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

From the ¹ From the Department of Biochemistry and Biophysics, University of California, Davis, California 95616

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.

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