Bicarbonate enhances the peroxidase activity of Cu,Zn-superoxide dismutase- Role of carbonate anion radical

We examined the effect of bicarbonate on the peroxidase activity of copper- zinc superoxide dismutase (SOD1), using the nitrite anion as a peroxidase p robe. Oxidation of nitrite by the enzyme-bound oxidant results in the forma tion of the nitrogen dioxide radical, which was measured by monitoring 5-ni tro-gamma-tocopherol formation. Results indicate that the presence of bicar bonate is not required for the peroxidase activity of SOD1, as monitored by the SOD1/H2O2-mediated nitration of gamma-tocopherol in the presence of ni trite, However, bicarbonate enhanced SOD1/H2O2-dependent oxidation of tocop herols in the presence and absence of nitrite and dramatically enhanced SOD 1/H2O2-mediated oxidation of unsaturated lipid in the presence of nitrite, These results, coupled with the finding that bicarbonate protects against i nactivation of SOD1 by H2O2, suggest that SOD1/H2O2 oxidizes the bicarbonat e anion to the carbonate radical anion. Thus, the amplification of peroxida se activity of SOD1/H2O2 by bicarbonate is attributed to the intermediary r ole of the diffusible oxidant, the carbonate radical anion. We conclude tha t, contrary to a previous report (Sankarapandi, S., and Zweier, J. L. (1999 ) J, Biol, Chem. 274, 1226-1282), bicarbonate is not required for peroxidas e activity mediated by SOD1 and H2O2. However, bicarbonate enhanced the per oxidase activity of SOD1 via formation of a putative carbonate radical anio n, Biological implications of the carbonate radical anion in free radical b iology are discussed.