Support of ristocetin-induced platelet aggregation by procoagulant-inactive and plasmin-cleaved forms of human factor VIII/von Willebrand factor.

Human factor VIII/von Willebrand factor (fVIII/vWf) was purified to homogeneity as defined by electrophoretic and immunologic criteria and tested for fVIII procoagulant and ristocetin cofactor activities. As little as 0.4 sg/mI of purified fVllI/vWf fully aggregated washed human platelets in the presence of ristocetin. Purified fVIlI/vWf. whether thrombinactivated or -inactivated. and fVIIl/vWf with its procoagulant activity abolished by a human inhibitor. supported ristocetin-induced platelet aggregation as effectively as native fVIII/vWf. Similarly. purified hemophilic fVlll/vWf protein and fVIII/vWf-Iike protein isolated from normal serum supported the aggregation of platelets in the presence of ristocetin. FVIII/vWf extensively degraded by human plasmin and exposed to denaturing solvents also supported platelet aggregation in the presence of ristocetin. Plasmin hydrolyzed fVIlI/vWf to yield noncovalently bonded fragments that could be separated into two pools by gel filtration in guanidine hydrochloride. After dialysis into dilute neutral buffers, the pool of high molecular weight fragments supported significant ristocetin-induced platelet aggregation and cross-reacted with an antibody to native fVlll/vWf. Hence. unlike fVIII procoagulant activity. the ristocetin cofactor activity and antigenicity of fVlII/vWf are retained, despite changes in the structure and conformation of the molecule. These results suggest caution about the interpretation of procoagulant activity:fVIII/vWf antigen ratios or procoagulant activity:ristocetin cofactor activity ratios in whole plasma. fVIIl/vWf concentrates or purified preparations of fVlll/vWf, since small amounts of minimally degraded. procoagulant-inactive fVlll/vWf cound markedly alter such values.