612 G. SCHMIDT 



take,^-2'2'''2*'^ by the spectrophotometric determination of reduced cyto- 

 chrome c, or by determination of the uric acid formed. 



Specificity P^ Xanthine oxidase catalyzes the oxidation of hypoxanthine, 

 xanthine, adenine (to dihydroxyadenine),^^^ 8-hydroxypurine, 6-amino-8- 

 hydroxypurine, 6-amino-2-hydroxypurine, 2-azaadenine,2^-^ and xanthop- 

 terin.-^^ It is noteworthy that the oxidation of the aminopurines is not 

 accompanied by the formation of ammonia. 



Even highly purified preparations of xanthine oxidase are active toward 

 a considerable number of aliphatic and aromatic aldehydes and toward 

 DPNH. 



Certain substances related to aldehydes such as chloral hydrate, butyryl- 

 chloraldehyde, paraldehyde, glucosone, glucose, mannose, and gluconate 

 are not oxidized by the enzyme. 



Inhibitors. Xanthine oxidase is completely and irreversibly inhibited by 

 low sodium cyanide concentrations (e.g., 0.0025 M^). 222 ,235.235a ^he reaction 

 of the cyanide with the enzyme protein takes an appreciable time and the 

 cyanide effect can only be demonstrated if the solutions of the enzyme and 

 of the cyanide are mixed before the addition of the substrate. Hypoxanthine 

 and other purines, e.g., uric acid, protect the enzyme against the effect of 

 cyanide, presumably because of their higher affinities to the enzyme. The 

 fact that not only the oxidation of hypoxanthine and xanthine, but also that 

 of aldehydes, is protected by purines against the effect of cyanide strongly 

 suggests the identity of xanthine oxidase and the Schardinger enzyme. 

 Xanthine oxidase is inhibited by borate ions, most likely owing to the 

 formation of borate complexes with the ribityl group of the riboflavin 

 moiety of the enzyme. In experiments of Roush and Norris,-^^ the inhibition 

 which is competitive was 50% at concentrations of 0.03 M borate and 

 0.034 mM xanthine. 



Effect of some substances of quinoid structure. Xanthine oxidase of liver is 

 practically completely inhibited by oxidized p-aminophenol at 1 X lO""* 

 jl^ 236a 'pi^g inhibition can be fully reversed by reductive removal of the 

 inhibitor from the enzymic digest. With the milk enzyme, the effects of 



"' D. A. Richert, S. Edwards, and W. W. Westerfeld, J. Biol. Chern. 181, 254 (1949). 



"la S. B. Dhungat and A. Sreenivasan, /. Biol. Chem. 208, 845 (1954). 



"2 H. Klenow, Biochem. J. 50, 404 (1952). 



2"a E. Shaw and D. W. Woolley, J. Biol. Chem. 194, 641 (1952). 



2" H. Wieland and R. Liebig, Ann. 555, 146 (1944). 



"^ H. M. Kalckar, N. O. Kjeldgaard, and H. Klenow, Biochim. el Biophys. Acta 5, 



575 (1950). 

 "5 A. Szent-Gyorgyi, Biochem. Z. 173, 275 (1926). 



"sa M. Dixon and D. Keilin, Proc. Roy. Soc. (London) B119, 159 (1936). 

 "6 A. Roush and E. R. Norris, Arch. Biochem. 29, 344 (1950). 

 23«a F. Bernheim and M. C. L. Bernheim, /. Biol. Chem.. 123, 307 (1938). 



