Fibrin moderates the catalytic action of heparin but not that of dermatan sulfate on thrombin inhibition in human plasma

Three recent studies have reported that fibrin in solution significantly inhibits the ability of heparin to catalyze the inhibition of thrombin by antithrombin III. In addition, heparin inhibits the release of fibrinopeptide A by clot-bound thrombin less effectively than it inhibits the release of fibrinopeptide A by thrombin in solution. We have also reported that dermatan sulfate, which catalyzes thrombin inhibition by heparin cofactor II, inhibits thrombus growth in rabbits more effectively than heparin. Because the results of these studies suggest that fibrin inhibits the reactivity of thrombin with antithrombin III-heparin but not with heparin cofactor II-dermatan sulfate, we compared the relative catalytic effects of heparin and dermatan sulfate on thrombin inhibition in plasma, both in the presence and absence of fibrin. We quantitated the rates of thrombin inhibition by antithrombin III and heparin cofactor II by specific enzyme-linked immunosorbent assays. When it was generated, fibrin was kept in solution by adding 2 mmol/L Gly-Pro-Arg-Pro to plasma. Fibrinogen-fibrin reduced the reactivity of thrombin with plasma antithrombin III, both in the presence of and in the absence of heparin. In contrast, the catalytic action of dermatan sulfate on thrombin inhibition by plasma heparin cofactor II was unimpaired by fibrinogen-fibrin. Based on the ability of dermatan sulfate to inhibit thrombus growth in rabbits, failure of fibrinogen-fibrin to moderate the catalytic action of dermatan sulfate may account for its greater antithrombotic effectiveness relative to that of heparin.     

Modulation of heparin cofactor II activity by histidine-rich glycoprotein and platelet factor 4.

Heparin cofactor II is a plasma protein that inhibits thrombin rapidly in the presence of either heparin or dermatan sulfate. We have determined the effects of two glycosaminoglycan-binding proteins, i.e., histidine-rich glycoprotein and platelet factor 4, on these reactions. Inhibition of thrombin by heparin cofactor II and heparin was completely prevented by purified histidine-rich glycoprotein at the ratio of 13 micrograms histidine-rich glycoprotein/microgram heparin. In contrast, histidine-rich glycoprotein had no effect on inhibition of thrombin by heparin cofactor II and dermatan sulfate at ratios of less than or equal to 128 micrograms histidine-rich glycoprotein/microgram dermatan sulfate. Removal of 85-90% of the histidine-rich glycoprotein from plasma resulted in a fourfold reduction in the amount of heparin required to prolong the thrombin clotting time from 14 s to greater than 180 s but had no effect on the amount of dermatan sulfate required for similar anti-coagulant activity. In contrast to histidine-rich glycoprotein, purified platelet factor 4 prevented inhibition of thrombin by heparin cofactor II in the presence of either heparin or dermatan sulfate at the ratio of 2 micrograms platelet factor 4/micrograms glycosaminoglycan. Furthermore, the supernatant medium from platelets treated with arachidonic acid to cause secretion of platelet factor 4 prevented inhibition of thrombin by heparin cofactor II in the presence of heparin or dermatan sulfate. We conclude that histidine-rich glycoprotein and platelet factor 4 can regulate the antithrombin activity of heparin cofactor II.     

Heparin cofactor II activity in plasma during pregnancy and oral contraceptive use

Inhibition of thrombin by heparin is mediated by at least two plasma proteins, antithrombin III, and heparin cofactor II. The plasma titer of heparin cofactor II was significantly elevated in both pregnant women and users of oral contraceptives.     

Purification and characterization of hereditary abnormal antithrombin III with impaired thrombin binding

Investigations of a family predisposed to recurrent venous thromboses disclosed a hereditary antithrombin III deficiency. The reactive antithrombin III concentration in plasma was reduced approximately 50%, and the antigen concentration of the inhibitor was normal. Antithrombin III from two members of this family was purified by dextran sulfate precipitation, affinity chromatography on heparin-Sepharose, and ion-exchange chromatography on DEAE-Sephadex A-50. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and crossed immunoelectrophoresis showed that only approximately half of the purified antithrombin III was capable of forming a complex with thrombin. This corroborated the finding that approximately twice as much purified antithrombin III from these patients compared with antithrombin III from normal humans was needed for titration of a given amount of thrombin. The nonreactive as well as the reactive population of antithrombin III bound heparin with the same affinity as normal antithrombin III. This was shown by crossed immunoelectrophoresis using heparin in the first dimension, by the elution pattern during salt gradient elution of antithrombin III from heparin-Sepharose, and by heparin enhancement of intrinsic fluorescence. Kinetic studies in the absence and in the presence of heparin indicated that the fraction of antithrombin III that could inactivate thrombin was functionally normal. The affected family members appeared to be heterozygotes with two autosomal codominant alleles that encode a normal and an abnormal antithrombin III protein, respectively.     

Identification in vitro of an endothelial cell surface cofactor for antithrombin III. Parallel studies with isolated perfused rat hearts and microcarrier cultures of bovine endothelium.

Two in vitro systems were used to identify an antithrombin III cofactor activity on vascular endothelium. Langendorff rat heart preparations or columns packed with endothelium cultured on microcarrier beads were perfused with mixtures of purified thrombin and antithrombin III. With each preparation, accelerated inhibition of thrombin by antithrombin III occurred during passage over endothelium. Platelet factor 4, protamine sulfate and diisopropylphosphoryl thrombin, all antagonists of the antithrombin III cofactor activity of heparin, significantly reduced the capacity of the preparation to inhibit thrombin. It is concluded that a substance with the functional properties of a stationary phase cofactor for antithrombin III is present on the microvascular endothelium and there catalyzes the inactivation of circulating free thrombin.     

刺参酸性粘多糖介导肝素辅因子Ⅱ对凝血酶活性的抑制

目的：进一步澄清刺参酸性粘多糖（Ｓｊａｍｐ）的抗凝血酶机制。方法：用国产Ｓｊａｍｐ作为激动剂，在正常混合血浆体系、纯化的肝素辅因子Ⅱ（ＨＣⅡ）体系以及纯化的抗凝血酶Ⅲ（ＡＴ－Ⅲ）体系研究Ｓｊａｍｐ的抗凝血酶作用机制。结果：Ｓｊａｍｐ对凝血酶的抑制主要呈ＨＣⅡ依赖性，当存在ＨＣⅡ时，Ｓｊａｍｐ抗凝血酶作用的二级速率常数Ｋ２＝１．５６×１０７ｍ－１·ｍｉｎ－１，其抑制速率常数是ＡＴ－Ⅲ的４．６倍。结论：在抗凝血酶作用方面，Ｓｊａｍｐ的效率（Ｋ２值）及机制（ＨＣⅡ依赖性）与硫酸皮肤素类似。     

Effect of protamine sulfate on antithrombin III activity

When protamine sulfate was added to heparinized plasma in vitro for neutralization of heparin, the activities on both thrombin and Xa known as heparin cofactor in antithrombin action were completely abolished. However, progressive activities on thrombin and Xa both recovered within 30 minutes after protamine sulfate addition. When equivalent heparin was again added, heparin cofactor activity was immediately restored. Based on the fact that protamine sulfate did not show any direct action on the antithrombin III molecule, the presence of AT III with progressive activity was considered to play an important role in the rebound phenomenon of heparin after heparin neutralization with protamine sulfate.     

A method of determining heparin in blood plasma based on its antithrombin activity]

The suggested method for measuring blood plasma heparin is based on heparin ability to enhance antithrombin activity of antithrombin III (AT-III), the major Xa and thrombin inhibitor. The method consists in measurement of blood plasma AT-III activity in the presence and absence of protamine sulfate that destroys the heparin--AT-III complex. Heparin content in U/ml is determined from the difference in the activities of heparin--AT-III complex and AT-III proper activity represented on the calibration curve. The method is sufficiently sensitive, it permits registration of heparin concentrations in a wide band (from 0.01 U/ml to 0.75 U/ml of plasma).     

高半胱氨酸对血管内皮细胞,血小板聚集和肝素辅助因子活性？…

目的 揭示高半胱氨酸（Ｈｃｙ）致血栓形成的机制。方法 （１）免疫荧光分析务管内皮细胞凝血酶调节蛋白（ＴＭ）抗原分布;（２）发色基质法测定肝素辅助因子Ⅱ（ＨＣⅡ）活性。结果 （１）Ｈｃｙ可下调血管内皮细胞表面ＴＭ的表达,且呈浓度依赖性;（２）Ｈｃｙ对血浆ＡＴ－Ⅲ和ＨＣⅡ的抗凝血酶活性无明显影响,在纯化的ＡＴ－Ⅲ的抗凝血酶作用也无影响。结论 Ｈｃｙ的致血栓效应与其对内皮细胞ＴＭ表达调节有关,且独立于血     

Abnormal antithrombin III with defective serine protease binding (antithrombin III "Denver")

A hereditary (three family members) deficiency of antithrombin III (AT-III) in which AT-III antigen (AT-III ag) is normal in spite of low heparin cofactor and antithrombin activity is described. Plasma levels were: AT-III ag, 0.92-0.96 U/ml; AT-III heparin cofactor activity, 0.54-0.62 U/ml; progressive antithrombin activity index, 0.13-0.18; anti-Xa activity, 0.50-0.56 U/ml. Plasma crossed immunoelectrophoresis (CIE) patterns performed with and without added heparin were normal, but serum CIE revealed a decreased complex peak. Purification of the patient's plasma AT-III by heparin-sepharose affinity chromatography showed a normal protein recovery and elution profile, but the purified AT-III fraction showed only 50% of the normal progressive thrombin neutralization and anti-Xa activity. When thrombin-antithrombin (TAT) complexes were formed by incubating with excess thrombin, SDS-polyacrylamide gel electrophoresis (PAGE) analysis revealed that half the patient AT-III formed TAT complexes while the remainder migrated as free AT-III. All the control AT-III formed TAT complexes. The patient's nonreacting AT-III (AT-III "Denver"), isolated by affinity chromatography, showed CIE and SDS-PAGE migration patterns characteristic of normal AT-III but failed to bind thrombin or Xa. Calculations from turnover studies in one patient and normal subjects with autologous 131I-AT-III suggested that AT-III "Denver" is removed from the plasma slightly more rapidly than normal. These studies indicate that the patients' variant AT-III molecule was characterized by normal heparin interaction but defective binding and inhibition of thrombin and Xa. These characteristics allow isolation of the nonreactive variant molecule by heparin-sepharose affinity chromatography.     

Structure-activity relationships of heparin. Independence of heparin charge density and antithrombin-binding domains in thrombin inhibition by antithrombin and heparin cofactor II.

To better understand how heparin structure affects its activity the relationships between the functional domains for inhibitor binding and charge density were investigated to determine how these domains affect heparin-mediated thrombin inhibition by two different heparin-dependent protease inhibitors, antithrombin (AT) and heparin cofactor II (HC II). A series of heparins, fractionated systematically by charge density, was further fractionated on antithrombin agarose to isolate more homogeneous subfractions that were either inactive or highly active with respect to thrombin inhibition by AT. With AT, the activities of the AT-active subfractions increased sharply with heparin charge density, while those with little or no affinity for AT were virtually inactive. In contrast, with HC II inhibitor, the activities of the heparins depended only upon their charge densities and were independent of AT affinity. At any given charge density, the heparin before fractionation by AT affinity and the fractions that were highly active and inactive with AT were all equally active with HC II. The two inhibitors also differed in their reactivity with heparan sulfate and dermatan sulfate. A charge-density effect with the subfractions having similar high affinity for AT demonstrates that charge density represents a heparin functional domain that is independent of the AT-binding domain. The behavior of the AT-inactive heparins, being fully active with HC II, demonstrates the functional domain necessary for AT binding is not needed to produce HC II activity.     

Ischemic stroke in congenital (type II C) defective antithrombin III

Summary Type II C is a subtype of defective antithrombin III deficiency in which there is a molecular defect in the heparin binding site. Usually, heterozygous patients with this defect have little or no thrombotic symptoms. Ischemic stroke has never been described. We report the case of a young woman who had an episode of right-sided hemiparesis. The computed tomographic scan showed an ischemic lesion in the left hemisphere. There were no previous thrombotic episodes in the patient’s history and the sole risk factor for thrombosis was taking of an oral contraceptive. Her family history was mildly positive for thrombosis (superficial thrombophlebitis in two relatives). Laboratory tests showed normal antithrombin III antigen, low heparin cofactor activity and an increased slow-moving peak in crossed-immunoelectrophoresis with heparin. The same pattern was found in 7 of 20 relatives. Thus, a diagnosis of congenital type II C defective antithrombin III was made. This report suggests that young patients with ischemic stroke should be screened for defective antithrombin III.     

An enzyme-linked immunosorbent assay for heparin cofactor II (HCII). Application to the measurement of HCII in clinical materials.

Heparin cofactor II (HCII) is a thrombin inhibitor in human plasma, the activity of which is enhanced by heparin and dermatan sulfate. To measure the plasma antigen concentration of HCII, an enzyme-linked immunosorbent assay (ELISA) has been developed, and this is based on the use of polyclonal anti-HCII IgG, both as the antigen-capturing antibody and as the labelled antibody. The intra- and inter-assay reproducibilities have been calculated to be less than 6%. HCII antigen concentrations evaluated with this ELISA technique in clinical plasma samples correlated well with those determined by electroimmunodiffusion and with HCII activities evaluated using a functional assay (r = 0.909 and r = 0.930, P less than 0.0001). Since the correlation between HCII deficiency and thrombosis is still a controversial issue, the ELISA technique described in this report could be a useful tool for the large-scale studies needed to determine the prevalence of HCII deficiency in healthy individuals and in patients with a history of thrombosis.     

Heparin, thrombin and Factor Xa inhibitors

Direct and indirect coagulation inhibitors are used to inhibit the activity of the serine proteases of the coagulation system. Indirect inhibitors act via antithrombin and heparin cofactor II. The main representatives are heparins, lowmolecular-weight heparins, fondaparinux, idraparinux and danaparoid. They bind to antithrombin and potentiate the inactivation of factor Xa and other serine proteases. Direct thrombin inhibitors bind reversibly to thrombin without cofactor. Anticoagulants are determined by global and specific anticoagulant methods. New anticoagulants are developed such as oral factor Xa inhibitors, oral thrombin inhibitors, antibody against activated factor VII, recombinant tissue pathway inhibitor to improve inhibition of blood coagulation or to induce nonanticoagulant effects (e. g. activated protein C in septicaemia). New anticoagulant methods are developed to improve and specify the anticoagulant effect of anticoagulants in thromboembolic diseases.     

A sensitive colorimetric assay for thrombin, prothrombin and antithrombin III in human plasma using a new synthetic substrate

Benzoyl-L-leucyl-L-alanyl-L-arginine-alpha-naphthylester (Bz-Leu-Ala-Arg-NE) was synthesized as a new substrate for use in the assay of thrombin. In the assay alpha-naphthol released by the enzyme reaction was measured colorimetrically. With Bz-Leu-Ala-Arg-NE as substrate, the minimum detectable concentration of human thrombin was 0.0025 U. This assay using Bz-Leu-Ala-Arg-NE is a highly sensitive method for detecting prothrombin, thrombin and antithrombin III in human plasma. Prothrombin could be determined with 0.2 microliter of human plasma using Echis carinatus venom (ECV) as activator. Antithrombin III activity could be determined with 2 microliter of human plasma using human thrombin and heparin as cofactor. A zymogram of human prothrombin was prepared with Bz-Leu-Ala-Arg-NE as substrate. The preparation gave one band (pI 4.9) on polyacrylamide disc gel isoelectrophoresis.     

Clot-bound thrombin is protected from inhibition by heparin-antithrombin III but is susceptible to inactivation by antithrombin III-independent inhibitors.

Propagation of venous thrombi or rethrombosis after coronary thrombolytic therapy can occur despite heparin administration. To explore potential mechanisms, we set out to determine whether clot-bound thrombin is relatively protected from inhibition by heparin-antithrombin III but susceptible to inactivation by antithrombin III-independent inhibitors. Using plasma fibrinopeptide A (FPA) levels as an index of thrombin activity, we compared the ability of thrombin inhibitors to block FPA release mediated by fluid-phase thrombin with their activity against the clot-bound enzyme. Incubation of thrombin with citrated plasma results in concentration-dependent FPA generation, which reaches a plateau within minutes. In contrast, there is progressive FPA generation when fibrin clots are incubated with citrated plasma. Heparin, hirudin, hirudin dodecapeptide (hirugen), and D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone (PPACK) produce concentration-dependent inhibition of FPA release mediated by fluid-phase thrombin. However, heparin is much less effective at inhibiting thrombin bound to fibrin because a 20-fold higher concentration is necessary to block 70% of the activity of the clot-bound enzyme than is required for equivalent inhibition of fluid-phase thrombin (2.0 and 0.1 U/ml, respectively). In contrast, hirugen and PPACK are equally effective inhibitors of fluid- and solid-phase thrombin, while hirudin is only 50% as effective against the clot-bound enzyme. None of the inhibitors displace bound 125I-labeled thrombin from the clot. These studies indicate that (a) clot-bound thrombin is relatively protected from inhibition by heparin, possibly because the heparin binding site on thrombin is inaccessible when the enzyme is bound to fibrin, and (b) clot-bound thrombin is susceptible to inactivation by antithrombin III-independent inhibitors because the sites of their interaction are not masked by thrombin binding to fibrin. For these reasons, antithrombin III-independent inhibitors may be more effective than heparin in certain clinical settings.     

Anticoagulant synergism of heparin and activated protein C in vitro. Role of a novel anticoagulant mechanism of heparin, enhancement of inactivation of factor V by activated protein C.

Interactions between standard heparin and the physiological anticoagulant plasma protein, activated protein C (APC) were studied. The ability of heparin to prolong the activated partial thromboplastin time and the factor Xa- one-stage clotting time of normal plasma was markedly enhanced by addition of purified APC to the assays. Experiments using purified clotting factors showed that heparin enhanced by fourfold the phospholipid-dependent inactivation of factor V by APC. In contrast to factor V, there was no effect of heparin on inactivation of thrombin-activated factor Va by APC. Based on SDS-PAGE analysis, heparin enhanced the rate of proteolysis of factor V but not factor Va by APC. Coagulation assays using immunodepleted plasmas showed that the enhancement of heparin action by APC was independent of antithrombin III, heparin cofactor II, and protein S. Experiments using purified proteins showed that heparin did not inhibit factor V activation by thrombin. In summary, heparin and APC showed significant anticoagulant synergy in plasma due to three mechanisms that simultaneously decreased thrombin generation by the prothrombinase complex. These mechanisms include: first, heparin enhancement of antithrombin III-dependent inhibition of factor V activation by thrombin; second, the inactivation of membrane-bound FVa by APC; and third, the proteolytic inactivation of membrane-bound factor V by APC, which is enhanced by heparin.     

Standard assays underestimate the concentration of heparin in neonatal plasma

Heparin is commonly used to treat neonatal thrombosis. Drug monitoring often involves assays that measure the inhibition of added factor Xa or thrombin by the antithrombin III (AT-III) heparin complex. We examined whether the neonatal AT-III deficiency affects heparin recovery in such assays at therapeutic drug concentrations (0.1 to 0.6 U/ml). The chromogenic anti-factor Xa assay (Teien AN, Lie M, Abildgaard U. Thromb Res 1976;8:413-6) and the protamine titration test were performed in eight pooled cord plasma samples and in normal adult plasma. Only 65% to 70% of heparin activity detected in adult plasma was recovered by the assay of Teien et al. In cord plasma; recovery by the protamine test was 80% or less. Heparin recovery in cord plasma was significantly improved by raising the AT-III concentration to normal adult levels in both assays. We conclude that heparin assays underestimate drug levels in neonatal plasma unless the neonatal AT-III deficiency is fully corrected in the test system. The use of standard assays may lead to over-heparinization of newborn infants, thereby placing them at a higher risk of bleeding.     

Human alpha2-macroglobulin and its antitrypsic and antithrombin activities in serum and plasma.

alpha2-Macroglobulin level, trypsin protein esterase and progressive antithrombin activities were measured in normal and nephrotic sera and plasma. Trypsin protein esterase activity was proportional to the alpha2-macroglobulin concentration in serum and plasma from both normal and nephrotic patients. The results were different, however, with progressive antithrombin activity: in normal plasma, antithrombin III is the main thrombin inhibitor, then alpha2-macroglobulin and alpha1-antitrypsin, whereas in nephrotic syndrome patients, alpha2-macroglobulin is the main thrombin inhibitor.     

Neutralization of the anticoagulant effects of glycosaminoglycans by serum amyloid P component: comparison with other plasma and platelet proteins.

Serum amyloid P protein (SAP) is a heparin-binding protein that is found in blood and connective tissues including some types of vascular basement membrane. In this article we present evidence that SAP is capable of blocking the anticoagulant effects of glycosaminoglycans. SAP neutralized the catalytic effect of heparin on the thrombin-antithrombin III reaction more effectively than vitronectin, histidine-rich glycoprotein, fibronectin, and high-molecular-weight kininogen and almost as effectively as platelet factor 4. SAP also blocked the effects of heparin and dermatan sulfate on the inhibition of thrombin by heparin cofactor II. We found evidence for the formation of a high-affinity 1:1 complex between SAP and heparin and for inhibition of binding of both thrombin and antithrombin III to heparin-Sepharose by SAP. We conclude that SAP may account for much of the heparin-neutralizing capacity of plasma under some conditions and that basement-membrane-bound SAP may modulate extravascular coagulation by blocking the anticoagulant effects of basement membrane glycosaminoglycans.