Binding of porcine von Willebrand factor to human platelets in the presence of ristocetin

The interaction of porcine von Willebrand factor (vWF) with human platelets in the presence of ristocetin was examined. Binding was rapid, specific, saturable and dependent on ristocetin concentration with half-maximal stimulation occuring at a ristocetin concentration of 0.5mM. Assuming vWF to be a tetramer with a MW of 9.5×10^?5, approximately 94,000 vWF binding sites per platelet were found with an average binding constant of 2.1×10^?8M. Humam vWF competed with the porcine protein for this site only in the presence of ristocetin. Binding of porcine vWF to platelets was found to be less sensitive to reduction with dithiothreitol than platelet agglutinating activity.