Effect of heparin on the interaction between thrombin and hirudin

The effect of heparin on the interaction between thrombin and hirudin has been examined by kinetic methods. Three forms of heparin fractionated on the basis of their affinity for antithrombin III and unfractionated heparin were found to act as noncompetitive inhibitors of the formation of the thrombin-hirudin complex. A three--four fold increase in the dissociation constant of the complex was observed at saturating heparin concentrations. This increase in the dissociation constant was due to a twofold decrease in the rate of association of thrombin and hirudin together with a similar increase in the rate of dissociation of the complex. Implications for the location of the heparin binding site on thrombin and the possible therapeutic use of the hirudin are discussed.