Anticoagulant mechanisms of covalent antithrombin-heparin investigated by thrombelastography

Helen M. Atkinson; Tracy A. Mewhort-Buist; Leslie R. Berry; Anthony K. C. Chan

doi:10.1160/TH08-11-0769

Thrombosis and Haemostasis, Table of Contents

Thromb Haemost 2009; 102(01): 62-68
DOI: 10.1160/TH08-11-0769

Blood Coagulation, Fibrinolysis and Cellular Haemostasis

Schattauer GmbH

Anticoagulant mechanisms of covalent antithrombin-heparin investigated by thrombelastography

Comparison with unfractionated heparin and low-molecular-weight heparin

Helen M. Atkinson

¹Henderson Research Centre, Hamilton, ON, Canada

²Department of Pediatrics, McMaster University, Hamilton, ON, Canada

,

Tracy A. Mewhort-Buist

¹Henderson Research Centre, Hamilton, ON, Canada

²Department of Pediatrics, McMaster University, Hamilton, ON, Canada

,

Leslie R. Berry

¹Henderson Research Centre, Hamilton, ON, Canada

²Department of Pediatrics, McMaster University, Hamilton, ON, Canada

,

Anthony K. C. Chan

¹Henderson Research Centre, Hamilton, ON, Canada

²Department of Pediatrics, McMaster University, Hamilton, ON, Canada

› Author Affiliations

Abstract

Summary

We have developed an antithrombin-heparin covalent complex (ATH) which inhibits coagulation enzymes by two mechanisms: directly, or by catalytic activation of plasma antithrombin (AT). Anticoagulation by ATH was compared to unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) using a blood-based, tissue factor (TF)-activated thrombelastography (TEG) assay. Simplified TEG assays with plasma or purified plasma components were used to determine the contribution of the direct and catalytic mechanisms to ATH efficacy. Low anti-Xa concentrations of UFH inhibited clot formation significantly more than equivalent concentrations of ATH or LMWH in blood and plasma. ATH had reduced ability to catalyse AT-mediated thrombin (IIa) inhibition compared to UFH. However, at high anti-Xa concentrations, ATH had similar anticoagulant activity to UFH. ATH and non-covalent AT+UFH directly inhibited clotting to a similar degree in AT-deficient plasma. IIa-ATH complexes, which are limited to catalytic inhibition, displayed impaired anticoagulation compared to free ATH, and the magnitude of this effect increased significantly as anticoagulant concentration increased. Kinetic experiments indicated that the rate of reaction of AT with IIa is lower when catalysed by ATH versus UFH. In conclusion, at low anti-Xa doses catalytic inhibition is the primary mechanism of ATH anticoagulation, and the catalytic potential of ATH is reduced relative to UFH. However, the direct inhibitory activity of ATH is comparable to noncovalent AT+UFH, and at high anti-Xa doses the direct inhibitory activity of ATH may play a larger role in anticoagulation.

Keywords

Antithrombin-heparin complex - thrombelastography - anticoagulant - heparin - catalysis

Full Text

References

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