CHLORINATION OF TAURINE BY MYELOPEROXIDASE - KINETIC EVIDENCE FOR AN ENZYME-BOUND INTERMEDIATE

The chlorination of taurine by the myeloperoxidase-H2O2-Cl- system was investigated under steady state conditions. By systematically varying the pH and the concentrations of H2O2, Cl-, and taurine such that chl oride inhibition and the unwanted formation of inactive compound II in termediate are minimized, rate data were found to fit a mechanism invo lving an enzyme-bound chlorinating intermediate. The mechanism we prop ose is as follows. [GRAPHICS] The kinetic parameters determined at pH 4.7 are: k(1) = (3.3 +/- 0.2) x 10(7) M(-1) S-1, k(2) = (2.8 +/- 1.2) x 10(6) M(-1) S-1, and k(3) = (4.4 +/- 0.2) x 10(5) M(-1) S-1. The rat e constant for compound I formation (k(1)) is of the same order of mag nitude as the value (1.8 x 10(7) M(-1) S-1) obtained using transient s tate techniques in a previous study by our group. The value of k(3) is 2 orders of magnitude greater than the nonenzymatic reaction between HOCl and taurine at the same pH. The results of this study indicate th at the chlorination reaction mediated by the myeloperoxidase system in vivo may involve an enzyme intermediate species rather than free HOCl . Not only does this mechanism offer the advantage of substrate specif icity but also of speed compared to the non-enzymatic reaction. This m echanism can also explain how the indiscriminate oxidation reactions b y HOCl are prevented in the leukocyte. The fast formation of taurine m onochloramine, a relatively nontoxic and stable compound compared to H OCl, is consistent with the proposed role of taurine in the neutrophil , that of protecting certain targets including myeloperoxidase from th e attack by potent chlorinated oxidants.