Interaction of Factor XIa and Antithrombin in the Presence

We have studied the interaction between purified human factor XIa and antithrombin in the presence and absence of well-characterized preparations of heparin. The concentrations of hemostatic enzyme. protease inhibitor. and mucopolysaccharide were 5.76 x iO8 mol/L. 5.76 x iO mol/L. and either 5.88 x iO mol/L or 0. respectively. Kinetic investigation of this process using a tritiated factor IX substrate demonstrated that the pseudo first-order rate constants of this reaction in the presence and absence of heparin are -i.O mm’ and -0.025 min . respectively.