Mechanism of reaction of myeloperoxidase with hydrogen peroxide and chloride ion

The reaction of myeloperoxidase compound I (MPO-I) with chloride ion is wid ely assumed to produce the bacterial killing agent after phagocytosis. Two values of the rate constant for this important reaction have been published previously: 4.7 x 10(6) M-1.s(-1) measured at 25 degrees C [Marquez, L.A. and Dunford, H.B. (1995) J. Biol. Chem. 270, 30434-30140], and 2.5 x 10(4) M-1.s(-1) at 15 degrees C [Furtmuller, P.G., Burner, U. & Obinger, C. (1998 ) Biochemistry 37, 17923-17930]. The present paper is the result of a colla boration of the two groups to resolve the discrepancy in the rate constants . It was found that the rate constant for the reaction of compound I, gener ated from myeloperoxidase (MPO) and excess hydrogen peroxide with chloride, decreased with increasing chloride concentration. The rate constant publis hed in 1995 was measured over a lower chloride concentration range; the 199 8 rate constant at a higher range. Therefore the observed conversion of com pound I to native enzyme in the presence of hydrogen peroxide and chloride ion cannot be attributed solely to the single elementary reaction MPO-I + C l- --> MPO + HOCl. The simplest mechanism for the overall reaction which fi t the experimental data is the following: [GRAPHICS] where MPO-I-Cl- is a chlorinating intermediate. We can now say that the 199 5 rate constant is k(2) and the corresponding reaction is rate-controlling at low [Cl-]. At high [Cl-], the reaction with rate constant k(3) is rate c ontrolling. The 1998 rate constant for high [Cl-] is a composite rate const ant, approximated by k(2)k(3)/k(-2) Values of k(1) and k(-1) are known from the literature. Results of this study yielded k(2) = 2.2 x 10(6) M-1.s(-1) k(-2) = 1.9 x 10(5) s(-1) and k(3) = 5.2 x 10(4) s(-1) Essentially identic al results were obtained using human myeloperoxidase and beef spleen myelop eroxidase.