Factor X, also known by the eponym Stuart-Prower factor or as thrombokinase, is an enzyme (EC 3.4.21.6) of the coagulation cascade. It is a serine endopeptidase (protease group S1).
Physiology
Factor X is synthetised in the liver and requires vitamin K for its synthesis.
Factor X is activated into factor Xa by both factor IX (in the presence of factor VIII) and factor VII. It is therefore the first member of the final common pathway or thrombin pathway.
It acts by cleaving prothrombin in two places (an arg-thr and then an arg-ile bond), which yields the active thrombin. This process requires factor V as a cofactor.
Factor Xa is inactived by protein Z-dependent protease inhibitor (ZPI), a serine protease inhibitor (serpin). The affinity of this protein for factor Xa is increased 1000-fold by the presence of protein Z, while it does not require protein Z for inactivation of factor XI. Defects in protein Z lead to increased factor Xa activity and a propensity for thrombosis.
Genetics
The human factor X gene is located on the thirteenth chromosome (13q34).
Role in disease
Inborn deficiency of factor X is very uncommon (1:500,000), and may present with epistaxis (nosebleeds), hemarthrosis (bleeding into joints) and gastrointestinal blood loss. Apart from congenital deficiency, low factor X levels may occur occasionally in a number of disease states.
Deficiency of vitamin K or antagonism by warfarin (or similar medication) leads to the production of an inactive factor X. In warfarin therapy, this is desireable to prevent thrombosis.
Therapeutic use
Factor X is not commerically available as a concentrate, but is part of fresh frozen plasma and prothrombin complex .
History
American and British scientists described deficiency of factor X independently in 1953 and 1956, respectively. As with some other coagulation factors, the factor was initially named after these patients, a Mr Rufus Stuart and a Miss Audrey Prower.
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