24 VITAMINS A AND CAROTENES 



diffuse light to all-fraws-/3-carotene. Its biological activity is about one- 

 half that of the all-^rans-jS-carotene.^* 



€i-Carotene (XL) was synthesized by Karrer and Eugster,^* It is an iso- 

 mer of OL- and jS-carotenes, but it contains two a-ionone rings and was found 

 to be completely devoid of biological activity. 



Four higher homologs of /3-carotene have also been synthesized: 16,- 

 16'-homo-i5-carotene (XLI),^^ biologically active; decapreno-j3-carotene 

 (XLII);^^ decapreno-€i-carotene (XLIII);" and dodecapreno-)3-carotene 

 (XXLIV).^^ The structural formulas of these carotenoids are given in 

 Table VIII. For further discussion of the synthesis and properties of these 

 carotenoids, the reader is referred to the origmal literature. 



6. Conversion of Provitamin A Carotenoids to Vitamin A 



The conversion of provitamins A in vivo is a complicated process and is 

 not well understood at present. It has already been mentioned^- ^ that the 

 conversion probably occurs in the intestinal tract of animals. If the con- 

 version is enzymatic, as it is generally believed,*^ the over-all shape of the 

 provitamin A molecule must be such that it fits the enzyme system. The 

 actual process, however, may be either hydrolytic or oxidative. Karrer 

 et al.^^ were the first to suggest that j8-carotene is cleaved symmetrically 

 by adding two molecules of water at the 15,15' double bond to give two 

 molecules of vitamin A. This simple hypothesis was not realized in prac- 



C40H66 + 2H2O -> 2C20H29OH 



tice, since on an equal weight basis /3-carotene had always a much lower 

 biological potency than vitamin A. However, recently Koehn^" has claimed 

 that in the presence of a-tocopherol the fission of /3-carotene to vitamin A 

 is accomplished almost quantitatively by the rat. 



Theoretically the addition of water to the double bond 15,15' will not 

 give vitamin A unless the water is first oxidized to hydrogen peroxide which 

 will then add to the double bond in the presence of the enzyme as two 

 hydroxyl groups to give the intermediate 15,15'-dihydroxy /3-carotene 

 (XLV). Both hydroxyl groups are allylic and of the same type as the hy- 

 droxyl group present in vitamin A. Furthermore, such a dihydroxy caro- 



^3 L. Zechmeister, H. J. Deuel, Jr., H. H. Inhoffen, J. Leemann, S. M. Greenberg, 



and J. Ganguly, Arch. Biochem. and Biophys. 36, 80 (1952). 

 «4 P. Karrer and C. H. Eugster, Helv. Chim. Ada 33, 1433 (1950). 

 " H. H. Inhoffen, F. Bohlmann, H. J. Aldag, S. Bork, and G. Leidner, Ann. 673, 1 



(1951). 

 66 p. Karrer and C. H. Eugster, Helv. Chim. Acta 34, 28, 1805 (1951). 

 6' P. Karrer, C. H. Eugster, and M. Faust, Helv. Chim. Acta 34, 823 (1951). 



68 P. Karrer and C. H. Eugster, Helv. Chim. Acta 34, 1805 (1951). 



69 P. Karrer, R. Morf, and K. Schopp, Helv. Chim. Acta 14, 1036, 1431 (1931). 

 76 C. J. Koehn, Arch. Biochem. 17, 337 (1948). 



