Thrombin Activation of Factor VIII: the Effect of Inhibitors

Abstract

Factor VIII [human] is a large glycoprotein which can be separated into a high MW component which retains factor VIII-related antigens and supports ristocetin-induced platelet agglutination, and a low MW component which has procoagulant activity. This separation, observed in high ionic strength buffers, may be the consequence of proteolysis by plasma proteins. Several proteolytic inhibitors were studied to determine their ability to inhibit factor VIII activation by thrombin. Under the current experimental conditions, diisopropylfluorophosphate (DFP) and hirudin inhibited the reaction, while heparin was an effective inhibitor only when plasma proteins were added. Benzamidine inhibited factor VIII activation when used at high concentrations, and phenylmethyl sulfonylfluoride and soybean trypsin inhibitor were ineffective. DFP and hirudin prevent thrombin alteration of factor VIII and will be useful in purification and characterization studies of the factor VIII molecule.