The ability of the two forms of activated Hageman factor (HFa) produced during contact activation of plasma to activate prekallikrein and factor XI was studied. alpha-HFa, defined as an 80,000 mol wt two-chain enzyme which remains bound to the surface was capable of cleaving surface-bound prekallikrein and factor XI. beta-HFa, a 28,000 mol wt single chain molecule, released from the surface during contact activation was able to cleave prekallikrein but showed no activity on factor XI. Cleavage of prekallikrein by beta-HFa occurred irrespective of whether the substrate was surface-bound or in solution. Cleavage of factor XI occurred only when it was surface bound and only the alpha-form of HFa was capable of this proteolytic action. Factor XI was found to remain bound to the surface while prekallikrein and kallikrein rapidly dissociated from the surface into the supernate. These findings suggest that the initiation of intrinsic coagulation through the activation factor XI is a localized event occurring at the site of contact activation and is the result of the action of alpha-HFa. By contrast, kinin generation and fibrinolysis resulting from the formation of kallikrein can be initiated either at the site of contact activation, by alpha-HFa action, or throughout the plasma, by beta-HFa; further dissemination of these activities is assured by the rapid dissociation of kallikrein itself from the surface.

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