A kinetic study on iron stimulation of the xanthine oxidase dependent oxidation of ascorbate

Xanthine oxidase reduces molecular oxygen to H2O2 and superoxide radicals during its catalytic action on xanthine, hypoxanthine or acetaldehyde. Ascorbate is catalytically oxidized by the superoxide radicals generated, when present in the reaction solution (Nishikimi 1975). The present study shows that iron ions markedly stimulate the enzyme dependent ascorbate oxidation, by acting as a red/ox-cycling intermediate between the oxidase and ascorbate. An apparent Km-value of 10.8 microM characterized the iron stimulatory effect on the reaction at pH 6.0. Reduced transition-state metals can be oxidized by H2O2 through a Fenton-type reaction. Catalase was found to reduce the effect of iron on the enzyme dependent ascorbate oxidation, strongly suggesting that H2O2, produced during catalysis, is involved in the oxidation of ferrous ions.