Heparin inhibits neutrophil-induced platelet activation via cathepsin G.

Activated human polymorphonuclear neutrophils (PMNs) induce platelet stimulation via cathepsin G (cat G), a platelet-activating cationic serine proteinase released from the azurophilic granules. Heparin inhibited up to 100% of aggregation and serotonin release triggered by 180 nmol/L of purified cat G in a concentration-dependent manner between 10 and 70 mU/ml. When tested against the enzymatic activity of 180 nmol/L cat G (hydrolysis of N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide), inhibition by heparin never exceeded 60% (up to 100 U/ml). Inhibition was most probably related to electrostatic interactions between the cationic cat G and the anionic heparin, because addition of 180 mU/ml of protamine sulfate restored platelet activation. Low molecular weight heparin, CY 216, used between 5 and 50 mU/ml, also inhibited cat G-induced platelet activation. When purified PMNs and washed platelets were mixed together and challenged with a PMN agonist (FMLP at 1 mumol/L), platelet activation was observed. Pretreatment of the mixed cell population with 300 mU/ml heparin prevented platelet activation. This was illustrated by electron microscopy studies. The present data, apart from confirming a participation of cat G in the PMN-platelet interaction, bring evidence that heparin has potent antiproteinase effect in a biologic model.