The Reactions of Thrombin with Fibrinogen and Plasma Antithrombin

SummaryThe effects of pH, salt and amino acid (β-alanine) concentration on the rate of thrombin formation and antithrombin activity in recalcified plasma was studied from the viewpoint of reaction rate kinetics, and units of thrombin and antithrombin activity defined on this basis. Optimal thrombin activity (fibrin formation) was observed between pH 6.5 and 8.0 and at 0.19 M NaCl. Maximal antithrombin activity occurred at pH 8.5 and 0.10 M sodium chloride.Antithrombin activity, examined under optimal conditions as well as under the influence of changes in pH, and of salt and amino acid concentrations proved to follow first-order reaction kinetics. The activities of thrombin and antithrombin changed in a diphasic manner with increasing concentrations of salt or amino acid and such change may represent subtle protein-protein or protein-salt interactions.The antithrombin activity of fifteen normal plasmas was determined from reaction rate studies of thrombin destroyed during recalcification of human plasma and compared to the inactivation of bovine thrombin by citrated plasma.A concept of “coagulability” of plasma based on the measurement of two independent parameters, fibrinogen and antithrombin, and their kinetic interrelationships, was developed as an approach to a functional concept of hemostasis.