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

Heparin-binding domains in vascular biology

Heparin is a major anticoagulant with activity mediated primarily through its interaction with antithrombin (AT). Heparan sulfate (HS), structurally related to heparin, binds a wide range of proteins of different functionality, taking part in various physiological and pathological processes. The heparin-AT complex, the most well understood facet of anticoagulation, serves as a prototypical example of the important role of heparin/HS in vascular biology. Extensive studies have identified common structural features in heparin/HS-binding sites of proteins. These include the elucidation of consensus sequences in proteins, patterns of clusters of basic and nonbasic residues, and common spatial arrangements of basic amino acids in the heparin-binding sites. Although these studies have provided valuable information, heparin/HS-binding proteins differ widely in structure. The prediction of heparin/HS-binding proteins from sequence information is not currently possible, and elucidation of protein-binding sites requires the individual study of each glycosaminoglycan-protein complex. Thus, x-ray crystallography and site-directed mutagenesis experiments are among the most powerful tools, providing accurate structural information, facilitating the characterization of heparin-protein complexes. Heparin and structurally related heparan sulfate bind a large number of proteins, taking part in a wide range of biological processes, particularly ones involved in vascular biology. Heparin-binding domains share certain common structural features, but there is no absolute dependency on specific sequences or protein folds.

Non-specific influence of chemical modification upon the properties of antithrombin III:modification of carboxyl groups

The ability of antithrombin III to inhibit thrombin was observed to be rapidly inactivated upon specific modification of carboxyl groups. The loss of activity, upon treatment with nitrotyrosyl ester in the presence of 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide metho-p-toluenesulfonate, was concomitant to the incorporation of 2 moles of nitrotyrosine per mole of inhibitor indicating the modification of only two carboxyl groups. Moreover, the modification occurred with loss, also, of the ability of the native protein to bind tightly to heparin. The modified antithrombin III retained a reduced affinity for heparin (eluting at 0.3M NaCl from heparin Agarose) and was observed to be a competitive inhibitor of the heparin-dependent rate of inhibition of thrombin by native antithrombin III. However, FAB-MS (fast atom bombardment mass spectroscopy) analysis of digests of modified material gave no indication that modification was localized to specific Asp or Glu residues. It is concluded that the loss of activity is due to deleterious change in conformation during modification. These findings, together with our previous report upon tryptophan modification of antithrombin III [1] suggest that the nature of the molecule is such that considerable care must be taken in interpretation of results when investigating the structure/function relationships of this protein by chemical modification.

Synthesis and bioactivity of copolymers with fragments of heparin

A new type of biocompatible copolymer comprising small fragments of heparin, (octa- to dodecasaccharides) copolymerized with a synthetic monomeric component, viz. acrylamide, has been prepared. The heparin fragments are produced by enzymatic or chemical means and are copolymerized, directly or after suitable derivatization, with acrylamide as the major polymerizable component. The polymeric material incorporates the heparin segments as pendant moieties such that their essential functional groups and structural features for specific binding with the selective serine protease coagulation factor inhibitor antithrombin III are preserved. An important feature of this copolymer is its biocompatibility which relates specifically to its antithrombotic and antithrombogenic activity derived from those of heparin fragments. The biological activity of heparin fragments and copolymers thereof are determined in terms of APTT and anti-Xa activity, their antithrombotic potential being expressed as a ratio of anti-Xa activity to APTT. The copolymers reported have biological activities similar to equivalent amounts of respective heparin fragments, and show higher antithrombotic activity compared to intact heparin or commercially available low-molecular-weight heparin (4,000-6,000 Da).

Evidence that arginine-129 and arginine-145 are located within the heparin binding site of human antithrombin III

Arginyl residues of human antithrombin III have been implicated to involve in the heparin binding site [Jorgensen, A. M., Borders, C. L., & Fish, W. W. (1985) Biochem, J. 231, 59-63]. We have performed chemical modification of antithrombin with (p-hydroxyphenyl)glyoxal (HPG) in order to determine the locations of these arginine residues. Antithrombin was modified with 12 mM HPG in the absence and presence of heparin (2-fold by weight to antithrombin). In the absence of heparin, about 3-4 mol of arginines/mol of antithrombin were modified within 60 min, and the modification led to the loss of 95% of the inhibitor's heparin cofactor activity as well as heparin-induced fluorescence enhancement and 50% of its progressive inhibitory activity. In the presence of heparin, the extent of modification was diminished by 30% and modified antithrombin retained approximately 70% of its heparin cofactor activity. Peptide mapping and subsequent sequence analysis revealed that selective HPG modification occurred at Arg129 and Arg145 and that their modifications were protected upon binding of heparin to antithrombin. We conclude that Arg129 and Arg145 are situated within the heparin binding site of human antithrombin III.

Re-formation of disulphide bonds in reduced antithrombin III

Human antithrombin III (AT-III) contains three disulphide linkages (Cys-8-Cys-128, Cys-21-Cys-95 and Cys-247-Cys-430), and two of them (Cys-8-Cys-128 and Cys-21-Cys-95) are situated near the heparin-binding domain of the inhibitor. We demonstrate in this paper that: (i) partially reduced AT-III (with Cys-8-Cys-128 and Cys-21-Cys-95 quantitatively reduced) could be re-oxidized in air to regain 70-80% of its heparin cofactor activity and thrombin-inhibitory activity; (ii) completely reduced AT-III was re-oxidized under similar conditions and recovered 30-35% of it biological activities. Structural analysis of refolded AT-IIIs indicates that restorations of their disulphide contents and conformations (evaluated by chemical modification) are congruent with recoveries of their biological activities.

Mechanism of heparin action

Heparin catalysis of clotting proteinase inactivation occurs most efficiently through the reaction of the proteinase with the antithrombin-heparin complex. The efficiency of a heparin molecule in this reaction depends on the presence of a specific pentasaccharide sequence in it, and its molecular weight. The mechanism by which such high affinity heparin acts when antithrombin III is the inhibitor is promotion of the formation of an intermediate proteinase-heparin-antithrombin complex. Heparin promotion of thrombin inactivation by heparin cofactor II may occur by a similar mechanism. The requirement for a specific oligosaccharide sequence within the heparin molecule does not, however, exist for heparin cofactor II. Binding of heparin to both thrombin and antithrombin III interferes with thrombin inactivation. This binding is very dependent on the ionic strength of the reaction mixture and may explain some of the discordant results and interpretations from early studies on the mechanism of heparin action. Low ionic strength in in vitro reactions also results in cleavage of antithrombin III by thrombin in the presence of heparin and effectively converts antithrombin III from an inhibitor to a substrate.

Intravenous nitroglycerin-induced heparin resistance: a qualitative antithrombin III abnormality

An ability of intravenous nitroglycerin to interfere with the anticoagulant properties of intravenous heparin would have profound clinical implications. To investigation nitroglycerin-heparin interactions, the following pilot study was performed. Patients (N = 18) admitted to the coronary care unit with a diagnosis of either acute myocardial infarction or unstable angina were divided into four treatment groups: (1) intravenous nitroglycerin and intravenous heparin; (2) intravenous nitroglycerin alone; (3) intravenous heparin alone; or (4) neither intravenous nitroglycerin nor intravenous heparin. Serial determinations of activated partial thromboplastin time (APTT), serum heparin concentration, antithrombin III (ATIII) antigen (ATA), and ATIII activity (ATC) were obtained over a 72-hour period. Overall, patients receiving intravenous nitroglycerin did not differ significantly from other patients in APTT, heparin dose, heparin concentration, ATA, ATC, or ATA/ATC ratio (ATR). However, patients receiving intravenous nitroglycerin at a rate exceeding 350 micrograms per minute had a lower APTT (p less than 0.05), lower ATC (p = 0.02), higher ATR (p = 0.004), and a larger heparin dose requirement than patients receiving lower infusion rates. ATR correlated directly (r = 0.91; p less than 0.05) and ATC inversely (r = -0.78; p less than 0.05) with the intravenous nitroglycerin dose. Serum heparin concentration did not correlate with the intravenous nitroglycerin dose. Intravenous nitroglycerin-induced heparin resistance occurs at a critical nitroglycerin dose. A nitroglycerin-induced qualitative ATIII abnormality may be the underlying mechanism.

Influence of chemical modification of tryptophan residues on the properties of human antithrombin III

According to the reaction conditions selected, chemical modification of tryptophan residues in antithrombin III by dimethyl (2-hydroxy-5 nitrobenzyl) sulfonium bromide (HNBSB) generated products with similar levels of modification (equivalent to 0.9 mole 2-hydroxy-5-nitrobenzyl (HNB) incorporated/mole of antithrombin III) but with high or low affinity for heparin. These products were subjected to digestion by cyanogen bromide and shown to be modified equivalently in fragment II containing Trp 189 and Trp 225 and fragment III containing Trp 49. The molar level of incorporation of HNB into these fragments was similar in the high and low affinity forms. Both high and low affinity forms showed loss of heparin cofactor activity. A recovery of heparin cofactor activity towards coagulation factor Xa was observed upon prolonged storage of low affinity forms at -70 degrees C. It is considered that the loss of high affinity for heparin upon modification of antithrombin III arises from change or stabilization of conformation associated with tryptophan modification and is not a singular property of modification of Trp 49.

The heparin and pentosan polysulfate binding sites of human antithrombin overlap but are not identical

Four sulfated polysaccharides (unfractioned heparin, low-molecular-mass heparin, heparan sulfate and pentosan polysulfate) were investigated for their abilities (a) to bind antithrombin, (b) to induce conformational change of the inhibitor and (c) to potentiate antithrombin inhibition of thrombin. The binding capacity was reflected by the shielding of the heparin binding site. This was characterized by the extent to which a polysaccharide could protect chemical modification of Lys-125 and Lys-136, two lysyl residues of antithrombin which have been implicated in heparin binding. The conformational change was measured by fluorescence enhancement and the increased accessibility of Lys-236 to chemical modification. Our results reveal that the events of polysaccharide binding, conformational change and the enhancement of inhibitory activity are not quantitatively interlinked. Compared to the unfractionated heparin on an equal mass basis, the low-molecular-mass heparin (molecular mass 4-6 kDa) binds more strongly to antithrombin, induces a greater conformational change (about twofold), but is less potent in accelerating the inhibitory activity. Both heparin and heparan sulfate shield Lys-125 and Lys-136 and induce a conformational change that leads to exposure of Lys-236 and an increased fluorescence. On the other hand, pentosan polysulfate protects only Lys-125 and causes no appreciable conformational change, although it is also capable of enhancing the antithrombin-thrombin interaction. These data clearly demonstrate that the heparin and pentosan polysulfate binding sites of antithrombin overlap (at Lys-125) but are not identical.

Structure-function relationships in heparin cofactor II: spectral analysis of aromatic residues and absence of a role for sulfhydryl groups in thrombin inhibition

This study characterizes the structural and functional significance of sulfhydryl residues in human plasma heparin cofactor II (HCII). For quantification of sulfhydryl groups, the extinction coefficient of HCII was redetermined and found to be 0.593 ml mg-1 cm-1 using second-derivative spectroscopy and multicomponent analysis assuming 4, 10, and 2 residues of tryptophan, tyrosine, and tyrosine-O-sulfate per mole of protein, respectively. The results show that tyrosine-O-sulfate residues in HCII and in cholecystokinin peptide fragments (as model compounds) do not significantly contribute to the absorbance spectrum from 280 to 300 nm. A total of three sulfhydryl groups per mole of HCII was detected by Ellman's reagent titration, with or without treatment with dithioerythritol, indicating the absence of intramolecular disulfide bonds. Incubation of HCII with 0.1-10 mM dithioerythritol did not diminish its heparin-enhanced thrombin inhibition activity. Treatment with various sulfhydryl-specific reagents, including p-mercuribenzoate, HgCl2, and N-substituted maleimide derivatives, inactivated HCII. Titration with Ellman's reagent after these reactions identified the modification site as a cysteinyl residue(s). However, complete methanethio derivatization of the sulfhydryl groups of HCII using methyl methanethiosulfonate did not alter heparin-catalyzed thrombin inhibition. These results indicate that the sulfhydryl groups of HCII are not essential for thrombin inhibition. HCII differs from antithrombin III, which contains an essential disulfide bond for heparin-dependent thrombin inhibition (Longas, M. O., et al. (1980) J. Biol. Chem. 255, 3436). Furthermore, within the "serpin" (serine proteinase inhibitor) superfamily, HCII resembles chicken ovalbumin in occurrence of sulfhydryl residues and reactivity with various sulfhydryl group-directed compounds.

Carboxylate polyanions accelerate inhibition of thrombin by heparin cofactor II

The heparin cofactor II (HCII)/thrombin inhibition reaction is enhanced by various carboxylate polyanions. In the presence of polyaspartic acid, the HCII/thrombin reaction is accelerated more than 1000-fold with the second-order rate constant increasing from 3.2 x 10(4) M-1 min-1 (in the absence of polyAsp) to 3.6 x 10(7) M-1 min-1 as the polyAsp concentration is increased from 1 to 250 micrograms/ml. This accelerating effect was observed for HCII/thrombin, though to varying degrees, with other carboxylate polyanions. In contrast to HCII, the rate of antithrombin III inhibition of thrombin was decreased in the presence of polyAsp. The HCII/thrombin complex is rapidly formed in the presence of 10 micrograms/ml polyAsp when 125I-labeled-thrombin is incubated with plasma. It is possible that at physiological sites rich in carboxylate polyanions, thrombin may be preferentially inhibited by HCII.

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.

Antithrombin III and its interaction with heparin. Comparison of the human, bovine, and porcine proteins by 1H NMR spectroscopy

1H NMR has been used to characterize and compare the structures of antithrombin III from human, bovine, and porcine plasma as well as to investigate the interactions of each of these proteins with heparin fragments of defined length. The amino acid compositions of the three proteins are very similar, which is reflected in the gross features of their 1H NMR spectra. In addition, aromatic and methyl proton resonances in upfield-shifted positions appear to be common to all three proteins and suggest similar tertiary structures. Human antithrombin III has five histidine residues, bovine has six, and porcine has five. The C(2) proton from each of these residues gives a narrow resonance and titrates with pH; the pKa's are in the range 5.15-7.25. It is concluded that all histidines in each protein are surface residues with considerable independent mobility. The carbohydrate chains in each protein also give sharp resonances consistent with a surface location and motional flexibility. The 1H spectra are sensitive to heparin binding. Although heparin resonances obscure protein resonances in the region 3.2-6.0 ppm, difference spectra between antithrombin III with and without heparin show clear perturbation of a small number of aromatic and aliphatic protein protons. These resonances include those of histidine C(2) and C(4) protons, of 10-20 other aromatic protons, of a methyl group, and also of protons with chemical shifts similar to those of lysine and/or arginine side chains. For human antithrombin III, it was shown that heparin fragments 8, 10, and 16 sugar residues in length result in almost identical perturbations to the protein. In contrast, tetrasaccharide results in fewer perturbations.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Structure-function relationships in heparin cofactor II: chemical modification of arginine and tryptophan and demonstration of a two-domain structure

Heparin cofactor II and antithrombin III are plasma proteins functionally similar in their ability to inhibit thrombin at accelerated rates in the presence of heparin. To further characterize the structural and functional properties of human heparin cofactor II as compared to antithrombin III, we studied the possible significance of arginyl and tryptophanyl residues and the changes in protein structure and activity during guanidinium chloride (GdmCl) denaturation. Both antithrombin and heparin cofactor activities of heparin cofactor II are inactivated by the arginine-specific reagent, 2,3-butanedione. Saturation kinetics are observed during modification and suggest formation of a reversible protease inhibitor-butanedione complex. Quantitation of arginyl residues following butanedione modification shows a loss of about four residues for total inactivation, one of which is essential for antithrombin activity. Arginine-modified heparin cofactor II did not bind to heparin-agarose and implies a role for the other modified arginyl residues during heparin cofactor activity. N-Bromosuccinimide oxidation (20 mol of reagent/mol of protein) of heparin cofactor II results in modification of approximately two tryptophanyl residues with no concomitant loss of heparin cofactor activity. Moreover, there is no enhancement of intrinsic protein fluorescence during heparin binding to the native inhibitor. Circular dichroism measurements show that the structural transition of heparin cofactor II during denaturation is distinctly biphasic, yielding midpoints at 0.6 and 2.6 M GdmCl. Functional protease inhibitory activities are affected to the same extent following denaturation-renaturation at various GdmCl concentrations. The results indicate that arginyl residues are critical for both antithrombin and heparin binding activities. In contrast, tryptophanyl residues are apparently not essential for heparin-dependent interactions. The results also suggest that heparin cofactor II contains two structural domains which unfold at different GdmCl concentrations.

Arginine residues are critical for the heparin-cofactor activity of antithrombin III

A dilution/quench technique was used to monitor the time course of chemical modification on the heparin-cofactor (a) and progressive thrombin-inhibitory (b) activities of human antithrombin III. Treatment of antithrombin III (AT III) with 2,4,6-trinitrobenzenesulphonate at pH 8.3 and 25 degrees C leads to the loss of (a) at 60-fold more rapid rate than the loss of (b). This is consistent with previous reports [Rosenberg & Damus (1973) J. Biol. Chem. 248, 6490-6505; Pecon & Blackburn (1984) J. Biol. Chem. 259, 935-938] that lysine residues are involved in the binding of heparin to AT III, but not in thrombin binding. Treatment of AT III with phenylglyoxal at pH 8.3 and 25 degrees C again leads to a more rapid loss of (a) than of (b), with the loss of the former proceeding at a 4-fold faster rate. The presence of heparin during modification with phenylglyoxal significantly decreases the rate of loss of (a). Full loss of (a) correlates with the modification of seven arginine residues per inhibitor molecule, whereas loss of (b) does not commence until approximately four arginine residues are modified and is complete upon the modification of approximately eleven arginine residues per inhibitor molecule. This suggests that (the) arginine residue(s) in AT III are involved in the binding of heparin in addition to the known role of Arg-393 at the thrombin-recognition site [Rosenberg & Damus (1973) J. Biol. Chem. 248, 6490-6505; Jörnvall, Fish & Björk (1979) FEBS Lett. 106, 358-362].

Changes of the proteinase binding properties and conformation of bovine alpha 2-macroglobulin on cleavage of the thio ester bonds by methylamine

Cleavage of the thio ester bonds of human alpha2-macroglobulin (alpha 2M) by methylamine leads to an extensive conformational change and to inactivation of the inhibitor. In contrast, cleavage of these bonds in bovine alpha 2M only minimally perturbs the hydrodynamic volume of the protein [Dangott, L. J., & Cunningham, L. W. (1982) Biochem. Biophys. Res. Commun. 107, 1243-1251], as well as its spectroscopic properties, as analyzed by ultraviolet difference spectroscopy, circular dichroism, and fluorescence in this work. A conformational change analogous to that undergone by human alpha 2M thus does not occur in the bovine inhibitor. However, changes of several functional properties of bovine alpha 2M are induced by the amine. The apparent stoichiometry of inhibition of trypsin thus is reduced from about 1.2 to about 0.7 mol of enzyme/mol of inhibitor. In spite of this decrease, the interaction with the proteinase induces similar conformational changes in methylamine-treated alpha 2M as in intact alpha 2M, as revealed by spectroscopic analyses, indicating that the mode of binding of the proteinase to the inhibitor is essentially unperturbed by thio ester bond cleavage. The reaction with methylamine also greatly increases the sensitivity of bovine alpha 2M to proteolysis by trypsin at sites other than the "bait" region. Moreover, the second-order rate constant for the reaction with thrombin is reduced by about 10-fold. These results indicate that the thio ester bonds of bovine alpha 2M, although not required per se for the binding of proteinases, nevertheless are responsible for maintaining certain structural features of the inhibitor that are of importance for full activity.

Denaturation behavior of antithrombin in guanidinium chloride. Irreversibility of unfolding caused by aggregation

The structural stability of the protease inhibitor antithrombin from bovine plasma was examined as a function of the concentration of guanidinium chloride (GdmCl). A biphasic unfolding curve at pH 7.4, with midpoints for the two phases at 0.8 and 2.8 M GdmCl, was measured by far-ultraviolet circular dichroism. Spectroscopic and hydrodynamic analyses suggest that the intermediate state which exists at 1.5 M GdmCl involves a partial unfolding of the antithrombin molecule that exposes regions of the polypeptide chain through which slow, intermolecular association subsequently takes place. The partially unfolded molecule can be reversed to its fully functional state only before the aggregation occurs. Upon return of the aggregated state to dilute buffer, the partially unfolded antithrombin remains aggregated and does not regain the spectroscopic properties, thrombin-inhibitory activity, or heparin affinity of the native inhibitor. This behavior indicates that the loss of the functional properties of the proteins is caused by the macromolecular association. Comparative experiments gave similar results for the human inhibitor. Analyses of bovine antithrombin in 6 M GdmCl indicated that the second transition reflects the total unfolding of the protein to a disulfide-cross-linked random coil. This transition is spectroscopically reversible; however, on further reversal to dilute buffer, the molecules apparently are trapped in the partially unfolded, aggregated, intermediate state. The results are consistent with the existence of two separate domains in antithrombin which unfold at different concentrations of GdmCl but do not support the contention that the thrombin-binding and heparin-binding regions of the protein are located in different domains [Villanueva, G. B., & Allen, N. (1983) J. Biol. Chem. 258, 14048-14053].

The role of tryptophan residues in heparin-antithrombin interactions

A single tryptophan residue on antithrombin has been modified with dimethyl-(2-hydroxy-5-nitrobenzyl)sulfonium bromide. This alteration led to a 500-fold reduction in the heparin-dependent acceleration of thrombin-modified antithrombin interactions, as well as a 10-fold decrease in the avidity of the modified protease inhibitor for mucopolysaccharide. Preincubation of antithrombin with the octasaccharide binding domain of heparin prior to treatment with dimethyl-(2-hydroxy-5-nitrobenzyl) sulfonium bromide was able to suppress modification of the critical tryptophan and preserve the functional capacities of the protease inhibitor. Fluorescence quenching experiments indicated that the modifiable tryptophan groups of antithrombin were exposed to the solvent environment. Based upon these data, it was proposed that the loss of "heparin cofactor" activity of antithrombin must be predominantly due to an inability of the modified protease inhibitor to undergo a conformational transition required for mucopolysaccharide-dependent "activation" of the macromolecule.

Characterization of gamma-aminobutyric acid-benzodiazepine receptor complexes by protection against inactivation by group-specific reagents

The chemical topography of the gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptors was investigated in a thoroughly washed cortical membrane preparation of the rat. Chemical modification by several amino- and tyrosyl-selective reagents and the protection from it by direct and allosteric ligands of the GABA-BZ receptor complex were used to identify the residues at the binding sites. Inhibition of specific GABA binding by p-diazobenzenesulfonic acid (DSA), tetranitromethane (TNM), and N-acetylimidazole and the selective and complete protection from it by GABA and muscimol suggest the presence of a tyrosine residue at the GABAA site. TNM, like DSA, selectively decreased the number of the low-affinity GABA receptors, and this could be completely protected only by GABA concentrations that can saturate the low-affinity sites. TNM pretreatment also abolished the muscimol enhancement of [3H]diazepam binding, which suggests that the low-affinity GABA receptor sites are responsible for this enhancement. Inhibition of GABA binding by pyridoxal-5-phosphate (PLP) and the selective protection by GABA and muscimol support the presence of a lysine residue at the GABAA receptor site. Complete and selective protection from diethylpyrocarbonate (DEP) inhibition of [3H]diazepam binding by flurazepam suggests the presence of a histidine residue at the BZ site. Flurazepam selectively protected from inhibition of [3H]diazepam binding by N-bromosuccinimide and N-acetylimidazole, but not that by DSA and TNM, which does not allow a unanimous conclusion regarding the presence of tyrosine or tryptophan residues at the BZ site.

Antithrombin in vertebrate species: conservation of the heparin-dependent anticoagulant mechanism

Heparin is thought to regulate the rate of mammalian blood clotting by enhancing the activity of antithrombin, an inhibitor of coagulation enzymes. The present study establishes that this same inhibitor is present in the blood plasma of each of the terrestrial vertebrate groups including mammals, birds, reptiles, and amphibians. In each case, an inhibitor with remarkably similar properties to human antithrombin was isolated by affinity chromatography on immobilized porcine heparin. The purified vertebrate inhibitors all show the following physical and functional homologies to human antithrombin: (i) heparin-enhanced inhibition of both bovine thrombin and human Factor Xa, (ii) molecular masses of approximately 60,000, and (iii) heparin-induced increases in ultraviolet fluorescence. Also, the heparin-binding interaction of vertebrate antithrombins is highly selective with each demonstrating the same rigid specificity for heparin species fractionated on the basis of their affinity for human antithrombin. This common ability of vertebrate antithrombins to discriminate among heparins is accomplished by a nearly unvarying equilibrium binding constant for the high-affinity heparin species. Thus, the present results suggest that the anticoagulant relationship of heparin and antithrombin was established at an early point in the evolution of the coagulation system and has been highly conserved since that time.

Isolation and characterization of a hereditary abnormal antithrombin III 'Antithrombin III Toyama'

A hereditary abnormal antithrombin III (AT-III) 'Antithrombin III Toyama' was purified from the plasma of a patient with recurrent thrombophlebitis by a procedure involving barium chloride and ammonium sulfate fractionations, affinity chromatography on anti-AT-III-Sepharose gel, and DEAE-Sephadex chromatography. Purified abnormal AT-III was shown to be the same as normal one in the molecular size, having the same molecular weight, amino-terminal sequence and carboxy-terminal amino acid. Abnormal AT-III gave the same UV spectrum as normal AT-III and both proteins were immunologically identical. Abnormal AT-III, however, showed the different electrophoretic mobility on agarose gel electrophoresis and immunoelectrophoresis. Abnormal AT-III was more electronegative than normal one, before and after a neuraminidase digestion of both proteins. These results suggest that in antithrombin III Toyama an amino acid residue at the heparin-binding site has been replaced by less basic or more acidic one which has no ability to interact with heparin, resulting in a loss of heparin cofactor activity of this protein.