Inactivation of Factor XIa by Plasma Protease Inhibitors

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Abstract
Factor XIa is a plasma protease that, by activating Factor IX, plays an important role in the early phase of the intrinsic pathway of blood coagulation. Four plasma protease inhibitors, α1-protease inhibitor, antithrombin III, C1-inhibitor, and α2-plasmin inhibitor, have been reported to inactivate human Factor XIa, but their quantitative contribution to the inactivation of Factor XIa in plasma has not been fully assessed. Using purified systems, we observed that the second-order rate constants for the reaction of Factor XIa with α1-protease inhibitor, antithrombin III, and CI-inhibitor were 4.08, 10, and 14.6 M−1 min−1 × 103, respectively. The pseudo-first-order rate constants, at plasma concentration of the inhibitors, were 1.86 × 10−1, 4.68 × 10−2, and 2.4 × 10−2 min−1, respectively. These kinetic data predict that α1-protease inhibitor should account for 68%, antithrombin III for 16%, and C1-inhibitor and the equipotent α2-plasmin inhibitor each for 8% of the total inhibitory activity of plasma against Factor XIa. The rate of inactivation of Factor XIa in various plasma samples specifically deficient in inhibitors was consistent with these predictions. Factor XI, the zymogen form of Factor XIa, circulates in plasma associated with the contact system cofactor, high molecular weight kininogen (HMW kininogen). Kinetic analysis indicated the existence of a reversible bimolecular Factor XIa-HMW kininogen complex with a dissociation constant (Kd) = 0.17 μM. The light chain derived from HMW kininogen decreased the inactivation rate of Factor XIa by C1-inhibitor with a Kd of 0.08 μM for a complex of Factor XIa and the light chain derived from HMW kininogen. The protective effect of HMW kininogen was confirmed by the finding that the inactivation rate of Factor XIa in kininogen-deficient plasma was increased over normal plasma. The present study confirms that α1-protease inhibitor is the major inhibitor of Factor XIa in plasma, and that the formation of a reversible complex between Factor XIa and HMW kininogen decreases the rate of inactivation of the enzyme by its inhibitors.