Recombinant fXI. (A) Recombinant fXI^C321S (321), fXI^C321S,L284A (321/284), and fXI^C321S,I290A (321/290) appear as 80-kDa monomers on 7.5% nonreducing SDS-PAGE because they lack the Cys³²¹-Cys³²¹ interchain disulfide bond that connects the 2 subunits of the dimer in fXI^WT (160-kDa band at right). Positions of molecular mass standards (kDa) are indicated at the left of the panel. Size-exclusion chromatography of (B) plasma fXI (pXI) and fXI^C321S (321) or (C) fXI^C321S,L284A (321/284), fXI^C321S,I290A (321/290) and plasma PK. Shown are continuous readouts of optical density (280 nm) of solutions exiting a Superose-12 size exclusion column (flow rate 1.0 mL/min). Protein retention volumes in milliliters are indicated below the curves.