Inhibition of thrombin by antithrombin III in the presence of certain glycosaminoglycans found in the mammalian aorta

Heparin mimetics with anticoagulant activity

Heparin, a sulfated polysaccharide belonging to the glycosaminoglycan family, has been widely used as an anticoagulant drug for decades and remains the most commonly used parenteral anticoagulant in adults and children. However, heparin has important clinical limitations and is derived from animal sources which pose significant safety and supply problems. The ever growing shortage of the raw material for heparin manufacturing may become a very significant issue in the future. These global limitations have prompted much research, especially following the recent well-publicized contamination scandal, into the development of alternative anticoagulants derived from non-animal and/or totally synthetic sources that mimic the structural features and properties of heparin. Such compounds, termed heparin mimetics, are also needed as anticoagulant materials for use in biomedical applications (e.g., stents, grafts, implants etc.). This review encompasses the development of heparin mimetics of various structural classes, including synthetic polymers and non-carbohydrate small molecules as well as sulfated oligo- and polysaccharides, and fondaparinux derivatives and conjugates, with a focus on developments in the past 10 years.

Syndromes of Thrombosis and Hypercoagulability: Congenital and Acquired Thrombophilias

This article stresses the common hereditary and acquired blood protein defects associated with thrombosis. The most common of the hereditary defects apear to be APC-R, SPS, antithrombin, protein C, and protein S deficiency, and the most common acquired defects are anticardiolipin antibodies and the lupus anticoagulant (antiphospholipid antibodies). Therefore, these are the defects that should first be looked for in an individual with unexplained thrombosis. If these more common defects are not found, then the rarer defects including HC II, plasminogen or TPA deficiency, dysfibrinogenemia, el evated PAI-1 and hyperhomocysteinemia should be sought. The importance of finding these defects has significant impli cations for therapy of the individual patient and for institutions of family studies to identify, inform, and possibly treat others at risk. It is expected that as knowledge of hemostasis expands, more hereditary and acquired defects, such as elevated lipopro tein (a) or defects of extrinsic (tissue factor) pathway inhibitor (EPI, TFPI), may be associated with enhanced risks of throm bosis. Finally, it must be recalled that a diagnosis of thrombo sis, like that of anemia, is only a generic and partial diagnosis; just as in the anemic patient, the etiology must be clearly de fined. Only in this manner can cost-effective and appropriate therapy for both primary treatment and secondary prevention be designed. In addition, the demonstration of a hereditary defect will allow primary prevention in afflicted family mem bers by allowing the choice of appropriate therapy.

Selective cleavage of heparin using aqueous 2-hydroxypyridine: Production of an aldose-terminating fragment with high anticoagulant activity

Unfractionated heparin (UFH) was partially depolymerized by heating at 115 degrees C with aqueous 2-hydroxypyridine. Compared to starting UFH, no significant loss of anticoagulant (anti-Xa) activity was observed. Products consisted of polysaccharide fragments and small quantities of ammonia, sulfate, and hexuronic acid. Fragments with aldose termini that reacted with [3H]NaBH4 (fragment A) were of relatively uniform size (6000 D) and increased as depolymerization time increased. Fragment A contained the anticoagulant activity, with 90-94% and 24-31% binding to Sepharose-thrombin and Sepharose-antithrombin, respectively. In contrast, a non-reducing fragment B that did not react with [3H]NaBH4 was more heterogeneous (6000-10,000 D) and did not have anticoagulant activity or Sepharose-antithrombin affinity. Given the polysaccharide 3H-incorporation, small release of monosaccharide products, and fragment A end-group analysis, thermolysis of UFH is likely limited to one site per molecule when protected by 2-hydroxypyridine. Thus, an anticoagulant fragment A is hydrolytically released from UFH leaving a variable-length fragment B complete with linkage region.

Structural Effects of a Covalent Linkage Between Antithrombin and Heparin: Covalent N-Terminus Attachment of Heparin Enhances the Maintenance of Antithrombin's Activated State

We have produced a molecule comprising of permanently-activated covalently linked antithrombin and heparin (ATH). This study was designed to elucidate the covalent linkage point(s) for heparin on antithrombin and conformational properties of the ATH molecule. ATH was produced using Schiff base/Amadori rearrangement by incubating antithrombin with unfractionated heparin for 14 d at 40 degrees C. ATH was then digested using Proteinase K, and the heparin-peptide was reacted with NaIO4/NaBH4/mild acid to degrade the heparin moiety. Sequencing of the remaining peptide was performed by Edman degradation with linkage point confirmation by LC-MS. The degree of insertion of the reactive center loop (RCL) of antithrombin into the A-sheet of ATH was examined using synthesized antithrombin RCL peptides. Binding between the peptides and ATH, and the formation of ATH in the presence of the peptides were tested. CD was used to further examine the secondary and tertiary structures of ATH. The results suggest that heparin is conjugated to the amino terminal of antithrombin in the majority of ATH molecules, proximal to the previously determined heparin binding domain of antithrombin. From the linkage data, a model is proposed for the structure of ATH. Studies using the RCL peptides and CD analysis of ATH support this model.

Mechanisms responsible for catalysis of the inhibition of factor Xa or thrombin by antithrombin using a covalent antithrombin-heparin complex

Covalent antithrombin-heparin (ATH) complexes, formed spontaneously between antithrombin (AT) and unfractionated standard heparin (H), have a potent ability to catalyze the inhibition of factor Xa (or thrombin) by added AT. Although approximately 30% of ATH molecules contain two AT-binding sites on their heparin chains, the secondary site does not solely account for the increased activity of ATH. We studied the possibility that all pentasaccharide AT-binding sequences in ATH may catalyze factor Xa inhibition. Chromatography of ATH on Sepharose-AT resulted in >80% binding of the load. Similar chromatographies of non-covalent AT + H mixtures lead to a lack of binding for AT and fractionation of H into unbound (separate from AT) or bound material. Gradient elution of ATH from Sepharose-AT gave 2 peaks, a peak containing higher affinity material that had greater anti-factor Xa catalytic activity (708 units/mg heparin) compared with the peak containing lower affinity material (112 units/mg). Sepharose-AT chromatography of the ATH component with short heparin chains (<or=12 monosaccharides) resulted in active unbound (40%) and bound fractions (190 and 560 units/mg, respectively). Factor Xa-ATH or thrombin-ATH inhibitor complexes gave chromatograms on Sepharose-AT with more unbound material compared with that of free ATH. Also, ATH did not bind to Sepharose-heparin, and the intrinsic fluorescence due to activation of AT in ATH by its heparin chain was reversed at higher [NaCl] than that required to dissociate non-covalent AT.H complexes. Thus, exogenous AT can compete with the AT moiety of ATH for binding to the covalently linked heparin chain, leading to catalytic inhibition of factor Xa or thrombin. These data may suggest that access to pentasaccharide units in non-covalent AT.H complexes by free AT may be facile.

Prothrombin G20210A mutation, antithrombin, heparin cofactor II, protein C, and protein S defects

These defects are not as common as factor V Leiden, but they are more common than many other hereditary procoagulant defects. The incidence of the prothrombin gene (G20210A) mutation is not yet known with certainty, but it may approach or even exceed that of factor V Leiden. These defects also seem less common than hereditary sticky platelet syndrome; however, they are all common enough that they always should be considered in any individual with unexplained thrombosis and should be part of the work-up for patients with thrombotic disorders. Of the defects discussed herein, prothrombin G20210A mutation seems, thus far, to be more common than AT, protein C, protein S, or HC-II defects. Assessment of prothrombin gene mutation should be part of the primary evaluation of patients with unexplained thrombosis.

Blood-compatible biomaterials by surface coating with a novel antithrombin-heparin covalent complex

Covalent antithrombin-heparin complex (ATH) was covalently grafted to a polycarbonate urethane (Corethane) endoluminal graft (a kind gift of Corvita Corporation) after being activated using 0.3% m/m NaOCl in 0.15 M phosphate pH 6.0. ATH graft density (1.98 x 10(-7) mol/m2) was 6 times the maximum amount of unfractionated heparin (UFH) that could be bound to polycarbonate urethane surfaces. Surface-bound ATH could be stored in sterile 0.15 M NaCl at 4 degrees C for at least 2 months with good antithrombotic activity before being implanted into rabbits. Analysis of ATH-coated tubing showed that it contained significant direct thrombin inhibitory activity. In vivo testing in a rabbit model was compared to non-activated non-coated surfaces, activated-non-coated surfaces, hirudin-coated surfaces and antithrombin (AT)-coated surfaces. The weight of the clot generated in the ATH-coated graft tubing was significantly less than the weight of the clot generated within the hirudin-coated graft (p = 0.03 with a 1-tailed Student's t test). The anticoagulant nature of ATH grafts in vivo was shown to be due to bound ATH because boththe AT-coated surfaces and non-coated but activated surfaces showed similar thromboresistant efficacy to that of untreated material (ANOVA; p < 0.05). Apart from the direct antithrombin activity that contributed to much of the prolonged patency in vivo, surface-bound ATH likely catalyzed AT inhibition of thrombin, as evidenced by a significant number of 125I-AT binding sites (> or = 1.5 x 10(-8) mol/m2). Thus, ATH appears to be a good candidate for coating cardiovascular devices, such as endoluminal grafts, with high levels of substitution and significant long-term blood-compatibility.

Exosites 1 and 2 are essential for protection of fibrin-bound thrombin from heparin-catalyzed inhibition by antithrombin and heparin cofactor II

Assembly of ternary thrombin-heparin-fibrin complexes, formed when fibrin binds to exosite 1 on thrombin and fibrin-bound heparin binds to exosite 2, produces a 58- and 247-fold reduction in the heparin-catalyzed rate of thrombin inhibition by antithrombin and heparin cofactor II, respectively. The greater reduction for heparin cofactor II reflects its requirement for access to exosite 1 during the inhibitory process. Protection from inhibition by antithrombin and heparin cofactor II requires ligation of both exosites 1 and 2 because minimal protection is seen when exosite 1 variants (gamma-thrombin and thrombin Quick 1) or an exosite 2 variant (Arg93 --> Ala, Arg97 --> Ala, and Arg101 --> Ala thrombin) is substituted for thrombin. Likewise, the rate of thrombin inhibition by the heparin-independent inhibitor, alpha1-antitrypsin Met358 --> Arg, is decreased less than 2-fold in the presence of soluble fibrin and heparin. In contrast, thrombin is protected from inhibition by a covalent antithrombin-heparin complex, suggesting that access of heparin to exosite 2 of thrombin is hampered when ternary complex formation occurs. These results reveal the importance of exosites 1 and 2 of thrombin in assembly of the ternary complex and the subsequent protection of thrombin from inhibition by heparin-catalyzed inhibitors.

Biology of endothelium

Endothelial cells form a multifunctional cell lining that covers all of the inner surface of blood vessels and regulates several important physiological and pathological reactions. These include inflammation/immune reaction, blood vessel tonus, hemostasis/thrombosis, angiogenesis and so on. Thus, abnormalities of endothelial function may play crucial roles in the development of angitis syndrome, thrombosis/embolism, bleeding disseminated intravascular coagulation (DIC), and neovascularization in some pathological states including tumor growth and diabetic retinopathy. Research on endothelial cells now forms a new frontier termed 'Endotheliology'. Recent advances of the functional and structural aspects of endothelial cells are reviewed here mainly from the viewpoint of endothelial regulation of coagulation and the fibrinolytic system. First we show that the natural endothelial membrane protein thrombomodulin is localized not only on apical endothelial surface but also in caveolae. Since it has been reported that such factors involved in coagulation/fibrinolysis as tissue factor, tissue factor pathway inhibitor (TFPI), thrombin receptor and urokinase receptor are also localized in the caveolae, this membrane structure may act as a special component to regulate coagulation/fibrinolysis on the endothelial membrane surface. Next we demonstrate the signaling pathway of the thrombin receptor. Thrombin cleaves the N-terminus of the receptor as a substrate, exposing a new N-terminus. This newly exposed N-terminus acts as a ligand and activates platelets, endothelial cells and vascular smooth-muscle cells. We have identified that the signal from the thrombin receptor activates NF-kappaB through the activation of protein C kinase, tyrosine kinase and MAP kinase, and results in proliferation of the cells. We have also shown that the receptor is over-expressed on platelets from diabetes patients.

Syndromes of thrombosis and hypercoagulability. Congenital and acquired causes of thrombosis

Blood coagulation protein and platelet defects are now known to account for up to ninety percent of unexplained venous thrombosis and up to seventy percent of unexplained arterial thrombotic or ischemic events. This article summarizes the common and uncommon blood protein and platelet defects which should be suspected, and searched for, in patients with such events. Defining such defects will have major impact on secondary prevention and duration of antithrombotic therapy in the afflicted patient and impact on primary prevention for identified family members in those harboring hereditary defects.

Endothelial Cell-Blood Interface Actions and the Procoagulant Response

Anesthesiologists and surgeons have focused on the problems of hypocoagulability and resulting hemorrhage after cardiopulmonary bypass. Recent work in endothelial cell biology has demonstrated that the interaction of inflammatory processes and coagulation dysfunction with the endothelium may contribute to either hypocoagulability (bleeding) or hypercoagulability (thrombosis). New work with endothelial cell function and intracellular signaling of procoagulant responses may allow for unique therapeutic interventions in the future.

Hypercoagulability and thrombosis

This article has stressed the common hereditary and acquired blood protein defects associated with thrombosis. The commonest hereditary defects appear to be antithrombin, protein C, and protein S deficiency, and the commonest acquired defects are anticardiolipin antibodies and the lupus anticoagulant. Therefore these are the defects that should first be looked for in an individual with unexplained thrombosis. If these commoner defects are not found, the rarer defects, including HC-II, plasminogen or t-PA deficiency, dysfibrinogenemia, or elevated PAI-1, should next be sought. The incidence of activated protein C cofactor deficiency is not yet clear but may also represent a common defect. Likewise, PAI-1 defects may, with time, be shown to be quite common. The importance of finding these defects has significant implications for therapy of the individual patient and for institution of family studies to identify, inform, and possibly treat others at risk. It is expected that as knowledge of hemostasis expands, more hereditary and acquired defects, such as elevated lipoprotein (a) or defects of extrinsic (tissue factor) pathway inhibitor may be associated with enhanced risks of thrombosis.

Thrombin in inflammation and healing: relevance to rheumatoid arthritis

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Interactions of thermally denatured fibrinogen on polyethylene with plasma proteins and platelets

During the investigation of fibrin deposition onto hydrophobic polymers in contact with human blood, a model was developed in which fibrinogen was denatured and irreversibly coated onto a polyethylene surface by heating to 70 degrees C for 10 min. The denatured fibrinogen-polyethylene surface is resistant to fluid wall shear rates of up to 550 s-1 and the fibrinogen does not desorb in the presence of plasma proteins. Compared to uncoated polyethylene, little albumin or fibrinogen adsorbs to heat-denatured fibrinogen. Thrombin binds to the denatured fibrinogen-coated polyethylene with low affinity and also acts on the surface-bound denatured fibrinogen and cleaves fibrinopeptide A (FPA) quantitatively. Washed, 51Cr-labeled platelets do not adhere to the thermally denatured fibrinogen at either low or high shear rates compared to surfaces coated with undenatured fibrinogen (p < 0.01). These observations support the role of the D domain of fibrinogen in platelet adhesion because this is the region that is denatured by heating. In contrast, the E domain of fibrinogen is not altered by heating to 70 degrees C and hence remains susceptible to thrombin and/or plasmin cleavage. The characteristics of this surface are such that it can be used to develop fibrin-coated surfaces for use in studies of thrombus formation on artificial surfaces.

Comparative behavior of thrombin and an inactive derivative, FPR-thrombin, toward the rabbit vascular endothelium. Heparin liberates FPR-thrombin from the endothelium in vivo

Thrombin rapidly binds to and saturates rabbit aorta endothelium in vitro, a process that depends on pericellular glycosaminoglycans and that is inhibited by heparin. To characterize the initial adsorption of thrombin to the endothelium in vivo, an enzymatically inactive derivative, FPR-thrombin (i.e., thrombin inactivated by D-Phe-Pro-Arg-chloromethyl ketone), was prepared. The binding characteristics of thrombin and FPR-thrombin to heparin-Sepharose and to the endothelial surface of rabbit aorta segments in vitro were compared. From these experiments, we concluded that FPR-thrombin mimicked, qualitatively, the binding of thrombin to the endothelium. When injected intravenously, 125I-FPR-thrombin was removed rapidly from the rabbit circulation (T1/2, approximately 1.4 minutes) and simultaneously was adsorbed by the vascular endothelium, particularly in the lung. By injecting heparin (1,000 units/kg i.v.) before 125I-FPR-thrombin, adsorption by the aorta endothelium at 30 minutes after injection was reduced by 90%, and T1/2 was increased to approximately 3.4 minutes. Heparin, administered at various times after 125I-FPR-thrombin, liberated a significant proportion of 125I-FPR-thrombin from the endothelial surface into the plasma compartment as shown by a pronounced "spike" on the plasma curve, a concomitant loss of radioactivity from the lung and from the aorta endothelium, and analysis of the radioactive components of plasma taken before and after heparin injection. Thus, FPR-thrombin was cleared rapidly from the circulation, and endothelium-bound FPR-thrombin was released into the circulation by heparin.

Thrombin binding to platelets from hypercholesterolaemic rats

Platelets from rats made hypercholesterolaemic with a diet enriched with milk fat and cholesterol and containing taurocholate to promote hypercholesterolaemia aggregated more extensively to a low concentration of thrombin than platelets from rats given a milk fat-enriched diet containing sitosterol. Total and specific binding of thrombin to platelets from hypercholesterolaemic rats was significantly greater than in controls when expressed per mg platelet protein, per mumol platelet cholesterol, or per unit relative surface area. Total and specific binding of thrombin per platelet were not different between the groups. However, platelets from hypercholesterolaemic rats had less protein and cholesterol, were smaller and had less surface area than control platelets; platelet cholesterol content expressed per mg platelet protein was not different. Thus, the increase in thrombin-binding to the smaller platelets from hypercholesterolaemic rats during the first 10 s after its addition may be responsible, at least in part, for the hypersensitivity of these platelets to thrombin.

Extraction and analysis of glycosaminoglycans from intima-media of single rabbit aortae: effect of balloon catheter de-endothelialization on the content and profile of glycosaminoglycans

A novel procedure is described for extracting, and simultaneously 3H-labelling, glycosaminoglycans from the intima-media of a single rabbit aorta. The procedure was used to compare contents and types of glycosaminoglycans isolated from uninjured (control) aortae, and partially re-endothelialized aortae at 11 weeks after de-endothelialization by a balloon catheter. Briefly, the isolated delipidated tissue was digested in 0.8 M NaOH containing NaB3H4 at room temperature. The neutralized digest was then degraded by a non-specific proteinase during dialysis. The 3H-labelled glycosaminoglycan fraction was recovered after a gel filtration step. The yield (10.5 micrograms of glycosaminoglycan/mg of dry, delipidated tissue) was within the range reported previously for rabbit aorta. Although the de-endothelialized (DEA) and re-endothelialized areas (REA) of all injured aortae contained a significantly thickened intima, the glycosaminoglycan concentration (DEA, 11.2 micrograms/mg; REA, 11.6 micrograms/mg) did not differ significantly from that of control aorta. The profile of [3H]glycosaminoglycan types was determined by the serial use of glycosaminoglycan-selective methods: greater than 86% of 3H was released as small molecular weight products by this analytical scheme. The glycosaminoglycan profile for control tissue compared well with several previous reports. Compared with control aortae, both DEA and REA contained relatively less chondroitin sulphate, whereas DEA contained more hyaluronic acid and REA contained more heparan sulphate. Future use of this procedure will improve measurement of the changes to the extracellular matrix which take place after injury to the vessel wall and which may precede atherogenesis.

Influence of blood flow and the effect of protamine on the thromboresistant properties of a covalently bonded heparin surface

Polyethylene tubings, 2-mm inner diameter and the length of 1 m, untreated or furnished with a covalently bonded heparin surface layer, were inserted as arteriovenous shunts bilaterally in dogs. By compressing the middle part, the initial blood flow was adjusted to 10 or 40 mL/min. The thrombogenicity of the tubings was assessed by the patency of the shunts and by assaying the generation of fibrinopeptide A (FPA) in arterial blood and in blood after its passage through the shunts. In untreated shunts clotting rapidly occurred preceded by high FPA generation in blood passing through the shunts. The blood flow in heparinized shunts remained unchanged throughout the test period. At the low flow rate a certain degree of FPA generation in the shunts occurred. At the high flow rate no changes in FPA levels occurred. The function of the heparin surface is thus flow rate dependent. Systematic heparinization and subsequent neutralization with protamine or administration of protamine alone did not interfere with the function of the heparin surface.

Permeability of a heparin-polyvinyl alcohol hydrogel to thrombin and antithrombin III

The diffusivities of thrombin and antithrombin III in a heparin-polyvinyl alcohol hydrogel were estimated and used to demonstrate that diffusion limits the effectiveness of the immobilized heparin in the interior of such hydrogels. Diffusivities were calculated from permeabilities and partition coefficients measured with films in a diffusion chamber apparatus. The diffusion coefficients were estimated to be 6 +/- 4 X 10(-8) cm2/s for thrombin and 4 +/- 2 X 10(-8) cm2/s for antithrombin III in 10% gel membranes with or without immobilized heparin. Using the diffusivity of thrombin and a Thiele-type modulus, the effectiveness factor of a spherical heparin-PVA bead used to accelerate the inactivation of thrombin by antithrombin III was found to be 4-9% (diameter range 250-150 micron). While indicating that diffusion of thrombin limited the full utilization of the immobilized heparin, these values for the effectiveness factor could not completely account for the low apparent heparin activity (0.2%) in a thrombin time test of heparin-PVA "beads" (J. Biomed. Mater. Res., 17, 359 (1983]. Other factors such as the immobilization chemistry or the diffusion of thrombin-antithrombin III complex must be considered for a full explanation of the thrombin time results.

Neutralization of thrombin by antithrombin III in the presence of cultured human fibroblasts

Recent evidence suggests that heparan sulfate on endothelial cell surfaces acts as a catalyst for the neutralization of thrombin by antithrombin III (AT III). Fibroblasts also produce heparan sulfate which is present on the cell surface and secreted into the extracellular matrix. We evaluated the ability of cultured human fibroblasts to catalyze the interaction between thrombin and AT III and found that heparan sulfate produced by post-confluent fibroblasts was anticoagulantly active. Furthermore, after initial binding of thrombin to cells, thrombin-heparan sulfate appeared in the fluid phase above the cells; this thrombin could be rapidly neutralized by AT III independent of the further presence of cells. These results indicate that fibroblasts do produce an anticoagulantly active species of heparan sulfate and that the normal interaction between AT III and thrombin may be driven by initial release of heparan sulfate from the cell surface by thrombin followed by AT III interaction with the soluble thrombin-heparan sulfate complex.

Uptake and inactivation of thrombin on the subendothelium: comparisons with endothelium

One aspect of the non-thrombogenicity of endothelium is the property to bind and inhibit thrombin. Loss of the endothelial layer gives rise to a thrombogenic surface. The present study concerns uptake and inactivation of thrombin on endothelium as well as subendothelium. Both the immediate subendothelial layer and the media were investigated. Binding and inhibition of thrombin was assayed using both a synthetic chromogenic substrate and fibrinogen. In the latter case thrombin enzymatic activity was assessed by measuring liberation of fibrinopeptide A. There was a linear relationship between thrombin in the solution and thrombin enzymatic activity recovered on the surface on both endothelium and media. 30-50 per cent of the surface bound thrombin measured with the chromogenic substrate was also active towards fibrinogen. Preincubation of endothelium with plasma gave rise to an increased loss of thrombin from the solution at the same time as less thrombin was recovered on the surface. This indicates increased inactivation. Increased inactivation could also be obtained by pretreatment with antithrombin (AT) but not with AT-depleted plasma. The same general pattern was seen on the subendothelium but not on the media. It is concluded that inhibition of thrombin occurs on the endothelium and can be augmented by prior binding of AT to the surface. This effect although less pronounced can also be obtained on the subendothelium. The deeply injured vessel is much less capable of inhibiting thrombin.

Immune endothelial-cell injury in heparin-associated thrombocytopenia

We studied the possibility that immune injury to endothelial cells may have a role in the development of thrombosis in some patients with heparin-associated thrombocytopenia. Serum samples from each of 27 patients who had this clinical diagnosis contained heparin-dependent platelet antibodies and deposited more than normal amounts of IgG, IgA, or IgM on endothelial cells, stimulating the production of tissue factor. Binding of immunoglobulins to endothelial cells was no longer detected when the patients were studied after heparin was withdrawn, but reappeared within several days upon reexposure to heparin in a patient who experienced a clinical recurrence. Binding of immunoglobulin to endothelial cells was partially reduced by the preadsorption of serum samples with heparin or heparan sulfate bound to Sepharose or by enzymatic cleavage of cell-bound heparan sulfate, and was augmented by the addition of heparan sulfate. Thus, serum from some patients with heparin-associated thrombocytopenia may contain antibodies that react with heparin bound to endothelial cells or with heparan sulfate synthesized by endothelial cells. Immune injury to both platelets and endothelial cells may play a part in the development of thrombosis in some patients after heparin therapy.

Natural proteinase inhibitors: blood coagulation inhibition and evolutionary relationships

1. Natural proteinase inhibitors are divided into polysaccharides, plasma proteinase inhibitors and natural non-plasma inhibitors. 2. Polysaccharides are antithrombin-III and heparin co-factor-II dependent or independent regarding their biological activity. Knowledge of the inhibitory mechanism at a molecular level was gained by the study of heparin. 3. Antithrombin-III, heparin-co-factor-II and alpha 2-macroglobulin are the most important plasma proteinase inhibitors involved in coagulation. alpha 2-macroglobulin has a particular inhibitory mechanism. 4. Non-plasma proteinase inhibitors were isolated from many species. They inhibit mainly the contact activation and fibrinolysis. 5. The evolutionary relationships are poorly understood.

Thrombin uptake and inhibition on heparinized polytetrafluoroethylene (PTFE) grafts and native sheep vessels

Non-heparinized and covalently-heparinized PTFE grafts, sheep aorta, carotid artery and jugular vein were evaluated according to their ability to adsorb and inactivate thrombin. All surfaces, except non-heparinized PTFE caused considerable losses of enzymatically active thrombin from the solution. Covalently-heparinized PTFE and jugular veins adsorbed thrombin, which was inactivated on subsequent contact with antithrombin III. Despite the same loss from the solution as was encountered with veins arteries adsorbed significantly smaller amounts of thrombin. Protamine treatment of covalently-heparinized PTFE almost totally abolished the ability to adsorb thrombin. It is concluded that 1) arteries possess a stronger thrombin inhibitory capacity than veins, possible explanations to this being a different glucosaminoglycan or the presence of antithrombin III in the arterial wall and 2) that heparinizing PTFE results in a pronounced ability to inactivate thrombin.

Thrombin uptake and inhibition on endothelium and surfaces with a stable heparin coating: a comparative in vitro study

The endothelial lining and two differently heparin-coated surfaces were compared in vitro regarding thrombin uptake and inhibition. One heparin surface was based on stabilized ionic binding of heparin, the other on covalent binding of partially degraded heparin. Both heparin surfaces have previously been shown to have pronounced thromboresistant properties. The two heparinized polyethylene surfaces and the endothelial surface of segments of the porcine aorta were studied. After exposure to the surfaces, thrombin disappeared from the solution and appeared bound to the surfaces. The disappearance rate of thrombin from the solution was the same on exposure to the endothelium and the covalently bonded heparin surface, but less following exposure to the ionically bonded heparin surface. The thrombin activity appearing on the endothelium was lower than on the heparin surfaces, indicating that the endothelium exerted a slow thrombin inhibiting capacity. On exposure of the thrombin-loaded endothelium to plasma, thrombin was rapidly inhibited. Thrombin bound to the covalently bonded heparin surface was inhibited at a slower rate than on the ionically bonded surface, but still faster than the rate at which free thrombin was inhibited in nonheparinized plasma. It is concluded that the endothelium and stabilized heparin coatings bind thrombin and accelerate its inhibition by plasma.

The interaction of platelet factor four and glycosaminoglycans

The interaction of platelet factor four (PF-4) with glycosaminoglycans (GAG) was evaluated using fluorescence spectroscopy, a radioligand binding assay, and a functional assay utilizing antithrombin III and factor Xa. In these studies, we have (i) characterized the binding parameters for PF-4 to several forms of heparin and to dextran sulfate; (ii) examined the structural features of these glycosaminoglycans which support PF-4 binding; and (iii) examined the effects of selective digestion of the carboxy terminus of PF-4 on binding. The binding of PF-4 to unfractionated porcine intestinal mucosal heparin ([Mr] = 11,000) was specific and saturable, with a molar stoichiometry of PF-4 to heparin of approximately 4:1 and an apparent estimated Kd of 3 X 10(-8) M. Heparin fractions ([Mr] = 6,000) with either low or high affinity for antithrombin III bound to PF-4 with a similar apparent Kd. PF-4 also bound to dextran sulfate ([Mr] = 22,500) with an estimated apparent Kd of 6 X 10(-8) M and a molar stoichiometry of approximately 16:1. Carboxypeptidase Y (CP-Y) digestion of PF-4 progressively decreased GAG binding. After 30 min of digestion, by which time all of the carboxyterminal serine and glutamate, both of the two leucines, and approximately one-quarter of the four lysines were removed, the IC50 for heparin binding shifted from 10 to 150 nM. These studies demonstrated the effect of GAG polymer size and degree of sulfation on the affinity and stoichiometry of PF-4 binding, and the critical importance of the carboxy-terminal amino acids of PF-4 for binding to natural and synthetic GAGs.

In vitro evaluation of a biologic graft surface. Effect of treatment with conventional and low molecular weight (LMW) heparin

Human umbilical vein grafts were treated with either conventional or LMW heparin, followed by exposure to alcohol. The grafts were investigated for their ability to adsorb and inactivate thrombin, and comparison was made with non-heparinized and saline-alcohol treated grafts and grafts supplied with a covalently bonded layer of conventional heparin. In addition, the effect of protamine exposure to heparin-alcohol and LMW heparin-alcohol treated grafts as well as native human umbilical veins (HUV) was studied. Native HUV and heparin treated graft surfaces adsorbed and inactivated thrombin, whereas non-heparinized and saline-alcohol treated grafts inactivated surface-bound thrombin to only a small degree. Surface-bound LMW heparin exhibited a significantly lower ability to inactivate thrombin as compared with conventional heparin, but LMW heparin-alcohol surfaces were better than non-heparinized ones. Protamine treatment of "heparinized" surfaces impaired the thrombin inhibiting ability of the heparin-alcohol surface, whereas this property was totally abolished for the LMW heparin-alcohol surface. The findings indicate that LMW heparin, despite its weaker thrombin inhibiting capacity, may be an alternative to conventional heparin, for "heparinizing" the human umbilical vein graft. Protamine exposure may be potentially harmful for a heparin treated surface, although protamine concentrations used in the present in vitro study may not be reached in vivo. The native HUV was not at all affected by protamine exposure regarding its ability to inactivate thrombin.

Anticoagulant properties in vitro of heparan sulphates

The anticoagulant properties in vitro of eight heparan sulphate preparations were studied using clotting (APTT, anti-Xa) and amidolytic (anti-Xa, anti-thrombin) assays. Activities ranged from very low levels (less than 5 iu/mg) up to values similar to those of heparin. Activities measured by APTT assay showed the best correlation with the sulphate to carboxylate ratio of the heparan sulphates. Highest activities were obtained in the anti-Xa clotting assay, these being approximately two-fold greater than activities in the anti-Xa amidolytic assay. Five of the heparan sulphate preparations were readily neutralised by protamine sulphate, whereas the three heparans with the lowest sulphate to carboxylate ratio were much more resistant to neutralisation. After fractionating each heparan sulphate into At III-binding and non-binding material, it was found that the anti-coagulant properties were associated only with the former. It is concluded that these properties are dependent on the activation of At III.

Measurement of human activated factor X-antithrombin complex by an enzyme-linked differential-antibody immunosorbent assay

An enzyme-linked immunoabsorbent assay (ELISA) has been developed for the measurement of the complex of human antithrombin and Factor Xa. Rabbit anti-human Factor X antibodies are adsorbed to ELISA plates, and samples containing Xa-antithrombin complex are added. This is followed by the addition of F(ab')2 fragments of rabbit antibodies against human antithrombin, previously labeled with alkaline phosphatase, and subsequent measurement of the bound labeled antibody by hydrolysis of p-nitrophenylphosphate. The minimum level of complex detectable in a sample is ca. 0.1 nM. The assay has been used to follow the generation of Xa-antithrombin complex in kinetic situations by the addition of 1 microM Ile-Glu-Gly-Arg-chloro- methylketone to the ELISA sampling buffer, and it has also been used in plasma systems, where a 20-fold reduction in the sensitivity of the assay is observed. This reduction was shown to be entirely caused by the plasma Factor X. The assay has been used to follow generation of the Xa-antithrombin complex in defibrinated plasma upon activation of the clotting system with the Factor X-activating protein of Russell's Viper venom, and has been compared with the total generation of Factor Xa, measured by a radiopeptide assay of Factor X activation in the same mixtures.

Effect of glycosaminoglycans and antithrombin III on uptake and inhibition of thrombin by the vascular wall

Thrombin binds to and is inactivated by the endothelium. The inactivation is potentiated by plasma. The present investigation was designed to clarify the role of vessel wall glycosaminoglycans (GAG) and plasma antithrombin III (AT III) in the inactivation and binding of thrombin by endothelium. Thrombin was shown to bind to vascular endothelium and artificial surfaces containing GAG:s. The binding could be inhibited on both types of surfaces by pretreating them with protamine. Thrombin bound to endothelium was rapidly inactivated in the presence of plasma but only slowly if the plasma was replaced by AT III, AT III-depleted plasma or a balanced salt solution. It is concluded that thrombin binds to vessel wall GAG:s and is inactivated by the endothelium. Potentiation of the inhibition of the endothelially bound thrombin by plasma is dependent upon presence of AT III but an additional plasma factor is also required.

Antithrombin III progressive function: a biochemical analysis

Antithrombin III (AT III) was isolated by two procedures using polyethylene glycol-400 (PEG) precipitation as the first stage. The PEG supernatant (PEG-sup) was applied to a heparin-affinity chromatographic system and AT III-heparin cofactor (AT III-HCF) was isolated. The PEG precipitate (PEG-ppt) was separated by a Sephacryl S-200 column. Fractions were collected and those demonstrating maximum AT III antigen and progressive thrombin inhibition were pooled and reapplied to the washed Sephacryl S-200 column. Fractions were again collected and assayed via specific antisera for AT III, alpha 1-antitrypsin (alpha 1 AG), alpha 2-macroglobulin (alpha 2 M), and alpha 1-acid glycoprotein (alpha 1 AT). AT III antigen (AT III AGN) and progressive function were confined primarily to one peak containing virtually no alpha 2 M, a low level of alpha 1 AT, and moderate quantities of alpha 1 AG. The PEG-sup, PEG-ppt, AT III-HCF, and the fraction obtained after two passes across Sephacryl S-200 (S#2) were similar in that they showed reactivity with specific AT III antisera and demonstrated heparin cofactor activity. They differed, however, in that the PEG-sup and AT III-HCF demonstrated considerably reduced progressive antithrombin function assessed over 30 min. This function was present in the PEG-ppt and S#2 fractions and this progressive activity was potentiated by EDTA. AT III two-dimensional immunoelectrophoresis (2-DIE) in the presence of heparin of both the Sephacryl and heparin-affinity purified components were very different, with the Sephacryl-purified AT III AGN showing both a fast peak and a very prominent slow-moving hump. The AT III heparin affinity fraction showed primarily a fast component. Dilution of the S#2 and PEG-ppt fractions resulted in considerable loss of the progressive activity and also the slow-moving component on 2-DIE. On the basis of these observations, it is postulated that AT III purified by PEG precipitation is in an aggregated form and that aggregate formation and dissolution is associated with AT III progressive activity.

Glycosaminoglycans and the control of cell surface proteinase activity

It is postulated that the metabolically variable fine structure of pericellular heparan glycosaminoglycans affects the ability of these molecules to influence cell proliferation-associated proteinase-catalysed reactions occurring at cell surfaces. Evidence suggesting the possibility of a wide repertoire of glycosaminoglycan-mediated positive and negative effects on such reactions is reviewed. It is suggested that clinical administration of compounds related chemically to heparins might usefully modulate cell proliferation-associated proteinase activity.

Identification of a heparin activated amidolytic enzyme in carp (Cyprinus carpio) plasma

A heparin activated amidolytic enzyme capable of cleaving synthetic thrombin sensitive chromogenic substrates was identified from the plasma of the carp (Cyprinus carpio). Activation was inhibited by KCl, protamine sulphate and human plasma. Heparin was not required for the continued action of the enzyme. Active enzyme was irreversibly inhibited by DFP. The pH and temperature optima was studied and the enzyme semi purified by Sephadex G100 superfine and Sephadex A50 chromatography. An approximate MW of 62,000 was found. Activity generated by as little as 0.002 units of heparin per ml carp plasma was detected. The trivial name carpamidase is proposed for the enzyme.

Heparin with low affinity to antithrombin III inhibits the activation of prothrombin in normal plasma

Standard unfractionated heparin is known to have two actions on blood clotting. Unfractionated heparin enhances the rates at which antithrombin III inactivates activated clotting factors, and inhibits the activation of both Factor X and prothrombin by disrupting the calcium and phospholipid dependent assembly of the Factor X and prothrombin activator complexes. This latter inhibitory action of heparin occurs independently of antithrombin III. A heparin fraction with low affinity to antithrombin III was prepared from standard heparin by affinity chromatography on antithrombin-III-Sepharose and its properties compared with unfractionated heparin. The low affinity heparin fraction and the unfractionated heparin had equivalent inhibitory effects on prothrombin activation in antithrombin III depleted plasma. In normal plasma, the low affinity fraction inhibited the activation of prothrombin. Unlike the unfractionated heparin, however, the fraction of heparin with low affinity to antithrombin III did not enhance the inactivation of either Factor Xa or thrombin. This antithrombin III independent inhibition of the activation of prothrombin was also evident when activated platelets were used as the source of the procoagulant phospholipids. The antithrombin III independent effect of heparin is unlikely to be important therapeutically, however, if this property of heparin is shared by other naturally occurring glycosaminoglycans, it could be important in maintaining the fluidity of blood under physiological conditions.

Comparison of the in vivo hemorrhagic and antithrombotic effects of a low antithrombin-III affinity heparin fraction

Antithrombin III (ATIII) affinity chromatography of commercial grade heparin yields fractions of high and low affinity for ATIII. In vitro, the high affinity fraction accounts for most of the anticoagulant effect while there is evidence that the low affinity material interferes with platelet function. We have studied the relative antithrombotic and hemorrhagic effects of low affinity heparin. The low affinity heparin fraction, specific activity 43 USP units/mg, was compared with standard heparin (150 USP units/mg) in rabbit experimental models. A residual 12.5% by weight of this low affinity heparin showed high affinity for ATIII. Inhibition of thrombosis in a stasis-hypercoagulability model was directly related to the weight (mg) of high affinity material in each of the heparins. In the bleeding model, when similar weights (mg) of high affinity material were infused, significantly more bleeding was demonstrated with the low affinity fraction which contained a 5-fold excess by weight of low affinity material. We have demonstrated that a low affinity heparin depleted of in vitro anticoagulant and in vivo antithrombotic activity significantly contributes to hemorrhage.

Uptake and inhibition of thrombin by the vascular wall

Thrombin activity and inhibition were assayed on the aortic surface of dogs and pigs. After sacrifice, the aortae were excised and thrombin activity was measured with an amidolytic assay. A small thrombin activity was found on the endothelium. Heparinization of the animals lowered endothelial thrombin activity. After exposure of the aortic endothelium to a thrombin/albumin solution in vitro, thrombin activity disappeared from the solution and was recovered on the surface. De-endothelialized vessels took up more thrombin than those with intact endothelium. Endothelium confined thrombin was rapidly inactivated when exposed to plasma. A slow inactivation was seen upon exposure to a modified Ringer's solution. Thrombin confined to de-endothelialized aortae, i.e. to medial structures, was always inactivated at a slow rate irrespective as to whether it was exposed to plasma or Ringer's solution. It is concluded that endothelium and subendothelium bind thrombin and subsequently inactivate it. The inactivation proceeds faster on endothelium when exposed to plasma. The possible role of glycosaminoglycans is discussed.

The anticoagulant effect of chondroitin-4-sulfate

Chondroitin-4-sulfate (chondroitin sulfate A, CSA) is a natural glycosaminoglycan which has been shown to have antithrombotic effects in vivo. This study investigates its anticoagulant effect in vitro. CSA was added to citrated plasma obtained from 16 human volunteers to yield concentrations ranging from 25 to 500 micrograms/ml. The anticoagulant effect of CSA was determined by activated partial thromboplastin time (APTT). The average baseline APTT was 32.5 +/- 3.4 sec. An excellent linear relationship was observed between the natural logarithms of APTT and plasma CSA concentrations; the slopes of these relationships (APTT-CSA slopes) were determined by linear regression. The average APTT-CSA slope was 0.986 +/- 0.145 ml/mg. Plasma concentrations of antithrombin III, alpha 1 antitrypsin, alpha 2 macroglobulin, fibrinogen, and alpha 1 acid glycoprotein were normal. There were no statistically significant correlations between baseline APTT and APTT-CSA slope values or between either of these parameters and plasma concentrations of any of the measured plasma proteins. Additional in vitro studies showed that the anticoagulant effect of CSA is only in part mediated by antithrombin III. The results of this study suggest that the antithrombotic effect of this biologically active substance are at least in part due to its anticoagulant effect.