Structural investigation of chondroitin/dermatan sulfate oligosaccharides from human skin fibroblast decorin

Hybrid chondroitin/dermatan sulfate (CS/DS) glycosaminoglycan chains, derived from decorin secreted by human skin fibroblasts, were shown to interact with FGF-2, as did oligosaccharides derived therefrom by chondroitin B lyase digestion. In a first attempt to identify the biologically active sequence, a novel protocol for structural analysis of enzyme-resistant oligosaccharides larger than standard trisulfated hexasaccharides was developed. The method bases on capillary electrophoresis (CE) for separating oversulfated species in offline combination with nanoelectrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoESI-QTOF-MS/MS) in the negative ion mode. Under optimized CE and ESI-MS conditions, up to 12-mer oligosaccharides with different degrees of sulfation were identified. A novel tandem MS protocol (CID-VE) was applied to elucidate the structure of a previously undescribed pentasulfated CS/DS hexasaccharide, Delta-4,5-IdoAGalNAc[GlcAGalNAc]2(5S). In this molecular species, detected as a triply charged ion at m/z 511.38, three sulfates are found in the IdoAGalNAcGlcA moiety offering two structural variants: one containing sulfated IdoA together with a disulfated GalNAc moiety and in the other one both uronic acids, that is, GlcA and IdoA and the amino sugar each carry a sulfate ester group.     

A defect in exodegradative pathways provides insight into endodegradation of heparan and dermatan sulfates

Within cells, dermatan sulfate (DS) and heparan sulfate (HS) are degraded in two steps. The initial endohydrolysis of these polysaccharides is followed by the sequential action of lysosomal exoenzymes to reduce the resulting oligosaccharides to monosaccharides and inorganic sulfate. Mucopolysaccharidosis (MPS) type II is a lysosomal storage disorder caused by a deficiency of the exoenzyme iduronate-2-sulfatase (I2S). Consequently, partially degraded fragments of DS and HS have been shown to accumulate in the lysosomes of affected cells and are excreted in the urine. Di- to hexadecasaccharides, isolated from the urine of a MPS II patient using anion exchange and gel filtration chromatography, were identified using electrospray ionization-tandem mass spectrometry (ESI-MS/MS). These oligosaccharides were shown to have non-reducing terminal iduronate-2-sulfate residues by digestion with recombinant I2S. A pattern of growing oligosaccharide chains composed of alternating uronic acid and N-acetylhexosamine residues was identified and suggested to originate from DS. A series of oligosaccharides consisting of hexosamine/N-acetylhexosamine alternating with uronic acid residues was also identified and on the basis of the presence of unacetylated hexosamine; these oligosaccharides are proposed to derive from HS. The presence of both odd and even-length oligosaccharides suggests both endo-beta-glucuronidase and endo-N-acetylhexosaminidase activities toward both glycosaminoglycans. Furthermore, the putative HS oligosaccharide structures identified indicate that heparanase activities are directed toward regions of both low and high sulfation, while the N-acetylhexosaminidase activity acted only in regions of low sulfation in this polysaccharide.     

Chimeric glycosaminoglycan oligosaccharides synthesized by enzymatic reconstruction and their use in substrate specificity determination of Streptococcus hyaluronidase

A method was developed for the reconstruction of glycosaminoglycan (GAG) oligosaccharides using the transglycosylation reaction of an endo-beta-N-acetylhexosaminidase, testicular hyaluronidase, under optimal conditions. Repetition of the transglycosylation using suitable combinations of various GAGs as acceptors and donors made it possible to custom-synthesize GAG oligosaccharides. Thus we prepared a library of chimeric GAG oligosaccharides with hybrid structures composed of disaccharide units such as GlcA-GlcNAc (from hyaluronic acid), GlcA-GalNAc (from chondroitin), GlcA-GalNAc4S (from chondroitin 4-sulfate), GlcA-GalNAc6S (from chondroitin 6-sulfate), IdoA-GalNAc (from desulfated dermatan sulfate), and GlcA-GalNAc4,6-diS (from chondroitin sulfate E). The specificity of the hyaluronidase from Streptococcus dysgalactiae (hyaluronidase SD) was then investigated using these chimeric GAG oligosaccharides as model substrates. The results indicate that the specificity of hyaluronidase SD is determined by the following restrictions at the nonreducing terminal side of the cleavage site: (i) at least one disaccharide unit (GlcA-GlcNAc) is necessary for the enzymatic action of hyaluronidase SD; (ii) cleavage is inhibited by sulfation of the N-acetylgalactosamine; (iii) hyaluronidase SD releases GlcA-GalNAc and IdoA-GalNAc units as well as GlcA-GlcNAc. At the reducing terminal side of the cleavage site, the sulfated residues on the N-acetylgalactosamines in the disaccharide units were found to have no influence on the cleavage. Additionally, we found that hyaluronidase SD can specifically and endolytically cleave the internal unsulfated regions of chondroitin sulfate chains. This demonstration indicates that custom-synthesized GAG oligosaccharides will open a new avenue in GAG glycotechnology.     

Analysis of novel over‐ and under‐sulfated glycosaminoglycan sequences by enzyme cleavage and multiple stage MS

We report on a novel strategy for identification of specific sulfation motifs in chondroitin/dermatan sulfate (CS/DS) chain derived from decorin (Dcn), based on enzyme cleavage and multistage MS (MSⁿ). Released CS/DS chains were digested with chondroitin B and in parallel with AC I lyases to obtain oligosaccharides of known hexuronic acid (HexA) epimerization. The depolymerized chains were separated by gel filtration, and collected di‐ and hexasaccharides were analyzed by ESI MSⁿ. MS² on bisulfated 4,5‐Δ‐HexAGalNAc revealed an additional sulfate ester group at 4,5‐Δ‐HexA. MS² data provided evidence upon GlcA sulfation in Dcn due to the fact that 4,5‐Δ‐HexA derived from GlcA after chondroitin AC I lyase treatment. Hexasaccharide screening in the MS¹ mode indicated direct correlation between the sulfate distribution and HexA epimerization. MSⁿ performed on ions that, according to mass calculation, correspond to pentasulfated [4,5‐Δ‐HexAGalNAc(GlcAGalNAc)₂], trisulfated [4,5‐Δ‐HexAGalNAc(GlcAGalNAc)₂] with IdoA‐derived 4,5‐Δ‐HexA at the nonreducing end, tetrasulfated [4,5‐Δ‐HexAGalNAc(IdoAGalNAc)₂] and monosulfated [4,5‐Δ‐HexAGalNAc(IdoAGalNAc)₂] with GlcA‐derived 4,5‐Δ‐HexA at the nonreducing end rendered fragmentation patterns confirming the presence of over‐, regular, and under‐sulfated regions as well as structural motifs having both types of HexA sulfated within Dcn CS/DS.     

Human salivary alpha-amylase Trp58 situated at subsite -2 is critical for enzyme activity.

The nonreducing end of the substrate-binding site of human salivary alpha-amylase contains two residues Trp58 and Trp59, which belong to beta2-alpha2 loop of the catalytic (beta/alpha)(8) barrel. While Trp59 stacks onto the substrate, the exact role of Trp58 is unknown. To investigate its role in enzyme activity the residue Trp58 was mutated to Ala, Leu or Tyr. Kinetic analysis of the wild-type and mutant enzymes was carried out with starch and oligosaccharides as substrates. All three mutants exhibited a reduction in specific activity (150-180-fold lower than the wild type) with starch as substrate. With oligosaccharides as substrates, a reduction in k(cat), an increase in K(m) and distinct differences in the cleavage pattern were observed for the mutants W58A and W58L compared with the wild type. Glucose was the smallest product generated by these two mutants in the hydrolysis oligosaccharides; in contrast, wild-type enzyme generated maltose as the smallest product. The production of glucose by W58L was confirmed from both reducing and nonreducing ends of CNP-labeled oligosaccharide substrates. The mutant W58L exhibited lower binding affinity at subsites -2, -3 and +2 and showed an increase in transglycosylation activity compared with the wild type. The lowered affinity at subsites -2 and -3 due to the mutation was also inferred from the electron density at these subsites in the structure of W58A in complex with acarbose-derived pseudooligosaccharide. Collectively, these results suggest that the residue Trp58 plays a critical role in substrate binding and hydrolytic activity of human salivary alpha-amylase.     

A tandem mass spectrometric approach to determination of chondroitin/dermatan sulfate oligosaccharide glycoforms

Dermatan sulfate (DS) chains are variants of chondroitin sulfate (CS) that are expressed in mammalian extracellular matrices and are particularly prevalent in skin. DS has been implicated in varied biological processes including wound repair, infection, cardiovascular disease, tumorigenesis, and fibrosis. The biological activities of DS have been attributed to its high content of IdoA(alpha1-3)GalNAc4S(beta1-4) disaccharide units. Mature CS/DS chains consist of blocks with high and low GlcA/IdoA ratios, and sulfation may occur at the 4- and/or 6-position of GalNAc and 2-position of IdoA. Traditional methods for the analysis of CS/DS chains involve differential digestion with specific chondroitinases followed by steps of chromatographic isolation of the products and di-saccharide analysis on the individual fraction. This work reports the use of tandem mass spectrometry to determine the patterns of sulfation and epimerization of CS/DS oligosaccharides in a single step. The approach is first validated and then applied to a series of skin DS samples and to decorins from three different tissues. DS samples ranged from 74 to 99% of CSB-like repeats, using this approach. Decorin samples ranged from 30% CSB-like repeats for those samples from articular cartilage to 75% for those from sclera. These values agree with known levels of glucuronyl C5-epimerase in these tissues.     

N-Acetylgalactosamine 4,6-O-sulfate residues mediate binding and activation of heparin cofactor II by porcine mucosal dermatan sulfate

Dermatan sulfate (DS) accelerates the inhibition of thrombin by heparin cofactor II (HCII). A hexasaccharide consisting of three l-iduronic acid 2-O-sulfate (IdoA2SO3)-->N-acetyl-D-galactosamine 4-O-sulfate (GalNAc4SO3) subunits was previously isolated from porcine skin DS and shown to bind HCII with high affinity. DS from porcine intestinal mucosa has a much lower content of this disaccharide but activates HCII with potency similar to that of porcine skin DS. Therefore, we sought to characterize oligosaccharides from porcine mucosal DS that interact with HCII. DS was partially depolymerized with chondroitinase ABC, and oligosaccharides containing 2-12 monosaccharide units were isolated. The oligosaccharides were then fractionated by anion-exchange and affinity chromatography on HCII-Sepharose, and the disaccharide compositions of selected fractions were determined. We found that the smallest oligosaccharides able to bind HCII were hexasaccharides. Oligosaccharides 6-12 units long that lacked uronic acid (UA)2SO3 but contained one or two GalNAc4,6SO3 residues bound, and binding was proportional to both oligosaccharide size and number of GalNAc4,6SO3 residues. Intact DS and bound dodecasaccharides contained predominantly IdoA but little D-glucuronic acid. Decasaccharides and dodecasaccharides containing one or two GalNAc4,6SO3 residues stimulated thrombin inhibition by HCII and prolonged the clotting time of normal but not HCII-depleted human plasma. These data support the hypothesis that modification of IdoA-->GalNAc4SO3 subunits in the DS polymer by either 2-O-sulfation of IdoA or 6-O-sulfation of GalNAc can generate molecules with HCII-binding sites and anticoagulant activity.     

Molecular cloning of the heparin/heparan sulfate delta 4,5 unsaturated glycuronidase from Flavobacterium heparinum,its recombinant expression in Escherichia coli,and biochemical determination of its unique substrate specificity

The soil bacterium Flavobacterium heparinum produces several enzymes that degrade heparan sulfate glycosaminoglycans (HSGAGs) in a sequence-specific manner. Among others, these enzymes include the heparinases and an unusual glycuronidase that hydrolyzes the unsaturated Delta4,5 uronic acid at the nonreducing end of oligosaccharides resulting from prior heparinase eliminative cleavage. We report here the molecular cloning of the Delta4,5 glycuronidase gene from the flavobacterial genome and its recombinant expression in Escherichia coli as a highly active enzyme. We also report the biochemical and kinetic characterization of this enzyme, including an analysis of its substrate specificity. We find that the Delta4,5 glycuronidase discriminates on the basis of both the glycosidic linkage and the sulfation pattern within its saccharide substrate. In particular, we find that the glycuronidase displays a strong preference for 1-->4 linkages, making this enzyme specific to heparin/heparan sulfate rather than 1-->3 linked glycosaminoglycans such as chondroitin/dermatan sulfate or hyaluronan. Finally, we demonstrate the utility of this enzyme in the sequencing of heparinase-derived HSGAG oligosaccharides.     

Fluorescent-tagged heparan sulfate precursor oligosaccharides to probe the enzymatic action of heparitinase I

Heparitinase I, a key lyase enzyme essential for structural analysis of heparan sulfate (HS), degrades HS domains that are undersulfated at glucuronyl residues through an elimination mechanism. Earlier studies employed viscosimetric measurements and electrophoresis to deduce the mechanism of action of heparitinase I and two other related lyases, heparitinase II and heparitinase III. However, these findings lack molecular evidence for the intermediates formed and could not distinguish whether the cleavage occurred from the reducing end or the nonreducing end. In the current study, 2-aminoacridone (2-AMAC)-labeled HS precursor oligosaccharides of various sizes were prepared to investigate the mechanism of heparitinase I-mediated depolymerization using sensitive and quantitative methodologies. Furthermore, fluorescent (2-AMAC) tagging of HS precursor oligosaccharides allowed us to distinguish fragments that result from cleavage of the substrates at various time intervals and sites farther away from the reducing and nonreducing ends of oligosaccharide substrates. This study provides the first direct molecular evidence for a predominantly random endolytic mechanism of cleavage of HS precursor oligosaccharides by heparitinase I. This robust strategy can be adapted to deduce the mechanism of action of other heparitinases and also to deduce structural information of complex HS oligosaccharides of biological importance.     

Enzymatic reconstruction of dermatan sulfate

We investigated the enzymatic reconstruction of dermatan sulfate (DS) using the transglycosylation reaction of testicular hyaluronidase. First, in order to insert the IdoA-GalNAc disaccharide unit into chondroitin sulfate chains consisting of GlcA-GalNAc disaccharide units, desulfated DS as a donor and pyridylaminated (PA) chondroitin 6-sulfate (Ch6S) hexasaccharide as an acceptor were subjected to a transglycosylation reaction using testicular hyaluronidase. The products were analyzed by HPLC, mass spectrometry, and enzymatic digestions, and the results indicated that one of the products was IdoA-GalNAc-(GlcA-GalNAc6S)(3)-PA. Next, when the resulting PA-Ch6S (hexa-)desulfated DS (di-)octasaccharide was used as an acceptor and chondroitin as a new donor, a decasaccharide having a GlcA-GalNAc-IdoA-GalNAc-(GlcA-GalNAc6S)(3) sequence was reconstructed. Using suitable combinations of donors and acceptors, it was possible to custom synthesize DS having any IdoA sequence as its uronic acid component. It is likely that application of this system would facilitate artificial reconstruction of variant DS having different specific functions.     

Ion-spray mass spectrometry for identification of the nonreducing terminal sugar of glycosaminoglycan

Various oligosaccharides from hyaluronic acid, which have glucuronic acid or N- acetylglucosamine at the nonreducing terminal, were prepared by digestion with a combination of testicular hyaluronidase and beta- glucuronidase. These oligo saccharides were analyzed by negative-mode ion-spray mass spectrometry (MS) with an atmospheric pressure ion source. Introduction of collisionally activated dissociation tandem mass spectrometry (CAD-MS/MS) produced ions derived from cleavage of the glycosidic bonds, allowing the structure to be analyzed. The CAD- MS/MS spectrum showed an intense and characteristic fragment ion at m/z 193 for oligosaccharides having glucuronic acid at the nonreducing terminal. On the other hand, this ion was not observed in the spectra of oligosaccharides having N- acetylglucosamine at the nonreducing terminal. Therefore, the fragmentation pattern revealed by CAD-MS/MS provides useful information for distinguishing glucuronic acid and N- acetylglucosamine at the nonreducing terminal of oligosaccharides derived from hyaluronic acid and other glycosaminoglycans. This ion- spray CAD-MS/MS technique was also applied successfully to the characterization of glycosaminoglycans reconstructed by glycotechnology.      

Purification of human liver microsomal epoxide hydrase. Differences in the properties of the human and rat enzymes.

Human liver microsomal epoxide hydrase has been highly purified to a specific activity (570 to 620 nmol/min/mg of protein) comparable to that of the rat enzyme using styrene oxide as substrate. Like the purified rat liver microsomal epoxide hydrase, the human enzyme has a minimum molecular weight of 49,000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and exhibits broad substrate specificity toward a variety of alkene and arene oxides. Despite these similarities, the human and rat enzymes are different proteins as judged by their immunochemical properties as well as their relative catalytic activities toward certain substrates.     

Heparin/Heparan Sulfate N-Sulfamidase from Flavobacterium heparinum: STRUCTURAL AND BIOCHEMICAL INVESTIGATION OF CATALYTIC NITROGEN-SULFUR BOND CLEAVAGE*

Sulfated polysaccharides such as heparin and heparan sulfate glycosaminoglycans (HSGAGs) are chemically and structurally heterogeneous biopolymers that that function as key regulators of numerous biological functions. The elucidation of HSGAG fine structure is fundamental to understanding their functional diversity, and this is facilitated by the use of select degrading enzymes of defined substrate specificity. Our previous studies have reported the cloning, characterization, recombinant expression, and structure-function analysis in Escherichia coli of the Flavobacterium heparinum 2-O-sulfatase and 6-O-sulfatase enzymes that cleave O-sulfate groups from specific locations of the HSGAG polymer. Building on these preceding studies, we report here the molecular cloning and recombinant expression in Escherichia coli of an N-sulfamidase, specific for HSGAGs. In addition, we examine the basic enzymology of this enzyme through molecular modeling studies and structure-function analysis of substrate specificity and basic biochemistry. We use the results from these studies to propose a novel mechanism for nitrogen-sulfur bond cleavage by the N-sulfamidase. Taken together, our structural and biochemical studies indicate that N-sulfamidase is a predominantly exolytic enzyme that specifically acts on N-sulfated and 6-O-desulfated glucosamines present as monosaccharides or at the nonreducing end of odd-numbered oligosaccharide substrates. In conjunction with the previously reported specificities for the F. heparinum 2-O-sulfatase, 6-O-sulfatase, and unsaturated glucuronyl hydrolase, we are able to now reconstruct in vitro the defined exolytic sequence for the heparin and heparan sulfate degradation pathway of F. heparinum and apply these enzymes in tandem toward the exo-sequencing of heparin-derived oligosaccharides.     

Computational analyses of the catalytic and heparin-binding sites and their interactions with glycosaminoglycans in glycoside hydrolase family 79 endo-β-D-glucuronidase (heparanase)

Mammalian heparanase is an endo-β-glucuronidase associated with cell invasion in cancer metastasis, angiogenesis and inflammation. Heparanase cleaves heparan sulfate proteoglycans in the extracellular matrix and basement membrane, releasing heparin/heparan sulfate oligosaccharides of appreciable size. This in turn causes the release of growth factors, which accelerate tumor growth and metastasis. Heparanase has two glycosaminoglycan-binding domains; however, no three-dimensional structure information is available for human heparanase that can provide insights into how the two domains interact to degrade heparin fragments. We have constructed a new homology model of heparanase that takes into account the most recent structural and bioinformatics data available. Heparin analogs and glycosaminoglycan mimetics were computationally docked into the active site with energetically stable ring conformations and their interaction energies were compared. The resulting docked structures were used to propose a model for substrates and conformer selectivity based on the dimensions of the active site. The docking of substrates and inhibitors indicates the existence of a large binding site extending at least two saccharide units beyond the cleavage site (toward the nonreducing end) and at least three saccharides toward the reducing end (toward heparin-binding site 2). The docking of substrates suggests that heparanase recognizes the N-sulfated and O-sulfated glucosamines at subsite +1 and glucuronic acid at the cleavage site, whereas in the absence of 6-O-sulfation in glucosamine, glucuronic acid is docked at subsite +2. These findings will help us to focus on the rational design of heparanase-inhibiting molecules for anticancer drug development by targeting the two heparin/heparan sulfate recognition domains.     

Oligosaccharide library-based assessment of heparan sulfate 6-O-sulfotransferase substrate specificity

Heparan sulfate mediates numerous complex biological processes. Its action critically depends on the amount and the positions of O-sulfate groups (iduronyl 2-O-sulfates, glucosaminyl 6-O- and 3-O-sulfates) that form binding sites for proteins. The structures and distribution of these protein-binding domains are influenced by the expression and substrate specificity of heparan sulfate biosynthetic enzymes. We describe a general approach to assess substrate specificities of enzymes involved in glycosaminoglycan metabolism, here applied to 6-O-sulfotransferases involved in heparan sulfate biosynthesis. To understand how 2-O-sulfation affects subsequent 6-O-sulfation reactions, the substrate specificity of 6-O-sulfotransferase 3 was probed using substrates from a heparin-based octasaccharide library. Purified 3H-labeled N-sulfated octasaccharides from a library designed to sample 2-O-sulfated motifs were used as sulfate acceptors, 3'-phosphoadenosine 5'-phosphosulfate as sulfate donor, and cell extract from 6-O-sulfotransferase 3-overexpressing 293 cells as enzyme source in the 6-O-sulfotransferase-catalyzed reactions. The first 6-O-sulfate group was preferentially incorporated at the internal glucosamine unit of the octasaccharide substrate. As the reaction proceeded, the octasaccharides acquired three 6-O-sulfate groups. The specificities toward competing octasaccharide substrates, for 6-O-sulfotransferase 2 and 6-O-sulfotransferase 3, were determined using overexpressing 293 cell extracts and purified octasaccharides. Both 6-O-sulfotransferases showed a preference for 2-O-sulfated substrates. The specificity toward substrates with two to three 2-O-sulfate groups was three to five times higher as compared with octasaccharides with no or one 2-O-sulfate group.     

Screening for hetero-transglycosylating activities in extracts from nasturtium (Tropaeolum majus)

Using combinations of different polysaccharides as glycosyl donors and of oligosaccharides fluorescently labeled by sulforhodamine (SR) as glycosyl acceptors, we screened for the presence of transglycosylating activities in extracts from nasturtium (Tropaeolum majus). Besides xyloglucan endotransglycosylase/hydrolase (XTH/XET, EC 2.4.1.207) activity, which transfers xyloglucanosyl residues from xyloglucan (XG) to XG-derived oligosaccharides (XGOs), a glycosyl transfer from XG to SR-labeled cellooligosaccharides and laminarioligosaccharides has been detected. The XGOs also served as acceptors for the glycosyl transfer from soluble cellulose derivatives carboxymethyl cellulose and hydroxyethylcellulose. The effectivity of these polysaccharides as glycosyl donors for transfer to XG-derived octasaccharide [1-3H]XXLGol decreased in the order XG > HEC > CMC. Isoelectric focusing in polyacrylamide gels showed that bands corresponding to hetero-transglycosylase activities coincided with zones corresponding to XTH/XET. These results can be explained as due either to substrate non-specificity of certain isoenzymes of XTH/XET or to existence of enzymes catalyzing a hetero-transfer, that is the formation of covalent linkages between different types of carbohydrate polymers.     

Characterization of chondroitin sulfate lyase ABC from Bacteroides thetaiotaomicron WAL2926

Chondroitin sulfate ABC lyase (ChonABC) is an enzyme with broad specificity that depolymerizes via beta-elimination chondroitin sulfate (CS) and dermatan sulfate (DS) glycosaminoglycans (GAGs). ChonABC eliminates the glycosidic bond of its GAG substrates on the nonreducing end of their uronic acid component. This lyase possesses the unusual ability to act on both epimers of uronic acid, either glucuronic acid present in CS or iduronic acid in DS. Recently, we cloned, purified, and determined the three-dimensional structure of a broad specificity chondroitin sulfate ABC lyase from Bacteroides thetaiotaomicron (BactnABC) and identified two sets of catalytic residues. Here, we report the detailed biochemical characterization of BactnABC together with extensive site-directed mutagenesis resulting in characterization of the previously identified active site residues. BactnABC's catalysis is stimulated by Ca(2+) and Mg(2+) cations, particularly against DS. It displays extremely low activity toward hyaluronic acid and no activity toward heparin/heparan sulfate. Degradation of CS and DS by BactnABC yields only disaccharide products, pointing to an exolytic mode of action. The kinetic evaluations of the active-site mutants indicate that CS and DS substrates bind in the same active site, which is accompanied by a conformational change bringing the two sets of active site residues together. Conservative replacements of key residues suggest that His345 plays the role of a general base, initiating the degradation by abstracting the C5 bound proton from DS substrates, whereas either Tyr461 or His454 perform the equivalent role for CS substrates. Tyr461 is proposed, as well, to serve as general acid, completing the degradation of both CS and DS by protonating the leaving group.     

Different acceptor specificities of two glucuronyltransferases involved in the biosynthesis of HNK-1 carbohydrate

The biosynthesis of HNK-1 carbohydrate is mainly regulated by two glucuronyltransferases (GlcAT-P and GlcAT-S) and a sulfotransferase (HNK-1 ST). To determine how the two glucuronyltransferases are involved in the biosynthesis of the HNK-1 carbohydrate, we prepared soluble forms of GlcAT-P and GlcAT-S fused with the IgG-binding domain of protein A and then compared the enzymatic properties of the two enzymes. Both GlcAT-P and GlcAT-S transferred glucuronic acid (GlcA) not only to a glycoprotein acceptor, asialoorosomucoid (ASOR), but also to a glycolipid acceptor, paragloboside. The activity of GlcAT-P toward ASOR was enhanced fivefold in the presence of sphingomyelin, but there were no effects on that of GlcAT-S. The activities of the two enzymes toward paragloboside were only detected in the presence of phospholipids such as phosphatidylinositol. Kinetic analysis revealed that the K(m) value of GlcAT-P for ASOR was 10 times lower than that for paragloboside. Furthermore, acceptor specificity analysis involving various oligosaccarides revealed that GlcAT-P specifically recognized N-acetyllactosamine (Galbeta1-4GlcNAc) at the nonreducing terminals of acceptor substrates. In contrast, GlcAT-S recognized not only the terminal Galbeta1-4GlcNAc structure but also the Galbeta1-3GlcNAc structure and showed the highest activity toward triantennary N-linked oligosaccharides. GlcAT-P transferred GlcA to NCAM about twice as much as to ASOR, whereas GlcAT-S did not show any activity toward NCAM. These lines of evidence indicate that these two enzymes have significantly different acceptor specificities, suggesting that they may synthesize functionally and structurally different HNK-1 carbohydrates in the nervous system.     

Examination of the substrate specificity of heparin and heparan sulfate lyases

We have examined the activities of different preparations of heparin and heparan sulfate lyases from Flavobacterium heparinum. The enzymes were incubated with oligosaccharides of known size and sequence and with complex polysaccharide substrates, and the resulting degradation products were analyzed by strong-anion-exchange high-performance liquid chromatography and by oligosaccharide mapping using gradient polyacrylamide gel electrophoresis. Heparinase (EC 4.2.2.7) purified in our laboratory and a so-called Heparinase I (Hep I) from a commercial source yielded similar oligosaccharide maps with heparin substrates and displayed specificity for di- or trisulfated disaccharides of the structure----4)-alpha-D-GlcNp2S(6R)(1----4)-alpha-L-IdoAp2S( 1----(where R = O-sulfo or OH). Oligosaccharide mapping with two different commercial preparations of heparan sulfate lyase [heparitinase (EC 4.2.2.8)] indicated close similarities in their depolymerization of heparan sulfate. Furthermore, these enzymes only degraded defined oligosaccharides at hexosaminidic linkages with glucuronic acid:----4)-alpha-D-GlcNpR(1----4)-beta-D-GlcAp(1----(where R = N-acetamido or N-sulfo). The enzymes showed activity against solitary glucuronate-containing disaccharides in otherwise highly sulfated domains including the saccharide sequence that contains the antithrombin binding region in heparin. A different commercial enzyme, Heparinase II (Hep II), displayed a broad spectrum of activity against polysaccharide and oligosaccharide substrates, but mapping data indicated that it was a separate enzyme rather than a mixture of heparinase and heparitinase/Hep III. When used in conjunction with the described separation procedures, these enzymes are powerful reagents for the structural/sequence analysis of heparin and heparan sulfate.     

Development of a functional bioassay for arylsulfatase B using the natural substrates of the enzyme

A functional bioassay has been developed for measuring the intracellular activity of recombinant human arylsulfatase B (rhASB) on its natural glycosaminoglycan (GAG) substrates, dermatan sulfate (DS), and chondroitin sulfate (CS) when the enzyme is taken up into cultured ASB-deficient human fibroblasts (GM00519). The enzyme ASB is a lysosomal exohydrolase, cleaving sulfate from the N-acetylgalactosamine-4-sulfate (GalNAc-4S) residue at the nonreducing terminal of GAG structures. ASB-deficient cells accumulate DS and CS, which may be partially hydrolyzed by other lysosomal hydrolases, with the reactions stopping if a GalNAc-4S residue is reached on the nonreducing end of the oligosaccharide. When rhASB is added to the culture medium, the enzyme is taken up and translocates to the lysosomes and the intracellular DS and CS are depleted, demonstrating that the uptake of rhASB is able to restore lysosomal function in an in vitro cell-based assay. The accumulation and depletion of DS and CS are measured by digesting the residual intracellular DS and CS content with chondroitin ABC lyase and monitoring a characteristic disaccharide digestion product by laser-induced fluorescence–capillary zone electrophoresis (LIF–CZE). In the proposed assay format, GM00519 cells are cultured 5 weeks postconfluence to accumulate DS/CS, followed by incubation with rhASB (1–20 pM) for 5 days, and the CS/DS depletion profiles are compared between samples. The assay measures depletion of DS/CS independently of their molecular size or processing state; in this approach, all DS- and CS-like substances accumulating in the absence of ASB activity are considered to be natural substrates of the enzyme.