lakes pike {Esox lucius) gives vitamin A2 ;^'*° it is there- 

 fore referred to as kitol2. Hickman"^ is of the opinion that kitol is a de- 

 toxication product of vitamin A. Mammals, such as the whale, ordinarily 

 are unable to tolerate as large quantities of vitamin A as can the fishes; 

 therefore, by converting excess vitamin A to kitol, it is detoxified. Hick- 

 man^^^ suggests that kitol is a pos^-vitamin A rather than a provitamin A. 



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

 Zechmeister, Arch. Biochem., 5, 365-371 (1944). 



"0 A. Lund, Beretn. Forsogslab Kobenh., No. 180, 1-295 (1938); Chem. Abst., 34, 3327 

 (1940); cited by R. Braude, A. D. Foot, K. M. Henry, S. K. Kon, S. Y. Thompson, and 

 T. H. Mead, Biochem. J., 35, 693-707 (1941), p. 693; Chimie & Industrie, 42, 569 (1939). 



"1 R. Braude, A. S. Foot, K. M. Henry, S. K. Kon, S. Y. Thompson, and T. H. Mead, 

 Biochem. J., 35, 693-707 (1941). 



"2 H. V. Euler, P. Karrer, and A. Zubrys, Helv. Chim. Acta, 17, 24-29 (1934). 



"3 p. Karrer and C. H. Eugster, Helv. Chim. Acta, 33, 1433-1437 (1950). 



"■* H. Pritchard, H. Wilkinson, J. R. Edisbury, and R. A. Morton, Biochem. J., 31, 

 258-265 (1937). 



"5 H. Willstaedt and H. B. Jensen, Natjire, 143, 474 (1939). 



"6 H. Kj-ingstad and J. Lee, Tidsskr. Kjemi Bergvesen Met., 1, 82-83 (1941). 



'" D. Libermann and I. Grundland, Co7npt. rend., 224, 1033-1035 (1947). 



