On the role of bicarbonate in peroxidations catalyzed by Cu,Zn superoxide dismutase

The interaction of Cu,ZnSOD with H2O2 generates an oxidant at the active site that can then cause either the inactivation of this enzyme or the oxidation of a variety of exogenous substrates. We show that the rate of inactivation, imposed by 10-mM H2O2 at 25°C and pH 7.2, is not influenced by 10-mM HCO3−; whereas the oxidation of 2,2′-azino-bis-[3-ethylbenzothiazoline sulfonate] (ABTS=) is virtually completely dependent upon HCO3−. The reduction of the active site Cu(II) by H2O2, which precedes inactivation of the enzyme, occurred at the same rate in phosphate buffer with or without bicarbonate added. These results indicate that HCO3− does not play a role in facilitating the interaction of H2O2 with the active site copper, but they can be accommodated by the proposal that HCO3− is oxidized to HCO3·, which then diffuses from that site and causes the oxidation of substrates, such as ABTS=, that are too large to traverse the solvent access channel to the Cu(II).