Activation of low molecular weight fragment of antihaemophilic factor (factor VIII) by thrombin.

Abstract

THE precise molecular function of plasma antihaemophilic factor (AHF, factor VIII), a glycoprotein with an apparent molecular weight of 2 × 106, is unknown. Recent experiments suggest that this function may be carried out by a low molecular weight (LMW) fragment which can be dissociated from plasma AHF by increased salt concentration1–3. The LMW fragment retains procoagulant activity, but is not reactive in immunoprecipitation assays for AHF-related antigen4. A high molecular weight (HMW) fragment, also identified using high salt conditions, has no procoagulant activity but retains AHF-related antigen in immunoprecipitation assays and von Willibrand factor activity in washed platelet assays4,5. We have now found that the LMW fragment, as well as AHF6–9, is susceptible to activation by thrombin. This observation suggests that the LMW fragment is not a product of thrombin activation of AHF and verifies the proposed role of the LMW fragment in AHF procoagulant activity.