Refinement of α1-antitrypsin Pittsburgh’s reactive center loop P1′ residue fully eliminates inhibition of thrombin and APC. (A-B,F) Inhibition of enzymes at a fixed SERPIN concentration; table insets show second-order rate constants (k₂: 10⁴ M⁻¹·s⁻¹). (A) Inhibition of 17.7 nM thrombin by 38.5 nM SERPIN. (B) Inhibition of 8.5 nM FXa by 192.3 nM SERPIN. (C) Inhibition of dilute PT clotting times in normal plasma in the presence of SERPINs. (D-E) Inhibition of TF-driven thrombin generation in the presence of 384 nM SERPIN. (F) Inhibition of 17.9 nM APC by 96.2 nM SERPIN. (G) Inhibition of APC in an aPTT clotting assay by 384 nM SERPIN. Data are expressed as a ratio of the APC-dependent increase in clotting times. Data represent the mean ± SD of 3 separate experiments, each performed in duplicate. #P < .005, †P < .0005, §P < .0001, compared with pdC1INH by 1-way ANOVA.