The effect of chloride on the redox and EPR properties of myeloperoxidase.

Myeloperoxidase was purified from human polymorphonuclear leukocytes and the effect of chloride upon the EPR and potentiometric properties was studied. The redox titration between the ferrous and ferric states of the enzyme yielded n = 1 Nernst plots between pH 9 and 4, with clear isosbestic points in the optical spectra during the redox change. The midpoint potential (Em) between the ferric and ferrous forms of the enzyme exhibited a pH-dependent change between pH 4 and 9, and the effect of added chloride ion indicated that Cl- competed with OH- for a binding site on the enzyme. Interestingly, the pH dependence of the Em indicated that the overall redox reactions of the enzyme was: ferric myeloperoxidase + 2e- + 1H+ = ferrous myeloperoxidase. Myeloperoxidase exhibited a rhombic high spin EPR signal which exhibited reduced rhombicity upon the binding of chloride. Our results strongly suggest that chloride binds to the sixth coordination position of the chlorin iron in myeloperoxidase by replacing the water which is the sixth ligand in the resting state. It is also concluded that the two iron centers are identical and that there is no interaction between them.