title: On the reduction potentials of Fe and Cu-Zn containing superoxide dismutases.
abstract: The reduction potentials of bovine erythrocyte copper-zinc superoxide dismutase and Escherichia coli iron superoxide dismutase were determined in EPR-monitored redox titrations in homogeneous solution. The copper-zinc enzyme is reduced and reoxidized with a midpoint potential of +120 mV versus standard hydrogen electrode (SHE) at pH 7.5. The iron enzyme can be reduced with an apparent midpoint potential of -67 mV versus SHE at pH 7.5. However, reaction with ferricyanide affords only slow, 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 electrode 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 relevance of this value is limited, since EPR experiments indicated that reduction of the copper-zinc enzyme at pH 4.0 is not reversible. Consequences of the irreversible behavior of the two dismutases for the previously reported studies on their redox properties are discussed.
authors.affiliation: Department of Biochemistry, Agricultural University, Wageningen, The Netherlands.
authors.forename: M F
authors.initials: MF
authors.lastname: Verhagen
authors.forename: E T
authors.initials: ET
authors.lastname: Meussen
authors.forename: W R
authors.initials: WR
authors.lastname: Hagen
mesh_terms: D000818:Animals; D002417:Cattle; D003300:Copper; D004578:Electron Spin Resonance Spectroscopy; D004912:Erythrocytes / Q000201:enzymology; D004926:Escherichia coli / Q000201:enzymology; D066298:In Vitro Techniques; D007501:Iron; D010084:Oxidation-Reduction; D013482:Superoxide Dismutase / Q000737:chemistry*; D015032:Zinc
chemical_list: D003300:Copper; D007501:Iron; D013482:Superoxide Dismutase; D015032:Zinc