Redox properties of the couple compound I/native enzyme of myeloperoxidaseand eosinophil peroxidase

The standard reduction potential of the redox couple compound I/native enzy me has been determined for human myeloperoxidase WO) and eosinophil peroxid ase (EPO) at pH 7.0 and 25 degreesC. This was achieved by rapid mixing of p eroxidases with either hydrogen peroxide or hypochlorous acid and measuring spectrophotometrically concentrations of the reacting species and products at equilibrium. By using hydrogen peroxide, the standard reduction potenti al at pH 7.0 and 25 degreesC was 1.16 +/- 0.01 V for MPO and 1.10 +/- 0.01 V for EPO, independently of the concentration of hydrogen peroxide and pero xidases. In the case of hypochlorous acid, standard reduction potentials we re dependent on the hypochlorous acid concentration used. They ranged from 1.16 V at low hypochlorous acid to 1.09 V at higher hypochlorous acid for M PO and from 1.10 V to 1.03 V for EPO. Thus, consistent results for the stan dard reduction potentials of redox couple compound I/native enzyme of both peroxidases were obtained with all hydrogen peroxide and at low hypochlorou s acid concentrations: possible reasons for the deviation at higher concent rations of hypochlorous acid are discussed. They include instability of hyp ochlorous acid, reactions of hypochlorous acid with different amino-acid si de chains in peroxidases as well as the appearance of a compound I-chloride complex.