Reactions of H(2)O(2) with superoxide dismutase were studied by e.p.r. (electron paramagnetic resonance) spectroscopy and other methods. In agreement with earlier work, the Cu(2+) of the enzyme is reduced by H(2)O(2), although the reaction does not go to completion and its kinetics are not simple. With dilute enzyme the time for half-reduction with 9mm-H(2)O(2) is about 150ms. It is suggested that the reaction is a one-electron reduction, involving liberation of O(2) (-). On somewhat more prolonged exposure to H(2)O(2), the enzyme is inactivated. For enzyme in dilute solution and over a limited range of H(2)O(2) concentrations, inactivation is first-order with respect to enzyme and reagent, with k=3.1m(-1).s(-1) at 20-25 degrees C. Inactivation is accompanied by marked changes in the e.p.r. and visible spectra and appears to be associated with destruction of one histidine residue per subunit. It is suggested that this histidine is close to the metal in the native enzyme and essential for its enzymic activity.

Amino Acids↗

PubMed1974

Mechanism of action of superoxide dismutase from pulse radiolysis and electron paramagnetic resonance. Evidence that only half the active sites function in catalysis.

1. Detailed studies on the mechanism of the enzymic reaction of bovine superoxide dismutase were carried out by using pulse radiolysis and electron paramagnetic resonance (e.p.r.). 2. The second-order rate constant for reaction between superoxide dismutase and the superoxide ion was redetermined as (2.37+/-0.18)x10(9)m(-1).s(-1) at 25 degrees C. This reaction governs the turnover, and any first-order steps must have rate constants higher than about 10(6)s(-1). Turnover has a low activation energy and is slowed substantially when the viscosity is increased with glycerol, confirming that the reaction rate is near the limit for diffusion control. In water a reversible conformation change to a less active form appears to take place above about 40 degrees C. 3. Pre-steady-state rates of reduction and reoxidation of copper in the enzyme are consistent with these processes being rate-limiting in enzyme turnover. 4. Examination, with the help of computer simulation, of the e.p.r. spectra at 9 and 35GHz of native superoxide dismutase indicated that, apart from 10-20% of impurities, only one species of Cu(2+) is distinguishable. Further, the specific activity of our enzyme preparations, measured by pulse radiolysis, is at least as high as that obtained by other workers. 5. Nevertheless, measurement of the proportion of copper present as Cu(2+) (determined both optically and by e.p.r. spectroscopy) in the steady states approached from both the oxidized and the reduced forms of the enzyme, indicates (after allowing for the impurities) that only half of the copper atoms participate in turnover. E.p.r. spectroscopy provided no evidence for differences between functioning and non-functioning Cu(2+) atoms. 6. It is suggested that the results may be best interpreted in terms of an allosteric type of mechanism, with two initially indistinguishable copper atoms in the enzyme. Reaction of one of these with a superoxide ion then renders the other, at least transiently, unreactive.

Animals↗

PubMed1974