610 G. SCHMIDT 



Ball'-^- and later Corran, Dewan, Gordon, and Green--^ succeeded in a 

 considerable purification of the enzyme obtained from milk and established 

 its nature as that of a conjugated protein containing a flavin adenine dinu- 

 cleotide as a prosthetic group which can be separated from the protein by 

 heat or treatment with acid or alcohol. The prosthetic group of xanthine 

 oxidase can replace the coenzyme of amino acid oxidase in Warburg and 

 Christian's^-^ test, and both flavin adenine dinucleotides have closely similar 

 absorption spectra. 



Evidence suggesting the participation of the flavin group of xanthine 

 oxidase in its catalytic activity was obtained by Ball, and his conclusions 

 were later supported by Horecker and Heppel'"^ and by MorelP^^ on much 

 purer enzyme preparations. Morell established the proportionality of the 

 catalytic activity of his preparations with their contents of a specific flavin 

 compound. This was inferred from the fact that only a part of the total 

 flavine of xanthine oxidase is reduced by the substrate. According to ob- 

 servations by Mackler et al.,"^^^ this behavior must be attributed to the 

 oxido-reduction equilibria between the enzyme- and the substrate-systems. 

 The standard potential of xanthine oxidase is more negative than those of 

 other flavoproteins. 



The essential role of the flavin adenine dinucleotide for the catalytic 

 action of xanthine dehydrogenase is further supported by the striking 

 parallelism between the riboflavin content of the diet and the xanthine 

 oxidase concentration in the tissues of growing rats--^ —a parallelism which 

 resembles the behavior of other respiratory enzymes. 



Methylene blue,^-" oxygen,^^- or cytochrome c-^^ can act as hydrogen ac- 

 ceptors. 



In addition to flavine adenine dinucleotide, xanthine oxidase has an- 

 other, possibly iron containing chromophoric group.--^^'^"^ The presence of 

 this group is the reason for the differences between the absorption spectrum 

 of xanthine oxidase and that of a flavoprotein. 



The complex nature of xanthine oxidase is impressively demonstrated 

 by the surprising identification of sodium molybdate as an additional 

 dietary factor required for maintaining normal xanthine oxidase levels in 



"2 E. G. Ball, /. Biol. Chem. 128, 51 (1939). 



2" H. S. Corran, J. G. Dewan, A. H. Gordon, and D. E. Green, Biochem. J. 33, 1694 



(1939). 

 "4 O. Warburg and W. Christian, Biochem. Z. 298, 150 (1938). 



225 B. L. Horecker and L. A. Heppel, J. Biol. Chem. 178, 683 (1949). 



226 D. B. Morell, Biochem. J., 51, 657 (1952). 



226" B. Mackler, H. B. Mahler, and D. E. Green, J. Biol. Chem. 210, 149 (1954). 



227 E. C. de Renzo, E. Kaleita, P. Heyther, J. J. Cleson, B. L. Hutchings, and J. H. 

 Williams, /. Am. Chem. Soc. 75, 753 (1953). 



22'" D. A. Richert and W. W. Westerfeld, J. Biol. Chem. 209, 179 (1954). 



