Serine Protease Inhibitor Dependence of Heparin Mediated Inhibition of Thrombin Activatable Fibrinolytic Inhibitor

Thrombin activatable fibrinolytic inhibitor (TAFIa) modifies fibrin making it resistant to lysis. Serine protease inhibitors (SERPINS) such as antithrombin (AT) and heparin cofactor II (HCII) are cofactors that mediate heparin’s activity. To test the hypothesis that TAFIa generation inhibition is dependent on structure and molecular weight of heparins, heparin, enoxaparin, delegoparin and 5 molecular fractions of heparin (MW = 2.1–15.7 kDa) were profiled for anti‐Xa, anti‐IIa and TAFI activities in normal human pooled plasma (NHP), AT deficient plasma (DP) and HCII depleted plasma (DP). Anti‐Xa and anti‐IIa activities were determined using amidolytic assays. TAFI activity was measured utilizing a TAFIa specific substrate (PentaPharm, Basel, Switzerland). In NHP, a MW dependent inhibition of Xa, IIa and TAFI activation was observed. Lower MW fractions exhibited weaker inhibition. In the AT DP, all heparins exhibited a weaker inhibition of Xa, IIa and TAFI activation (~7.5 fold) with IC50’s > 50 ug/ml for the anti‐Xa and anti‐IIa activities for all fractions except for the 15.7 kDa (48.8 ± 0.92) and heparin (30 ± 3). When the heparins were supplemented to HCII DP, the IC50’s were similar to those in NHP (3.5±0.06 to >100 μg/ml). Delegoparin exhibited a decreased IC50 for TAFI activation. Inhibition of TAFI activation was dependent on the MW of the heparin and AT. HCII did not contribute to TAFI activation inhibition. These studies suggest that heparin mediated inhibition of TAFI activation is primarily mediated by SERPIN‐dependent inhibition of thrombin.