Activation of Factor XII in Human Plasma: Protection by Benzamidine of the Cofactor Function of High Molecular Weight Kininogen

: By incubation of human citrated plasma with acetone 25% v/v kallikrein inhibitors were destroyed and prekallikrein activated to kallikrein. When the incubation was carried out in the presence of benzamidine 7 mM, the cofactor capacity of high molecular weight kininogen (HMrK) was protected against destruction by a serine protease which was not plasma kallikrein. By analogy with studies in rat plasma this protease might be a plasminogen activator (Berstad & Briseid 1982; Johansen & Briseid 1983). Factor XII in the plasma preparation was activated to unfragmented factor XIIa by adsorption to kaolin, and assayed as prekallikrein activator (PKA). The extent of activation of factor XII was only insignificantly influenced by the 1 + 1 (v/v) dilution of the plasma preparation with a suspension of kaolin. When, however, the preparation was diluted > 1 + 5 (v/v) before incubation with the suspension, a stoichiometric HMrK concentration‐effect curve could be established, allowing the assay of cofactoractive HMrK. Assays of HMrK in plasma preparations from healthy men and women demonstrated an average lower level of cofactor‐active HMrK in the preparations from women. It is suggested that benzamidine is not capable of providing a complete protection of HMrK during the procedure in all plasma samples.