One-electron transfer reactions in biochemical systems IV. A mixed mechanism in the reaction of milk xanthine oxidase with electron acceptors

Abstract

Xanthine oxidase (xanthine: O 2 oxidoreductase, EC 1.2.3.2) catalyzes one-electron and two-electron reduction of p-benzoquinone, both at the same time. The ratio is greatly dependent on the concentration of p-benzoquinone. The mechanism is analyzed quantitatively with electron spin resonance technique. The quantitative identification of perhydroxyl radical formed during xanthine oxidase reaction is made with a new method trapping the radical as lactoperoxidase Compound III. It is concluded that the mechanisms of electron transfer from xanthine oxidase to p-benzoquinone and molecular oxygen are identical. The reduction of cytochromes mediated by p-benzoquinone and molecular oxygen in the xanthine oxidase system is explained on the basis of an one-electron transfer mechanism, in which free-radical intermediates such as semiquinone and perhydroxyl radical are direct reductants for the cytochromes.