Assessment of the protein interaction between coagulation factor XII and corn trypsin inhibitor by molecular docking and biochemical validation

Essentials Corn Trypsin Inhibitor (CTI) is a selective inhibitor of coagulation Factor XII (FXII). Molecular modelling of the CTI-FXIIa complex suggested a canonical inhibitor binding mode. Mutagenesis revealed the CTI inhibitory loop and helices α1 and α2 mediate the interaction. This confirms that CTI inhibits FXII in canonical fashion and validates the molecular model.

SUMMARY

Background Corn trypsin inhibitor (CTI) has selectivity for the serine proteases coagulation factor XII and trypsin. CTI is in widespread use as a reagent that specifically inhibits the intrinsic pathway of blood coagulation but not the extrinsic pathway. Objectives To investigate the molecular basis of FXII inhibition by CTI. Methods We performed molecular docking of CTI, using its known crystal structure, with a model of the activated FXII (FXIIa) protease domain. The interaction model was verified by use of a panel of recombinant CTI variants tested for their ability to inhibit FXIIa enzymatic activity in a substrate cleavage assay. Results The docking predicted that: (i) the CTI central inhibitory loop P1 Arg34 side chain forms a salt bridge with the FXIIa S1 pocket Asp189 side chain; (ii) Trp22 from CTI helix α1 interacts with the FXIIa S3 pocket; and (iii) Arg43 from CTI helix α2 forms a salt bridge with FXIIa H1 pocket Asp60A. CTI amino acid substitution R34A negated all inhibitory activity, whereas the G32W, L35A, W22A and R42A/R43A substitutions reduced activity by large degrees of 108-fold, 41-fold, 158-fold, and 100-fold, respectively; the R27A, W37A, W39A and R42A substitutions had no effect. Synthetic peptides spanning CTI residues 20-44 had inhibitory activity that was three-fold to 4000-fold less than that of full-length CTI. Conclusions The data confirm the validity of a canonical model of the FXIIa-CTI interaction, with helix α1 (Trp22), central inhibitory loop (Arg34) and helix α2 (Arg43) of CTI being required for effective binding by contacting the S1, S3 and H1 pockets of FXIIa, respectively.

The risk of a first and a recurrent venous thrombosis associated with an elevated D-dimer level and an elevated thrombin potential: results of the THE-VTE study

BACKGROUND

D-dimer and thrombin generation have been associated with the risk of recurrent venous thrombosis. However, for both measurements, different assays are available, and in vitro thrombin generation may be affected by the problem of contact activation during blood sampling.

OBJECTIVES

To determine the association between hypercoagulability and first and recurrent thrombosis by the use of different D-dimer and thrombin generation assays, to assess whether the addition of corn trypsin inhibitor (CTI) prior to blood sampling to inhibit contact activation improved the association between thrombin generation and thrombosis risk, and to calculate the DASH score with two different D-dimer assays.

METHODS

A case-control study (626 patients and 361 controls) with subsequent follow-up of the cases was performed (2987 patient-years after stopping of anticoagulant therapy). Blood was drawn 2-3 months after discontinuation of anticoagulation for the first event in citrate tubes with and without CTI.

RESULTS/CONCLUSIONS

An elevated D-dimer level and elevated thrombin generation were associated with an increased risk of a first event regardless of the assay used (odds ratios: 1.8-3.4). An elevated D-dimer level but not elevated thrombin generation was associated with the risk of recurrence. Patients with elevated D-dimer levels had a more than two-fold increased recurrence rate (Vidas - hazard ratio [HR] 2.3, 95% confidence interval [CI] 1.4-3.8; HemosIL - HR 2.4, 95% CI 1.5-3.9; Thrombinoscope and Technoclone assay - HR 1.3). Elimination of contact factor activation did not improve the predictive value of thrombin generation. In patients with unprovoked first events, the DASH score had a similar predictive value for deep vein thrombosis and pulmonary embolism, both when calculated with Vidas D-dimer and when calculated with HemosIL D-dimer.

The effect of corn trypsin inhibitor and inhibiting antibodies for FXIa and FXIIa on coagulation of plasma and whole blood

BACKGROUND

Corn trypsin inhibitor (CTI), an inhibitor of FXIIa, 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) XIIa 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 μm CTI (assay concentration 2.4 μm). No effect was found on the prothrombin time (PT) when high TF concentrations were used. Also, with specific antibodies for FXIIa and for FXIa 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 FXIIa and a competitive inhibitor of FXIa with Ki  = 8.1 ± 0.3 μm, without effect on the coagulation factors FVIIa, FIXa, FXa 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 μm CTI, 25 resp. 100 mg L(-1) in blood) was found with ≥ 1 pm TF. At ≤ 0.1 pm 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 FXIa 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 μm) and in plasma below 3 μm.

Failure of corn trypsin inhibitor to affect the thrombin generation assay in plasma from severe hemophiliacs

BACKGROUND

The thrombin generation assay (TGA) is an important global coagulation assay; however, it suffers from the lack of preanalytical standardization. The addition of corn trypsin inhibitor (CTI) to blood collection tubes before TGA has been previously advocated to block the contact activation pathway. Emerging data, however, suggest that CTI may only be necessary when minimal tissue factor (TF) concentrations < 1 pmol are used.

OBJECTIVES

To determine whether blood collection tubes containing CTI influenced TGA parameters.

METHODS

This cross-sectional, observational study performed the TGA using TF 1 pmol L(-1) in 15 healthy volunteers, 14 severely factor VIII (FVIII)-deficient patients, and 15 severely FVIII-deficient patients with documented FVIII inhibitors. TGA was conducted using blood tubes that contained CTI 33 μg mL(-1) and no CTI.

RESULTS

CTI markedly reduced peak thrombin (P = 0.002) and endogenous thrombin potential (P < 0.001) in the healthy volunteers but had no significant effect on TGA parameters in severely FVIII-deficient patients or those with inhibitors.

CONCLUSIONS

This lack of effect raises additional questions regarding the need for CTI as a preanalytical addition to blood collection tubes during TGA in severe hemophiliacs, particularly when activating samples with TF 1 pmol L(-1) .

Platelets do not express the oxidized or reduced forms of tissue factor

BACKGROUND

Expression of tissue factor (TF) antigen and activity in platelets is controversial and dependent upon the laboratory and reagents used. Two forms of TF were described: an oxidized functional form and a reduced nonfunctional form that is converted to the active form through the formation of an allosteric disulfide. This study tests the hypothesis that the discrepancies regarding platelet TF expression are due to differential expression of the two forms.

METHODS

Specific reagents that recognize both oxidized and reduced TF were used in flow cytometry of unactivated and activated platelets and western blotting of whole platelet lysates. TF-dependent activity measurements were used to confirm the results.

RESULTS

Western blotting analyses of placental TF demonstrated that, in contrast to anti-TF#5, which is directed against the oxidized form of TF, a sheep anti-human TF polyclonal antibody recognizes both the reduced and oxidized forms. Flow cytometric analyses demonstrated that the sheep antibody did not react with the surface of unactivated platelets or platelets activated with thrombin receptor agonist peptide, PAR-1. This observation was confirmed using biotinylated active site-blocked factor (F)VIIa: no binding was observed. Likewise, neither form of TF was detected by western blotting of whole platelet lysates with sheep anti-hTF. Consistent with these observations, no FXa or FIXa generation by FVIIa was detected at the surface of these platelets. Similarly, no TF-related activity was observed in whole blood using thromboelastography.

CONCLUSION AND SIGNIFICANCE

Platelets from healthy donors do not express either oxidized (functional) or reduced (nonfunctional) forms of TF.