Inhibition of the Thrombin‐Fibrinogen Reaction by Heparin in the Absence of Cofactor

The influence of heparin on the reaction between purified human thrombin and fibrinogen was studied. The appearance of N‐terminal glycine, which reflects conversion of fibrinogen to fibrin, was measured. At low thrombin concentrations, the reaction was inhibited by a wide range of heparin concentrations. The inhibition was immediate and reversible. It increased as the fibrinogen concentration was increased. It is suggested that the inhibition is caused by the formation of thrombin‐heparin‐fibrinogen complexes.