Factor XI or plasma thromboplastin antecent is one of the enzymes (EC 3.4.21.27) of the coagulation cascade. Like many other coagulation factors, it is a serine protease.
The gene for factor XI is located on the 4th chromosome (4q35).
Factor XI (FXI) is produced by the liver and circulates in its inactive form. It is activated into factor XIa by factor XIIa (FXIIa), thrombin, and it is also autocatalytic, and FXI is a member of the "contact pathway" due to activation by FXIIa (with includes HMWK, prekallikrein, factor XII, factor XI and factor IX).
Factor XIa activates factor IX by selectively cleaving arg-ala and arg-val peptide bonds. Factor IXa, in turn, activates factor X.
Inhibitors of factor XIa include protein Z-dependent protease inhibitor (ZPI, a member of the serine protease inhibitor/serpin class of proteins), which is independent of protein Z (its action on factor X, however, is protein Z-dependent, hence its name).
Deficiency of factor XI causes the rare Hemophilia C; this mainly occurs in Ashkenazi Jews and is believed to affect approximately 8% of that population. The condition has been described in other populations at around 1% of cases. It is an autosomal recessive disorder. There is little spontaneous bleeding, but surgical procedures may cause excessive blood loss, and prophylaxis is required.
Low levels of factor XI also occur in many other disease states, including Noonan syndrome.
High levels of factor XI have been implicated in thrombosis, although it is uncertain what determines these levels and how serious the procoagulant state is.
VisuLize FXI Antigen kit for the measurement of FXI antigen in plasma.
Sources: DiaPharma, Wikipedia