Abstract
Activation of rabbit blood coagulation Factor XI by purified rabbit Hageman factor, prekallikrein and high MW kininogen in the presence of kaolin was measured by clotting assay. Hageman factor and kaolin were absolute requirements for Factor XI activation. Prekallikrein and high MW kininogen enhanced the rate of Factor XI activation. Factor XI a generation was temporally associated with limited proteolytic cleavage of the molecule into disulfide-linked fragments of 50,000 and 33,000 molecular weight. Identical cleavage fragments were also seen when radiolabelled Factor XI was present in kaolin-activated rabbit plasma. The surface-binding and cleavage of rabbit 125I-Factor XI in normal human plasma and in plasmas deficient in high MW kininogen and Hageman factor were studied. The binding of Factor XI in plasma to kaolin was dependent upon the presence of high MW kininogen while the limited proteolysis of Factor XI during contact activation required the presence of both high MW kininogen and Hageman factor. These observations are consistent with the hypothesis that high MW kininogen links Factor XI to negatively charged surfaces where activated Hageman factor activates Factor XI by limited proteolysis.
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