α1-Antitrypsin variants SMTR/V and SLLR/V are powerful inhibitors of contact system enzymes. (A-D) Inhibition of enzymes at a fixed SERPIN concentration; table insets show second-order rate constants (k₂: 10⁴ M⁻¹·s⁻¹). (A) Inhibition of 25 nM αFXIIa by 96.2 nM SERPIN. (B) Inhibition of 2.3 nM PKa by 28.9 nM SERPIN. (C) Inhibition of 24.1 nM plasmin by 24.1 nM SERPIN. (D) Inhibition of 2.5 nM FXIa by 9.6 nM SERPIN. Effect of 384 nM SERPIN on dilute aPTT clotting times (E) or kaolin-driven thrombin generation (F). Data represent the mean ± SD of 3 separate experiments, each performed in duplicate. #P < .005; §P < .0001, compared with pdC1INH by 1-way ANOVA.