A, non-reducing gel of plasma fXI (lane 1), fXISer362–482 (lane 2), plasma fXIa (lane3), and fXIaSer362–482 (lane 4). B, same proteins as in panel A but under reducing conditions. C, fXIa catalytic domain (CD) purified from activated fXISer362–482 by antibody affinity chromatography. The flow-through of the affinity column contains dimers of fXIa heavy chain (HC). Positions of molecular mass standards in kilodaltons are shown at the left of each panel. Proteins were run on 10% SDS-PAGE and stained with GelCode Blue.