INTERACTION OF DISINTEGRINS WITH THE ALPHA(IIB)BETA(3) RECEPTOR ON RESTING AND ACTIVATED HUMAN PLATELETS

Viper venom disintegrins contain the RGD/KGD motif. They inhibit platelet aggregation and cell adhesion, but show structural and functional heterogeneity. We investigated the interaction of four prototypic disintegrins with alpha(IIb)beta(3) expressed on the surface of resting and activated platelets. The binding affinity (K-d) of I-125-albolabrin, I-125-echistatin, I-125-bitistatin and I-125-eristostatin toward resting platelets was 294, 153, 48 and 18 nM respectively. The K-d value for albolabrin decreased 3-fold and 6-fold after ADP- or thrombin-induced activation. The K-d values for bitistatin and echistatin also decreased with ADP, but there was no further decrease with thrombin. In contrast, eristostatin bound with the same high affinity to resting and activated platelets. The pattern of fluorescein isothiocyanate (FITC)-eristostatin and FITC-albolabrin binding to resting and activated platelets was consistent with observations using radiolabeled material. Eristostatin showed faster and more irreversible binding to platelets, and greater potency compared with albolabrin in inducing conformational neo-epitopes in beta(3). The anti-alpha(IIb)beta(3) monoclonal antibody OP-G2 that is RGD-dependent inhibited disintegrin binding to activated platelets more strongly than binding to resting platelets and it inhibited the binding to platelets of albolabrin more strongly than eristostatin. The specificity of disintegrin interaction with alpha(IIb)beta(3), was confirmed by demonstrating cross-linking of these peptides to alpha(IIb)beta(3) on normal platelets, but not to thrombasthenic platelets deficient in alpha(IIb)beta(3).