Protein C Activity: Clotting assay

The complex orchestration of cellular and molecular participants of haemostasis achieves a crucial yet fine balance of procoagulant and anticoagulant mechanisms. Deficiencies of natural anticoagulant regulators of secondary haemostasis, such as antithrombin, protein C and protein S, and gain of function mutations in genes for FII and FV, are heritable disorders that can shift this balance and increase the risk of venous thromboembolic disease. Protein C (PC) is the vitamin K dependent zymogen of the serine protease activated protein C (APC), which functions as a regulator of secondary haemostasis by inactivating FVa & FVIIIa within a forming clot. 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 & its non-enzymatic cofactor protein S (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.Deficiency of PC can be qualitative or quantitative and reduces the effectiveness of haemostasis regulation, thus tipping the balance towards an increased risk of venous thromboembolism.