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The neutrophils are the first cellular responders to invading microorganisms and exert most of their antimicrobial activity in phagosomes, specialized membrane-bound intracellular compartments formed by the ingestion of microorganisms by human neutrophils. The cytotoxic substances generated de novo by the phagocyte NADPH oxidase and delivered to the phagosome via granule fusion create conditions that kill and inactivate the microorganisms. The azurophilic granules of human neutrophils contain a number of proteins that promote this process, including MPO.
In the phagosome, MPO777 catalyzes the halogenation reaction (Fig. 3). Chloride, which is available in the phagosome through a combination of pinocytosis during phagocytosis and transport through one or more chloride channels, is oxidized by H2O2 to the bactericidal species hypochlorous acid (HOCl). This forms a covalent bond with the thiolate molecule in the protein methionine sulfoxide or tyrosine residues in other proteins, to form a cytotoxic compound known as protein chloramine or monochloramine [60].
Other reactions catalyzed by MPO are less understood but also contribute to bacterial killing. MPO oxidizes several classic peroxidase substrates to produce a variety of reactive oxygen species, such as superoxide and hydroxyl radicals, which in turn react with other molecules to form cross-links or deactivate enzymes. The MPO-peroxidase reaction is especially important, as it provides a mechanism for producing the high levels of oxidants required to kill bacteria.
Similarly, MPO can oxidize the thiolate molecule to the chloride ion, which reacts with the thiolate group in cysteine residues in protein chains to form a sulfhydryl radical. This radical is capable of inactivating a number of important enzymes, including the alpha-hydroxylases of collagen and fibronectin (Fig. 4).
The cytotoxic action of MPO is the primary mechanism by which most bacteria are killed. However, a small proportion of bacteria are resistant to MPO-dependent killing. Studies of mice with a genetic absence of the neutrophil serine proteases elastase or cathepsin G have stimulated a re-examination of these components, which are thought to contribute to resistance against certain pathogens. Although the elimination of these two genes results in reduced susceptibility to challenge by a broad range of microorganisms, mutant mice remain susceptible to most infections, and therefore do not represent a complete model for understanding the role of these neutrophil defense mechanisms.
