Variation of one unit of activity with oxidation rate of organic substrate in indirect superoxide dismutase assays.

Although numerous reviews and articles have been written about indirect superoxide dismutase assays, little has been reported about the variation of the amount of superoxide dismutase giving 50% inhibition (SOD50), defined as one unit of dismutase activity, with variations in assay conditions. This report indicates that there is a linear dependence of the SOD50 on the magnitude of the uninhibited rate of oxidation of the organic substance for two indirect assays, one involving epinephrine autoxidation and the other NADH oxidation. For one indirect assay involving an acceleration of the oxidation rate of an organic substrate, the amount of superoxide dismutase giving a doubling of the oxidation rate, defined as one unit of dismutase activity, also shows this linear dependence on the autoxidation rate of the organic substance. The significance of this dependence and its implications for experimenters using these assays is discussed.