Quantitative assay of a plasma factor deficient in von Willebrand's disease that is necessary for platelet aggregation. Relationship to factor VIII procoagulant activity and antigen content

Abstract

In the present paper an assay is described for a factor, the von Willebrand factor [VIII(VWF)], based on the observation that a log log relationship exists between the amount of ristocetin induced aggregation of washed, normal platelets and the concentration of normal plasma present in the test system. The activity of VIII(VWF) as well as antihemophilic factor procoagulant activity [VIII(AHF)] and factor VIII antigen [VIII(AGN)] was assayed in 15 patients with von Willebrand's disease and 20 normal subjects. A highly significant correlation (r 0.80) between VIII(VWF) and both VIII(AGN) and VIII(AHF) was found in normal subjects and in patients with von Willebrand's disease. This finding, in addition to the observation that agarose gel chromatography fractions that have VIII(AHF) procoagulant activity also have VIII(VWF) activity, strongly suggests that the von Willebrand factor is associated with the factor VIII molecule. VIII(VWF) in normal plasma was not inhibited by human anti VIII, and VIII(VWF) levels were normal in hemophilic plasma. Thus, the VIII(VWF) site on the factor VIII molecule appears to be different from that determining VIII(AHF). Finally, the activity of VIII(VWF) appeared to correlate better with the bleeding time than either VIII(AHF) or VIII(AGN). This suggests that VIII(VWF) assayed in this study may be the 'anti bleeding factor' that is deficient in von Willebrand's disease. These findings are consistent with a decreased synthesis of the factor VIII molecule in von Willebrand's disease and suggest the possibility of additional abnormalities of the site on the molecule that determines the activity of VIII(VWF).