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 plate let aggregation and cell adhesion, but show structural and functional heterogeneity. We investigated the interaction of four prototypic disi ntegrins with alpha(IIb)beta(3) expressed on the surface of resting an d activated platelets. The binding affinity (K-d) of I-125-albolabrin, I-125-echistatin, I-125-bitistatin and I-125-eristostatin toward rest ing platelets was 294, 153, 48 and 18 nM respectively. The K-d value f or albolabrin decreased 3-fold and 6-fold after ADP- or thrombin-induc ed activation. The K-d values for bitistatin and echistatin also decre ased with ADP, but there was no further decrease with thrombin. In con trast, eristostatin bound with the same high affinity to resting and a ctivated platelets. The pattern of fluorescein isothiocyanate (FITC)-e ristostatin and FITC-albolabrin binding to resting and activated plate lets was consistent with observations using radiolabeled material. Eri stostatin showed faster and more irreversible binding to platelets, an d greater potency compared with albolabrin in inducing conformational neo-epitopes in beta(3). The anti-alpha(IIb)beta(3) monoclonal antibod y OP-G2 that is RGD-dependent inhibited disintegrin binding to activat ed platelets more strongly than binding to resting platelets and it in hibited the binding to platelets of albolabrin more strongly than eris tostatin. The specificity of disintegrin interaction with alpha(IIb)be ta(3), was confirmed by demonstrating cross-linking of these peptides to alpha(IIb)beta(3) on normal platelets, but not to thrombasthenic pl atelets deficient in alpha(IIb)beta(3).