Inhibition of thrombin and APC by PCI variants. Nonreducing SDS-PAGE of the reactions of (A) human plasma thrombin and (B) APC with PCI variants (FRS is wt, KRS is P2 Lys, FRK is P1′ Lys, and KRK is the combined P2 and P1′ Lys variant). Native serpin is indicated by a solid arrow, cleaved serpin by an open arrow, and the covalent serpin:protease complex by a shaded arrow. In each gel, lane 1 (M) is molecular weight markers, lane 2 is the protease alone, lane 3 is serpin alone, and the other lanes are the reactions of serpin and protease at the indicated times.