H2O2-MPO-halide system
Chapter: 2
This is an absolutely critical oxygen-dependent killing system, without which leukocytes have great difficulty killing bacteria in phagolysosomal vacuoles by halogenation. It is dependent on the oxidative burst that follows ingestion of a bacterium. superoxide anion is formed as a result, which is converted to hydrogen peroxide by the superoxide dismutase reaction. Meanwhile, the lysosomal enzyme, myeloperoxidase (MPO), has entered the phagolysosome when lysosomal contents are emptied into the phagocytic vacuole (phagosome). In the presence of a halide ion, MPO catalyzes the formation of an hypohalous (usually hypochlorous) acid, which kills the bacterium, either by binding the halide ion to intracellular constituents (a process called halogenation) or by oxidation of bacterial lipids and proteins (hypohalous acids are more potent oxidants than hydrogen peroxide).