Reaction of compound III of myeloperoxidase with ascorbic acid

A relatively pure and stable compound III of bovine spleen myeloperoxidase was prepared from native enzyme using the aerobic oxidation of dihydroxyfumarate to generate O2-(.). Spectral scans show well defined peaks at 450 and 625 nm and an isosbestic point between compound III and native enzyme at 440 nm. Compound III decayed to native enzyme without any detectable intermediate. The rate of decay was faster at alkaline pH values and also in the presence of superoxide dismutase. Ascorbic acid reduces compound III to native enzyme with a second order rate constant of (4.0 +/- 0.1) x 10(2) M-1 s-1. The ascorbic acid reduction of compound III has potential physiological relevance since it could help maintain the catalytic cycle of myeloperoxidase to generate the bactericidal agent hypochlorous acid.