Myeloperoxidase (MPO) is an important component of the neutrophil's antimicrobial armory and has been implicated in promoting tissue damage in numerous inflammatory diseases. For the first time the standard reduction potential of the redox couple compound II/native enzyme has been determined to be (0.97+/-0.01)V at pH 7.0 and 25 degrees C. This was achieved by rapid mixing of preformed compound II with either tyrosine or nitrite by using the sequential-mixing stopped-flow technique and measuring spectrophotometrically the concentrations of the reacting species and products at equilibrium. Using the recently determined standard reduction potential for the couple compound I/native enzyme (1.16 V), the reduction potential of the couple compound I/compound II was calculated to be 1.35 V at pH 7 and 25 degrees C. These data reveal substantial differences between the two known heme peroxidase superfamilies and reflect the dramatic differences observed in the oxidisability of substrates by the MPO redox intermediates compound I and compound II.