Reaction of myeloperoxidase compound I with chloride, bromide, iodide, andthiocyanate

Myeloperoxidase plays a fundamental role in oxidant production by neutrophi ls, The enzyme uses hydrogen peroxide to oxidize chloride (Cl-), bromide (B r-), iodide (I-), and the pseudohalide thiocyanate (SCN-) to their respecti ve hypohalous acids. This study for the first time presents transient kinet ic measurements of the oxidation of these halides and thiocyanate by the my eloperoxidase intermediate compound I, using the sequential mixing stopped- flow technique. At pH 7 and 15 degrees C, the two-electron reduction of com pound I to the native enzyme by Cl- has a second-order rate constant of (2. 5 +/- 0.3) x 10(4) M-1 s(-1), whereas reduction of compound I by SCN- has a second-order rate constant of (9.6 +/- 0.5) x 10(6) M-1 s(-1). Iodide [(7. 2 +/- 0.7) x 10(6) M-1 s(-1)] is shown to be a better electron donor for co mpound I than Br- [(1.1 +/- 0.1) x 10(6) M-1 s(-1)]. The pH dependence stud ies suggest that compound I reduction by (pseudo-)halides is controlled by a residue with a pK(a) of about 4.6. The protonation of this group is neces sary for optimum (pseudo-)halide anion oxidation. These transient kinetic r esults are underlined by steady-state spectral and kinetic investigations. SCN- is shown to be most effective in shifting the system myeloperoxidase/h ydrogen peroxide from the peroxidatic cycle to the halogenation cycle, wher eas iodide is shown to be more effective than bromide which in turn is much more effective than chloride. Decreasing pH increases the rate of this tra nsition. Our results show that thiocyanate is an important substrate of mye loperoxidase in most environments and that hypothiocyanate is likely to con tribute to leukocyte antimicrobial activity.