Nature of von Willebrand Factor: A New Assay and a Specific Inhibitor

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Abstract
Platelet-active “von Willebrand factor” is a poorly characterized activity of a plasma-protein macromolecular complex. A new simple assay for von Willebrand factor is based on the dose response relation of the factor and the ristocetin platelet aggregation time. This assay uses the “snowstorm” macroscopic endpoint. A multiply transfused subject with von Willebrand's disease was observed to have a circulating inhibitor that blocks normal ristocetin aggregation of platelets, but not ADP-, epinephrine-, or collagen-induced aggregation. The inhibitor was not adsorbed by normal platelets, and was stable to heating at 56° for 30 min and to repeated freezings and thawings. This inhibitor also prevents action on human platelets by platelet-aggregating factor of bovine plasma, indicating this bovine factor activity is a function of von Willebrand factor. Three inhibitors were compared: ( 1 ) the von Willebrand factor inhibitor specifically blocked von Willebrand factor activity, ( 2 ) a human antibody from a hemophiliac inhibited only antihemophilic factor activity, and ( 3 ) a rabbit antiserum to a preparation of human antihemophilic factor inhibited both activities. The active site for von Willebrand factor on the macromolecular complex appears to be spaced some distance from the antihemophilic factor site.