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The inhibition of thrombin by isolated bovine antithrombin III (1, 200 units/mg of protein) was studied under a variety of experimental conditions. This inactivation reaction follows a second-order reaction and the rate constant depends on a number of parameters. The second-order rate constant decreases with an increase of thrombin concentration. However, at a constant initial concentration of thrombin, the measured amount of antithrombin III present in a sample is directly proportional to the aliquot of the sample. The rate of inactivation was investigated with changes of temperature. For example, at an initial thrombin concentration of 6. 8 units/ml and antithrombin III concentration of 4. 2 units/ml, the reaction proceeds with Ea of 25 Kcal/mole and Δ S‡ of 42 e. u. /mole. In the presence of 0. 0015 unit of heparin/ml and with similar concentrations of thrombin and antithrombin III, the reaction proceeds with Ea of 16 Kcal/mole and Δ S‡ of 14 e.u. /mole. Therefore, the presence of heparin causes a lowering of the activation energy and a concomitant decrease in the entropy of activation. This change in the thermodynamic parameters allows the inactivation reaction to proceed much faster in the presence of heparin. The effect of heparin may be the result of solvent macromolecular interaction in this inhibition reaction.

Changes in Thermodynamic Parameters in the Inhibition of Thrombin by Bovine Antithrombin III