*6.2.3. Protein S*

of thrombosis by 2.1 fold. Plasma thrombin levels are reported to increase due to polymorphic variations. FV leiden is one the well-known FV variant that causes high risk of thrombosis. Koster et al. reported increase in fibrinogen levels increases the risk of thrombosis by 2.8 fold [105].

Activated Protein C, Protein S, Protein Z and Tissue factor pathway inhibitor are natural anticoagulants that help in preventing the accidental or pathological thrombi formation. Defects

PC is a vitamin K dependent serine protease majorly synthesized by liver and its expression has also been identified in epididymis, kidney, lung, brain and male reproductive organ. PC is a single polypeptide of 461 amino acids, consist one Gla domain, a helical aromatic segment, two epidermal growth factor (EGF)-like domains, an activation peptide and a trypsin-like serine protease domain [117]. In the presence of Calcium, PC binds to the endothelial membrane through its Gla domain and interacts with its receptor (endothelial PC receptor: EPCR) [117]. The complex of PC-EPCR facilitates the activation of PC by thrombin, thrombomodulin complex where, thrombin cleaves PC at Arg169-Leu170. This cleavage removes activation peptide from PC. Activated Protein C (APC) cleaves FV and FVIII, and inactivates them. Zymogen PC circulates in the blood at a concentration of 63 nM with a half-life of 2–3 hours whereas, plasma APC concentration is 40 pM with a half-life of 20 min. APC function is increased in presence of Protein S (PS) as PS acts as a cofactor for APC. Reduction of plasma APC antigen levels or loss of APC function is one of the causes for thrombosis. Causes for PC deficiency include congenital/ hereditary deficiencies due to mutations in PC gene. Till date 380 mutations are reported in PC gene (http://www.hgmd.cf.ac.uk/ac/gene.php?gene=PROC). Hereditary PC deficiency is treated by a protein C zymogen concentrate derived from human plasma known as Protexel® (Raosevich et al. 2003). Low plasma PC antigen levels (<10 IU/dl) are also caused by acquired PC deficiency. Acquired PC deficiency is caused by consumption of vitamin K antagonist or severe hepatic dysfunction. A recombinant analogue to the physiologic human activated PC (Drotrecogin alpha activated/ Xigris®) is used to treat the acquired PC deficiency. Thrombosis is also observed due to loss of APC function (APC resistance). APC resistance is observed due to mutations in FV (FV

leiden) or APC resistance is acquired by smoking, chronic alcoholism and obesity [118].

TFPI is a single chain polypeptide with specialized domains called Kuntz domains. It is primarily synthesized in endothelial cells, liver and macrophages [7]. TFPI is mainly bound to the endothelial cell surface through glycosaminoglycans. TFPI circulates in the plasma at a concentration of 1.0–2.5 nM with a half-life of 60–120 min. Major portion of plasma TFPI is bound to LDL and levels of TFPI are regulated by thyroid hormones. TFPI is cleared from the system by liver and kidney. TFPI directly binds to FVIIa, FXa complex and inhibits their function. Inhibitory function of TFPI is enhanced in presence of Protein S [7]. Low levels of TFPI

**6.2. Anticoagulants - thrombosis**

140 Hematology - Latest Research and Clinical Advances

*6.2.1. Protein C (PC)*

*6.2.2. TFPI*

in these clotting factors lead to thrombosis [10].

increases the risk of thrombosis by 2 fold [119].

PS is a vitamin K dependent single chain polypeptide consist of one Gla domain, four EGF like domains and two Laminin G domains. PS is primarily synthesized in liver and it circulates in the plasma at a concentration of 450 nM. 60% of circulatory PS is bound to compliment component binding protein 4b (C4BP) and only 40% of the circulatory PS is free [6, 120]. PS acts as a cofactor for APC and TFPI in inhibiting FVIIIa, FVa, and TF-FVIIa-FXa complex [121, 122]. PS was reported to directly interact with procoagulants such as FV, FIXa and FX and inhibit their function [121, 122]. PS plays a key role in regulating inflammation and clearing the apoptotic bodies from the system. PS deficiency enhances the risk of thrombosis and PS deficiency is classified as hereditary PS deficiency and acquired PS deficiency. Hereditary PS is caused by mutations in the PS gene and till date ~200 mutations are reported in PS gene. Acquired PS deficiency is caused by several factors such as, usage of oral contraceptive pills, pregnancy, consumption of vitamin K antagonists and pathogen infections.
