PLASMIN-PLATELET INTERACTION INVOLVES CLEAVAGE OF FUNCTIONAL THROMBINRECEPTOR

We tested the hypothesis that the inhibition of thrombin-induced plate let activation by plasmin is mediated via the enzymatic action of plas min on the functional thrombin receptor. We monitored the binding of t he anti-thrombin receptor antibody [anti-TR-(34-46)] to platelets; thi s binding is sensitive to the cleavage of the thrombin receptor at ami no acid residues Arg-41 to Ser-42. Plasmin inhibited anti-TR-(34-46) b inding in dose- and time-dependent manners. The inactive synthetic pep tide with the amino acid sequence 40-55 of the thrombin receptor (D-FP RSFLLRNPNDKYEPF) was similarly cleaved by thrombin and plasmin to an a ctive peptide (SFLLRNPNDKYEPF) that produced robust cytosolic Ca2+ res ponses. At high concentrations, plasmin itself can activate platelets. We explored this effect with the use of anti-TR-(1-160). This antibod y abolished the cytosolic Ca2+ responses to thrombin and to the thromb in receptor-activating peptide SFLLRN but did not attenuate the plasmi n-induced cytosolic Ca2+ response. Thus plasmin inhibits thrombin-evok ed platelet activation by cleaving the thrombin receptor, but the plas min-induced cytosolic Ca2+ response is not due to the generation of th e tethered peptide of the thrombin receptor.