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The effects of two hydrazine derivatives (isoniazid and hydralazine) on the inactivation of myeloperoxidase by H2O2 were investigated. Incubation of 20 nM myeloperoxidase with 0.25 mM H2O2 alone caused a time-dependent irreversible loss of tetramethylbenzidine oxidation activity with a pseudo-first order inactivation rate constant of 0.057 min -1 . The hydrazine derivatives increased the inactivation rate in a concentration-dependent manner. Inactivation of the enzyme by H2O2 with or without the hydrazides showed a saturation kinetics pattern. Steady state kinetics analysis suggests that the hydrazides likely inactivate myeloperoxidase using a similar inactivating species as does H2O2. A bimolecular rate constant, specific inactivation rate enhancement factor (k*enh) is proposed as a formal description of the inactivation rate stimulation by the hydrazides. This parameter potentially avoids confounding the finite inactivation due to H2O2 with that caused by the presence of the hydrazides. The relevance of these findings and the constants derived to the analysis of suicide inactivation of peroxidases by reductant substrates are discussed.

Mechanism-Based Inhibition of Myeloperoxidase by Hydrogen Peroxide: Enhancement of Inactivation Rate by Organic Donor Substrates