Much evidence exists for the increased peroxidase activity of copper, zinc superoxide dismutase (SOD1) in oxidant-induced diseases. In this study, we measured the peroxidase activity of SOD1 by monitoring the oxidation of dichlorodihydrofluorescein (DCFH) to dichlorofluorescein (DCF). Bicarbonate dramatically enhanced DCFH oxidation to DCF in a SOD1/H(2)O(2)/DCFH system. Peroxidase activity could be measured at a lower H(2)O(2) concentration ( approximately 1 microm). We propose that DCFH oxidation to DCF is a sensitive index for measuring the peroxidase activity of SOD1 and familial amyotrophic lateral sclerosis SOD1 mutants and that the carbonate radical anion (CO(3)) is responsible for oxidation of DCFH to DCF in the SOD1/H(2)O(2)/bicarbonate system. Bicarbonate enhanced H(2)O(2)-dependent oxidation of DCFH to DCF by spinal cord extracts of transgenic mice expressing SOD1(G93A). The SOD1/H(2)O(2)/HCO(3)(-)-dependent oxidation was mimicked by photolysis of an inorganic cobalt carbonato complex that generates CO(3). Metalloporphyrin antioxidants that are usually considered as SOD1 mimetic or peroxynitrite dismutase effectively scavenged the CO(3) radical. Implications of this reaction as a plausible protective mechanism in inflammatory cellular damage induced by peroxynitrite are discussed.