Effect of heparin on thrombin inactivation by antithrombin-III

• Published in: Biochemical journal Vol. 173; no. 3; pp. 869 - 875
• Main Authors: Machovich, R, Arányi, P
• Format: Journal Article
• Language: English
• Published: England 01-09-1978
• Subjects: Antithrombin III - pharmacology; Heparin - pharmacology; Kinetics; Temperature; Thermodynamics; Thrombin - antagonists & inhibitors
• ISSN: 0264-6021; 1470-8728
• DOI: 10.1042/bj1730869

The inactivation of thrombin by heat and by its physiological inhibitor, antithrombin-III, shows quite different dependence on heparin concentration. Heparin at 250 microgram/ml protects thrombin against heat inactivation, and thrombin behaves heterogeneously in this reaction. In the absence of heparin, the thermodynamic activation parameters change with temperature (deltaH+ = 733 kJ/mol and 210 kJ/mol at 50 and 58 degrees C respectively). When heparin is present, heat inactivation of the protected thrombin species proceeds with deltaH+ = 88 kJ/mol and is independent of temperature in the same range. On the other hand, heparin at 0.125-2.5 microgram/ml accelerates the thrombin-antithrombin-III reaction. Thrombin does not show heterogeneity in this reaction and the time courses at any heparin concentration and any temperature between 0 and 37 degrees C appear to follow first-order kinetics. Activation enthalpy is independent of heparin concentration or temperature, deltaH+ = 82-101 kJ/mol, varying slightly with antithrombin-III concentration and thrombin specific activity. Heparin seems to exert its effect by increasing activation entropy. On the basis of these data we suggest a mechanism of action of heparin in the thrombin-antithrombin-III reaction which accounts for all the important features of the latter and seems to unify the different hypotheses that have been advanced.