Ascorbic acid, metal ions and the superoxide radical

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Abstract
No evidence could be found for production of the superoxide radical, O2.- [a highly toxic species formed in living organisms], during autoxidation of ascorbic acid at alkaline pH values. Indeed, ascorbate may be important in protection against O2.- generated in vivo. Oxidation of ascorbate at pH 10.2 was stimulated by metal ions. Stimulation by Fe2+ was abolished by superoxide dismutase [EC 1.15.1.1], probably because of generation of O2.- during reduction of O2 by Fe2+, followed by reaction of O2.- with ascorbate. EDTA changed the mechanism of Fe2+-stimulated ascorbate oxidation. Stimulation of ascorbate oxidation by Cu2+ was also decreased by superoxide dismutase, but this appears to be an artifact, since apoenzyme or bovine serum albumin showed similar effects.