Oxidation of clozapine and ascorbate by myeloperoxidase

The kinetics and spectra of the reactions of clozapine with compounds I and II of myeloperoxidase were investigated using both single- and sequential-mixing stopped-flow techniques, steady-state kinetics, and spectrophotometric measurements. The results show conclusively that both compounds I and II are reduced in one-electron reactions with clozapine, At pH 7.0 the rate constant for compound I reacting with clozapine is (1.5 +/- 0.1) x 10(6) M-1 s(-1) and for compound II (4.8 +/- 0.1) x 10(4) M-1 s(-1). The physiological pH of 7.4 was found to be optimal for the oxidation of clozapine by compound I. The rate constant for compound I reacting with ascorbate is (1.1 +/- 0.1) x 10(6) M-1 s(-1) and for compound II (1.1 +/- 0.2) x 10(4) M-1 s(-1), both obtained at pH 7.0, Experiments with both clozapine and ascorbate present showed that ascorbate acts both as a competitive inhibitor and free radical scavenger. (C) 1999 Academic Press.