ON THE REDUCTION POTENTIALS OF FE AND CU-ZN CONTAINING SUPEROXIDE DISMUTASES

The reduction potentials of bovine erythrocyte copper-zinc superoxide dismutase and Escherichia coli iron superoxide dismutase were determin ed in EPR-monitored redox titrations in homogeneous solution. The copp er-zinc enzyme is reduced and reoxidized with a midpoint potential of +120 mV versus standard hydrogen electrode (SHE) at pH 7.5. The iron e nzyme can be reduced with an apparent midpoint potential of -67 mV ver sus SHE at pH 7.5. However, reaction with ferricyanide affords only sl ow, partial re-oxidation. Cyclic voltammetry of the copper-zinc enzyme in the presence of 50 mM Sc3+, at pH 4.0 using a glassy carbon electr ode results in asymmetric voltammograms. The midpoint potential of the enzyme at this pH value, calculated as the average of the anodic and cathodic peak potentials, is +400 mV versus SHE. The physiological rel evance of this value is limited, since EPR experiments indicated that reduction of the copper-zinc enzyme at pH 4.0 is not reversible. Conse quences of the irreversible behavior of the two dismutases for the pre viously reported studies on their redox properties are discussed.