Catalysis of the Direct Transfer of Oxygen from Nicotinamide N-Oxide to Xanthine by Xanthine Oxidase

The role of the oxidizing agent in the xanthine oxidase reaction has been evaluated. The long standing notion that oxygen normally functions only as an electron acceptor in the xanthine oxidase reaction has been confirmed. When molecular oxygen was the oxidizing agent and the reaction was carried out at pH 7.5, 18O was incorporated into uric acid from 18O-labeled water; no 18O was found in uric acid when xanthine was oxidized in an atmosphere of 18O2. However, at pH 8.9 there was a small amount of direct oxygen transfer from 18O2 into uric acid. In experiments in which nicotinamide N-oxide was used in place of O2, the N-oxide appeared to function to a large extent by direct transfer of oxygen. It acted as an electron acceptor as well. Xanthine oxidase catalyzed the transfer of 18O from nicotinamide 18ON-oxide to xanthine in the course of the formation of uric acid. 18O from 18O-labeled water was also incorporated into uric acid when nicotinamide N-oxide was used as the oxidant in the xanthine oxidase reaction. Essentially identical results were obtained with both milk and liver xanthine oxidases. The possibility was raised that heterocyclic N-oxides may be capable of acting as general biological oxygenating agents.