Superoxide ion as active intermediate in the autoxidation of ascorbate by molecular oxygen. Effect of superoxide dismutase

The oxidation of ascorbic acid by molecular oxygen was investigated by optical and magnetic resonance methods in the presence and in the absence of copper and iron complexes. The results show that one superoxide ion is generated for each oxidized molecule of ascorbate. The effect of addition of Cu,Zn superoxide dismutase, which at concentrations higher than 10(-7) M halves the oxidation rate, is discussed and a reaction mechanism is proposed. An upper limit for the kinetic rate constant of the reaction between the ascorbate radical and the superoxide ion is reported.