Steady-State Study of Inhibitory Effect of Nitrite on Myeloperoxidase Catalytic Activity by Hydrogen Peroxide Biosensor

Abstract

Myeloperoxidase (MPO) is a neutrophil enzyme that employs hydrogen peroxide (H2O2) to catalyze the oxidation of halides and thiocyanate to their respective hypohalous acids. In this study, the inhibitory effect of nitrite (NO2 ) on MPO-catalytic activity was investigated electrochemically. H2O2 consumption during steady-state catalysis was monitored amperometrically by a carbon fiber based H2O2-biosensor at 25 o C. Optimized initial concentrations were 50 nM MPO, 10 μM H2O2, and a selected halide or thiocyanate concentration from physiological range. Under these conditions, reactions were monophasic and rapid (complete H2O2 consumption occurs in < 10 s). As concentration of NO2 increases, reactions change to biphasic (rapid step followed by a slow step) and both steps have been inhibited by NO2 . Our results confirmed the inhibitory effect of NO2 and demonstrated for the first time that NO2 is a strong inhibitor towards MPO-catalyzed oxidation of iodide and bromide; and a weak inhibitor towards MPO-catalyzed oxidation of chloride and thiocyanate.