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

: Factor XII has been assayed as kaolin‐activated prekallikrein activator in rat citrated plasma pretreated with acetone (Briseid et al. 1978 & 1979; Briseid & Berstad 1979). In the present work benzamidine added during blood collection increased the extent of activation by a factor of 6. Rat high molecular weight kininogen (HMWK) added to acetone‐treated citrated plasma likewise increased the activation, providing evidence of the protection by benzamidine of the cofactor function of HMWK. All cofactor capacity was retained after the removal of the kinin part of HMWK. Experiments carried out with plasminogen‐free plasma showed that plasmin could hardly be the factor responsible for the destruction of HMWK. The stoichiometric factor XII concentration‐effect curve obtained by diluting acetone‐treated rat plasma with acetone‐treated human factor XII deficient plasma showed that factor XII is present in functional excess, the concentration of HMWK deciding the extent of activation. By diluting acetone‐treated rat plasma with buffer, HM WK concentration‐effect curves were obtained which were approximately linear over a range of 0.03–0.40 μg (bradykinin equivalents) per ml kaolin incubate. No further activation of factor XII was obtained at 0.80 μg/ml.