Binding of von Willebrand factor to glycoproteins Ib and IIb/IIIa complex : affinity is related to multimeric size

We have separated von Willebrand factor (vWF) multimers of different size into several fractions which were characterized by SDS-agarose gel electrophoresis and by measuring the ratio between ristocetin cofactor activity (Ricof) and von Willebrand antigen (vWF:Ag) content. The pooled fractions contained vWF with multimeric structures and Ricof similar to those in plasma. The pool was labelled with 125I and used for inhibition binding studies with individual fractions to calculate the dissociation constants (Kd values expressed in mol/l) of the individual fractions for ristocetin-dependent binding to GP Ib and thrombin-induced binding to GP IIb/IIIa. Direct binding studies of the 125I-vWF pool gave mean Kd values of 2.02 +/- 0.05 x 10(-8) for GP Ib and 1.15 +/- 0.02 x 10(-8) for the GP IIb/IIIa complex. Inhibition binding studies gave Kd mean values one third to one tenth as high for larger multimers and 3-10 times higher for smaller multimers, for both GP Ib and IIb/IIIa complex. Similar results were observed when binding studies were carried out in the presence of platelets from a patient with afibrinogenaemia. These data on binding correlated very well with ristocetin- and thrombin-induced aggregation of afibrinogenaemic platelets, since equal concentrations of the higher molecular weight forms gave significantly higher aggregation rates. Based on these results, we conclude that the affinity of the vWF molecule for its two platelet receptors is greater for the largest multimers