Structures of five sulfated hexasaccharides prepared from porcine intestinal heparin using bacterial heparinase. Structural variants with apparent biosynthetic precursor-product relationships for the antithrombin III-binding site

Synthetic Disaccharide Standards Enable Quantitative Analysis of Stored Heparan Sulfate in MPS IIIA Murine Brain Regions

Heparan sulfate (HS) is a complex polysaccharide from the glycosaminoglycan (GAG) family that accumulates in tissues in several neurological lysosomal storage diseases known as mucopolysaccharidosis (MPS) disorders. The quantitation of HS in biological samples is important for studying MPS disorders but is very challenging because of its high molecular weight and heterogeneity. Recently, acid-catalyzed butanolysis followed by LC-MS/MS analysis has emerged as a promising method for the determination of HS. Butanolysis of HS produces fully desulfated disaccharide cleavage products which are detected by LC-MS/MS. Herein we describe the synthesis of butylated HS disaccharide standards and their use for determining the identity of major product peaks in LC-MS chromatograms from butanolysis of HS as well as the related GAGs heparin and heparosan. Furthermore, synthesis of a d₉-labeled disaccharide internal standard enabled the development of a quantitative LC-MS/MS assay for HS. The assay was utilized for the analysis of MPS IIIA mouse brain tissues, revealing significant differences in abundance and in the regional accumulation of the various HS disaccharides in affected mice.

Downstream Products are Potent Inhibitors of the Heparan Sulfate 2-O-Sulfotransferase

Heparan Sulfate (HS) is a cell signaling molecule linked to pathological processes ranging from cancer to viral entry, yet fundamental aspects of its biosynthesis remain incompletely understood. Here, the binding preferences of the uronyl 2-O-sulfotransferase (HS2ST) are examined with variably-sulfated hexasaccharides. Surprisingly, heavily sulfated oligosaccharides formed by later-acting sulfotransferases bind more tightly to HS2ST than those corresponding to its natural substrate or product. Inhibition assays also indicate that the IC50 values correlate simply with degree of oligosaccharide sulfation. Structural analysis predicts a mode of inhibition in which 6-O-sulfate groups located on glucosamine residues present in highly-sulfated oligosaccharides occupy the canonical binding site of the nucleotide cofactor. The unexpected finding that oligosaccharides associated with later stages in HS biosynthesis inhibit HS2ST indicates that the enzyme must be separated temporally and/or spatially from downstream products during biosynthesis in vivo, and highlights a challenge for the enzymatic synthesis of lengthy HS chains in vitro.

Synthesis and mass spectrometric analysis of disaccharides from methanolysis of heparan sulfate

Heparan sulfate (HS) disaccharides were synthesized to identify HS methanolysis products by LC-MS/MS with applications for mucopolysaccharidosis disorders.

Discovery of a heparan sulfate 3-O-sulfation specific peeling reaction

Heparan sulfate (HS) 3-O-sulfation determines the binding specificity of HS/heparin for antithrombin III and plays a key role in herpes simplex virus (HSV) infection. However, the low natural abundance of HS 3-O-sulfation poses a serious challenge for functional studies other than the two cases mentioned above. By contrast, multiple distinct isoforms of 3-O-sulfotranserases exist in mammals (up to seven isoenzymes). Here we describe a novel peeling reaction that specifically degrades HS chains with 3-O-sulfated glucosamine at the reducing-end. When HS/heparin is enzymatically depolymerized for compositional analysis, 3-O-sulfated glucosamine at the reducing ends appears to be susceptible to degradation under mildly basic conditions. We propose a 3-O-desulfation initiated peeling reaction mechanism based on the intermediate and side-reaction products observed. Our discovery calls for the re-evaluation of the natural abundance and functions of HS 3-O-sulfation by taking into consideration the negative impact of this novel peeling reaction.

A simple method for early age phenotype confirmation using toe tissue from a mouse model of MPS IIIA

RATIONALE

Determination of genotype can be difficult, especially during the early stages of developing an animal model, e.g. when PCR primers are not yet available. An increase or decrease in specific metabolites can be used as a surrogate marker for genotype; for instance, in homozygous MPS IIIA mice heparan sulphate (HS) is increased.

METHODS

A simple method was developed for extracting and depolymerising HS from mouse toe tissue using methanolysis under acidic conditions. The sample was lyophilised and resuspended in methanolic HCl. The reaction products are desulphated disaccharides and readily analysable by liquid chromatography/tandem mass spectrometry (LC/MS/MS) in positive ion multiple reaction monitoring mode. Measurements were normalised to a spiked deuterated HS internal standard and to endogenous chondroitin sulphate (CS).

RESULTS

HS was measured in toe tissue taken from 30 mice in three groups of 10 (normal controls, MPS IIIA homozygotes and heterozygotes). A significant difference was observed between the MPS IIIA homozygotes and the other two groups, making it possible to identify mice with the MPS IIIA genotype based on the measurement of HS. Normalisation to CS was shown to correct for sample variability and reaction efficiency.

CONCLUSIONS

Analysis of toe tissue provides a simple and rapid way of determining a storage phenotype at 5 to 7 days of age. Significantly, this method does not require any additional samples to be taken from animals, as it utilises tissue that is a by-product of toe clipping, a method that is routinely used to permanently identify mice.

Heparan sulfate 3-O-sulfation: a rare modification in search of a function

Many protein ligands bind to heparan sulfate, which results in their presentation, protection, oligomerization or conformational activation. Binding depends on the pattern of sulfation and arrangement of uronic acid epimers along the chains. Sulfation at the C3 position of glucosamine is a relatively rare, yet biologically significant modification, initially described as a key determinant for binding and activation of antithrombin and later for infection by type I herpes simplex virus. In mammals, a family of seven heparan sulfate 3-O-sulfotransferases installs sulfate groups at this position and constitutes the largest group of sulfotransferases involved in heparan sulfate formation. However, to date very few proteins or biological systems have been described that are influenced by 3-O-sulfation. This review describes our current understanding of the prevalence and structure of 3-O-sulfation sites, expression and substrate specificity of the 3-O-sulfotransferase family and the emerging roles of 3-O-sulfation in biology.

Determination of the primary structure and carboxyl pK (A)s of heparin-derived oligosaccharides by band-selective homonuclear-decoupled two-dimensional (1)H NMR

Determination of the structure of heparin-derived oligosaccharides by ¹H NMR is challenging because resonances for all but the anomeric protons cover less than 2 ppm. By taking advantage of increased dispersion of resonances for the anomeric H¹ protons at low pD and the superior resolution of band-selective, homonuclear-decoupled (BASHD) two-dimensional ¹H NMR, the primary structure of the heparin-derived octasaccharide ∆UA(2S)-[(1 → 4)-GlcNS(6S)-(1 → 4)-IdoA(2S)-]₃-(1 → 4)-GlcNS(6S) has been determined, where ∆UA(2S) is 2-O-sulfated ∆^4,5-unsaturated uronic acid, GlcNS(6S) is 6-O-sulfated, N-sulfated β-d-glucosamine and IdoA(2S) is 2-O-sulfated α-l-iduronic acid. The spectrum was assigned, and the sites of N- and O-sulfation and the conformation of each uronic acid residue were established, with chemical shift data obtained from BASHD-TOCSY spectra, while the sequence of the monosaccharide residues in the octasaccharide was determined from inter-residue NOEs in BASHD-NOESY spectra. Acid dissociation constants were determined for each carboxylic acid group of the octasaccharide, as well as for related tetra- and hexasaccharides, from chemical shift–pD titration curves. Chemical shift–pD titration curves were obtained for each carboxylic acid group from sub-spectra taken from BASHD-TOCSY spectra that were measured as a function of pD. The pK_As of the carboxylic acid groups of the ∆UA(2S) residues are less than those of the IdoA(2S) residues, and the pK_As of the carboxylic acid groups of the IdoA(2S) residues for a given oligosaccharide are similar in magnitude. Relative acidities of the carboxylic acid groups of each oligosaccharide were calculated from chemical shift data by a pH-independent method.

Novel approach for preparation of heparins specific to factor Xa using affinity chromatography coupled with synthetic antithrombin III-related peptides

In the blood coagulation cascade, heparin activates human plasma antithrombin III (hAT III), resulting in the inhibition of factor Xa. This polysaccharide also exhibits hemorrhagic tendency mediated by the inhibition of thrombin in heparinotherapy. Therefore, attention has focused on the development of low molecular weight heparins (LMW-heparins) that inhibit factor Xa but not thrombin. In this investigation, we examined the biochemical and physicochemical properties of hAT III-derived heparin-binding peptides (HBPs). Of all the tested HBPs, hAT III (123-139) exhibited the highest affinity with heparin and showed an inhibitory effect on the heparin-induced enhancement of hAT III activity toward factor Xa, indicating that hAT III (123-139) specifically interacts with the active region in heparin. We prepared a synthetic hAT III (123-139)-coupled affinity chromatography system, and demonstrated that this novel affinity chromatography is useful for fractionation of highly active moieties in LMW-heparins.

Structural recognition by recombinant human heparanase that plays critical roles in tumor metastasis. Hierarchical sulfate groups with different effects and the essential target disulfated trisaccharide sequence

Human heparanase is an endo-beta-d-glucuronidase that degrades heparan sulfate/heparin and has been implicated in a variety of biological processes, such as inflammation, tumor angiogenesis, and metastasis. Although the cloned enzyme has been demonstrated to have a critical role in tumor metastasis, the substrate specificity has been poorly understood. In the present study, the specificity of the purified recombinant human heparanase was investigated for the first time using a series of structurally defined oligosaccharides isolated from heparin/heparan sulfate. The best substrates were deltaHexUA(+/-2S)-GlcN(NS,6S)-GlcUA-GlcN(NS,6S)-GlcUA-GlcN(NS,6S) and deltaHexUA(2S)-GlcN(NS,6S)-GlcUA-GlcN(NS,6S) (where deltaHexUA, GlcN, GlcUA, NS, 2S, and 6S represent unsaturated hexuronic acid, d-glucosamine, d-glucuronic acid, 2-N-sulfate, 2-O-sulfate, and 6-O-disulfate, respectively). Based on the percentage conversion of the substrates to products under identical assay conditions, several aspects of the recognition structures were revealed. 1) The minimum recognition backbone is the trisaccharide GlcN-GlcUA-GlcN. 2) The target GlcUA residues are in the sulfated region. 3) The -GlcN(6S)-GlcUA-GlcN(NS)- sequence is essential but not sufficient as the cleavage site. 4) The IdoUA(2S) residue, located two saccharides away from the target GlcUA residue, claimed previously to be essential, is not indispensable. 5) The 3-O-sulfate group on the GlcN is dispensable and even has an inhibitory effect when located in a highly sulfated region. 6) Based on these and previous results, HexUA(2S)-GlcN(NS,6S)-IdoUA-GlcNAc(6S)-GlcUA-GlcN(NS,+/-6S)-IdoUA(2S)-GlcN(NS,6S) (where HexUA represents hexuronic acid) has been proposed as a probable physiological target octasaccharide sequence. These findings will aid establishing a quantitative assay method using the above tetrasaccharide and designing heparan sulfate-based specific inhibitors of the heparanase for new therapeutic strategies.

Hexasaccharides from the histamine-modified depolymerization of porcine intestinal mucosal heparin

Specific sequences in heparin are responsible for its modulation of the biological activity of proteins. As part of a program to characterize heparin-peptide and heparin-protein binding, we are studying the interaction of chemically discrete heparin-derived oligosaccharides with peptides and proteins. We report here the isolation and characterization, by one- and two-dimensional 1H NMR spectroscopies, of ten hexasaccharides, one pentasaccharide, and one octasaccharide serine that were isolated from depolymerized porcine intestinal mucosal heparin. Hexasaccharides were chosen for study because they fall within the size range, typically tetra- to decasaccharide in length, of heparin sequences that modulate the activity of proteins. The depolymerization reaction was catalyzed by heparinase I (EC 4.2.2.7) in the presence of histamine, which binds site specifically to heparin. Histamine increases both the rate and extent of heparinase I-catalyzed depolymerization of heparin. It is proposed that oligosaccharides produced by heparinase I-catalyzed depolymerization can inhibit the enzyme by binding to the imidazolium group of histidine-203, which together with cysteine-135 forms the catalytic domain of heparinase I. The increased rate and extent of depolymerization are attributed to competitive binding of the oligosaccharides by histamine.

Determination of the primary structures of heparin- and heparan sulfate-derived oligosaccharides using band-selective homonuclear-decoupled two-dimensional 1H NMR experiments

Band-selective homonuclear-decoupled (BASHD) two-dimensional NMR experiments are applied to the assignment of 1H NMR spectra of oligosaccharides, using as an example a heparin-derived hexasaccharide. The anomeric (H1) region of the 1H NMR spectrum is band-selected in the F1 dimension. With the increased resolution that results from less truncation of interferograms in the t1 dimension, finer digital resolution in the F1 dimension, and collapse of multiplets to singlets in the F1 dimension, cross-peaks to the anomeric protons of the two iduronic acid residues, which overlap in normal two-dimensional total correlation spectroscopy (TOCSY) and rotating frame Overhauser enhancement spectroscopy (ROESY) spectra of the hexasaccharide, are resolved in BASHD-TOCSY and BASHD-ROESY spectra, leading to an unequivocal assignment of the 1H NMR spectrum of the hexasaccharide. Incorporation of the water attenuation by transverse relaxation method for the complete and selective elimination of the water resonance into two-dimensional BASHD experiments makes it possible to observe oligosaccharide resonances at the frequency of the water resonance, as demonstrated with the observation of cross-peaks to resonances at the frequency of the water resonance in BASHD-TOCSY spectra of a second heparin-derived hexasaccharide.

Microanalysis of glycosaminoglycan-derived oligosaccharides labeled with a fluorophore 2-aminobenzamide by high-performance liquid chromatography: application to disaccharide composition analysis and exosequencing of oligosaccharides

A series of disaccharides derived from chondroitin sulfate and heparin/heparan sulfate were derivatized at their reducing ends with a fluorophore 2-aminobenzamide to develop a sensitive microanalytical method for glycosaminoglycans. The resulting labeled compounds derived from chondroitin sulfate or heparin/heparan sulfate were well-separated and quantified by HPLC equipped with a fluorescence detector. The detection limit was a low picomole level. This method was applied to the analysis of the disaccharide composition of tetra- and hexasaccharides derived from chondroitin sulfate and heparin/heparan sulfate as well as these glycosaminoglycan polysaccharides. The method was also successfully applied to the exosequencing of chondrohexasaccharides, where the fluorophore-labeled oligosaccharides were degraded exolytically from the nonreducing ends using bacterial eliminases. The resultant labeled fragments were identified by HPLC.

Structural studies of octasaccharides derived from the low-sulfated repeating disaccharide region and octasaccharide serines derived from the protein linkage region of porcine intestinal heparin

Four octasaccharide serines and three octasaccharides were isolated after heparinase treatment of porcine intestinal heparin. Their structures were characterized by enzymatic digestion in conjunction with HPLC and 500 MHz 1H NMR spectroscopy. Three of the four octasaccharide serines were structurally identical with those isolated previously, whereas one has the unreported structure DeltaHexA(2-sulfate)alpha1-4GlcN(N-sulfate)alpha1-4GlcAbe ta1-4GlcNAca lpha1-4GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta 1-O-Ser (DeltaHexA, GlcN, IdceA, and GlcA represent 4-deoxy-alpha-L-threo-hex-4-enepyranosyluronic acid, D-glucosamine, L-iduronic acid, and D-glucuronic acid, respectively). The other three octasaccharides were isolated for the first time as discrete structures and shared the common core hexasulfated sequence DeltaHexA(2-sulfate)alpha1-4GlcN(N-sulfate)alpha1-4IdceAa lpha1-4GlcNA calpha1-4GlcAbeta1-4GlcN(N-sulfate)alpha1-4IdceA (2-sulfate)alpha1-4Gl cN(N,6-disulfate) with one or two additional sulfate groups. The octasaccharides which were derived from the low-sulfated repeating disaccharide region of heparin contained the common trisaccharide sequence -4IdceAalpha1-4GlcNAcalpha1-4GlcAbeta1- [Yamada, S., Yamane, Y., Tsuda, H., Yoshida, K., and Sugahara, K. (1998) J. Biol. Chem. 273, 1863-1871], suggesting the programmed biosynthesis of heparin. These octasaccharides are the largest oligosaccharides isolated so far from the low-sulfated irregular region of heparin. Since oligosaccharides larger than a pentasaccharide appear to potentially exhibit binding activities toward growth factors or other functional proteins, they will be useful for investigating the structural requirement for molecular interactions between heparin and/or heparan sulfate and biologically active proteins. During the course of the present structural studies, we evaluated the NMR data accumulated thus far on heparin oligosaccharides and found several interesting rules on chemical shifts of proton signals affected by the neighboring sugar residues and their sulfation, which will be in turn useful for determining structures of unknown heparin and/or heparan sulfate oligosaccharides based on the proton resonances.

Conversion of N-sulfated glucosamine to N-sulfated mannosamine in an unsaturated heparin disaccharide by non-enzymatic, base-catalyzed C-2 epimerization during enzymatic oligosaccharide preparation

A novel disaccharide was isolated beside the predominant trisulfated disaccharide, delta HexA(2-O-sulfate)(alpha 1-4)GlcN(2-N-,6-O-disulfate) (delta HexA and GlcN represent 4-deoxy-alpha-L-threo-hex-4-enepyranosyluronic acid and D-glucosamine, respectively) after treatment of porcine intestinal heparin with Flavobacterium heparinase. It accounted for 18% of total disaccharides. The structure was characterized by secondary ion mass spectrometry, enzymatic digestions, amino sugar analysis, and 500 MHz one- and two-dimensional 1H NMR spectroscopy as delta HexA(2-O-sulfate) (alpha 1-4)ManN(2-N-,6-O-disulfate), where ManN represents D-mannosamine. The C-2 epimerization from delta HexA(2-O-sulfate) (alpha 1-4)GlcN(2-N-,6-O-disulfate) to delta HexA(2-O-sulfate) (alpha 1-4)ManN(2-N-,6-O-disulfate) was also demonstrated to take place in vitro under very mild alkaline conditions. Hence, the latter compound is not a biosynthetic product, but is most likely an artifact generated by non-enzymatic, base-catalyzed C-2 epimerization during enzymatic preparation of heparin oligosaccharides. The present results warn that the formation of the C-2 epimerized compound has to be circumvented in the structural analysis of heparin/heparan sulfate.

Characterization of heparin oligosaccharide mixtures as ammonium salts using electrospray mass spectrometry

Among glycosaminoglycans, polysulfated heparin chains provide the greatest challenge to characterization due to high polarity, structural diversity, and sulfate lability. The present report demonstrates how electrospray mass spectrometry (ESMS) can be used to derive compositional information from pure and mixed fractions of heparin tetra- to decasaccharide fragments prepared by controlled digestion of heparin with heparinase I. It also describes an improved procedure for fractionation of heparin oligosaccharides up to octadecasaccharides. Ammonium salts prove to be superior to sodium salts, particularly for analysis of mixed components. In the mass spectrum of a hexasaccharide fraction, the identification of six major mass peaks that represent the known hexasaccharide structures confirms that ESMS analysis of heparin oligosaccharide fragments gives a close representation of their constituent composition. In addition to the previously identified components, several unreported oligosaccharides were detected in the spectra of octa- and decasaccharide fractions. The ESMS identification of the three major species in a decasaccharide fraction was confirmed after HPLC subfractionation and reanalysis. ESMS detection sensitivity of low picomole amounts of oligosaccharides can be readily achieved.

A major common trisulfated hexasaccharide core sequence, hexuronic acid(2-sulfate)-glucosamine(N-sulfate)-iduronic acid-N-acetylglucosamine-glucuronic acid-glucosamine(N-sulfate), isolated from the low sulfated irregular region of porcine intestinal heparin

The major structure of the low sulfated irregular region of porcine intestinal heparin was investigated by characterizing the hexasaccharide fraction prepared by extensive digestion of the highly sulfated region with Flavobacterium heparinase and subsequent size fractionation by gel chromatography. Structures of a tetrasaccharide, a pentasaccharide, and eight hexasaccharide components in this fraction, which accounted for approximately 19% (w/w) of the starting heparin representing the major oligosaccharide fraction derived from the irregular region, were determined by chemical and enzymatic analyses as well as 1H NMR spectroscopy. Five compounds including one penta- and four hexasaccharides had hitherto unreported structures. The structure of the pentasaccharide with a glucuronic acid at the reducing terminus was assumed to be derived from the reducing terminus of a heparin glycosaminoglycan chain and may represent the reducing terminus exposed by a tissue endo-beta-glucuronidase involved in the intracellular post-synthetic fragmentation of macromolecular heparin. Eight out of the 10 isolated oligosaccharides shared the trisaccharide sequence, -4IdceA alpha 1-4GlcNAc alpha 1-4GlcA beta 1-, and its reverse sequence, -4GlcA beta 1-4GlcNAc alpha 1-4IdceA alpha 1-, was not found. The latter has not been reported to date for heparin/heparan sulfate, indicating the substrate specificity of the D-glucuronyl C-5 epimerase. Furthermore, seven hexasaccharides shared the common trisulfated hexasaccharide core sequence delta HexA(2-sulfate)alpha 1-4GlcN(N-sulfate)alpha 1-4IdceA alpha 1-4GlcNAc alpha 1-4GlcA beta 1-4GlcN(N-sulfate) which contained the above trisaccharide sequence (delta HexA, IdceA, GlcN, and GlcA represent 4-deoxy-alpha-L-threo-hex-4-enepyranosyluronic acid, L-iduronic acid, D-glucosamine, and D-glucuronic acid, respectively) and additional sulfate groups. The specificity of the heparinase used for preparation of the oligosaccharides indicates the occurrence of the common pentasulfated octasaccharide core sequence, -4GlcN(N-sulfate)alpha 1-4HexA(2-sulfate)1-4GlcN(N-sulfate) alpha 1-4IdceA alpha 1-4GlcNAc alpha 1-4GlcA beta 1-4 GlcN(N-sulfate)alpha 1-4HexA(2-sulfate)1-, where the central hexasaccharide is flanked by GlcN(N-sulfate) and HexA(2-sulfate) on the nonreducing and reducing sides, respectively. The revealed common sequence constituted a low sulfated trisaccharide representing the irregular region sandwiched by highly sulfated regions and should reflect the control mechanism of heparin biosynthesis.

Enzymatic preparation of heparin oligosaccharides containing antithrombin III binding sites

Two new oligosaccharides were prepared from heparin by its partial depolymerization using heparin lyase I (EC 4.2.2.7) in an attempt to prepare oligosaccharides having intact antithrombin III binding sites. The oligosaccharides were purified by chromatography on the basis of both size and charge and demonstrated a high level of purity by capillary electrophoresis. One- and two-dimensional 1H NMR spectroscopy at 500 MHz revealed the structure of each oligosaccharide. The octasaccharide and decasaccharide are DeltaUAp2S(1-->4)-alpha-DGlcNpS6S(1-->4)-alpha-L-IdoAp (1-->4)-alpha-D -GlcNpAc6S(1-->4)-betaD-GlcAp(1-->4)-alpha-D-GlcNpS 3S6S(1-->4)-alpha- L-IdoAp2S(1-->4)alpha-D-GlcNpS6S (where DeltaUAp is 4-deoxy-alpha-L-threo-hex-enopyranosyluronic acid, GlcNp is 2-amino-2-deoxy-glucopyranose, GlcAp is glucopyranosyluronic acid, S is sulfate and Ac is acetate) and DeltaUAp2S(1-->4)-alpha-D-GlcNpS6S(1-->4)-alpha-L-IdoAp++ +(1-->4)-alpha- D-GlcNpAc6S (1-->4)-beta-D-GlcAp(1-->4)-alpha-D-GlcNpS3S6S(1-->4)-alpha- L-IdoAp2S (1-->4)-alpha-D-GlcNpS6S(1-->4)-alpha-L-IdoAp2S(1-->4)-alpha -D-GlcNpS 6S, respectively. A hexasaccharide containing a similar structural motif to that found in the antithrombin III binding site and having greatly reduced anticoagulant activity was also isolated. The structure of the hexasaccharide is DeltaUAp2S(1-->4)-alpha-D-GlcNpAc6S(1-->4)-beta-D-GlcAp++ +(1-->4)-alpha- D-GlcNpS3S6S(1-->4)-alpha-L-IdoAp(1-->4)-alpha-D-GlcNpS6S . The octasaccharide and decasaccharide correspond to the predominant structural motif found in porcine intestinal mucosal heparin. Sufficient quantities of the decasaccharide were obtained to examine its interaction with antithrombin III using microtitration calorimetry. This decasaccharide bound to antithrombin III with similar avidity as heparin and showed comparable anticoagulant activity, as determined using an antithrombin III dependent anti-factor Xa assay. Interestingly, while both decasaccharide and heparin bound to antithrombin with nanomolar affinity, very little heat of binding was observed.