The effect of corn trypsin inhibitor and inhibiting antibodies for FXI a and FXII a on coagulation of plasma and whole blood

Summary Background Corn trypsin inhibitor ( CTI ), an inhibitor of FXII a, is used to prevent plasma coagulation by contact activation, to specifically investigate tissue factor ( TF )‐initiated coagulation. Objective In the present work the specificity of CTI for factor (F) XII a is questioned. Methods and results In the commercial available plasma coagulation assays CTI was found to double activated partial thromboplastin time ( APTT ) at a plasma concentration of 7.3 ± 1.5 μ m CTI (assay concentration 2.4 μ m ). No effect was found on the prothrombin time ( PT ) when high TF concentrations were used. Also, with specific antibodies for FXII a and for FXI a only APTT was found to be extended but not PT . With specific enzyme assays using chromogenic substrates CTI was shown to be a strong inhibitor of FXII a and a competitive inhibitor of FXI a with K i  = 8.1 ± 0.3 μ m , without effect on the coagulation factors FVII a, FIX a, FX a and thrombin. In thrombin generation and coagulation (free oscillation rheometry, FOR ) assays, initiated with low TF concentrations, no effect of CTI (plasma concentrations of 4.4 and 13.6 μ m CTI , 25 resp. 100 mg L −1 in blood) was found with ≥ 1 p m TF . At ≤ 0.1 p m TF in the FOR whole blood assay the coagulation time ( CT ) concentration dependently increased while the plasma CT became longer than the observation time. Conclusion To avoid inhibition of FXI a and the thrombin feedback loop we recommend that for coagulation assays the concentration of CTI in blood should be below 20 mg L −1 (1.6 μ m ) and in plasma below 3 μ m .