Binding of human thrombin to human factor VIII:RAg

The binding capacity of radiolabelled human alpha-thrombin (alpha-thrombin), inactivated with phenyl-methyl sulphonyl fluoride (PMSF), to factor VIII related antigen (VIII:RAg) multimers isolated by immune precipitation and resolved by sodium dodecyl sulphate (SDS) agarose gel electrophoresis has been examined. 125I-PMSF alpha-thrombin bound predominantly to four low molecular weight multimers (LMW) of VIII:RAg prepared from either normal or haemophilia A plasma. Two of these VIII:RAg multimers present in serum had lost the capacity to bind thrombin. No bands were observed when either plasma or serum from three patients with severe von Willebrand's disease (vWd) was processed in an identical manner. This binding was not dependent on the presence of an intact catalytic site on the thrombin molecule. A similar pattern of binding to VIII:RAg multimers was observed with 125I-labelled human anti-FVIII antibodies as 125I-alpha-thrombin and appears to involve the same multimers.