The inhibition of activated factor XII (hageman factor) by antithrombin III: The effect of other plasma proteinase inhibitors

Human factor XII was activated by adsorption onto kaolin in the presence of high molecular weight kininogen. The washed kaolin-containing precipitates activate prekallikrein to kallikrein. When antithrombin III was added to the reaction mixture, the conversion of prekallikrein to kallikrein was inhibited, the degree of inhibition depending on the concentration of antithrombin and the time of incubation. Heparin had a slight enhancing effect with low concentrations of antithrombin and short incubation times. However, the inhibition of the generated kallikrein by antithrombin III was markedly enhanced by heparin. Antithrombin III inhibited also the effect of activated factor XII on the partial thromboplastin time, using factor XII-deficient plasma. Of other plasma proteinase inhibitors used (α 1 -antitrypsin, α 2 -macroglobulin, C l -inactivator) only C l -inactivator inhibited activated factor XII.