Here, we investigated the effect of bicarbonate anion (HCO3-) on the peroxidase activity stimulated by the thiol oxidase activity of copper, zinc superoxide dismutase (SOD1) using electron spin resonance (ESR) and optical techniques. Low temperature direct ESR revealed that cysteine (Cys) caused the reduction of copper(II) to copper(I) that was reoxidized by molecular oxygen to copper(II) at the active site of SOD1. The addition of HCO3- to aerobic incubations containing SOD1, Cys, and DTPA in phosphate buffer enhanced the peroxidase activity of SOD1, as measured by hydroxylation of cyclic nitrone spin traps, dichlorodihydrofluorescein oxidation to dichlorofluorescein, and oxidation of tyrosine to dityrosine. The addition of catalase inhibited the SOD1 peroxidase activity stimulated by the thiol oxidase actvity, implicating an intermediary role for H2O2 in SOD1/Cys/HCO3(-)-mediated oxidation and hydroxylation reactions. Using a competitive kinetic method, rate constants for the reaction between the oxidant formed in the SOD1/Cys/HCO3- system and selected inhibitors were measured. On the basis of these rate constants, we conclude that the thiol oxidase activity of SOD1 stimulates carbonate anion radical (CO3*-) formation in the presence of HCO3- and that the CO3*- formed in the SOD1/Cys/ HCO3- system is responsible for oxidation and hydroxylation reactions. Biological implications of this finding are discussed.