Fibrinolytic properties of activated factor XII

Activated factor XII (FXIIa), the initiator of the contact activation system, has been shown to activate plasminogen in a purified system. However, the quantitative role of FXIIa as a plasminogen activator in (factor XH-dependent) fibrinolysis in plasma is still unclear. The aim of our study was to investigate plasminogen activator activity (PAA) of both FXIIa in a purified system and endogenous FXIIa in plasma by measuring fibrinolysis in a clot lysis assay. During activation of purified FXII by trypsin, PAA was generated and remained stable even when FXIIa was completely cleaved to smaller fragments. Far more PAA was generated during autoactivation of purified FXII in the presence of dextran sulphate (DXS), a negatively charged component. However, PAA of autoactivated FXIIa decreased when it was cleaved to smaller fragments and reached a plateau comparable to PAA after activation by trypsin, while amidolytic activity of FXIIa remained constant. The same result was found when purified FXII was activated by kallikrein in the presence of DXS, while PAAwas reduced even more rapidly due to faster cleavage of FXII. This finding suggested that DXS may potentiate PAA of FXIIa in the clot lysis assay and not of the FXIIa fragments, missing the binding domain for DXS. Indeed, DXS was found to potentiate PAA of FXIIa more than tenfold according to a template model. Factor XII-dependent fibrinolysis is routinely measured in a dextran sulphate euglobulin fraction (DEF) of plasma and accounts for about 50% of total PAA. Since DXS was found to potentiate PAA of purified FXIIIa, FXI1 itself may be an important candidate for factor XIIdependent PAA in the DEF, a milieu in which DXS is present. Therefore, a DEF was prepared from normal human plasma and immunodepleted of FXIIa using an anti-FXII IgG Sepharose column. About 20% of factor XII-dependent PAA in the DEF could be ascribed to FXIIa. This study demonstrates that the fibrinolytic activity of FXIIa is substantially potentiated by DXS. Due to this potentiation, FXIIa significantly contributes to factor XII-dependent fibrinolysis in plasma as measured in a dextran sulphate euglobulin fraction.