Microcalorimetric studies on the dismutation of superoxide anion catalyzed by superoxide dismutase

Thermokinetics of the dismutation of superoxide anion (O2.-) catalyzed by superoxide dismutase (SOD), in which the decomposition of hydrogen peroxide catalyzed by catalase is utilized as a source of oxygen and the autoxidation of pyrogallod as a source of the substrate (O2.-), has been studied by microcalorimetry. The molar reaction enthalpies of the SOD reaction and the pyrogallol autoxidation have been measured as - 160.1 and - 218 kJ · mol-1, respectively, at 298. 15 K and pH 7.0. A novel method for the determination of SOD activity, the microcalorimetric assay for SOD, has been proposed. The experimental results showed that SOD had no effect on both kinetic parameters and the mechanism for the pyrogallol autoxidation. This autoxidation followed the second-order reaction kinetics in the presence of limited oxygen (the first order with respect to both pyrogallol and O2), and the second-order rate constants were hermined at 298.15 K and pH 8.0 to be 1.25 and 1.30 L·mol-1·s-1 in the absence and presence of SOD, respectively. A possible mechanism for the autoxidation of pyrogallol inhibited by SOD was also suggested.