"Bridged" peptide of fibrinogen binds to platelets activated by RGD peptide as well as by ADP.

We have evaluated the binding of a bridged peptide of Y-HHLGGAKQAGDV(G)9RGDS to RGDS-activated platelets. The bridged peptide saturably bound to platelets activated by RGDS peptide and by ADP with binding sites of 38,600 +/- 4800 molecules/platelet (Kd = 8.9 +/- 1.3 x 10(-7) M) and 43,600 +/- 9,200 molecules/platelet (Kd = 3.1 +/- 0.5 x 10(-7) M), respectively. Native fibrinogen inhibited the binding of the bridged peptide to platelets activated by RGDS peptide and by ADP with IC50s of 1.2 +/- 0.4 microM and 1.8 +/- 0.6 microM, respectively. In addition, the RGDS peptide and the dodecapeptide (gamma 400-411; HHLGGAKQAGDV) completely inhibited the binding of the bridged peptide to platelets activated by RGDS peptide with IC50s of 32 +/- 2.6 microM for RGDS and 102 +/- 4.1 microM for gamma 400-411, respectively. The anti-GPIIb/IIIa monoclonal antibody, LJ-CP8, also inhibited these bindings. The IC50s were 75 +/- 8.6 micrograms/ml for RGDS activation and 56 +/- 6.3 micrograms/ml for ADP stimulation. These results suggest that the flexible hinge region may allow assumption of the appropriate structure in the presence of the receptor complex. It indicates that both RGDS and agonist stimulation may play an important role in inducing activation of platelets that results in further platelet aggregation and tight thrombus formation.