Circular dichroism spectroscopy of heparin-antithrombin interactions

Near-Infrared Fluorescent Dye-Decorated Nanocages to Form Grenade-like Nanoparticles with Dual Control Release for Photothermal Theranostics and Chemotherapy

Recently, nanoparticles (NPs) have been widely investigated for delivery of anticancer drugs. Here, a dual control drug-release modality was developed that uses naturally occurring protein apoferritin loaded with doxorubicin (DOX) and ADS-780 near-infrared (NIR) fluorescent dye-decorated NPs (ADNIR NPs). ADNIR NPs act as a grenade to detonate the targeted tumor site following laser irradiation (photothermal therapy, PTT) and explode into cluster warheads (apoferritin-loaded DOX nanocages, AF-DOX NCs) that further destroy the tumor cells (chemotherapy). Light was shown to disrupt the grenade-like structure of NPs to release AF-DOX NCs as well as DOX from NCs in low-pH intercellular environments. In vitro and in vivo studies showed that the structure of AF-DOX NCs was disassembled to release DOX, which then killed the cancer cells in organelles with acidic environments. In vivo studies showed that the ADNIR NP-decorated with NIR dye facilitated tracking of the accumulated NPs at the tumor site using an IVIS imaging system. Overall, targeted ADNIR NPs with dual-release mechanisms were developed for use in photothermal theranostic and chemotherapy. This modality has high potential for application in cancer treatment and clinical translation for drug delivery and imaging.

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.

The allosteric mechanism of activation of antithrombin as an inhibitor of factor IXa and factor Xa: heparin-independent full activation through mutations adjacent to helix D

Allosteric conformational changes in antithrombin induced by binding a specific heparin pentasaccharide result in very large increases in the rates of inhibition of factors IXa and Xa but not of thrombin. These are accompanied by CD, fluorescence, and NMR spectroscopic changes. X-ray structures show that heparin binding results in extension of helix D in the region 131-136 with coincident, and possibly coupled, expulsion of the hinge of the reactive center loop. To examine the importance of helix D extension, we have introduced strong helix-promoting mutations in the 131-136 region of antithrombin (YRKAQK to LEEAAE). The resulting variant has endogenous fluorescence indistinguishable from WT antithrombin yet, in the absence of heparin, shows massive enhancements in rates of inhibition of factors IXa and Xa (114- and 110-fold, respectively), but not of thrombin, together with changes in near- and far-UV CD and (1)H NMR spectra. Heparin binding gives only ∼3-4-fold further rate enhancement but increases tryptophan fluorescence by ∼23% without major additional CD or NMR changes. Variants with subsets of these mutations show intermediate activation in the absence of heparin, again with basal fluorescence similar to WT and large increases upon heparin binding. These findings suggest that in WT antithrombin there are two major complementary sources of conformational activation of antithrombin, probably involving altered contacts of side chains of Tyr-131 and Ala-134 with core hydrophobic residues, whereas the reactive center loop hinge expulsion plays only a minor additional role.

New targets for glycosaminoglycans and glycosaminoglycans as novel targets

Biological functions of a variety of proteins are mediated via their interaction with glycosaminoglycans (GAGs). The structural diversity within the wide GAG landscape provides individual interaction sites for a multitude of proteins involved in several pathophysiological processes. This 'GAG angle' of such proteins as well as their specific GAG ligands give rise to novel therapeutic concepts for drug development. Current glycomic technologies to elucidate the glycan structure-function relationships, methods to investigate the selectivity and specificity of glycan-protein interactions and existing therapeutic approaches to interfere with GAG-protein interactions are discussed.

Potentiation of C1-esterase inhibitor by heparin and interactions with C1s protease as assessed by surface plasmon resonance

BACKGROUND

Human C1-esterase inhibitor (C1-INH) is a multifunctional plasma protein with a wide range of inhibitory and non-inhibitory properties, mainly recognized as a key down-regulator of the complement and contact cascades. The potentiation of C1-INH by heparin and other glycosaminoglycans (GAGs) regulates a broad spectrum of C1-INH activities in vivo both in normal and disease states. SCOPE OF RESEARCH: We have studied the potentiation of human C1-INH by heparin using Surface Plasmon Resonance (SPR), circular dichroism (CD) and a functional assay. To advance a SPR for multiple-unit interaction studies of C1-INH we have developed a novel (consecutive double capture) approach exploring different immobilization and layout.

MAJOR CONCLUSIONS

Our SPR experiments conducted in three different design versions showed marked acceleration in C1-INH interactions with complement protease C1s as a result of potentiation of C1-INH by heparin (from 5- to 11-fold increase of the association rate). Far-UV CD studies suggested that heparin binding did not alter C1-INH secondary structure. Functional assay using chromogenic substrate confirmed that heparin does not affect the amidolytic activity of C1s, but does accelerate its consumption due to C1-INH potentiation.

GENERAL SIGNIFICANCE

This is the first report that directly demonstrates a significant acceleration of the C1-INH interactions with C1s due to heparin by using a consecutive double capture SPR approach. The results of this study may be useful for further C-INH therapeutic development, ultimately for the enhancement of current C1-INH replacement therapies.

Multiple mechanisms for exogenous heparin modulation of vascular endothelial growth factor activity

Heparin and heparin-like molecules are known to modulate the cellular responses to vascular endothelial growth factor-A (VEGF-A). In this study, we investigated the likely mechanisms for heparin's influence on the biological activity of VEGF-A. Previous studies have shown that exogenous heparin's effects on the biological activity of VEGF-A are many and varied, in part due to the endogenous cell-surface heparan sulfates. To circumvent this problem, we used mutant endothelial cells lacking cell-surface heparan sulfates. We showed that VEGF-induced cellular responses are dependent in part on the presence of the heparan sulfates, and that exogenous heparin significantly augments VEGF's cellular effects especially when endogenous heparan sulfates are absent. Exogenous heparin was also found to play a cross-bridging role between VEGF-A(165) and putative heparin-binding sites within its cognate receptor, VEGFR2 when they were examined in isolation. The cross-bridging appears to be more dependent on molecular weight than on a specific heparin structure. This was confirmed by surface plasmon resonance binding studies using sugar chips immobilized with defined oligosaccharide structures, which showed that VEGF-A(165) binds to a relatively broad range of sulfated glycosaminoglycan structures. Finally, studies of the far-UV circular dichroism spectra of VEGF-A(165) showed that heparin can also modulate the conformation and secondary structure of the protein.

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.

Molecular cloning and expression of mouse and human cDNAs encoding heparan sulfate D-glucosaminyl 3-O-sulfotransferase

The cellular rate of anticoagulant heparan sulfate proteoglycan (HSPGact) generation is determined by the level of a kinetically limiting microsomal activity, HSact conversion activity, which is predominantly composed of the long sought heparan sulfate D-glucosaminyl 3-O-sulfotransferase (3-OST) (Shworak, N. W., Fritze, L. M. S., Liu, J., Butler, L. D., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27063-27071; Liu, J., Shworak, N. W., Fritze, L. M. S., Edelberg, J. M., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27072-27082). Mouse 3-OST cDNAs were isolated by proteolyzing the purified enzyme with Lys-C, sequencing the resultant peptides as well as the existing amino terminus, employing degenerate polymerase chain reaction primers corresponding to the sequences of the peptides as well as the amino terminus to amplify a fragment from LTA cDNA, and utilizing the resultant probe to obtain full-length enzyme cDNAs from a lambda Zap Express LTA cDNA library. Human 3-OST cDNAs were isolated by searching the expressed sequence tag data bank with the mouse sequence, identifying a partial-length human cDNA and utilizing the clone as a probe to isolate a full-length enzyme cDNA from a lambda TriplEx human brain cDNA library. The expression of wild-type mouse 3-OST as well as protein A-tagged mouse enzyme by transient transfection of COS-7 cells and the expression of both wild-type mouse and human 3-OST by in vitro transcription/translation demonstrate that the two cDNAs directly encode both HSact conversion and 3-OST activities. The mouse 3-OST cDNAs exhibit three different size classes because of a 5'-untranslated region of variable length, which results from the insertion of 0-1629 base pairs (bp) between residues 216 and 217; however, all cDNAs contain the same open reading frame of 933 bp. The length of the 3'-untranslated region ranges from 301 to 430 bp. The nucleic acid sequence of mouse and human 3-OST cDNAs are approximately 85% similar, encoding novel 311- and 307-amino acid proteins of 35,876 and 35,750 daltons, respectively, that are 93% similar. The encoded enzymes are predicted to be intraluminal Golgi residents, presumably interacting via their C-terminal regions with an integral membrane protein. Both 3-OST species exhibit five potential N-glycosylation sites, which account for the apparent discrepancy between the molecular masses of the encoded enzyme (approximately 34 kDa) and the previously purified enzyme (approximately 46 kDa). The two 3-OST species also exhibit approximately 50% similarity with all previously identified forms of the heparan biosynthetic enzyme N-deacetylase/N-sulfotransferase, which suggests that heparan biosynthetic enzymes share a common sulfotransferase domain.

Lysine residue 114 in human antithrombin III is required for heparin pentasaccharide-mediated activation

Recombinant native antithrombin III (ATIII) and two genetic variants with glutamine substitutions at lysine residues 114 and 139 were expressed in insect cells using a baculovirus-driven expression system. The purified proteins were used to evaluate the potential role(s) of these residues in the pentasaccharide-mediated activation of ATIII. The second order rate constants for the inhibition of factor Xa by both of the genetic variants were nearly identical to those of recombinant native ATIII, indicating that the glutamine substitutions did not result in serious protein conformational changes. The glutamine substitution at lysine 139 had no effect on the pentasaccharide-mediated activation of ATIII toward factor Xa. In contrast, lysine 114 was found to be critical in the activation of ATIII toward factor Xa. No activation was observed, even at a pentasaccharide concentration 10 times higher than that required to activate recombinant native ATIII. These data are the first to demonstrate a pivotal role for lysine 114 in the pentasaccharide-mediated activation of ATIII.

Purification of heparan sulfate D-glucosaminyl 3-O-sulfotransferase

The cellular generation of proteoglycans with anticoagulant heparan sulfate (HSPGact) is determined by microsomal "HSact conversion activity" that functions in concert with the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to convert nonanticoagulant heparan sulfate (HSinact) to anticoagulant heparan sulfate (HSact) (Shworak, N. W., Fritze, L. M. S., Liu, J., Butler, L. D., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27063-27071). Suspension cultures of L-33(+) cells in serum-free medium produce HSPGact and secrete HSact conversion activity. The secreted protein exhibiting HSact conversion activity was isolated by subjecting large volumes of conditioned suspension culture medium to heparin-AF Toyopearl affinity chromatography, Mono Q-FPLC, TSK SW3000-HPLC, and 3',5'-ADP-agarose affinity chromatography. The final product was purified approximately 700,000-fold relative to cellular material with a 5% overall recovery of HSact conversion activity. The isolated protein migrated on SDS-polyacrylamide gel electrophoresis as a broad band of Mr = 46,000 and co-migrated on nondenaturing acidic pH gel electrophoresis with HSact conversion activity. The purified component was identified as heparan sulfate D-glucosaminyl 3-O-sulfotransferase because it transferred sulfate from [35S]PAPS to the 3-O-position of D-glucosamine and D-glucosamine 6-O-sulfate of HSact precursor and HSinact precursor to produce nearly equivalent amounts of labeled HSact and HSinact. The exhaustive modification of wild-type LTA cell [35S]HS with either microsomal HSact conversion activity or purified enzyme increased HSact content from 9 to approximately 36%, which indicates that microsomal HSact conversion activity predominantly reflects the level of a 3-O-sulfotransferase that converts HSact precursor into HSact. The kinetic parameters of purified 3-O-sulfotransferase were determined for modification of HSact precursor and HSinact precursor. The apparent KM* and Vmax* with respect to PAPS concentration for sulfation of HSact precursor and HSinact precursor were 2.4 microM and 23 fmol of sulfate/min/ng of enzyme and 2.1 microM and 38 fmol of sulfate/min/ng of enzyme, respectively. There was substrate inhibition of the sulfation reaction at elevated HS concentration. The apparent KM* and Vmax* with respect to GAG concentration for sulfation of HSact precursor and HSinact precursor were 16 nM and 120 fmol of sulfate/min/ng of enzyme and 17 nM and 240 fmol of sulfate/min/ng of enzyme, respectively. The observation that purified 3-O-sulfotransferase catalyzes sulfation of HSact precursor and HSinact precursor in conjunction with a documented discordant regulation of 3-O-sulfate content in HSinact and HSact suggests that two discrete forms of the enzyme may exist.

Requirement of lysine residues outside of the proposed pentasaccharide binding region for high affinity heparin binding and activation of human antithrombin III

Variant forms of human antithrombin III with glutamine or threonine substitutions at Lys114, Lys125, Lys133, Lys136, and Lys139 were expressed in insect cells to evaluate their roles in heparin binding and activation. Recombinant native ATIII and all of the variants had very similar second order rate constants for thrombin inhibition in the absence of heparin, ranging from 1.13 x 10(5) M-1min-1 to 1.66 x 10(5) M-1min-1. Direct binding studies using 125I-flouresceinamine-heparin yielded a Kd of 6 nM for the recombinant native ATIII and K136T, whereas K114Q and K139Q bound heparin so poorly that a Kd could not be determined. K125Q had a moderately reduced affinity. Heparin binding affinity correlated directly with heparin cofactor activity. Recombinant native ATIII was nearly identical to plasma-purified ATIII, whereas K114Q and K139Q were severely impaired in heparin cofactor activity. K125Q and K136T were only slightly impaired. Based on these data, Lys114 and Lys139, which are outside of the putative pentasaccharide binding site, play pivotal roles in the high affinity binding of heparin to ATIII and the activation of thrombin inhibitory activity.

Mechanism of acceleration of antithrombin-proteinase reactions by low affinity heparin. Role of the antithrombin binding pentasaccharide in heparin rate enhancement

The role of the sequence-specific pentasaccharide region of high affinity heparin (HAH) in heparin acceleration of antithrombin-proteinase reactions was elucidated by determining the accelerating mechanism of low affinity heparin (LAH) lacking this sequence. LAH was shown to be free of HAH (< 0.001%) from the lack of exchange of added fluorescein-labeled HAH into LAH after separating the polysaccharides by antithrombin-agarose chromatography. Fluorescence titrations showed that LAH bound to antithrombin with a 1000-fold weaker affinity (KD 19 +/- 6 microM) and 5-6-fold smaller fluorescence enhancement (8 +/- 3%) than HAH. LAH accelerated the antithrombin-thrombin reaction with a bell-shaped dependence on heparin concentration resembling that of HAH, but with the bell-shaped curve shifted to approximately 100-fold higher polysaccharide concentrations and with a approximately 100-fold reduced maximal accelerating effect. Rapid kinetic studies indicated these differences arose from a reverse order of assembly of an intermediate heparin-thrombin-antithrombin ternary complex and diminished ability of LAH to bridge antithrombin and thrombin in this complex, as compared to HAH. By contrast, LAH and HAH both accelerated the antithrombin-factor Xa reaction with a simple saturable dependence on heparin or inhibitor concentrations which paralleled the formation of an antithrombin-heparin binary complex. The maximal accelerations of the two heparins in this case correlated with the inhibitor fluorescence enhancements induced by the polysaccharides, consistent with the accelerations arising from conformational activation of antithrombin. 1H NMR difference spectroscopy of antithrombin complexes with LAH and HAH and competitive binding studies were consistent with LAH accelerating activity being mediated by binding to the same site on the inhibitor as HAH. These results demonstrate that LAH accelerates antithrombin-proteinase reactions by bridging and conformational activation mechanisms similar to those of HAH, with the reduced magnitude of LAH accelerations resulting both from a decreased antithrombin affinity and the inability to induce a full activating conformational change in the inhibitor.

Lysine-heparin interactions in antithrombin. Properties of K125M and K290M,K294M,K297M variants

Lysine residues in two different regions of antithrombin have been proposed to be involved in heparin binding and heparin-mediated acceleration of proteinase inhibition. Lysine 125 has been implicated as an essential heparin binding residue from chemical modification studies [Peterson, C. B., Noyes, C. M., Pecon, J. M., Church, F. C., & Blackburn, M. N. (1987) J. Biol. Chem. 262, 8061-8065] whereas lysines 290, 294, and 297 have been proposed from model building studies to constitute the heparin binding site [Villanueva, G. B. (1984) J. Biol. Chem. 259, 2531-2536]. To evaluate both of these proposals, we have prepared two variant human antithrombins, K125M and K290M,K294M,K297M, in which these lysines have been changed by site-directed mutagenesis to methionines. The K290M,K294M,K297M variant had properties very similar to those of wild-type recombinant antithrombin in affinity for heparin, and in rates of inhibition of thrombin and factor Xa. In contrast, K125M antithrombin had reduced affinity for both heparin pentasaccharide and full-length heparin, corresponding to delta delta Gs of 3.1 and 2.0 kcal mol-1, respectively. However, this variant was still able to inhibit both thrombin and factor Xa. Whereas the rate of thrombin inhibition was similar to that of wild-type antithrombin, the rate of factor Xa inhibition was enhanced between 2- and 3-fold, suggesting a role for lysine 125 in the allosteric coupling between the heparin binding site and the reactive center region. At saturation with either heparin pentasaccharide or full-length high-affinity heparin, the rates of inhibition of both proteinases were similar to those of wild-type antithrombin for both the K125M and K290M,K294M,K297M variants.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of conformational change in serpin structure and function

Serpins are members of a family of structurally related protein inhibitors of serine proteinases, with molecular masses between 40 and 100kDa. In contrast to other, simpler, proteinase inhibitors, they may interact with proteinases as inhibitors, as substrates, or as both. They undergo conformational interconversions upon complex formation with proteinase, upon binding of some members to heparin, upon proteolytic cleavage at the reactive center, and under mild denaturing conditions. These conformational changes appear to be critical in determining the properties of the serpin. The structures and stabilities of these various forms may differ significantly. Although the detailed structural changes required for inhibition of proteinase have yet to be worked out, it is clear that the serpin does undergo a major conformational change. This is in contrast to other, simpler, families of protein inhibitors of serine proteinases, which bind in a substrate-like or product-like manner. Proteolytic cleavage of the serpin can result in a much more stable protein with new biological properties such as chemo-attractant behaviour. These structural transformations in serpins provide opportunities for regulation of the activity and properties of the inhibitor and are likely be important in vivo, where serpins are involved in blood coagulation, fibrinolysis, complement activation and inflammation.

New approaches for defining sequence specific synthesis of heparan sulfate chains

Mammalian cells synthesize heparan sulfate proteoglycans (HSPG) which consist of core proteins with covalently linked glycosaminoglycans (GAGs) of 50-150 disaccharide units. The GAGs exhibit great structural diversity which arise from differing arrangements of alternate disaccharide units. It has been hypothesized that HSPG may be involved in regulating the most basic aspects of cell biologic systems such as adhesion, proliferation and differentiation. However, considerable doubt exists about the specific nature of the above interactions because of a failure to isolate GAGs of unique monosaccharide sequence with appropriate biologic activities. We have demonstrated that mouse LTA cells synthesize cell surface heparan sulfate proteoglycans with regions of defined monosaccharide sequence that specifically interact with antithrombin (HSPGact). However, it remains unclear how HSPGact can be generated by a biosynthetic pathway with no simple template for directing the ordered assembly of monosaccharide units. To examine this issue, we treated LTA cells with ethylmethane sulfonate and then identified mutants that exhibit decreased antithrombin binding to heparan sulfate chains but possess no gross defects in glycosaminoglycan biosynthesis. After screening 40,000 colonies, we isolated 7 stable mutants which synthesize 8-27% of the wild type HSPGact but produce normal amounts of other HSPG. These mutants are recessive in nature, and fall into at least two different complementation groups. The delineation of the molecular basis of these defects should greatly improve our understanding of how cells synthesize HSPG with regions of defined monosaccharide sequence.

One- and two-dimensional 13C-n.m.r. characterization of two series of oligosaccharides derived from porcine intestinal mucosal heparin by degradation with heparinase

Two tetrasaccharides, two hexasaccharides, and a disaccharide have been purified from heparinase digests of porcine intestinal mucosal heparin in sufficient quantities to permit 13C-n.m.r. characterization of the species. The two tetrasaccharides are the sulfated iduronic acid-containing 4en-HexpA2SO3-(1----4)-alpha-D-GlcpNSO3;6SO3-(1- ---4)-alpha-L- IdopA2SO3-(1----4)-D-GlcpNSO3;6SO3 and the non-sulfated glucuronic acid-containing 4en-HexpA2SO3-(1----4)-alpha-D-GlcpNSO3;6SO3-(1- ---4)-beta-D-GlcpA-(1----4)-D- GlcpNSO3;6SO3. The two hexasaccharides are related to the two tetrasaccharides by the insertion of alpha-linked L-IdopA2SO3-(1----4)-D-GlcpNSO3;6SO3 after the non-reducing end sulfated glucosamine residue. The disaccharide is 4en-HexpA2SO3-(1----4)-alpha-D-GlcpNSO3;6SO3. The disaccharide, together with each of the iduronate-containing oligosaccharides, form one series of related di-, tetra-, and hexa-saccharides, while the disaccharide together with the glucuronate-containing oligosaccharides form a second series. Using inverse detection as a means of increasing sensitivity, two-dimensional n.m.r. 13C-1H heterocorrelation spectra have been obtained for all five oligosaccharides. The use of two-dimensional heterocorrelation n.m.r. spectroscopy offers a much less ambiguous means of making 13C resonance assignments than do traditional one-dimensional methods, while the use of inverse detection gives both greater sensitivity than direct detection, as well as values for the one-bond 13C-1H coupling constants. From a knowledge of the assignments of resonances in the 1H spectra of these species, it has been possible to assign almost all of the 13C resonances of these five oligosaccharides. Some corrections to previously published assignments for the tetrasaccharides have been made. In addition, one-bond 13C-1H coupling constant data have been obtained for all of the anomeric protons.

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).

New approaches for defining the molecular basis of anticoagulantly active heparan sulfate production

Mammalian cells synthesize heparan sulfate proteoglycans, which consist of core proteins with covalently linked glycosaminoglycans of 50-150 disaccharide units. The GAGs exhibit great structural diversity, which arises from differing arrangements of alternate disaccharide units. It has been hypothesized that HSPG may be involved in regulating the most basic aspects of cell biologic systems, such as adhesion, proliferation, and differentiation. However, considerable doubt exists about the specific nature of the above interactions because of a failure to isolate GAGs of unique monosaccharide sequence with appropriate biologic activities. We have demonstrated that mouse LTA cells synthesize cell surface heparan sulfate proteoglycans with regions of defined monosaccharide sequence that specifically interact with antithrombin (HSPGact). However, it remains unclear how HSPGact can be generated by a biosynthetic pathway with no simple template for directing the ordered assembly of monosaccharide units. To examine this issue, we treated LTA cells with ethylmethane sulfonate and then identified mutants that exhibit decreased antithrombin binding to heparan sulfate chains but possess no gross defects in glycosaminoglycan biosynthesis. After screening 40,000 colonies, we isolated seven stable mutants that synthesize 8-27% of the wild type HSPGact but produce normal amounts of other HSPG. These mutants are recessive in nature and fall into at least two different complementation groups. The delineation of the molecular basis of these defects should greatly improve our understanding of how cells synthesize HSPG with regions of defined monosaccharide sequence.

Cell mutants defective in synthesizing a heparan sulfate proteoglycan with regions of defined monosaccharide sequence

We have demonstrated that mouse LTA cells synthesize cell-surface heparan sulfate proteoglycans (HSPGs) with regions of defined monosaccharide sequence that specifically interact with antithrombin (HSPGact). It remains unclear how HSPGact can be generated by a biosynthetic pathway with no simple template for directing the ordered assembly of monosaccharide units. To examine this issue, we treated LTA cells with ethyl methanesulfonate and then isolated seven stable mutants that synthesize only 8-27% of the wild-type HSPGact but produce normal amounts of other HSPGs. These mutants are recessive in nature and fall into at least two different complementation groups. The delineation of the molecular basis of these defects should help to elucidate the manner by which cells synthesize HSPGs with regions of defined monosaccharide sequence.

Effect of a heparan sulphate with high affinity for antithrombin III upon inactivation of thrombin and coagulation factor Xa

The kinetics of inhibition of human alpha-thrombin and coagulation Factor Xa by antithrombin III were examined under pseudo-first-order reaction conditions as a function of the concentration of heparan sulphate with high affinity for antithrombin III. The maximum observed second-order rate constant was, for the antithrombin III-thrombin reaction, 1.2 x 10(9) M-1.min-1 compared with 2.4 x 10(9) M-1.min-1 in the presence of high-affinity heparin. However, the maximum rate was catalysed by much higher concentrations of heparan sulphate (1.3 microM) than of heparin (0.025 microM). Differences were also observed in the maximal acceleration of the antithrombin III-Factor Xa interaction: 1.2 x 10(9) M-1.min-1 at 0.2 microM-heparin sulphate compared with 2.2 x 10(9) M-1.min-1 at 0.04 microM-heparin. The differences in properties of heparan sulphate and heparin were analysed by using the random bi-reactant model of heparin action [Griffith (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 5460-5464]. It was observed that the apparent binding affinity for thrombin was higher for heparan sulphate (180 nM) than for heparin (14 nM). The rate constant for transformation of the antithrombin III-Factor Xa complex into irreversible product differed between heparan sulphate (96 min-1) and heparin (429 min-1). These properties of the high-affinity heparan sulphate may be of importance in consideration of a putative role in the control of intravascular haemostasis.

The molecular-mass dependence of dextran sulfate enhancement of inactivation of thrombin and fibrinogen and on factor Xa neutralization by antithrombin III

To study molecular-mass dependence of dextran sulfate (DS) for interactions with several plasma proteins, a commercial preparation of the sulfated polysaccharide was fractionated by gel filtration chromatography into six subfractions with relatively different molecular masses. Simple two-component systems were available to measure the interactions of the proteins with the subfractions of DS. These were done to determine the rates of time-dependent changes in intrinsic fluorescence of thrombin and fibrinogen, and the enzyme inactivation in the presence of DS. Their interactions were also confirmed in three-component systems, in which the interactions of DS with thrombin and fibrinogen were measured by the displaced binding by FTC-heparin, and DS-enhanced proteolysis by chymotrypsin, respectively. Moreover, the neutralization of factor Xa by antithrombin III (AT III) depended on the molecular mass of DS. All the results obtained indicate that most of the general interactions of thrombin, fibrinogen, and probably AT III increased with increasing molecular mass of DS.

Monoclonal antibodies against antithrombin III. Identification of their epitopes and effects on antithrombin III activities

Four monoclonal antibodies with distinct epitopes were prepared against antithrombin III. None of them is directed against the heparin-binding region nor the active site, yet two mAb namely A36 and B108, interfere with antithrombin III inhibition of thrombin. The epitope of monoclonal antibody A36 is located within amino acid residues 1-393, at a site different from the active site since it recognizes antithrombin III and antithrombin-III-thrombin complexes with the same affinity. A36 partially prevents the intrinsic antithrombin III activity and has no effect on the heparin-enhanced antithrombin III activity when added to the antithrombin-III--heparin complex. If A36 is first reacted with antithrombin III and then heparin is added to the reaction mixture, A36 fixes the conformation of antithrombin III so that heparin binds to antithrombin III, but is not able to induce the conformational change in the antithrombin III molecule required for the enhanced activity. The epitope for monoclonal antibody B108 is located within residues 282-393, close to the active site. It does not recognize antithrombin-III-thrombin complexes by solid-phase radioimmunoassay. Its binding to antithrombin III induces a conformational change that enhances antithrombin III activity in a manner that resembles the heparin effect, but its effect is additive to the heparin effect, since when it was added to a reaction mixture which contained a saturating amount of heparin, inhibition of thrombin was enhanced. The epitope for monoclonal antibody A5 is located within residues 1-393, and its recognition of antithrombin III or antithrombin-III-thrombin is strongly dependent on the integrity of the disulfide bonds. A5 has no effect on antithrombin III activities. The epitope for monoclonal antibody A10 is well defined within a narrow range of 55 amino acid residues, 339-393, on the antithrombin III molecule, close to the active site, yet it has no effect on antithrombin III inhibitory activity. These monoclonal antibodies may be developed for various diagnostic or clinical purposes and offer a powerful tool for studying the conformational changes and structure/activity relationships in the antithrombin III molecule.

Interaction of heparin and antithrombin III. The role of O-sulfate groups

A synthetic pentasaccharide corresponding to the sequence involved in heparin for binding and activation of antithrombin III contains eight sulfate groups. The role of some of them in the interaction with the protein has been demonstrated through the study of fragments obtained from heparin. An approach based on the total chemical synthesis of heparin fragments allows us to provide new information on the O-sulfate groups borne by the iduronic acid and the glucosamine units that constitute the reducing-end disaccharide of the above pentasaccharide sequence. Although not strictly necessary for a weak interaction to take place, these two sulfates co-operate to express maximal activity. This suggests that they belong to a secondary sub-region of interaction with antithrombin III, the primary one being accounted for by other critical parts of the structure and particularly the trisaccharide sequence placed at the non-reducing end of the pentasaccharide.

Bovine endothelial cell plasminogen activator inhibitor. Purification and heat activation

A plasminogen activator inhibitor (PAI) was purified from bovine endothelial cell conditioned medium by a simple procedure in the absence of protein denaturant. The yield was 2.2 mg from 1.61 conditioned medium in a typical experiment. The purified inhibitor showed a single band on sodium dodecyl sulfate/polyacrylamide gel electrophoresis and reverse fibrin autography with an apparent molecular mass of 45 kDa. The amino-terminal 40-amino-acid sequence was determined and found to be 70% similar to the reported corresponding sequence of human PAI-1. The amino acid composition also revealed a close relationship between bovine PAI and human PAI-1. The purified PAI was substantially inactive (570 U/mg) but it could be activated by treatment with protein denaturants such as 1% SDS (1.8 X 10(5) U/mg) and 4 M guanidine-HCl (1.5 X 10(5) U/mg). A more effective activation of this latent PAI was achieved by heat treatment at 100 degrees C for 2.5 min, generating the specific activity of 1.0 X 10(6) U/mg. The heat-activated PAI lost its activity during incubation at 56 degrees C for 30 min, but repeated heat at 100 degrees C for 2.5 min could regenerate about 70% of the initial activity. Treatment at 37 degrees C, 56 degrees C and 80 degrees C, however, failed to activate the latent PAI at all. These findings suggest that the buried reactive site of the latent PAI is exposed as a result of a heat-induced, specific conformational change, but tends to be masked again during renaturation under mild conditions, i.e. the PAI protein takes on preferentially a latent form.

Expression of functionally active human antithrombin III

Human antithrombin III cDNA was cloned into an expression vector suitable for transient expression in COS cells. Upon transfection COS cells secreted a single immunoreactive 58-kDa protein. Quantitation of secretion levels by ELISA indicated that at 44 hr posttransfection cells were secreting 48 +/- 5 ng of antithrombin III per 10(6) cells per 24 hr. Heparin-agarose chromatography resulted in the elution of the COS-derived protein as a broad band between 0.3 and 1.0 M NaCl. 35S-labeled medium from transfected cells reacted with human thrombin (1.5 ng/ml) in the absence of heparin. In 40 min, greater than 80% of the immunoreactive material was found as a higher molecular weight species, consistent with stoichiometric covalent complex formation. In a two-stage chromogenic thrombin inactivation assay, under pseudo-first-order conditions, at 16 nM antithrombin III the t1/2 was 74 min and 50 min for plasma and COS cell-derived antithrombin III, respectively, in the absence of heparin. In the presence of 17.4 nM high-affinity heparin, the t1/2 was 5.2 min and 2.2 min, respectively.

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)

Thrombosis in inherited deficiencies of antithrombin, protein C, and protein S

The coagulation cascade can be pictured as a series of reactions in which a zymogen, a cofactor, and a converting enzyme interact to form a multimolecular complex on a natural surface. In each case, the four reactants must be present if the conversion of a zymogen to the corresponding serine protease is to take place at any significant rate. The principal natural anticoagulant systems that are able to exert damping effects on the various steps of the cascade are the heparin-antithrombin and protein C-thrombomodulin mechanisms that regulate the serine proteases and the cofactors or activated cofactors, respectively. Inherited thrombotic disorders associated with specific deficiencies of antithrombin, protein C, and protein S have been described. This review describes the biochemistry and physiology of these endogenous anticoagulant systems. The development of specific radioimmunoassay techniques for prothrombin activation fragment F1 + 2, fibrinopeptide A, and protein C activation peptide has allowed us to carry out studies of these endogenous regulatory mechanisms involved in thrombin generation in patients with deficiencies of antithrombin or protein C. This information is then used to construct a framework for understanding the pathophysiology of the prethrombotic and actively thrombotic states in humans with these clinical disorders.

The relative molecular mass dependence of the anti-factor Xa properties of heparin

The effect of heparin fractions of various Mr, with high affinity for antithrombin III, on the kinetics of the reaction between factor Xa and antithrombin III have been studied using purified human proteins. Each of the heparin fractions, which varied between pentasaccharide and Mr 32,000, accelerated the inhibition of factor Xa although an increasing rate of inhibition was observed with increasing Mr. The chemically synthesized pentasaccharide preparation (Mr 1714) gave a maximum inhibition rate constant of 1.2 X 10(7) M-1 X min-1, compared with 6.3 X 10(4) M-1 X min-1 in the absence of heparin, and this rose progressively to 4.2 X 10(8) M-1 X min-1 with the two fractions of highest Mr (22,500 and 32,000). The 35-fold difference in inhibition rates observed with the high-affinity fractions was virtually abolished by the presence of 0.3 M-NaCl. The disparity in these rates of inhibition was shown to be due to a change in the Km for factor Xa when a two-substrate model of heparin catalysis was used. The Km for factor Xa rose from 28 nM for the fraction of Mr 32,000 to 770 nM for the pentasaccharide, whilst 0.3 M-NaCl also caused an increase in Km with the high-Mr fraction. These data suggest that the increased rates of inhibition observed with heparins of higher Mr may be due to an involvement of heparin binding to factor Xa as well as to antithrombin III.

Biological implications of the structural, antithrombin affinity and anticoagulant activity relationships among vertebrate heparins and heparan sulphates

We analysed the distribution, structural characteristics, antithrombin-III-binding properties and anticoagulant activities of heparins and heparan sulphates isolated from the tissues of a wide range of vertebrates. Heparin has a curiously limited distribution, since it was absent from lower aquatic vertebrate species, present in only certain organs such as intestine in many higher vertebrates, and completely absent from the rabbit among mammals examined. The heparins were structurally diverse, and they exhibited a broad range of anticoagulant activities, from approx. 50% to 150% of average commercial heparins. Although there was a rough correlation between the anticoagulant potency of the starting isolate and the proportional content of material exhibiting high-affinity binding to the proteinase inhibitor antithrombin III, activities of high-affinity fractions from heparins low in activity overlapped those of low-affinity fractions from highly active heparins. Heparan sulphates, which in contrast were isolated from nearly all vertebrate organs, contained high-affinity subfractions constituting up to 5% of the starting material and possessing anticoagulant potencies of 2-30 units/mg. In consideration of the heparin data, we infer that its biological function is either species-specific or may be served by other molecular elements, and that there exists considerable diversity in the antithrombin-III-binding sequence of heparin. The more-generally distributed glycosaminoglycan heparan sulphate possesses within its variable structure a small high-affinity subfraction with low anticoagulant potency, whether isolated from aorta or other tissues. Although heparan sulphate appears to have an essential function at the cellular level, we suggest that this is probably not that of providing heparin-like antithrombotic effects on vascular surfaces.

Sequence variation in heparin octasaccharides with high affinity for antithrombin III

We have isolated from nitrous acid cleavage products of heparin two major octasaccharide fragments which bind with high affinity to human antithrombin. Octasaccharide S, with the predominant structure iduronic acid----N-acetylglucosamine 6-O-sulfate----glucuronic acid-----N-sulfated glucosamine 3,6-di-O-sulfate----iduronic acid 2-O-sulfate----N-sulfated glucosamine 6-O-sulfate----iduronic acid 2-O-sulfate----anhydromannitol 6-O-sulfate, is sensitive to cleavage by Flavobacterium heparinase as well as platelet heparitinase and binds to antithrombin with a dissociation constant of (5-15) X 10(-8) M. Octasaccharide R, with the predominant structure iduronic acid 2-O-sulfate----N-sulfated glucosamine 6-O-sulfate----iduronic acid----N-acetylglucosamine 6-O-sulfate----glucuronic acid----N-sulfated glucosamine 3,6-di-O-sulfate----iduronic acid 2-O-sulfate----anhydromannitol 6-O-sulfate, is resistant to degradation by both enzymes and binds antithrombin with a dissociation constant of (4-18) X 10(-7) M. The occurrence of a 15-17% replacement of N-sulfated glucosamine 3,6-di-O-sulfate with N-sulfated glucosamine 3-O-sulfate and a 10-12% replacement of iduronic acid with glucuronic acid in both octasaccharides indicates that these substitutions have little or no effect on the binding of the oligosaccharides to the protease inhibitor. When bound to antithrombin, both octasaccharides produce a 40% enhancement in the intrinsic fluorescence of the protease inhibitor and a rate of human factor Xa inhibition of 5 X 10(5) M-1 s-1 as monitored by stopped-flow fluorometry. This suggests that the conformation of antithrombin in the region of the factor Xa binding site is similar when the protease inhibitor is complexed with either octasaccharide.