Formation of Intrinsic Factor‐X‐Activator Activity, with Special Reference to the Role of Thrombin

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Abstract
Summary When thrombin‐activated purified human factor VIII (factor VIIIt), purified human factor IXa, lipid and calcium were combined, a very rapid reaction occurred involving the lipid and at least one of the plasma clotting factors. No other interaction between the components of intrinsic factor‐X activator could be demonstrated. Full activity developed so rapidly as to suggest an instantaneous reaction. In contrast, when native factor VIII was used instead of factor VIIIt, insignificant factor‐X activator was found over 15 min. Further experiments with hirudin confirmed that a preliminary activation of factor VIII is an absolute requirement for the development of human intrinsic factor‐X activator activity. Both thrombin and trypsin effectively activated factor VIII, which suggests that activation results from proteolysis. Factor IXa, factor XIa, factor Xa, collagen, connective tissue, platelet fractions, plasmin and Reptilase ‘R’could not activate factor VIII within a physiologically significant time interval.

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