Multinuclear magnetic resonance studies of the interaction of inorganic cations with heparin

Metal Binding to Sodium Heparin Monitored by Quadrupolar NMR

Heparins and heparan sulfate polysaccharides are negatively charged glycosaminoglycans and play important roles in cell-to-matrix and cell-to-cell signaling processes. Metal ion binding to heparins alters the conformation of heparins and influences their function. Various experimental techniques have been used to investigate metal ion-heparin interactions, frequently with inconsistent results. Exploiting the quadrupolar 23Na nucleus, we herein develop a 23Na NMR-based competition assay and monitor the binding of divalent Ca2+ and Mg2+ and trivalent Al3+ metal ions to sodium heparin and the consequent release of sodium ions from heparin. The 23Na spin relaxation rates and translational diffusion coefficients are utilized to quantify the metal ion-induced release of sodium ions from heparin. In the case of the Al3+ ion, the complementary approach of 27Al quadrupolar NMR is employed as a direct probe of ion binding to heparin. Our NMR results demonstrate at least two metal ion-binding sites with different affinities on heparin, potentially undergoing dynamic exchange. For the site with lower metal ion binding affinity, the order of Ca2+ > Mg2+ > Al3+ is obtained, in which even the weakly binding Al3+ ion is capable of displacing sodium ions from heparin. Overall, the multinuclear quadrupolar NMR approach employed here can monitor and quantify metal ion binding to heparin and capture different modes of metal ion-heparin binding.

Investigating the Role of Sulfate Groups for the Binding of Gd3+ Ions to Glycosaminoglycans with NMR Relaxometry

Glycosaminoglycans (GAGs) are highly negatively charged macromolecules with a large cation binding capacity, but their interaction potential with exogeneous Gd³⁺ ions is under‐investigated. These might be released from chelates used as Gadolinium‐based contrast agents (GBCAs) for clinical MR imaging due to transmetallation with endogenous cations like Zn²⁺. Recent studies have quantified how an endogenous GAG sequesters released Gd³⁺ ions and impacts the thermodynamic and kinetic stability of some GBCAs. In this study, we investigate and compare the chelation ability of two important GAGs (heparin and chondroitin sulfate), as well as the homopolysaccharides dextran and dextran sulfate that are used as models for alternative macromolecular chelators. Our combined approach of MRI‐based relaxometry and isothermal titration calorimetry shows that the chelation process of Gd³⁺ into GAGs is not just a long‐range electrostatic interaction as proposed for the Manning model, but presumably a site‐specific binding. Furthermore, our results highlight the crucial role of sulfate groups in this process and indicate that the potential of a specific GAG to engage in this mechanism increases with its degree of sulfation. The transchelation of Gd³⁺ ions from GBCAs to sulfated GAGs should thus be considered as one possible explanation for the observed long‐term deposition of Gd³⁺in vivo and related observations of long‐term signal enhancements on T₁‐weighted MR images.

An NMR relaxometry approach for quantitative investigation of the transchelation of gadolinium ions from GBCAs to a competing macromolecular chelator

Gadolinium-based contrast agents (GBCAs) have been used in clinical Magnetic Resonance Imaging (MRI) for more than 30 years. However, there is increasing evidence that their dissociation in vivo leads to long-term depositions of gadolinium ions in the human body. In vitro experiments provide critical insights into kinetics and thermodynamic equilibria of underlying processes, which give hints towards the in vivo situation. We developed a time-resolved MRI relaxometry-based approach that exploits distinct relaxivities of Gd3+ in different molecular environments. Its applicability to quantify the transmetallation of GBCAs, the binding of Gd3+ to competing chelators, and the combined transchelation process is demonstrated. Exemplarily, the approach is applied to investigate two representative GBCAs in the presence of Zn2+ and heparin, which is used as a model for a macromolecular and physiologically occurring chelator. Opposing indirect impacts of heparin on increasing the kinetic stability but reducing the thermodynamic stability of GBCAs are observed. The relaxivity of resulting Gd-heparin complexes is shown to be essentially increased compared to that of the parent GBCAs so that they might be one explanation for observed long-term MRI signal enhancement in vivo. In forthcoming studies, the presented method could help to identify the most potent Gd-complexing macromolecular species.

Enhanced Antiviral Function of Magnesium Chloride-Modified Heparin on a Broad Spectrum of Viruses

Previous studies reported on the broad-spectrum antiviral function of heparin. Here we investigated the antiviral function of magnesium-modified heparin and found that modified heparin displayed a significantly enhanced antiviral function against human adenovirus (HAdV) in immortalized and primary cells. Nuclear magnetic resonance analyses revealed a conformational change of heparin when complexed with magnesium. To broadly explore this discovery, we tested the antiviral function of modified heparin against herpes simplex virus type 1 (HSV-1) and found that the replication of HSV-1 was even further decreased compared to aciclovir. Moreover, we investigated the antiviral effect against the new severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) and measured a 55-fold decreased viral load in the supernatant of infected cells associated with a 38-fold decrease in virus growth. The advantage of our modified heparin is an increased antiviral effect compared to regular heparin.

Clot activators and anticoagulant additives for blood collection. A critical review on behalf of COLABIOCLI WG-PRE-LATAM

In the clinical laboratory, knowledge of and the correct use of clot activators and anticoagulant additives are critical to preserve and maintain samples in optimal conditions prior to analysis. In 2017, the Latin America Confederation of Clinical Biochemistry (COLABIOCLI) commissioned the Latin American Working Group for Preanalytical Phase (WG-PRE-LATAM) to study preanalytical variability and establish guidelines for preanalytical procedures to be applied by clinical laboratories and health care professionals. The aim of this critical review, on behalf of COLABIOCLI WG-PRE-LATAM, is to provide information to understand the mechanisms of the interactions and reactions that occur between blood and clot activators and anticoagulant additives inside evacuated tubes used for laboratory testing. Clot activators - glass, silica, kaolin, bentonite, and diatomaceous earth - work by surface dependent mechanism whereas extrinsic biomolecules - thrombin, snake venoms, ellagic acid, and thromboplastin - start in vitro coagulation when added to blood. Few manufacturers of evacuated tubes state the type and concentration of clot activators used in their products. With respect to anticoagulant additives, sodium citrate and oxalate complex free calcium and ethylenediaminetetraacetic acid chelates calcium. Heparin potentiates antithrombin and hirudin binds to active thrombin, inactivating the thrombin irreversibly. Blood collection tubes have improved continually over the years, from the glass tubes containing clot activators or anticoagulant additives that were prepared by laboratory personnel to the current standardized evacuated systems that permit more precise blood/additive ratios. Each clot activator and anticoagulant additive demonstrates specific functionality, and both manufacturers of tubes and laboratory professional strive to provide suitable interference-free sample matrices for laboratory testing. Both manufacturers of in vitro diagnostic devices and laboratory professionals need to understand all aspects of venous blood sampling so that they do not underestimate the impact of tube additives on laboratory testing.

Tools for the Quality Control of Pharmaceutical Heparin

Heparin is a vital pharmaceutical anticoagulant drug and remains one of the few naturally sourced pharmaceutical agents used clinically. Heparin possesses a structural order with up to four levels of complexity. These levels are subject to change based on the animal or even tissue sources that they are extracted from, while higher levels are believed to be entirely dynamic and a product of their surrounding environments, including bound proteins and associated cations. In 2008, heparin sources were subject to a major contamination with a deadly compound-an over-sulphated chondroitin sulphate polysaccharide-that resulted in excess of 100 deaths within North America alone. In consideration of this, an arsenal of methods to screen for heparin contamination have been applied, based primarily on the detection of over-sulphated chondroitin sulphate. The targeted nature of these screening methods, for this specific contaminant, may leave contamination by other entities poorly protected against, but novel approaches, including library-based chemometric analysis in concert with a variety of spectroscopic methods, could be of great importance in combating future, potential threats.

Heparin Modulates the Kinetics of Zinc-Induced Aggregation of Amyloid-β Peptides

Zinc-induced aggregation of amyloid-β peptides (Aβ) is considered to contribute to the pathogenesis of Alzheimer's disease. While glycosaminoglycans (GAGs) that are commonly present in interneuronal space are known to enhance Aβ self-aggregation in vitro, the impact of GAGs on the formation of zinc-induced amorphous Aβ aggregates has not yet been thoroughly studied. Here, employing dynamic light scattering, bis-ANS fluorimetry, and sedimentation assays, we demonstrate that heparin serving as a representative GAG modulates the kinetics of zinc-induced Aβ42 aggregation in vitro by slowing the rate of aggregate formation and aggregate size growth. By using synthetic Aβ16 peptides to model the Aβ metal-binding domain (MBD), heparin was found to effectively interact with MBDs in complex with zinc ions. We suggest that heparin adsorbs to the surface of growing zinc-induced Aβ42 aggregates via electrostatic interactions, thus creating a steric hindrance that inhibits further inclusion of monomeric and/or oligomeric zinc-Aβ42 complexes. Furthermore, the adsorbed heparin can interfere with the zinc-bridging mechanism of Aβ42 aggregation, requiring the formation of two zinc-mediated interaction interfaces in the MBD. As revealed by computer simulations of the zinc-Aβ16 homodimer complexed with a heparin chain, heparin can interact with the MBD via polar contacts with residues Arg-5 and Tyr-10, resulting in a conformational rearrangement that hampers the formation of the second zinc-mediated interaction in the MBD interface. The findings of this study suggest that GAGs, which are common in the in vivo macromolecular environment, may have a substantial impact on the time course of zinc-induced Aβ aggregation.

Solution Conformation of Heparin Tetrasaccharide. DFT Analysis of Structure and Spin⁻Spin Coupling Constants

Density functional theory (DFT) has provided detailed information on the molecular structure and spin⁻spin coupling constants of heparin tetrasaccharide (GlcNS,6S-IdoA2S-GlcNS,6S-IdoA2S-OMe) representing the predominant heparin repeating-sequence. The fully optimised molecular structures of two tetrasaccharide conformations (differing from each other in the conformational form of the sulphated iduronic acid residue⁻one ¹C₄ and the other ²S₀) were obtained using the B3LYP/6-311+G(d,p) level of theory and applying explicit water molecules to simulate the presence of a solvent. The theoretical data provided insight into variations of the bond lengths, bond angles and torsion angles, formations of intra- and intermolecular hydrogen bonds and ionic interactions. Optimised molecular structures indicated the formation of a complex hydrogen bond network, including interresidue and intraresidue bonds. The ionic interactions strongly influence the first hydration shell and, together with hydrogen bonds, play an important role in shaping the 3D tetrasaccharide structure. DFT-derived indirect three⁻bond proton⁻proton coupling constants (³J_H-C-C-H) showed that the best agreement with experiment was obtained with a weighted average of 67:33 (¹C₄:²S₀) of the IdoA2S forms. Detailed analysis of Fermi-contact contributions to ³J_H-C-C-H showed that important contributions arise from the oxygen lone pairs of neighbouring oxygen atoms. The analysis also showed that the magnitude of diamagnetic spin⁻orbit contributions are sufficiently large to determine the magnitude of some proton⁻proton coupling constants. The data highlight the need to use appropriate quantum-chemical calculations for a detailed understanding of the solution properties of heparin oligosaccharides.

Critical Influence of Cosolutes and Surfaces on the Assembly of Serpin-Derived Amyloid Fibrils

Many proteins and peptides self-associate into highly ordered and structurally similar amyloid cross-β aggregates. This fibrillation is critically dependent on properties of the protein and the surrounding environment that alter kinetic and thermodynamic equilibria. Here, we report on dominating surface and solution effects on the fibrillogenic behavior and amyloid assembly of the C-36 peptide, a circulating bioactive peptide from the α₁-antitrypsin serine protease inhibitor. C-36 converts from an unstructured peptide to mature amyloid twisted-ribbon fibrils over a few hours when incubated on polystyrene plates under physiological conditions through a pathway dominated by surface-enhanced nucleation. In contrast, in plates with nonbinding surfaces, slow bulk nucleation takes precedence over surface catalysis and leads to fibrillar polymorphism. Fibrillation is strongly ion-sensitive, underlining the interplay between hydrophilic and hydrophobic forces in molecular self-assembly. The addition of exogenous surfaces in the form of silica glass beads and polyanionic heparin molecules potently seeds the amyloid conversion process. In particular, heparin acts as an interacting template that rapidly forces β-sheet aggregation of C-36 to distinct amyloid species within minutes and leads to a more homogeneous fibril population according to solid-state NMR analysis. Heparin's template effect highlights its role in amyloid seeding and homogeneous self-assembly, which applies both in vitro and in vivo, where glycosaminoglycans are strongly associated with amyloid deposits. Our study illustrates the versatile thermodynamic landscape of amyloid formation and highlights how different experimental conditions direct C-36 into distinct macromolecular structures.

Investigating the relationship between temperature, conformation and calcium binding in heparin model oligosaccharides

Glycosaminoglycans such as heparan sulfate (HS) are major components of the cell surface and extracellular matrix (ECM) of all multicellular animals, connecting cells to each other as well as to their environment. The ECM must, therefore, both sense and accommodate changes to external conditions. Heparin, a model compound for HS, responds to increased temperatures, involving changes in the populations of conformational states with implications for the binding of HS to proteins, cations and, potentially, for its activity. A fully ¹³C and ¹⁵N labelled model octasasccharide; D-GlcNS6S α(1-4) L-IdoA2S [α(1-4) D-GlcNS6S α(1-4) L-IdoA2S]₂ α(1-4) D-GlcNS6S α(1-4) L-IdoA1,6an, was studied by ¹H, ¹³C and ¹⁵N NMR, revealing complex changes in chemical shifts and conformation, over temperatures (280-305 K), comfortably within the range relevant to terrestrial biology. These complex conformational changes indicated an interaction between the carboxylate group of L-iduronate and D-glucosamine residues that was susceptible to temperature changes in this range, while the well-documented hydrogen bond between the N-sulfamido group of glucosamine and the hydroxyl group at position-3 of iduronate remained intact. Unexpectedly, despite the presence of similar thermally-induced conformational changes in a heparin octasaccharide fraction in the sodium ion form, its subsequent binding to calcium ions and their resulting conformation was stringently maintained, as judged by comparisons of ¹H NMR chemical shifts.

Heparan sulfate and heparin interactions with proteins

Heparan sulfate (HS) polysaccharides are ubiquitous components of the cell surface and extracellular matrix of all multicellular animals, whereas heparin is present within mast cells and can be viewed as a more sulfated, tissue-specific, HS variant. HS and heparin regulate biological processes through interactions with a large repertoire of proteins. Owing to these interactions and diverse effects observed during in vitro, ex vivo and in vivo experiments, manifold biological/pharmacological activities have been attributed to them. The properties that have been thought to bestow protein binding and biological activity upon HS and heparin vary from high levels of sequence specificity to a dependence on charge. In contrast to these opposing opinions, we will argue that the evidence supports both a level of redundancy and a degree of selectivity in the structure-activity relationship. The relationship between this apparent redundancy, the multi-dentate nature of heparin and HS polysaccharide chains, their involvement in protein networks and the multiple binding sites on proteins, each possessing different properties, will also be considered. Finally, the role of cations in modulating HS/heparin activity will be reviewed and some of the implications for structure-activity relationships and regulation will be discussed.

Interaction of the Heparin-Binding Consensus Sequence of β-Amyloid Peptides with Heparin and Heparin-Derived Oligosaccharides

Alzheimer's disease (AD) is characterized by the presence of amyloid plaques in the AD brain. Comprised primarily of the 40- and 42-residue β-amyloid (Aβ) peptides, there is evidence that the heparan sulfate (HS) of heparan sulfate proteoglycans (HSPGs) plays a role in amyloid plaque formation and stability; however, details of the interaction of Aβ peptides with HS are not known. We have characterized the interaction of heparin and heparin-derived oligosaccharides with a model peptide for the heparin- and HS-binding domain of Aβ peptides (Ac-VHHQKLV-NH2; Aβ(12-18)), with mutants of Aβ(12-18), and with additional histidine-containing peptides. The nature of the binding interaction was characterized by NMR, binding constants and other thermodynamic parameters were determined by isothermal titration calorimetry (ITC), and relative binding affinities were determined by heparin affinity chromatography. The binding of Aβ(12-18) by heparin and heparin-derived oligosaccharides is pH-dependent, with the imidazolium groups of the histidine side chains interacting site-specifically within a cleft created by a trisaccharide sequence of heparin, the binding is mediated by electrostatic interactions, and there is a significant entropic contribution to the binding free energy as a result of displacement of Na(+) ions from heparin upon binding of cationic Aβ(12-18). The binding constant decreases as the size of the heparin-derived oligosaccharide decreases and as the concentration of Na(+) ion in the bulk solution increases. Structure-binding relationships characterized in this study are analyzed and discussed in terms of the counterion condensation theory of the binding of cationic peptides by anionic polyelectrolytes.

Heparan sulfate phage display antibodies recognise epitopes defined by a combination of sugar sequence and cation binding

Phage display antibodies are widely used to follow heparan sulfate (HS) expression in tissues and cells. We demonstrate by ELISA, that cations alter phage display antibody binding profiles to HS and this is mediated by changes in polysaccharide conformation, demonstrated by circular dichroism spectroscopy. Native HS structures, expressed on the cell surfaces of neuroblastoma and fibroblast cells, also exhibited altered antibody binding profiles following exposure to low mM concentrations of these cations. Phage display antibodies recognise conformationally-defined HS epitopes, rather than sequence alone, as has been assumed, and resemble proteins in being sensitive to changes in both charge distribution and conformation following binding of cations to HS polysaccharides.

Nanoparticle-based highly sensitive MRI contrast agents with enhanced relaxivity in reductive milieu

A gadolinium containing nanoparticle exhibiting a 10-fold higher r₁ relaxivity than Dotarem® and further increase in relaxivity in reductive milieu is proposed.

MnO2 nanosheet-based heparin and OSCS fluorescent biosensor with lowered background and amplified hybridization chain reaction

A “turn-on” ultrasensitive detection of heparin or OSCS was reported through MnO₂ nanosheet quenched-ultralow background and HCR amplification strategy.

Effects of calcium complexation on heparin-like disaccharides. A combined theoretical, tandem mass spectrometry and ultraviolet experiment

RATIONALE

In order to shed light on the influence of the Ca(2+) metal cation on the structure of heparin-like (Hp) disaccharides, we have explored the gas-phase structures of both [Hp, -2H](2-) and [Ca(Hp), -3H](-) ions by coupling experimental and theoretical methods.

METHODS

The goal of this work was to (i) provide new evidence of the metal influence on the Hp structure, which can have important biological consequences, and (ii) to study the usefulness of metal complexation for the analytical distinction of Hp isomers. Collision-induced dissociation (CID) and ultraviolet photodissociation (UVPD) fragments, as well as optical spectra recorded in the gas phase for both [Hp, -2H](2-) and [Ca(Hp), -3H](-) complexes were compared for I-H, II-S and III-S isomers of Hp.

RESULTS

In the case of CID fragmentation, a change in the fragmentation pattern was observed upon calcium complexation, with respect to deprotonated Hp.

CONCLUSIONS

Remarkably, when optical spectra are compared in the UV range, the metal effect on the carboxylic group absorption can be detected by an unambiguous blue-shift (~20 nm).

Inhibition of urinary macromolecule heparin on aggregation of nano-COM and nano-COD crystals

Purpose: This research aims to study the influences of heparin (HP) on the aggregation of nano calcium oxalate monohydrate (COM) and nano calcium oxalate dihydrate (COD) with mean diameter of about 50 nm. Method: The influences of different concentrations of HP on the mean diameter and Zeta potential of nano COM and nano COD were investigated using a nanoparticle size Zeta potential analyzer. Results: HP could be adsorbed on the surface of nano COM and nano COD crystals, leading to an increase in the absolute value of Zeta potential on the crystals and an increase in the electrostatic repulsion force between crystals. Consequently, the aggregation of the crystals is reduced and the stability of the system is improved. The strong adsorption ability of HP was closely related to the -OSO₃⁻ and -COO⁻ groups contained in the HP molecules. X-ray photoelectron spectroscopy confirmed the coordination of HP with Ca²⁺ ions of COM and COD crystals. Conclusion: HP could inhibit the aggregation of nano COM and nano COD crystals and increase their stability in aqueous solution, which is conducive in inhibiting the formation of calcium oxalate stones.

NMR and DFT analysis of trisaccharide from heparin repeating sequence

NMR and density functional theory (DFT) have afforded detailed information on the molecular geometry and spin-spin coupling constants of a trisaccharide from the heparin repeating-sequence. The fully optimized molecular structures of two trisaccharide conformations (differing from each other in the form of the central iduronic acid residue) were obtained using the B3LYP/6-311+G(d,p) level of theory in the presence of solvent, the latter included as either explicit water molecules or via a continuum solvent model. NMR spin-spin coupling constants were also computed using various basis sets and functionals and then compared with measured experimental values. Optimized structures for both conformers showed differences in geometry at the glycosidic linkages and in the formation of intramolecular hydrogen bonds. Three-bond proton-proton coupling constants ((3)JH-C-C-H), based on fully optimized geometry computed using the B3LYP/6-311+G(d,p)/UFF level of theory and hydrated with 57 water molecules, showed that the best agreement with experiment was obtained with the 6-311+G(d,p) basis set and a weighted average of 55:45 ((1)C4:(2)S0) of the IdoA2S forms. Other basis sets, DGDZVP and TZVP, also gave acceptable data for most coupling constants, with DGDZVP outperforming the TZVP. Detailed analysis of Fermi-contact contributions to (3)JH-C-C-H showed that important contributions arise from oxygen at both glycosidic linkages, as well as from oxygen atoms on the neighboring monosaccharide units. Their contribution to the Fermi term cannot be neglected and must be taken into account for a correct description of coupling constants. The analysis also showed that the magnitude of paramagnetic (PSO) and diamagnetic (DSO) spin-orbit contributions is comparable to the magnitude of the Fermi-contact contribution in some coupling constants in the IdoA2S residue. Calculations of the localized molecular orbital contributions to the DSO terms from separate conformational residues showed that the contribution from adjacent residues is not negligible and can be important for the spin-spin coupling constants between protons located close to the geometrical center of the molecule. These contributions should be taken into account when interpreting DSO terms in spin-spin coupling constants especially in large molecules.

On the catalytic mechanism of polysaccharide lyases: evidence of His and Tyr involvement in heparin lysis by heparinase I and the role of Ca2+

The structurally diverse polysaccharide lyase enzymes are distributed from plants to animals but share common catalytic mechanisms. One, heparinase I (F. heparinum), is employed in the production of the major anticoagulant drug, low molecular weight heparin, and is a mainstay of cell surface proteoglycan analysis. We demonstrate that heparinase I specificity and efficiency depend on the cationic form of the substrate. Ca(2+)-heparin, in which α-L-iduronate-2-O-sulfate residues adopt (1)C4 conformation preferentially, is a substrate, while Na(+)-heparin is an inhibitor. His and Tyr residues are identified in the catalytic step and a model based on molecular dynamics and docking is proposed, in which deprotonated His203 initiates β-elimination by abstracting the C5 proton of the α-L-iduonate-2-O-sulfate residue in the substrate, and protonated Tyr357 provides the donor to the hexosamine leaving group.

The Effects of Metal Ions on Heparin/Heparin Sulfate-Protein Interactions

Heparin/heparin sulfate (HS) interacts with a number of proteins thereby playing an essential role in the regulation of many physiological processes. The understanding of heparin/HS-protein interactions at the molecular level is of fundamental importance to biology and will aid in the development of highly specific glycan-based therapeutic agents. The heparin-binding proteins (HBPs) interact with sulfated domains of heparin/HS chains primarily through ionic attraction between negatively charged groups in HS/heparin chains and basic amino acid residues within the protein. Reports in literature have been shown that heparin molecules have a high affinity for a wide range of metal ions. In the present study, we used surface plasmon resonance (SPR) to study the effects of metal ions (under physiological and non-physiological concentrations) on heparin/HS-protein interactions. The results showed that under non-physiological of metal ion concentration, different metal ions showed different effects on heparin binding to fibroblast growth factor-1 (FGF1) and interleakin-7 (IL7). While the effects of individual metal ion at physiological concentrations had little impact on protein binding, the mixed metal ions reduced the FGF1/heparin or IL7/heparin binding affinity, changing its binding profile.

The interaction of enoxaparin and fondaparinux with calcium

The main sites of calcium binding were determined for the low molecular weight heparin drug enoxaparin and the synthetic pentasaccharide Arixtra (fondaparinux). [(1)H,(13)C] HSQC pH titrations were carried out to characterize the acid-base properties of these samples both in the presence and absence of calcium. The differences in the titration curves were used to determine the structural components of enoxaparin and fondaparinux responsible for Ca(2+) binding. In enoxaparin both unsubstituted and 2-O-sulfated iduronic acid residues are important in calcium binding and the presence of the 2-O-sulfo group does not seem to influence the Ca(2+) binding capability of the iduronate ring. In fondaparinux changes in chemical shifts upon Ca(2+) binding were smaller than observed for enoxaparin, and were observed for both the glucuronic acid and 2-O-sulfated iduronic acid residues. In enoxaparin significant perturbations of the chemical shift of the N-sulfoglucosamine anomeric carbon in residues connected to 2-O-sulfated iduronic acid were detected on Ca(2+) binding, however it was not possible to determine whether these changes reflect direct involvement in calcium complexation or result from through space interactions or conformational changes.

Bio-functionalization of ligand-free upconverting lanthanide doped nanoparticles for bio-imaging and cell targeting

We report on the functionalization of ligand-free NaGdF(4):Er(3+), Yb(3+) upconverting nanoparticles with heparin and basic fibroblast growth factor (bFGF). These upconverting nanoparticles are used to obtain high-contrast images of HeLa cells. These images reveal that the heparin-bFGF functionalized nanoparticles show specific binding to the cell membrane.

Bone marrow-derived heparan sulfate potentiates the osteogenic activity of bone morphogenetic protein-2 (BMP-2)

Lowering the efficacious dose of bone morphogenetic protein-2 (BMP-2) for the repair of critical-sized bone defects is highly desirable, as supra-physiological amounts of BMP-2 have an increased risk of side effects and a greater economic burden for the healthcare system. To address this need, we explored the use of heparan sulfate (HS), a structural analog of heparin, to enhance BMP-2 activity. We demonstrate that HS isolated from a bone marrow stromal cell line (HS-5) and heparin each enhances BMP-2-induced osteogenesis in C2C12 myoblasts through increased ALP activity and osteocalcin mRNA expression. Commercially available HS variants from porcine kidney and bovine lung do not generate effects as great as HS5. Heparin and HS5 influence BMP-2 activity by (i) prolonging BMP-2 half-life, (ii) reducing interactions between BMP-2 with its antagonist noggin, and (iii) modulating BMP2 distribution on the cell surface. Importantly, long-term supplementation of HS5 but not heparin greatly enhances BMP-2-induced bone formation in vitro and in vivo. These results show that bone marrow-derived HS effectively supports bone formation, and suggest its applicability in bone repair by selectively facilitating the delivery and bioavailability of BMP-2.

Biologically Relevant Metal-Cation Binding Induces Conformational Changes in Heparin Oligosaccharides as Measured by Ion Mobility Mass Spectrometry

Heparin interacts with many proteins and is involved in biological processes such as anticoagulation, angiogenesis, and antitumorigenic activities. These heparin-protein interactions can be influenced by the binding of various metal ions to these complexes. In particular, physiologically relevant metal cations influence heparin-protein conformations through electronic interactions inherent to this polyanion. In this study, we employed ion mobility mass spectrometry (IMMS) to observe conformational changes that occur in fully-sulfated heparin octasaccharides after the successive addition of metal ions. Our results indicate that binding of positive counter ions causes a decrease in collision cross section (CCS) measurements, thus promoting a more compact octasaccharide structure.

Voltammetric extraction of heparin and low-molecular-weight heparin across 1,2-dichloroethane/water interfaces

Heparin and low-molecular-weight heparin are voltammetrically extracted across 1,2-dichloroethane/water interfaces for the detection of these highly sulfated polysaccharides widely used as anticoagulants/antithrombotics in many medical procedures. A new heparin ionophore, 1-[4-(dioctadecylcarbamoyl)butyl]guanidinium, is the first to enable the voltammetric extraction of various polyanionic heparins with average molecular weights of up to approximately 20 kDa including those in commercial preparations (i.e., Arixtra (1.5 kDa), Lovenox (4.5 kDa), and unfractionated heparin (15 kDa), as well as chromatographically fractionated heparins (7, 9, 15, and 20 kDa)). Facilitated Arixtra extraction is fully and quantitatively characterized by micropipet voltammetry to propose that cooperative effects from strong heparin-binding capability and high lipophilicity of this ionophore are required for the formation of an electrically neutral and highly lipophilic complex of a heparin molecule with multiple ionophore molecules to be extracted into the nonpolar organic phase. At the same time, the participation of multiple ionophore molecules in interfacial complexation with a heparin molecule slows down its extraction across the interface. This kinetic limitation is enhanced by fast mass transfer at a micropipet-supported interface to compromise thermodynamically favorable selectivity for heparin and an important contaminant, oversulfated chondroitin sulfate, thereby requiring a macroscopic interface for sensing applications. Another highly lipophilic guanidinium ionophore, N,N-dioctadecylguanidinium, cannot completely extract even Arixtra, which indicates the importance of elaborate ionophore design for heparin extraction.

Interaction of heparin and heparin-derived oligosaccharides with synthetic peptide analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein

BACKGROUND

Although protamine is effective as an antidote of heparin, there is a need to replace protamine due to its side effects. HIP peptide has been reported to neutralize the anticoagulant activity of heparin. The interaction of HIP analog peptides with heparin and heparin-derived oligosaccharides is investigated in this paper.

METHODS

Seven analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein (HIP) were synthesized, and their interaction with heparin was characterized by heparin affinity chromatography, isothermal titration calorimetry, and NMR.

RESULTS

NMR results indicate the imidazolium groups of the His side chains of histidine-containing Hip analog peptide interact site-specifically with heparin at pH 5.5. Heparin has identical affinities for HIP analog peptides of opposite chirality. Analysis by counterion condensation theory indicates the peptide AC-SRPKAKAKAKAKDQTK-NH2 makes on average approximately 3 ionic interactions with heparin that result in displacement of approximately 2 Na+ ions, and ionic interactions account for approximately 46% of the binding free energy at a Na+ concentration of 0.15 M.

CONCLUSIONS

The affinity of heparin for the peptides is strongly dependent on the nature of the cationic side chains and pH. The thermodynamic parameters measured for the interaction of HIP peptide analogs with heparin are strongly dependent on the peptide sequence and pH.

GENERAL SIGNIFICANCE

The information obtained in this research will be of use in the design of new agents for neutralization of the anticoagulant activity of heparin. The site-specific binding of protonated histidine side chains to heparin provides a molecular-level explanation for the pH-dependent binding of beta-amyloid peptides by heparin and heparan sulfate proteoglycan and may have implications for amyloid formation.

Residual dipolar coupling investigation of a heparin tetrasaccharide confirms the limited effect of flexibility of the iduronic acid on the molecular shape of heparin

The solution conformation of a fully sulfated heparin-derived tetrasaccharide, I, was studied in the presence of a 4-fold excess of Ca²⁺. Proton–proton and proton–carbon residual dipolar couplings (RDCs) were measured in a neutral aligning medium. The order parameters of two rigid hexosamine rings of I were determined separately using singular value decomposition and ab initio structures of disaccharide fragments of I. The order parameters were very similar implying that a common order tensor can be used to analyze the structure of I. Using one order tensor, RDCs of both hexosamine rings were used as restraints in molecular dynamics simulations. RDCs of the inner iduronic acid were calculated for every point of the molecular dynamics trajectory. The fitting of the calculated RDCs of the two forms of the iduronic acid to the experimental values yielded a population of ¹C₄ and ²S_o conformers of iduronic acid that agreed well with the analysis based on proton–proton scalar coupling constants. The glycosidic linkage torsion angles in RDC-restrained molecular dynamics (MD) structures of I are consistent with the interglycosidic three-bond proton–carbon coupling constants. These structures also show that the shape of heparin is not affected dramatically by the conformational flexibility of the iduronic acid ring. This is in line with conclusions of previous studies based on MD simulations and the analysis of ¹H-¹H NOEs. Our work therefore demonstrates the effectiveness of RDCs in the conformational analysis of glycosaminoglycans.

Interaction of heparin with two synthetic peptides that neutralize the anticoagulant activity of heparin

Two synthetic analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein (Ac-SRGKAKVKAKVKDQTK-NH2) and the all-d-amino acid version of the same peptide (l-HIPAP and d-HIPAP, respectively) were synthesized, and their efficacy as agents for neutralization of the anticoagulant activity of heparin was assayed. The two analogue peptides were found to be equally effective for neutralization of the anticoagulant activity of heparin, as measured by restoration of the activity of serine protease factor Xa by the Coatest heparin method. The finding that l-HIPAP and d-HIPAP are equally effective suggests that d-amino acid peptides show promise as proteolytically stable therapeutic agents for neutralization of the anticoagulant activity of heparin. The interaction of l-HIPAP and d-HIPAP with heparin was characterized by 1H NMR, isothermal titration calorimetry (ITC), and heparin affinity chromatography. The two peptides were found to interact identically with heparin. Analysis of the dependence of heparin-peptide binding constants on Na+ concentration by counterion condensation theory indicates that, on average, 2.35 Na+ ions are displaced from heparin per peptide molecule bound and one peptide molecule binds per hexasaccharide segment of heparin. The analysis also indicates that both ionic and nonionic interactions contribute to the binding constant, with the ionic contribution decreasing as the Na+ concentration increases.

The activities of heparan sulfate and its analogue heparin are dictated by biosynthesis, sequence, and conformation

The glycosaminoglycan heparan sulfate (HS), is expressed on the surface of virtually all mammalian cells and is implicated in many crucial biological activities. The activities of HS and its close structural analogue heparin are mediated through interactions with proteins. However, the relationship between structure and activity is not simple, because the structure and conformation of HS and heparin are complex. This review surveys some of the relevant findings in HS/heparin chemistry, biochemistry, and biology.

Conformational structure of some trimeric and pentameric structural units of heparin

The molecular structure of trimeric units (D-E-F and F-G-H) and the pentamer D-E-F-G-H of heparin (sodium salts and their anionic forms) was studied using the B3LYP/6-31G(d) method. The equilibrium structure of the sodium salts of the trimers and pentamer investigated in the isolated state was determined by multidentate coordination of the sodium cations with oxygen atoms of the sulfate, carboxyl, and hydroxyl (hydroxymethyl) groups, respectively. The displacement of Na+ ions from the binding sites in the sodium salt of oligosaccharides studied resulted in the appreciable change of the overall conformation of the corresponding anion. Upon dissociation, a large change in both the position of the sulfate groups and the conformation across the glycosidic bonds was observed. The stable energy conformations around the glysosidic bonds found for the pentamer investigated are compared and discussed with the available experimental X-ray structural data for the structurally related heparin-derived pentasaccharides in cocrystals with proteins.

Influence of substitution pattern and cation binding on conformation and activity in heparin derivatives

As model compounds for the biologically important heparan sulfate, eight systematically modified heparin derivatives were studied by synchrotron radiation circular dichroism (SRCD), which is sensitive to uronic acid conformation. Substitution pattern altered uronic acid conformation, even when structural changes were made in adjacent glucosamine residues (e.g. 6-O-desulfation) and did not involve a chromophore. SRCD spectra of these derivatives following conversion to the Na+, K+, Mg2+, Ca2+, Mn2+, Cu2+ and Fe3+ cation forms revealed that almost all substitution/cation combinations resulted in unique spectra, showing that each was structurally distinct. The detailed effects that binding Na+, K+, Mg2+ and Ca2+ ions had on a 2-de-O-sulfated derivative was also studied by NMR, revealing that subtle changes in conformation (by NOE) and flexibility (by T2 measurements) resulted. Conversion to the K+ and Cu2+ ion forms also drastically modified biological activity, from inactive to active, in a cell-based assay of fibroblast growth factor-receptor (FGF2/FGFR1c) signalling and this effect was not reproduced by free cations. These observations could explain the often-contradictory data concerning structure-activity relationships for these derivatives in the literature and, furthermore, argue strongly against the established trend of considering sequence as a complete structural definition. It also provides additional means of modifying the activity of these polysaccharides and suggests a possible additional level of control in biological systems. There are also obvious potential applications for these findings in the biotechnology sphere.

One-step surface modification of poly(lactide-co-glycolide) microparticles with heparin

PURPOSE

The aim of this study was to modify the surface of poly(lactide-co-glycolide) (PLGA) microparticles with heparin. The heparin-coated PLGA may enhance blood and tissue compatibility of PLGA devices and provide a novel approach to deliver growth factors.

MATERIALS AND METHODS

A one-step method using heparin to replace traditional emulsifiers (e.g., PVA) during emulsion-solvent evaporation process was employed to surface-entrap heparin in PLGA microspheres. The emulsifying activity of heparin was modified via varying counter ion form, including univalent (Na(+), K(+), Li(+), and [Formula: see text]) and divalent (Ca(2+), Mg(2+), Ba(2+), and Zn(2+)) cations, and complexation with amino acids (Arg, Lys, Leu, Val, Gly and Glu). Surface accessible and total heparin loading were determined by a modified toluidine blue assay and elemental analysis, respectively.

RESULTS

Heparin bound with univalent counter ions and amino acids exhibited emulsifying activity to varying degrees, whereas divalent heparin salts tended to cause complete aggregation of the PLGA o/w emulsion. Increasing pH (>or=7.4) of hardening medium enhanced heparin adsorption and significantly stabilized the PLGA o/w emulsion. The initial surface density of heparin on the PLGA microspheres prepared using univalent heparin salts was around 8-33 mg/m(2). Surface associated heparin desorbed quickly; potassium heparin showed the best retention, with approximately 0.2 and 0.1 mg/m(2) detected on PLGA microsphere surface following 1- and 14-day incubation in PBST at 37 degrees C, respectively.

CONCLUSIONS

PLGA microparticles were successfully surface-modified with heparin. Univalent salts and amino acid complexes of heparin, as effective emulsifiers, can become surface-immobilized in PLGA microspheres.

Voltammetric detection of heparin at polarized blood plasma/1,2-dichloroethane interfaces

Heparin, a highly negatively charged polysaccharide, which has been used widely as an anticoagulant and antithrombotic, was detected by ion-transfer voltammetry at the interface between 1,2-dichloroethane and an aqueous buffer solution or undiluted blood plasma. Quaternary ammoniums with different numbers of methyl and long alkyl groups were examined as positively charged heparin ionophores using pipet electrodes filled with the organic electrolyte solutions of their tetrakis(4-chlorophenyl)borate salts. It was shown that octadecyltrimethylammonium most selectively facilitates interfacial heparin adsorption without interference from potential-dependent ionophore transfer into the aqueous phase. Water-filled pipet electrodes were also used to study the stoichiometry of the interfacial complex between a heparin molecule and multiple ionophore molecules, which is discussed as a counterion condensation effect. Stripping voltammetry based on facilitated heparin adsorption and desorption gives a detection limit of 0.012 unit/mL in 0.12 M NaCl buffered at pH 7.2, which is 1 order of magnitude lower than therapeutic heparin concentrations (>0.2 unit/mL) and is comparable to a detection limit of the most sensitive heparin sensor reported so far. The biomedical utility of ion-transfer voltammetry was demonstrated for the first time in an undiluted blood sample. Despite interferences by Na+, Cl-, and plasma proteins such as serum albumin, a detection limit of 0.13 unit/mL was obtained in sheep blood plasma with the stripping method.

B3LYP/6-311++G* * study of structure and spin-spin coupling constant in heparin disaccharide

Structures of heparin disaccharide have been analyzed by DFT using the B3LYP/6-311++G( * *) method. The optimized geometries of two forms of this disaccharide, differing in the conformation ((1)C(4) and (2)S(0)) of the IdoA2S residue, confirmed considerable influences of the sulfate and the carboxylate groups upon the pyranose ring geometries. The computed energies showed that disaccharide having the (1)C(4) form of the IdoA2S residue is more stable than that with the (2)S(0) form. Interatomic distances, bond and torsion angles showed that interconversion of the IdoA2S residue results in geometry changes in the GlcN,6S residue as well. Three-bond proton-proton and proton-carbon spin-spin coupling constants computed for both forms agree with the experimental data and indicate that only two chair forms contribute to the conformational equilibrium in disaccharide. Influences of the charged groups upon the magnitudes of spin-spin coupling constants are also discussed.

B3LYP/6-311++G∗∗ study of structure and spin–spin coupling constant in methyl 2-O-sulfo-α-l-iduronate

Structures of three most stable conformers ((1)C4, (4)C1, (2)S0) of methyl 2-O-sulfo-alpha-L-iduronate monosodium salt have been analyzed by DFT using the B3LYP/6-311++G** method. The optimized geometries confirmed the influence of both 2-O-sulfate and carboxylate groups upon the pyranose ring geometry. The computed energies showed that the chair (1)C4 form is the most stable one. Time-averaged DFT-calculated proton-proton and proton-carbon spin-spin coupling constants agree with the experimental data and indicate that only two chair forms contribute to the conformational equilibrium of methyl 2-O-sulfo-alpha-L-iduronate monosodium salt. The influence of the charged groups upon the magnitudes of spin-spin coupling constants is also discussed.

A new radiochemical method to investigate ion binding with polyelectrolytes

A new method for investigating the binding of ions with polyelectrolytes has been developed. This method, based on Donnan equilibrium and an isotope exchange between the electrolyte and polyelectrolyte, can distinguish territorial from specific binding of ions and can determine fractions of ions bound with the polyion. This method can determine ion binding with polyelectrolytes in a wide range of polyelectrolyte concentrations in multicomponent solutions. The method was tested with radioactive tracers 22Na+, 36Cl- and heparin sodium salt. The influence of the ionic strength on the Na+ binding with heparin was investigated at 310 K. In the limit of zero ionic strength, all Na+ ions are bound to heparin, but only 45% of them are exchangeable. Thus Na+ ions can be bound both territorially and specifically. The fraction of bound ions decreases rapidly with increasing ionic strength. The fraction of the specifically bound ions becomes negligible when the ionic strength exceeds 0.01 M, whereas the fraction of territorially bound ions can be neglected at ionic strengths higher than 0.45 M.

Conformation of glycosaminoglycans by ion mobility mass spectrometry and molecular modelling

We have performed conformational analyses of heparin-derived oligosaccharide ions in the gas phase using a combination of ion-mobility mass spectrometry and molecular modelling. Negative mode electrospray ionisation was used to generate singly (disaccharide, [C12H15NO19S3Na3]-) and doubly charged (tetrasaccharides, [C24H30N2O38S6Na6]2- and [C24H31N2O35S5Na5]2-) ions containing three and six Na+ ions, respectively. Good agreement was obtained between the experimental and theoretical cross sections. The latter were obtained using modelled structures generated by the AMBER-based force field. Analysis of the conformations of the oligosaccharide ions shows that sodium cations play a major role in stabilizing these ions in the gas phase. This was seen in the formation of oligomers of the disaccharide ion and "compact" structures of tetrasaccharide ions. Interestingly, the gas phase conformations of the three tetrasaccharide ions with different primary structures were significantly different.

The heparin–Ca2+ interaction: the influence of the O-sulfation pattern on binding

The specific binding of Ca(2+) to synthetic hexasaccharide models of modified heparin has been investigated by NMR and molecular modeling and compared with previous results on a model of regular heparin. These two models represent the regular region of heparin lacking one type of O-sulfate group, either at C-6 of glucosamine or at C-2 of iduronate. The NMR experiments show different responses to the presence of Ca(2+). In the case of the compound lacking O-sulfate groups at C-2, the results are indicative of specific binding similar to that observed for the regular heparin, while the model lacking sulfate groups in position 6 interacts more weakly with Ca(2+). In order to understand the basis of this difference, a molecular modeling study based on a rigid body docking approach of the interaction of these carbohydrates with Ca(2+) and Na(+) was performed. We have found that the results are strongly dependent on the starting orientation of the lateral side chains of the charged groups of the carbohydrate, and that the best agreement with the experimental results is obtained when the starting conformations are taken from previous simulations in the presence of Ca(2+).