Purification and characterization of platelet factor XI

Factor XI activity and antigen was purified about 300 fold from human platelets through chromatography on Con-A Sepharose, SP-Sephadex C-50, immobilized goat anti-factor XI, and SP-Sephadex. The partially purified platelet factor XI (Pt-XI) could be activated by activated factor XII generated in situ from single chain factor XI in a reaction requiring high molecular weight kininogen (HMWK) and a surface. Native Pt-XI migrated as a molecule of Mr = 245,000 on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) as identified by Western blotting. On reduction, Pt-XI appeared to have a Mr = 52,000. Neither form was affected by exposure to trypsin. Incubation of Pt-XI with purified factor XII, HMWK, and kaolin produced activated platelet factor XI clotting activity and, concomitantly, the generation over time of a new chain on reduced SDS-PAGE of Mr = 44,500. The coagulant activity of the activated form could be neutralized by diisopropyl flurophosphate (DFP). Incubation of the activated mixture with 3H-DFP followed by reduced SDS-PAGE showed the active site to be associated with a unit of Mr = 44,500. The adsorption domain as defined by adsorption to kaolin was localized to the Mr = 44,500 chain containing the active site. Hence, both active site and adsorption functions, properties of separate chains in plasma factor XI, reside in the same chain of Mr = 44,500 of platelet factor XI.