The role of heparin in the thrombin-antithrombin III reaction

The effect of heparin on the kinetics of inactivation of thrombin by antithrombin III (AT) has been investigated in order to distinguish between two possible mechanisms. Either (1) heparin activates AT to make it a (kinetically) more effective inhibitor, or (2) heparin makes thrombin more susceptible to inhibition by AT. The results were consistent only with mechanism 1. The experimental approach was to premix heparin with either thrombin or AT and then to measure the rate of association of the two proteins in the rapid-mixing stop-flow spectrophotometer. Reactions were followed spectrophotometrically by observing displacement of the dye proflavine from the active site of thrombin as AT binds. Only premixing AT with heparin accelerated the reaction compared to control (no heparin); the observed second-order rate constant was enhanced by a factor of 200-400. Premixing of thrombin with heparin was without effect on the rate of association with AT. If heparin was premixed with both proteins before reaction, the rate was as slow as the control, indicating that heparin bound to thrombin is actually inhibitory to the association of enzyme with activated AT.