STUDIES ON MODE OF OXIDATION OF PYRAZOLO(3,4-D)PYRIMIDINE BY ALDEHYDE OXIDASE AND XANTHINE OXIDASE

Enzymic oxidation of the purine isomer pyrazolo(3,4-d)pyrimidine to the xanthine isomer 4,6-dihydroxypyrazolo(3,4-d)pyrimidine is catalyzed by the metalloflavoproteins xanthine oxidase and aldehyde oxidase. The aldehyde oxidase-satalyzed reaction proceeds conventionally, utilizing molecular oxygen as electron acceptor. The xanthine oxidase-catalyzed reaction is anomalous in type, proceeding rapidly in the presence of artificial electron acceptors for the enzyme (phenazine methosulfate, 2, 6-dichlorophenolindophenol, ferricyanide), but extremely slowly with the acceptors molecular oxygen and cytochrome c. Anaerobic incubation of pyrazolo ((3,4-d) pyrimidine with enzyme results in substrate-induced reduction of the flavin moiety of aldehyde oxidase, but not in reduction of the flavin moiety of xanthine oxidase. The latter observation, together with the anomalous acceptor requirements, support the possibility that the xanthine oxidase-catalyzed reaction occurs by direct electron transfer to an external acceptor, by-passing the internal electrontransport chain of the enzyme. © 1969.