Interaction of porcine von Willebrand factor (platelet aggregating factor) with human platelets

The binding of 125I-labeled porcine von Willebrand factor to washed human platelets was examined. In the absence of ristocetin, binding was found to be rapid, specific and saturable and was decreased in the presence of urea or at high ionic strength. Platelets were found to contain approx. 4760 binding sites for porcine von Willebrand factor with an average binding constant of 2.92 x 10(-7) M assuming the von Willebrand factor to be a tetramer with a molecular weight of 9 x 10(5). Although ristocetin was not absolutely required for binding, in its presence binding was increased.