The agglutination of human platelets by bovine factor VIII: R

The binding of bovine factor VIII: R to human platelets causes an agglutination of the platelets and subsequent responses that may serve as a model for the interaction of platelets with damaged vascular subendothelium. Purified bovine factor VIII: R was radiolabeled and its binding to formalin-fixed human platelets measured. Binding was rapid and readily reversible. At saturation, approximately 6 micrograms of factor VIII: R was bound per 10(9) platelets. Optimal agglutination of platelets by bovine factor VIII: R occurred in the pH range of 6.5 to 7.5, although binding was more extensive at lower pH. The isoelectric point of factor VIII: R was 4.3, indicating that it has a net negative charge at physiological pH. Binding and agglutination were both inhibited by increasing the ionic strength of the medium and by polycations like Polybrene and polylysine. Ristocetin promoted the binding of bovine factor VIII: R to platelets but did not promote the agglutination of platelets by bovine factor VIII: R. Concentrations of dextran sulfate that strongly inhibited agglutination actually increased the binding of factor VIII: R to platelets.