Inhibition of the Thrombin-fibrinogen Reaction by Antithrombin III, Studied by N-terminal Analysis

The influence of antithrombin III on the thrombin-fibrinogen reaction was studied with reagents purified from human plasma. The N-terminal changes reflecting fibrinopeptide release during the fibrinogen-fibrin conversion were measured.Antithrombin III inhibited progressively the release of fibrinopeptides A and B. Fibrin polymerization was not directly influenced. The amount of fibrin formed depended on antithrombin III concentration and initial thrombin concentration. The inactivation rate depended mainly on the antithrombin III conconcentration. In the presence of a physiological antithrombin III concentration, thrombin half life was about 40 seconds, and fibrin formation was decreased if initial thrombin concentration was below 0.5 NIH units/ml. The inhibition was maximal at 37°-45°C, and at pH 8.0–8.5. It is suggested that antithrombin III is the main inactivator of thrombin in plasma.