Studies on the mechanism of the rate-enhancing effect of heparin on the thrombin-antithrombin III reaction

Structure and physicochemical characterisation of heparin

Heparin is a member of the heparan sulphate family of glycosaminoglycans, a linear polysaccharide with a complex sequence resulting from the action of post-polymerisation enzymes on a regular repeating disaccharide background. Its overall conformation is rod-like in solution as well as in the solid state, but the conformational fluctuations of iduronate residues give rise to considerable internal motion and variation in local three-dimensional structure. Structure/function relationships and their relation to sequence are still the subject of argument, but new methodologies to tackle the subject are emerging. Heparin as a therapeutic agent and as the object of research may be characterised by numerous physico-chemical techniques. These include chromatographic methods for measurement of molecular weight; a variety of spectroscopic techniques; separation methods for whole polysaccharides, as well as for oligo- and monosaccharides; and mass spectrometric methods for mapping and sequence analysis. The impetus provided by the discovery of heparin contamination with oversulphated chondroitin sulphate has been influential in bringing combinations of many old and new techniques into use to ensure that heparin is sufficiently consistent and pure to be used safely. Synthetic and semi-synthetic heparins are in development and may become reality in the relatively near future.

Exact molecular mass determination of various forms of native and de-N-glycosylated human plasma-derived antithrombin by means of electrospray ionization ion trap mass spectrometry

Human plasma-derived antithrombin was characterized in both the native and de-N-glycosylated forms (without separation of isoforms) by means of electrospray ionization ion trap mass spectrometry (ESI-ITMS). In order to determine the limits of the instrument set-up, the molecular mass precision and accuracy of the ESI-ITMS analysis was evaluated with the standard protein enolase and some instrumental data acquisition parameters were optimized. Mass precision was determined as a function of the number of averaged mass spectra (= scans) and data acquisition time. For this study, 20 and 50 scans were averaged and the data acquisition time was chosen to be between 0.5 and 5 min. It turned out that data acquisition times longer than approximately 2 min show no significant differences of the standard deviation of the determined molecular mass. Furthermore, the ion trap scan rate was varied at constant acquisition time of 2 min and the number of averaged scans was set to 20. At the scan rate of 13,000 u s(-1) a mass precision of +/-1.8 Da and a mass accuracy of +0.026% were determined. On reducing the scan rate to 5500 u s(-1), better agreement with the theoretical molecular mass was obtained, showing a mass accuracy of +0.012% but with a decrease in the mass precision to +/-3.0 Da. Using the optimized scan rate of 13,000 u s(-1) and a data acquisition time of 2 min, the exact molecular mass was determined of the three forms of antithrombin, namely the alpha-form, the beta-form and the natural mixture (present in human plasma) containing both forms. The protonated molecular masses were found to be 57,854 and 55,664 Da for the affinity chromatography-isolated alpha-and beta-form, respectively. The mass difference of 2190 Da is attributed to the known difference in carbohydrate content at one specific site. The protonated molecular mass of the dominating species of the natural mixture in human plasma was shown to be 57,850 Da, corresponding to the alpha-form, the major component in native plasma. In this mixture the beta-form was also detected, exhibiting a protonated molecular mass of 55,655 Da, but showing a much lower abundance, as expected. To obtain a complete release of the N-glycan residues by means of PNGase F, a denaturation, reduction and alkylation step of the glycoproteins was performed before the enzymatic reaction. After enzymatic removal of all N-glycans, the protonated molecular masses obtained were 49,399, 49,380 and 49,391 Da for the alpha-form, the beta-form and the unseparated natural mixture, respectively. These values are in good agreement (+0.026% for the alpha-form, -0.012% for the beta-form and +0.010% for the unseparated mixture) with the calculated molecular mass based on the SwissProt data. The determined molecular masses after reduction/alkylation and de-N-glycosylation of the alpha-and beta-forms are almost equal, indicating that no major differences exist between the three preparations on the amino acid level.

Glycosaminoglycans and the regulation of blood coagulation

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Dissociation of the thrombin/antithrombin III reaction from amidolytic activity of the enzyme at high salt concentration

Amidolytic activity of thrombin increased proportionally with NaCl concentrations up to 1 M NaCl, due to an increase in the turnover rate with no change in the Km of the enzyme. Neutralization of thrombin by antithrombin III (AT III) was maximum in 0.1 to 0.15 M NaCl, but completely absent at high NaCl concentrations. Moreover, ratio of rates of the amidolytic activity of thrombin to of the enzyme/AT III reaction was not constant at several pHs. The rate of the thrombin/AT III reaction appeared to be inversely correlated with the enzyme's stability. Therefore, it was concluded that some factor(s) other than the active site of the thrombin molecule was more important in its primary interaction with AT III.

Pharmacology and special clinical applications of low-molecular-weight heparins

In this overview, the rationale of the development of low-molecular-weight (LMW) heparins and their toxicological, anticoagulant, fibrinolytic, lipolytic, and protamine interactions are summarized. Clinical experiences are reviewed on the benefit of LMW heparin for anticoagulation in patients with bleeding and other complications on conventional anticoagulants and during pregnancy. It is concluded that animal experiments have demonstrated the safety of LMW heparins, that the pharmacologic profile is improved compared with normal heparin, and that the simple and safe applicability of LMW heparins gives rise to new indications for the long-term prophylaxis of thromboembolism.

Heparin-like tubings. I. Preparation, characterization and biological in vitro activity assessment

In order to prepare tubular materials which could be used in blood-circulating medical devices, polystyrene was grafted by irradiation on to polyethylene tubings. A chemical surface treatment was used which resulted in the functionalization of the inner face of the tubing. This procedure is described and the chemical assessment of the constitution of the functionalized polymer has been completed. Tubing, the inner face of which is made of polyethylene-polystyrene copolymer in which polystyrene moieties were substituted with sulphonate and aspartic acid sulphamid groups, was tested for antithrombic properties in a circulating device under controlled transport conditions and by use of purified proteins.

Probing the heparin-binding domain of human antithrombin III with V8 protease

From structural analysis on genetically abnormal and chemically modified human antithrombin III [Koide, T., Odani, S., Takahashi, K., Ono, T. and Sakuragawa, N. (1984) Proc. Natl Acad. Sci. USA 81, 289-293; Chang, J.-Y. and Tran, T. H., (1986) J. Biol. Chem. 261, 1174-1176; Blackburn, M. N., Smith, R. L., Carson, J. and Sibley, C. C. (1984) J. Biol. Chem. 259, 939-941], the heparin-binding site of antithrombin III has been suggested to be in the region of Pro-41, Arg-47 and Trp-49. In this study the heparin-binding site was probed by preferential cleavage of V8 protease on heparin-treated and non-treated native antithrombin III. The study has been based on the presumption that the heparin-binding site of antithrombin III is situated at exposed surface domain and may be preferentially attacked during limited proteolytic digestion. Partially digested antithrombin III samples were monitored by quantitative amino-terminal analysis and amino acid sequencing to identify the preferential cleavage sites. 1-h-digested antithrombin III was separated on HPLC and peptide fragments were isolated and characterized both qualitatively and quantitatively. The results reveal that Glu-Gly (residues 34-35), Glu-Ala (residues 42-43) and Glu-Leu (residues 50-51) are three preferential cleavage sites for V8 protease and their cleavage, especially the Glu-Ala and the Glu-Leu sites, was drastically inhibited when antithrombin III was preincubated with heparin. Both high-affinity and low-affinity antithrombin-III-binding heparins were shown to inhibit the V8 protease digestion of native antithrombin III, but the high-affinity sample exhibited a higher inhibition activity than the low-affinity heparin. These findings (a) imply that the segment containing residues 34-51 is among the most exposed region of native antithrombin III and (b) support the previous conclusions that this region may play a pivotal role in the heparin binding.

Asthma and diabetes mellitus: a biochemical basis for antithetical features

Diabetes mellitus and asthma have many antipodal features. Although both are common disorders, concurrence occurs less often than would be predicted. When co-existence does occur, the cases are generally mild, and effective treatment of one disease frequently exacerbates the other. The hypothesis is advanced that basilar membrane concentrations of heparan sulfate differ in these two diseases and that this difference may account for the antithetical features. An experimental basis for postulating increased concentrations of extracellular heparan sulfate in asthma and diminished concentrations in diabetes is cited. A rationale for tying these differences to the polar activities of cholinergic transmission and atherogenesis in the two diseases is advanced. Diminished heparan sulfate concentrations in diabetes may down-regulate the transmission of vagal impulses to insulin-producing pancreatic cells, and thereby impair both the continued vitality of these cells, and the acetylcholine modulated potentiation of glucose-induced insulin release.

Anticoagulant hydrogels derived from crosslinked dextran. Part II: Mechanism of thrombin inactivation

Sephadex derivatives bearing carboxymethyl, sulphonated benzylamine and amino acid groups exhibit heparin-like behaviour as demonstrated by the kinetic study of the thrombin inactivation in the presence of antithrombin III. Furthermore, whatever the chemical composition of these hydrogels, the diffusion coefficient of thrombin remained approximately constant and could not be connected with the variation of the antithrombin activity of the resins. Hence, in the heparin-like mechanism, the diffusion rate of thrombin inside the beads of hydrogels was not the limiting step. In fact, the swelling ratio (varying according to the chemical composition of these biomaterials) was involved in the anticoagulant properties of the resins.

Effect of NaC1 on inactivation of bovine thrombin by antithrombin III in the presence of low affinity-heparin or dextran sulfate

Heparin with low affinity (LA-heparin) to antithrombin III (AT III) enhanced the rate of inactivation of thrombin by AT III. The enhancement of the rate was saturable with AT III and was proportional to the LA-heparin concentration. Although the rate-enhancement in the presence of LA-heparin decreased with increase in NaC1 concentration, it was comparable with that in the presence of high affinity-heparin (HA-heparin) in the absence of NaC1. Inactivation of thrombin by AT III in the presence of dextran sulfate (DS) was also sensitive to NaC1 concentration. These findings indicate that free AT III is favorable for binding to the complexes of thrombin and highly sulfated polysaccharides having low affinities to AT III in the absence of NaC1.

Catalytic and regulatory functions of N-bromosuccinimide-modified bovine thrombin

At pH 4.1, bovine thrombin reacts rapidly with N-bromo-succinimide to yield modified enzyme containing oxidized tryptophan residue. Both fibrinogen clotting activity and esterase activity are reduced considerably when three moles of tryptophan residues per mole of thrombin are oxidized, but the Michaelis constants for synthetic substrates are not appreciably altered. Reaction of NBS also results in a decrease in the affinity of thrombin for heparin. The dissociation constant for heparin-thrombin complex is increased by 2.6-fold due to the modification of one tryptophan residue. However, the magnitude of the increase in the dissociation constant remains the same for modified enzymes containing approximately two or three oxidized tryptophan residues. The rate constant for the inactivation of thrombin by antithrombin III is increased by 2.5-fold due to the modification of a single tryptophan residue. This increase in rate constant is not further amplified when more than one tryptophan residue is oxidized. In contrast, in the presence of heparin the rate of inactivation of modified and unmodified thrombins by antithrombin III are not significantly different. Thus, the heparin-sensitized inactivation of thrombin by antithrombin III is affected by the modification of one tryptophan residue. Spectrophotometric titrations of the phenolic hydroxyl groups suggest that the structural environments of tyrosyl groups for both unmodified and modified thrombin containing one oxidized tryptophan residue, are similar. The temperature for half loss of catalytic activity of control and NBS-modified thrombin, containing one oxidized tryptophan, are 52 and 51.5 degrees C respectively. It appears that the one tryptophan residue of thrombin is situated at or close to the binding site of heparin.

Anticoagulant activities of heparin oligosaccharides and their neutralization by platelet factor 4

Oligosaccharides of well-defined molecular size were prepared from heparin by nitrous acid depolymerization, affinity chromatography on immobilized antithrombin III (see footnote on Nomenclature) and gel chromatography on Sephadex G-50. High affinity (for antithrombin III) octa-, deca-, dodeca-, tetradeca-, hexadeca- and octadeca-saccharides were prepared, as well as oligosaccharides of larger size than octadecasaccharide. The inhibition of Factor Xa by antithrombin III was greatly accelerated by all of these oligosaccharides, the specific anti-Factor Xa activity being invariably greater than 1300 units/mumol. The anti-Factor Xa activity of the decasaccharide was not significantly decreased in the presence of platelet factor 4, even at high platelet factor 4/oligosaccharide ratios. Measurable but incomplete neutralization of the anti-Factor Xa activities of the tetradeca- and hexadeca-saccharides was observed, and complete neutralization of octadeca- and larger oligo-saccharides was achieved with excess platelet factor 4. The octa-, deca-, dodeca-, tetradeca- and hexadeca-saccharides had negligible effect on the inhibition of thrombin by antithrombin III, whereas specific anti-thrombin activity was expressed by the octadeca-saccharide and by the larger oligosaccharides. An octadecasaccharide is therefore the smallest heparin fragment (prepared by nitrous acid depolymerization) that can accelerate thrombin inhibition by antithrombin III. The anti-thrombin activities of the octadecasaccharide and larger oligosaccharides were more readily neutralized by platelet factor 4 than were their anti-Factor Xa activities. These findings are compatible with two alternative mechanisms for the action of platelet factor 4, both involving the binding of the protein molecule adjacent to the antithrombin III-binding site. Such binding results in either steric interference with the formation of antithrombin III-proteinase complexes or in displacement of the antithrombin III molecule from the heparin chain.

Calcium inhibits the heparin-catalyzed antithrombin III/thrombin reaction by decreasing the apparent binding affinity of heparin for thrombin

The present study has shown that calcium inhibits the heparin-catalyzed antithrombin III/thrombin reaction. The initial rate of thrombin (4.0 nM) inhibition by antithrombin III (200 nM) in the presence of heparin (2.5 ng/ml) decreased from 3.6 nM/min (in the absence of calcium) to 0.12 nM/min in the presence of 10 mM calcium. In the absence of heparin, the initial rate of thrombin inhibition by antithrombin III was not affected by calcium. The heparin-catalyzed antithrombin III/thrombin reaction is described by the general rate equation for a random-order, bireactant, enzyme-catalyzed reaction (M. J. Griffith (1982) J. Biol. Chem. 257, 13899-13902). As such, the reaction is saturable with respect to both thrombin and antithrombin III. The apparent kinetic parameters for the heparin-catalyzed antithrombin III/thrombin reaction were determined in the presence and absence of calcium. The apparent heparin/antithrombin III dissociation constant values were not measurably different in the presence of 0, 1.0, and 3.0 mM calcium. The apparent heparin/thrombin dissociation constant value increased from 7.0 nM, in the absence of calcium, to 10 and 30 nM in the presence of 1.0 and 3.0 mM calcium, respectively. The maximum reaction velocity, at saturation with respect to both proteins, was not affected by calcium. It is concluded that calcium binds to functional groups within the heparin molecule which are required for thrombin binding.

Heparin-catalyzed inhibitor/protease reactions: kinetic evidence for a common mechanism of action of heparin

Three different heparin-catalyzed inhibitor/protease reactions were studied: antithrombin III/thrombin, heparin cofactor II/thrombin, antithrombin III/factor Xa. The three reactions were saturable with respect to both inhibitor and protease. The initial reaction velocity, for each reaction, could be described by the general rate equation for a random-order bireactant enzyme-catalyzed reaction. The kinetic parameters for the heparin-catalyzed antithrombin III/thrombin and antithrombin III/factor Xa reactions differed in terms of apparent maximum velocity (Vmax) and apparent heparin-protease dissociation constant values. The apparent heparin-antithrombin III dissociation constant values were the same for both reactions. The kinetic parameters for the heparin-catalyzed antithrombin III/thrombin and heparin cofactor II/thrombin reactions differed in terms of apparent Vmax and apparent heparin-inhibitor dissociation constant values. The apparent heparin-thrombin dissociation constant values were the same for both reactions. The results are consistent with a general mechanism of action of heparin for the three reactions that, in its simplest form, requires only that both protease and inhibitor bind to heparin for catalysis to occur.

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.

Further characterization of the antithrombin-binding sequence in heparin

An octasaccharide with high affinity for antithrombin, isolated after partial deaminative cleavage of heparin and previously found to have the following predominant structure (see formula in text) has been studied further. High-voltage, paper electrophoresis of the 3H-labelled disaccharides obtained by deamination with HNO2 (pH 1.5) followed by reduction with Na[3H]BH4 showed approximately 25% of mono-O-sulfated components, in addition to L-iduronic acid(2-O-SO3)-2,5-anhydro-D-[3H]mannitol (6-O-SO3). The monosulfated disaccharides were identified by high pressure, ion-exchange chromatography as L-iduronic acid(2-O-SO3)-2,5-anhydro-D-[3H]mannitol, L-Iduronic acid-2,5-anhydro-D-[3H]mannitol(6-O-SO3). and D-glucuronic acid-2,5-anhydro-D-[3H]-mannitol L, iduronic acid 2,5-anhydro-D-[3H]mannitol(6-O-SO3), and D-glucuronic acid-2,5-anhydro-D-[3H]-mannitol. These components originated from the reducing, terminal disaccharide residue (units 7 and 8), as indicated by selective labelling with Na[3H]-BH4. The structural variability within this region suggests that it is not part of the antithrombin-binding sequence. Neither enzymic removal of the non-sulfated L-iduronic acid unit 1 nor N-deacetylation (by hydrazinolysis) at unit 2 had any significant effect on the affinity of the octasaccharide for antithrombin. However, removal of the disaccharide corresponding to units 1 and 2, by selective deamination of the N-deacetylated octasaccharide, yielded a low-affinity hexasaccharide. In addition, a high-affinity deamination product was formed, presumably an octasaccharide containing a 6-sulfated 2-deoxy-2-C-formyl-D-pentofuranosyl unit due to ring contraction in unit 2. These results suggest that the 6-sulfate group in unit 2 may be involved in antithrombin binding. It is concluded that the antithrombin binding site in heparin is represented by the pentasaccharide sequence extending from unit 2 to unit 6 of the octasaccharide studied.

The characteristics of anticoagulation by covalently immobilized heparin

The reactions of covalently immobilized heparin, abbreviated as I-Hep, with thrombin or Factor Xa were investigated both in the presence and absence of antithrombin III, AT III. Although I-Hep was able to bind to thrombin, the complex formation of thrombin and I-Hep did not affect the thrombin activity when measured by using a small artificial substrate, a peptide-MCA. Similarly, Factor Xa bound to I-Hep, but the activity of Factor Xa was not decreased in the absence of AT III, when a peptide-MCA was used for Factor Xa assay. Thrombin bound to I-Hep in much larger amounts than Factor Xa. Thrombin and Factor Xa were instantaneously inhibited by AT III in the presence of soluble heparin. However, when I-Hep was used instead of soluble heparin, instantaneous inhibition was not observed. When a natural, high-molecular-weight substrate was used for assay, the results were dependent on the structure of the immobilization carrier. Heparin immobilized on Sepharose 4B or Poly HEMA showed considerable prolongation of plasma recalcification time. However, heparin immobilized on the surface of PVA fiber did not prolong plasma recalcification time.

Anticoagulant activity of artificial biomedical materials with co-immobilized antithrombin III and heparin

An approach to providing anticoagulant activity to biomedical materials was presented, applying an immobilization technique. Antithrombin III (AT III) inactivates the activated coagulation factors including Factor Xa and thrombin. Heparin stimulates the inactivation of Factor Xa and thrombin by AT III. Thus AT III and heparin were co-immobilized on Sepharose 4B, polyvinyl alcohol, polyhydroxy-ethyl methacrylate and silicone-coated nylon by the cyanogen bromide procedure. Those co-immobilized preparations, abbreviated as I-AT III . Hep, actively neutralized both Factor Xa and thrombin. The activity of I-AT III . Hep was much higher than immobilized heparin and/or immobilized AT III. I-AT III . Hep, like soluble AT III and heparin, instantaneously neutralized both thrombin and Factor Xa. When two enzymes, thrombin and Factor Xa, were present, I-AT III . Hep neutralized Factor Xa in preference to thrombin : The neutralization of thrombin was inhibited by the presence of Factor Xa, but neutralization of Factor Xa was independent of the presence of thrombin. The amount of Factor Xa neutralized was higher than that of thrombin.

Evidence by chemical modification for the involvement of one or more tryptophanyl residues of bovine antithrombin in the binding of high-affinity heparin

Tryptophanyl residues of bovine antithrombin were modified with N-bromosuccinimide at near-neutral pH. The reaction was found to be specific for tryptophan at low levels of modification, i.e. when only up to 1--1.3 mol tryptophan/mol protein were oxidized. Further modification led to extensive side reactions. Modification of an average of about one tryptophanyl residue per protein molecule did not affect antithrombin activity measured in the absence of heparin, but decreased the activity assayed in the presence of heparin to about half the value given by unmodified antithrombin. Addition of an excess of high-affinity heparin to a similarly modified antithrombin sample resulted in much smaller circular dichroism, ultraviolet absorption and fluorescence changes than those observed with the intact protein. Modification experiments in the presence of excess high-affinity heparin gave a definitely lower extent of modification than when heparin was excluded. These studies thus reinforce the conclusion from previous spectroscopic analyses that one or more tryptophanyl residues of antithrombin are involved in the binding of high-affinity heparin, presumably by being located at or close to the heparin binding site.