When neutrophils phagocytose bacteria, they release myeloperoxidase (MPO) into phagosomes to catalyse the
conversion of superoxide to the potent antimicrobial oxidant hypochlorous acid (HOCl). Here we show that
within neutrophils, MPO is inactivated by HOCl. In this study, we aimed to identify the effects of HOCl on the
structure and function of MPO, and determine the enzyme’s susceptibility to oxidative inactivation during
phagocytosis. When hydrogen peroxide was added to a neutrophil granule extract containing chloride, MPO
activity was rapidly lost in a HOCl-dependent reaction. With high concentrations of hydrogen peroxide, western
blotting demonstrated that MPO was both fragmented and converted to high molecular weight aggregates. Using
the purified enzyme, we showed that HOCl generated by MPO inactivated the enzyme by destroying its prosthetic heme groups and releasing iron. MPO protein was additionally modified by forming high molecular weight
aggregates. Before inactivation occurred, MPO chlorinated itself to convert most of its amine groups to
dichloramines. When human neutrophils phagocytosed Staphylococcus aureus, they released MPO that was
largely inactivated in a process that required production of superoxide. Enzyme inactivation occurred inside
neutrophils because it was not blocked when extracellular HOCl was scavenged with methionine. The inactivated
enzyme contained a chlorinated tyrosine residue, establishing that it had reacted with HOCl. Our results
demonstrate that MPO will substantially inactivate itself during phagocytosis, which may limit oxidant production inside phagosomes. Other neutrophil proteins are also likely to be inactivated. The chloramines formed
on neutrophil proteins may contribute to the bactericidal milieu of the phagosome
