Human neutrophilic myeloperoxidase (MPO) is involved in the defence mechanism of the body against micro-organisms. The enzyme catalyses the generation of the strong oxidant hypochlorous acid (HOCl) from hydrogen peroxide and chloride ions. In normal neutrophils MPO is present in the dimeric form (140 kDa). The disulphide-linked protomers each consist of a heavy subunit and a light one. Reductive alkylation converts the dimeric enzyme into two promoters, 'hemi-myeloperoxidase'. We studied the initial activities of human dimeric MPO and hemi-MPO at the physiological pH of 7.2 and found no significant differences in chlorinating activity. These results indicate that, at least at neutral pH, the protomers of MPO function independently. The absorption spectra of MPO compounds II and III, both inactive forms concerning HOCl generation, and the rate constants of their formation were the same for dimeric MPO and hemi-MPO, but hemi-MPO required a slightly larger excess of H2O2 for complete conversion. Hemi-MPO was less stable at a high temperature (80 degrees C) as compared to the dimeric enzyme. Furthermore, the resistance of the chlorinating activity of hemi-MPO against its oxidative product hypochlorous acid was somewhat lower (IC50 = 32 microM HOCl) compared to dimeric MPO (IC50 = 50 microM HOCl). The higher stability of dimeric MPO in the presence of its oxidative product compared to that of monomeric MPO might be the reason for the occurrence of MPO as a dimer.