Myeloperoxidase-Mediated Platelet Release Reaction

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Abstract
The ability of the neutrophil myeloperoxidase-hydrogen peroxide-halide system to induce the release of human platelet constituents was examined. Both lytic and nonlytic effects on platelets were assessed by comparison of the simultaneously measured release of a dense-granule marker, [3H]serotonin, and a cytoplasmic marker, [14C]adenine. Incubation of platelets with H2O2 alone (20 μM H2O2 for 10 min) resulted in a small, although significant, release of both serotonin and adenine, suggesting some platelet lysis. Substantial release of these markers was observed only with increased H2O2 concentrations (>0.1 mM) or prolonged incubation (1-2 h). Serotonin release by H2O2 was markedly enhanced by the addition of myeloperoxidase and a halide. Under these conditions, there was a predominance of release of serotonin (50%) vs. adenine (13%), suggesting, in part, a nonlytic mechanism. Serotonin release by the complete peroxidase system was rapid, reaching maximal levels in 2-5 min, and was active at H2O2 concentrations as low as 10 μM. It was blocked by agents which inhibit peroxidase (azide, cyanide), degrade H2O2 (catalase), chelate Mg2+ (EDTA, but not EGTA), or inhibit platelet metabolic activity (dinitrophenol, deoxyglucose). These results suggest that the myeloperoxidase system initiates the release of platelet constituents primarily by a nonlytic process analogous to the platelet release reaction. Because components of the peroxidase system (myeloperoxidase, H2O2) are secreted by activated neutrophils, the reactions described here may have implications for neutrophilplatelet interaction in sites of thrombus formation.