PREFERENTIAL ANTAGONISM OF THE INTERACTIONS OF THE INTEGRIN ALPHA(IIB)BETA(3) WITH IMMOBILIZED GLYCOPROTEIN LIGANDS BY SNAKE-VENOM RGD (ARG-GLY-ASP) PROTEINS - EVIDENCE SUPPORTING A FUNCTIONAL-ROLE FOR THE AMINO-ACID-RESIDUES FLANKING THE TRIPEPTIDE RGD IN DETERMINING THE INHIBITORY PROPERTIES OF SNAKE-VENOM RGD PROTEINS

The inhibitory properties of a panel of snake-venom-derived RGD (Arg-Gly-Asp) proteins, including the disintegrins kistrin, elegantin and albolabrin, and the neurotoxin homologue dendroaspin, were investigated in a platelet-adhesion assay using three immobilized ligands of the glycoprotein IIb-IIIa complex (alpha(IIb)beta(3)), namely fibrinogen, fibronectin and von Willebrand factor (VWF). The snake-venom proteins preferentially inhibited the adhesion of ADP-treated platelets to one or more of the immobilized ligands. Kistrin and dendroaspin exhibited similar inhibitory characteristics, abrogating platelet adhesion to fibrinogen and VWF at nanomolar concentrations, but poorly inhibiting adhesion to fibronectin. Kistrin and dendroaspin share little overall amino-acid-sequence identity, but a considerable level of sequence similarity exists around the RGD tripeptide. Synthetic cyclic peptides corresponding to these regions of kistrin and dendroaspin inhibited platelet adhesion to both fibrinogen and fibronectin with approximately equal potency, but were 100-fold weaker antagonists of the interactions of the alpha(IIb)beta(3) complex with fibrinogen than their parent proteins. The disintegrins elegantin and albolabrin, which share approx. 60% overall amino-acid-sequence similarity with kistrin but have different residues around the RGD tripeptide, exhibited different antagonistic preferences. Elegantin inhibited platelet adhesion to immobilized VWF and fibronectin, but was significantly less effective at disrupting adhesion to fibrinogen. Albolabrin selectively inhibited platelet adhesion to immobilized VWF and was less effective with fibrinogen and fibronectin as adhesive ligands. In contrast with the behaviour of these venom proteins, the adhesion of ADP-treated platelets to immobilized fibrinogen, fibronectin and vWF was inhibited non-selectively by a range of monoclonal antibodies with specificity for the alpha(IIb)beta(3) complex. These observations, therefore, define antagonistic preferences in this panel of venom proteins towards the interactions of the alpha(IIb)beta(3) complex with three immobilized glycoprotein ligands.