[52] Antimicrobial activity of myeloperoxidase

Peroxidases when combined with H2O2 and a halide (chloride, bromide, iodide, and pseudohalide thiocyanate) form a potent cytotoxic system, which contributes to the host defense against invading microorganisms and possibly tumor cells. Neutrophils and monocytes contain the same peroxidase (myeloperoxidase, MPO), and eosinophils a different peroxidase (eosinophil peroxidase, EPO), in cytoplasmic granules, and these enzymes are discharged into the phagosome following particle ingestion. Phagocytosis also is associated with a respiratory burst and much of the added oxygen consumed is converted to H2O2. Peroxidase, H2O2, and a halide interact in the phagosome to destroy the ingested organism. The components of the peroxidase system can also be released extracellularly where they may attack adjacent normal or malignant cells, uningested organisms, or soluble mediators. A variety of methods has been employed for the measurement of the toxicity of the peroxidase system. These methods depend on the nature of the target cell and include the measurement of replication in growth medium, Cr release, metabolic activity, and morphologic changes. This chapter focuses on bactericidal activity as measured by decrease in colony-forming units, using Escherichia coli as the target, MPO as the peroxidase, and chloride as the halide.