INTERACTION OF VON-WILLEBRAND-FACTOR WITH PLATELETS ACTIVATED BY THROMBIN OR A SYNTHETIC 7-AMINO ACID PEPTIDE DERIVED FROM THE CLEAVED THROMBIN RECEPTOR

Thrombin and the 7-mer agonist peptide from its receptor (SFLLRNP) wer e compared for their ability to promote the binding of vWF to platelet s. Identical Ca2+-dependence and kinetics of activation were observed. Studies of inhibition of the binding by a series of monoclonal antibo dies to GPIb, GPIIb/IIIa and vWF and experiments performed using plate lets from patients with Glanzmann thrombasthenia or Bernard-Soulier sy ndrome enabled to identify GPIIb/IIIa as the receptor of vWF. Binding isotherms of vWF in the presence of an excess of either agonist yielde d a similar number of binding sites but an apparent dissociation const ant slightly but consistently higher with the 7-mer peptide than with thrombin. The latter point was confirmed by studying the binding of li miting amounts of vWF to platelets as a function of the agonist concen tration. The lower affinity in the presence of 7-mer peptide was not c orrected by adding increasing amounts of FPR-thrombin, a derivative wi th irreversibly blocked active site but retaining the binding properti es of the active enzyme. Conversely, the higher affinity observed with thrombin was decreased when platelets were treated with Serratia prot ease which selectively cleaved GPIb but did not affect the function of the thrombin receptor and GPIIb/IIIa. Our data thus suggest that both the 7-mer peptide and thrombin are able to induce the assembly of fun ctional GPIlb/IIIa. However the cleavage of the thrombin receptor foll owing the binding of thrombin to it and to another high affinity recep tor creates a new functional N-terminus which may be more efficient th an the free 7-mer agonist peptide in promoting platelet activation whe n maintained close to its interaction site by covalent linkage to the thrombin receptor.