A KINETIC-STUDY ON THE SUICIDE INACTIVATION OF PEROXIDASE BY HYDROGEN-PEROXIDE

In the absence of reductant substrates, and with excess H2O2, peroxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) shows the kinetic behaviour of a suicide inactivation, H2O2 being the suicide substrate. From the complex (compound I-H2O2), a competition is established between two catlytic pathways (the catalase pathway and the compound III-forming pathway), and the suicide inactivation pathway (formation of inactive enzyme). A kinetic analysis of this system allows us to obtain a value for the inactivation constant, ki = (3.92 ± 0.06) s ̇ 10-3 s ̇s-1. Two partition ratios (r), defined as the number of turnovers given by one mol of enzyme before its inactivation, can be calculated: (a) one for the catalase pathway, rc = 449 ± 47; (b) the other for the compound III-forming pathway, rCoIII = 2.00 ± 0.07. Thus, the catalase activity of the enzyme and, also, the protective role of compound III against an H2O2-dependent peroxidase inactivation are both shown to be important. © 1990.