Recombinant FXI-Ala⁵⁵⁷. (A) Shown are nonreducing (left panel) and reducing (right panel) sodium dodecyl sulfate-polyacrylamide gels of human FXI from plasma (pXI), recombinant WT FXI, and FXI with an alanine substitution for Ser⁵⁵⁷. FXI is a disulfide-linked homodimer, which accounts for the decrease in apparent molecular mass on reduction. (B) Western blot of FXI-Ala⁵⁵⁷ (200nM) incubated for 1 hour with 50nM α-IIa (IIa), α-kallikrein (Ka), FXIIa (XIIa), FIXaβ (IXa), FXa (Xa), or FVIIa with tissue factor (VIIa). The position of the zymogen FXI band (Z) and the heavy chain (HC) and light chain (LC) of FXIa are shown at the right of the gel. Lanes labeled XI and XIa contain samples of purified FXI and FXIa, respectively. The blot was developed with a goat polyclonal anti–human FXI antibody. For both panels, positions of molecular mass standards in kilodaltons are shown to the left of the gels.