ENZYMES ATTACKING NUCLEIC ACIDS 613 



quinone imine are variable, presumably because of the presence of a sub- 

 stance which reacts with quinone imine. Xanthine oxidase of milk is in- 

 hibited to an extent of 50% by pyrogallol at 1 X lO""* M concentration.-^^'' 

 Other known oxidases of liver are not appreciably inhibited by quinone 

 imine. Many other phenols such as p-nitrophenol, p-bromophenol, phloro- 

 glucinol, anthraquinone, and a-naphthol do not significantly influence the 

 action of xanthine oxidase. 



Effect of pterin derivatives. Kalckar et al.-^^'^ found that 2-amino-4;-hydrox3^- 

 6-formylpterin inhibits xanthine oxidase practically completely at a 

 concentration of 10""* M. 2-Amino-4-hydroxy-6-carboxypterine"^^'* and 2- 

 amino-4-hydroxy-6-hydroxymethylpterin'^*'^ inhibit the enzyme at higher 

 concentrations. Other respiratory enzymes tested were not inhibited by 

 these pterins. 



It is of interest that these specific inhibitors of xanthine oxidase also have 

 strong depressing effects on the endogenous oxygen consumption of liver 

 homogenates. This raises the question of the role of xanthine oxidase in the 

 endogenous respiration of isolated tissue preparations. 



L-Ascorbic acid-^^^ at 1 X 10~^ M inhibits irreversibly most of the activ- 

 ity of xanthine oxidase purified from cream. 



Antabuse-''' (tetraethylthiuram disulfide, (C2H5)2N(CS) -S-S- (SO)X- 

 (C2H5)2 , has a considerable inhibitory effect on crude and purified prepara- 

 tions of xanthine oxidase from rat liver, but not on preparations from milk. 

 The inhibition of the liver enzyme can be overcome by methylene blue. 



The differences in the behavior of xanthine oxidase preparations from 

 milk and from tissues toward inhibitors are ascribed to the association of 

 the enzyme with different carrier substances depending on the source of the 

 preparations. An interesting example of such an association is furnished by 

 the observation of Philpot,-^^^ who found that the xanthine oxidase activity 

 of milk is small until it was exposed to temperatures below 15°. The solidifi- 

 cation of the fats releases the enzyme which was associated with the fat 

 globules of the milk. 



Copper ions inactivate xanthine oxidase in verj' small concentrations.-^^*' 

 The use of water distilled from copper \-essels must therefore be avoided 

 during experiments with this enzyme. 



""b s. J. Gray and R. Z. Felsher, Proc. Soc. Exptl. Biol. Med. 59, 287 (1947). 



2"c H. M. Kalckar, N. O. Kjeldgaard, and H. Klenow, J. Biol. Chem. 174, 771 (1940). 



"6d H. G. Petering and J. A. Schmitt, /. Am. Chem. Soc. 72, 2995 (1950). 



236ep Feigelson, /. Biol. Chem. 197, 843 (1952). 



"6fD. A. Richert, R. Vanderlinde, and W. W. Westerfeld, /. Biol. Chem. 186, 261 



(1950). 

 "«8F. J. Philpot, Bioc.hem. J. 32, 2013 (1938). 

 "6h M. R. Stetten and C. L. Fox, Jr., J. Biol. Chem. 161, 333 (1945). 



