Binding of myeloperoxidase to bacteria: Effect on hydroxyl radical formation and susceptibility to oxidant-mediated killing

Neutrophils form superoxide anion (O₂^.-) and hydrogen peroxide (H₂O₂) and release myeloperoxidase (MPO) during ingestion of microbial pathogens, MPO, which adheres to some bacteria, catalyzes the formation of HOCl from H₂O₂, thereby enhancing H₂O₂/O₂^.- microbicidal activity, Hydroxyl radical (HO^.), also is an important contributor to H₂O₂ and O₂^.- microbicidal activity. MPO decreases iron-catalyzed HO^. production but also leads to HO^. production through the reaction of O₂^.- and HOCl. We hypothesized that binding of MPO to bacteria could alter the magnitude and site of HO^. production upon organism exposure to O₂^.-/H₂O₂. Incubation of MPO with Escherichia coli and Pseudomonas aeruginosa resulted in stable association of MPO with the bacteria which enhanced their susceptibility to killing by O₂^.-/H₂O₂. In the absence of MPO preincubation exposure of E. coli, but not P. aeruginosa to O₂^.-/H₂O₂, led to iron-catalyzed HO^. generation. This was associated with different amounts of redox active iron in the two types of bacteria. MPO preincubation slightly decreased HO^. detected with E. coli, but markedly increased HO^. formation with P. aeruginosa. This likely resulted from decreased iron-catalyzed HO^. production counterbalanced by increased iron-independent HO^. formation. MPO preincubation did not effect bacterial killing by a system which generated only H₂O₂, precluding MPO-dependent HO^. formation. These data are consistent with a possible role for MPO-derived HO^. in the augmentation of bacterial killing by this enzyme.