Inhibition of activated haceman factor (factor XIIa) by an inhibitor of the plasminogen activation (PA inhibitor)

Abstract Purified preparations of an inhibitor which prevents one or more reactions leading to the activation of plasminogen (PA inhibitor) but with no effect on plasmin have been tested for their effects on factors XII, XIIa, IXa and Xa. The PA inhibitor was found to be a fast reacting inhibitor of factor XIIa but had no effects on factors XII, XIa, IXa or Xa. The PA inhibitor also blocked the kaolin induced generation of fibrinolytic activity initiated by factor XIIa and a crude euglobulin fraction prepared from factor XII deficient plasma. These data indicate that the PA inhibitor of human plasma readily inactivates factor XIIa as assayed in two biological systems.