Antioxidant activity of hemocyanin; a pulse radiolysis study

In order to determine the reactivity on hemocyanin from Androctonus australis, the reaction of superoxide anion has been investigated using pulse radiolysis. The kinetics of O2- decays have been studied in aqueous buffered media at various basic pH (8, 8.5 and 9), first in the absence and then in the presence of hemocyanin (in oxygenated solutions containing formate anion 0.16 mol.l-1). We have shown that, in the presence of hemocyanin, O2- decay is a first-order process whose apparent rate constant is proportional to protein concentration (10(-7) to 10(-6) mol.l-1) and pH independent between 8 to 9. A second-order rate constant of 3.5 +/- 0.1.10(7) mol-1.l.s-1, has been deduced for the catalytic rate constant of hemocyanin with O2-. Meanwhile, this activity is smaller than that described for free copper, eukaryotic Cu-Zn-SOD or some copper chelates. We have verified that apohemocyanin--the copper deprived protein--does not exhibit such an activity vs. SOD (superoxide dismutase).