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 chloride inhibition and the unwanted formation of inactive compound II intermediate are minimized, rate data were found to fit a mechanism involving an enzyme-bound chlorinating intermediate. The mechanism we propose is as follows. [formula: see text] The kinetic parameters determined at pH 4.7 are: k1 = (3.3 +/- 0.2) x 10(7) M-1 S-1, k2 = (2.8 +/- 1.2) x 10(6) M-1 S-1, and k3 = (4.4 +/- 0.2) x 10(5) M-1 S-1. The rate constant for compound I formation (k1) is of the same order of magnitude as the value (1.8 x 10(7) M-1 S-1) obtained using transient state techniques in a previous study by our group. The value of k3 is 2 orders of magnitude greater than the non-enzymatic reaction between HOCl and taurine at the same pH. The results of this study indicate that 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 specificity but also of speed compared to the non-enzymatic reaction. This mechanism can also explain how the indiscriminate oxidation reactions by HOCl are prevented in the leukocyte. The fast formation of taurine monochloramine, a relatively non-toxic and stable compound compared to HOCl, is consistent with the proposed role of taurine in the neutrophil, that of protecting certain targets including myeloperoxidase from the attack by potent chlorinated oxidants.