THE INACTIVATION OF HORSERADISH-PEROXIDASE BY M-CHLOROPEROXYBENZOIC ACID, A XENOBIOTIC HYDROPEROXIDE

m-Chloroperoxybenzoic acid (m-CPBA) acts as an oxidant substrate of pe roxidase (EC 1.11.1.7) and, at the same time, is a powerful suicide su bstrate of the enzyme. A Value for the partition ratio (r) between the catalytic and the inactivating routes is calculated in the absence of the typical reductant substrates of peroxidase: One mole of enzyme gi ves around two turnovers (r=1.8+/-0.1). The kinetic analysis allows us to calculate a value for the inactivation constant k(i)=(4.80+/-0.40) . 10(-3) s(-1), being very similar to that obtained for H2O2. These re sults suggest that, contrary to H2O2, in the case of m-CPBA a catalase -like reaction is not active and so the enzyme is not protected. Also, the calculated value for K-2 (6.54 mu M) indicates a high affinity of Compound I for m-CPBA.