Inhibition of Thrombin by Antithrombin III and Heparin Cofactor II In Vivo

Abstract

SummaryThe critical role of thrombin in the pathogenesis of venous and arterial thrombosis, and the effectiveness of glycosaminoglycans as antithrombotic drugs are well known. Antithrombin III is a major inhibitor of thrombin and augmentation of its inhibitory actions by heparin is the basis for the clinical uses of heparin. Recent clinical and experimental studies have demonstrated that another glycosaminoglycan, dermatan sulfate, is an effective antithrombotic drug. Dermatan sulfate catalyses the inhibition of thrombin by heparin cofactor II. The concentrations of heparin cofactor II are higher in the plasmas of individuals with congenital antithrombin III deficiency and pregnant women than controls. The role of heparin cofactor II as a physiologic thrombin inhibitor is unknown. Enzyme-linked immunosorbent assays were used to quantify thrombin-heparin cofactor II and thrombin-antithrombin III endogenous to the plasmas of adult antithrombin III-Hamilton deficient subjects, their siblings with normal antithrombin III levels, pregnant women at term and 3 to 5 days after delivery. Both thrombin-antithrombin III and thrombin-heparin cofactor II complexed with vitronectin were detected in all the plasmas. Significantly, the concentrations of thrombin-heparin cofactor II-vitronectin were higher in the plasmas of congenital antithrombin III deficient subjects and in pre- and post-delivery plasmas than those of normal subjects. In addition, the concentrations of thrombin-heparin cofactor II decreased 3 to 5 days after delivery, reflecting the disappearance of the catalytically active dermatan sulfate elaborated by the placenta. Thus, heparin cofactor II normally inactivates thrombin in vivo, with its role increasing in conditions associated with high levels of heparin cofactor II and/or dermatan sulfate.

In Vivo Clearance of Ternary Complexes of Vitronectin-Thrombin-Antithrombin Is Mediated by Hepatic Heparan Sulfate Proteoglycans

“…The protein concentration of the purified VN-TAT was determined with the Bio-Rad protein assay kit. The relative amounts of AT and HCII in the VN-TAT preparations were determined by enzyme-linked immunosorbent assay (23). Purified human AT concentrate was the generous gift of Bayer Canada Inc. (Toronto, Canada).…”

“…For example, TAT in serum or plasma has been found to exist in the form of a covalent ternary complex with VN (20,21). Other VN-SECs have also been identified, including VN-thrombin-HCII (22,23), VN-thrombin-PN-1 (24), VN-factor Xa-AT (25), VN-thrombin-PAI-1 (26), and VN-thrombin-protein C inhibitor (27) as well as ternary complexes between VN and thrombin with the mutant ␣ 1 -proteinase inhibitor Pittsburgh (27). The formation of ternary complexes with VN suggests that VN may be important physiologically in the metabolism of various SECs.…”

Heparin Cofactor II Modulates the Response to Vascular Injury

“…36 Cultured fibroblasts and vascular smooth muscle cells accelerate inhibition of thrombin by HCII, but endothelial cells do not. 37,38 In the case of fibroblasts, which synthesize both heparan sulfate and DS, a small DS proteoglycan is responsible for the stimulatory effect.…”

Placental dermatan sulfate: isolation, anticoagulant activity, and association with heparin cofactor II

“…During the third trimester, plasma HCII levels are approximately 140% of normal, [16][17][18] and thrombin-HCII complexes are elevated up to 4-fold. 19 At the same time, both maternal and fetal plasma contain trace amounts of a DS proteoglycan that stimulates thrombin inhibition by HCII. 20 The placenta is rich in DS 21 and may be the source of this proteoglycan.…”