A kinetic analysis of the catalase activity of myeloperoxidase

The predominant physiological activity of myeloperoxidase is to convert hyd rogen peroxide and chloride to hypochlorous acid. However, this neutrophil enzyme also degrades hydrogen peroxide to oxygen and water. We have underta ken a kinetic analysis of this reaction to clarify its mechanism. When myel operoxidase was added to hydrogen peroxide in the absence of reducing subst rates, there was an initial burst phase of hydrogen peroxide consumption fo llowed by a slow steady state loss. The kinetics of hydrogen peroxide loss were precisely mirrored by the kinetics of oxygen production. Two mols of h ydrogen peroxide gave rise to 1 mot of oxygen. With 100,muM hydrogen peroxi de and 6 mM chloride, half of the hydrogen peroxide was converted to hypoch lorous acid and the remainder to oxygen. Superoxide and tyrosine enhanced t he steady-state loss of hydrogen peroxide in the absence of chloride. We pr opose that hydrogen peroxide reacts with the ferric enzyme to form compound I, which in turn reacts with another molecule of hydrogen peroxide to rege nerate the native enzyme and liberate oxygen. The rate constant for the two -electron reduction of compound I by hydrogen peroxide was determined to be 2 x 10(6) M-1 s(-1). The burst phase occurs because hydrogen peroxide and endogenous donors are able to slowly reduce compound I to compound II, whic h accumulates and retards the loss of hydrogen peroxide. Superoxide and tyr osine drive the catalase activity because they reduce compound II back to t he native enzyme. The two-electron oxidation of hydrogen peroxide by compou nd I should be considered when interpreting mechanistic studies of myeloper oxidase and may influence the physiological activity of the enzyme.