An on-line calorimetric study of the dismutation of superoxide anion catalyzed by SOD in batch reactors

Thermodynamics and kinetics of the dismutation of superoxide anion (O-2(.-) ) catalyzed by superoxide dismutase (SOD) in batch reactors has been studie d by on-line reaction calorimetry. The decomposition of hydrogen peroxide c atalyzed by catalase is utilized as a source of oxygen and the autoxidation of pyrogallol as a source of the substrate (O-2(.-)) for SOD. The molar re action enthalpies of the SOD reaction and the pyrogallol autoxidation have been measured as -160.1 and -218 kJ/mol, respectively, at 298.15 K and pH 8 .0. The experimental results showed that SOD had no effect on the kinetic p arameters or the mechanism for pyrogallol autoxidation. This autoxidation f ollowed second-order reaction kinetics in the presence of limited oxygen (f irst order with respect to both pyrogallol and O-2), and the second-order r ate constants were determined at 298.15 K and pH 8.0 to be 1.25 and 1.30 1/ mol s in the absence and presence of SOD, respectively. A possible mechanis m for the autoxidation of pyrogallol inhibited by SOD was also suggested. ( C) 2000 Elsevier Science Ltd. All rights reserved.