Myeloperoxidase-Mediated Platelet Release Reaction

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.

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