Studies on the Mechanism of Ristocetin‐induced Platelet Aggregation: Binding of Factor VIII to Platelets

The effect of ristocetin on the binding of [125I]factor VIII to platelets was studied. High and low affinity F.VIII binding sites exist on platelets. The high affinity sites bind 13 times more F.VIII than the low affinity sites. Ristocetin increased the binding of F.VIII to both types of binding sites by increasing the affinity of F.VIII for the platelet and increasing the total number of platelet binding sites. Chymotrypsin-treated platelets were not aggregated by ristocetin and F.VIII: these platelets have less of the major platelet membrane glycoproteins and bind much less [125I]F.VIII than do buffer-treated platelets with and without ristocetin.