What is factor V?

Updated: Feb 05, 2020
  • Author: Bishnu Prasad Devkota, MD, MHI, FRCS(Edin), FRCS(Glasg), FACP; Chief Editor: Eric B Staros, MD  more...
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Answer

Answer

Factor V is a large glycoprotein with a molecular weight of 330,000 daltons and a plasma half-life of about 12 hours, with some reports of a half-life of up to 36 hours. [1] It functions as a cofactor in converting factor II to active factor II. It is proteolyzed by protein C/S complex. [2]

Factor V deficiency has been called parahemophilia because hemarthrosis may occur with severe deficiency of factor V. This also increases the bleeding time. [4] It is also called Owren disease after Dr Paul Owren, who identified the defect first and published it in Lancet in 1947. [5]

The gene for factor V is located on chromosome 1. Factor V circulates in plasma as a single-chain molecule. Platelets contribute approximately 20% of the factor V present in whole blood, with nearly all of it in granules. [6] Activated platelet factor V is also more resistant to inactivation by activated protein C. [7, 8] Platelet factor V appears to be sufficient for hemostatic function, at least in mice. [9]

Factor V is believed to be primarily activated by thrombin in vivo, although it can be activated by factor Xa as well. [10] Factor Xa appears to be the preferred activator of factor V released from platelet granules. [7] A common Arg 506 Gln mutation in factor V leads to resistance to inactivation by activated protein C (factor V Leiden) and is associated with an increased risk of venous thromboembolism. [11] Disruption of the factor V gene leads to either intrauterine death or death from massive bleeding within 2 hours of birth in experimental animals (mice). [12]

Factor V has anticoagulant and procoagulant properties. It enhances the anticoagulant action of activated protein C against factor VIIIa in a reaction in which protein S acts synergistically with factor V. [13, 14] Evidence from patients with inhibitors and deficiencies of plasma and platelet factor V indicates that platelet-derived factor V has an important role in hemostasis. [15, 16] Platelets undergo microvesiculation when activated, and the microvesicles, which are rich in factor V, are potent promoters of coagulation. [15]

A study by Ellery et al indicated that in people with hemophilia, decreased factor VIII or IX levels may give rise to a compensatory procoagulant response, including an increase in platelet factor V. The investigators found that plasma protein S, platelet protein S, and plasma tissue factor pathway inhibitor-α were, respectively, one-third, 26%, and 9% lower than in controls, while platelet factor V was 50% higher. [17]

A study by Link et al, using a mathematic model of flow-mediated coagulation, indicated that in patients with hemophilia A, thrombin generation is boosted by low-normal factor V levels. The investigators suggested that a reduction in the level of factor V weakens the protein’s ability to compete with factor VIII for factor Xa on activated platelet surfaces, leading to enhanced factor VIII activation and, consequently, improved thrombin production. [18]

A study by Sridharan et al of patients with factor V inhibitors found that inhibitor titers and/or factor V activity did not correlate with clinical bleeding. In addition, laboratory data suggested that inhibitor effects are not time dependent. [19]


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