Binding of eosinophil peroxidase to mast cell granules with retention of peroxidatic activity

Eosinophil peroxidase (EPO) and to a lesser degree neutrophil peroxidase (myeloperoxidase, MPO) bound tightly to mast cell granules (MCG), particularly when the latter were depleted of histamine by suspension in physiologic salt solutions. The bound EPO was localized on the surface of the granule, and its dissociation required salt concentrations of high enough ionic strength (greater than 0.75 M) to solubilize the MCG matrix. Elution of MPO from the complex occurred at a lower salt concentration. The MCG/EPO complex retained the capacity of the isolated EPO to catalyze the iodination reaction when supplemented with iodide, H2O2, and a protein acceptor and to kill microorganisms when supplemented with H2O2 and a halide (iodide, chloride). Indeed, the MCG/EPO complex had significantly greater iodinating and bactericidal activity than the free enzyme when standardized to equal guaiacol units of peroxidase activity. Thus, in areas of inflammation where mast cells and phagocytic leukocytes coexist, there is the potential for the formation of active complexes extracellularly between mast cell granules and molecules such as EPO (or MPO) that can affect the inflammatory response.