Inhibition by heparin of platelet activation induced by neutrophil-derived cathepsin G

Heparin is the most widely used anticoagulant drug for prevention and treatment of thrombosis. However, inhibition of blood coagulation might not fully explain the antithrombic activity of this drug. The present study shows that different heparin preparations (50 nM) completely human platelet aggregation, serotonin release and thromboxane B 2 production induced by purified neutrophil-derived cathepsin G (E.C. No. 3.4.21.20). This inhibitory effect was not related to the anticoagulant property of the compounds, since a heparin preparation with an activated active site for antithrombin II was also effective. Heparins inhibited the protease activity of the enzyme over the same range of concentrations. Since the effect of cathepsin G on platelets requires an intact proteolytic active site, the inhibitory effect of the drugs on cathepsin G-induced platelet activation may be explained by a blockade activity. Heparins were also shownto reduce platelet activation induced by cathepsin G released from activated polymorphonuclear leucocytes in mixed cell suspensions. As polymorphonuclear leucocytes might contribute to both arterial and venous thrombosis through platelet activation induced by the release of cathepsin G, this novel property of heparin could be used its antithrombotic efficacy.