IV. BIOCHEMICAL SYSTEMS 349 



ill the presence of air, H2O2 is formeil. Dixoii*^ has reported on the relative 

 velocity with which nine different purines can be oxidized. Xanthine oxi- 

 dase can catalyze the anaerobic dismutation of xanthine to hypoxanthine 

 and uric acid:*^ 



2 Xanthine ^ Hypoxanthine + uric acid 



There is no specificity toward aldehydesJ" Any non-toxic aldehyde can 

 be oxidized by this flavoprotein. 



R OH O 



\ / /- 



RCHO + HoO -» C -^- RC + 2H 



/ \ \ 



H OH OH 



d. Xanthopterin Oxidase 



Wieland and Liebig'^ described the presence in liver and milk of an en- 

 zyme which catalyzes the oxidation of xanthopterin (2-amino-4,C)-dihy- 

 droxypteridine). Kalckar and Klenow"- found that xanthine and xanthop- 

 terin oxidases of cream are inhibited by certain preparations of folic acid. 

 Later, Kalckar et alP showed that this inhibition was due to 2-amino-4- 

 hydroxypteridine-6-aldehyde, a fission product of folic acid formed by 

 irradiation. Studies of the inhibitory effects of this and other compounds 

 by Hofstee/* Lowry et al.,''^ and Williams and Elvehjem^^ showed that 



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