Enzymatic and Bactericidal Activity of Monomeric and Dimeric Forms of Myeloperoxidase

Abstract

Enzymatic and bactericidal activities of mature, dimeric myeloperoxidase (MPO) and its monomeric form have been compared. Dimeric MPO was isolated from HL-60 cells. Hemi-MPO obtained from dimeric MPO by reductive cleavage of a disulfide bond between protomeric subunits was used as the monomeric form. Both peroxidase and halogenating (chlorinating) activities of MPO were assayed, each by two methods. Bactericidal activity of the MPO/Н₂О₂/Cl‾ system was tested using the Escherichia coli laboratory strain DH5α. No difference in the enzymatic and bactericidal activity between dimeric MPO and hemi- MPO was found. Both forms of the enzyme also did not differ in the resistance to HOCl, the main product of MPO. HOCl caused a dose-dependent decrease in peroxidase and chlorinating activity, and the pattern of this decrease was identical for dimeric MPO and hemi-MPO. At the equal heme concentration, the hemi- MPO/Н₂О₂/Cl‾ system demonstrated a somewhat higher bactericidal effect than the dimeric MPO/Н₂О₂/Cl‾ system. This is most likely explained by higher probability of contacts between the bacterial surface and hemi-MPO molecules, since at the same heme concentration the number of hemi-MPO molecules is 2-fold higher than that of dimeric MPO molecules. Using Western-blotting with antibodies to MPO, we have shown, for the first time, that the dimeric molecule of MPO could be cleaved into two monomeric subunits by HOCl, most probably due to oxidation of the disulfide bond between these subunits. This suggests that appearance in blood of MPO with mass corresponding to its monomer may result from the damage of dimeric MPO by reactive halogen species, especially upon their overproduction inducing oxidative/halogenative stress in inflammatory diseases.