Free protein S antigen
Protein S (PS) is the vitamin K dependent, non-enzymatic cofactor for the serine protease activated protein C (APC) which functions as a regulator of secondary haemostasis by inactivating FVa & FVIIIa within a forming clot. Zymogen protein C (PC) attaches to vessel endothelium via a specific receptor, endothelial PC receptor (EPCR) and then migrates across the endothelial surface 'in search' of its activator. If a clot is being formed, some of the thrombin leaks onto the endothelial surface and encounters membrane-bound thrombomodulin (TM), whereupon they form a complex that induces a conformational change in thrombin such that it loses procoagulant activity and instead adopts an anticoagulant role by activating PC. The APC/EPCR complex then dissociates from the TM-throbmin complex and the APC is released to migrate onto the surface of an activated platelet. PC & PS are vitamin K dependent and so bind to surface phospholipid and PS orientates the active site of PC above the platelet surface to facilitate inactivation of substrates by cleavage of a small number of peptide bonds. Inactivation of FVIIIa additionally requires zymogen FV to operate synergistically with PS in a cofactor role.
Approximately 60% of PS circulates bound to C4b-binding protein and is largely unavailable for functioning as a cofactor for APC. The remainder is termed free protein S (FPS) and is directly available for APC cofactor function. PS also operates as a cofactor for tissue factor pathway inhibitor. Deficiency of PS reduces the effectiveness of haemostasis regulation and tips the balance towards an increased risk of venous thromboembolism. "
"Male 80.2 - 137.4 Female 63.7 - 132.0"
450µL x 1 aliquot
Internal requests: please refer to EPR label
The sample should be analysed or manipulated & stored in the laboratory within 4 hours of venepuncture. Please ensure sample tubes are filled exactly to the fill-line as underfilling creates a dilution error and leads to inaccurate results.
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH
Laboratory opening times
Last updated: 09/03/2017