Myeloperoxidase-catalyzed taurine chlorination: initial versus equilibrium rate

Myeloperoxidase (MPO) catalyzes the two-electron oxidation of chloride, thereby producing hypochlorous acid (HOCl). Taurine (2-aminoethane-sulfonic acid, Tau) is thought to act as a trap of HOCl forming the long-lived oxidant monochlorotaurine (N-Cl)-Tau], which participates in pathogen defense. Here, we amend and extend previous studies by following initial and equilibrium rate of formation of (N-Cl)-Tau mediated by MPO at pH 4.0-7.0, varying H(2)O(2) concentration. Initial rate studies show no saturation of the active site under assay conditions (i.e. H(2)O(2)] > or = 2000 MPO]). Deceleration of Tau chlorination under equilibrium is quantitatively described by the redox equilibrium established by H(2)O(2)-mediated reduction of compound I to compound II. At equilibrium regime the maximum chlorination rate is obtained at H(2)O(2)] and pH values around 0.4mM and pH 5. The proposed mechanism includes known acid-base and binding equilibria taking place at the working conditions. Kinetic data ruled out the currently accepted mechanism in which a proton participates in the molecular step (MPO-I+Cl(-)) leading to the formation of the chlorinating agent. Results support the formation of a chlorinating compound I-Cl(-) complex (MPO-I-Cl) and/or of ClO(-), through the former or even independently of it. ClO(-) diffuses away and rapidly protonates to HOCl outside the heme pocket. Smaller substrates will be chlorinated inside the enzyme by MPO-I-Cl and outside by HOCl, whereas bulkier ones can only react with the latter.