Mass Spectrometry Analysis of Glycopeptides Enriched by Anion Exchange-Mediated Methods Reveals PolyLacNAc-Extended N-Glycans in Integrins and Tetraspanins in Melanoma Cells

Glycosylation is a key modulator of the functional state of proteins. Recent developments in large-scale analysis of intact glycopeptides have enabled the identification of numerous glycan structures that are relevant in pathophysiological processes. However, one motif found in N-glycans, poly-N-acetyllactosamine (polyLacNAc), still poses a substantial challenge to mass spectrometry-based glycoproteomic analysis due to its relatively low abundance and large size. In this work, we developed approaches for the systematic mapping of polyLacNAc-elongated N-glycans in melanoma cells. We first evaluated five anion exchange-based matrices for enriching intact glycopeptides and selected two materials that provided better overall enrichment efficiency. We then tested the robustness of the methodology by quantifying polyLacNAc-containing glycopeptides as well as changes in protein fucosylation and sialylation. Finally, we applied the optimal enrichment methods to discover glycopeptides containing polyLacNAc motifs in melanoma cells and found that integrins and tetraspanins are substantially modified with these structures. This study demonstrates the feasibility of glycoproteomic approaches for identification of glycoproteins with polyLacNAc motifs.

In silico analysis of the human milk oligosaccharide glycome reveals key enzymes of their biosynthesis

Human milk oligosaccharides (HMOs) form the third most abundant component of human milk and are known to convey several benefits to the neonate, including protection from viral and bacterial pathogens, training of the immune system, and influencing the gut microbiome. As HMO production during lactation is driven by enzymes that are common to other glycosylation processes, we adapted a model of mucin-type GalNAc-linked glycosylation enzymes to act on free lactose. We identified a subset of 11 enzyme activities that can account for 206 of 226 distinct HMOs isolated from human milk and constructed a biosynthetic reaction network that identifies 5 new core HMO structures. A comparison of monosaccharide compositions demonstrated that the model was able to discriminate between two possible groups of intermediates between major subnetworks, and to assign possible structures to several previously uncharacterised HMOs. The effect of enzyme knockouts is presented, identifying β-1,4-galactosyltransferase and β-1,3-N-acetylglucosaminyltransferase as key enzyme activities involved in the generation of the observed HMO glycosylation patterns. The model also provides a synthesis chassis for the most common HMOs found in lactating mothers.

UDP-Gal: GlcNAc-R beta1,4-galactosyltransferase--a target enzyme for drug design. Acceptor specificity and inhibition of the enzyme

Galactosyltransferases are important enzymes for the extension of the glycan chains of glycoproteins and glycolipids, and play critical roles in cell surface functions and in the immune system. In this work, the acceptor specificity and several inhibitors of bovine beta1,4-Gal-transferase T1 (beta4GalT, EC 2.4.1.90) were studied. Series of analogs of N-acetylglucosamine (GlcNAc) and GlcNAc-carrying glycopeptides were synthesized as acceptor substrates. Modifications were made at the 3-, 4- and 6-positions of the sugar ring of the acceptor, in the nature of the glycosidic linkage, in the aglycone moiety and in the 2-acetamido group. The acceptor specificity studies showed that the 4-hydroxyl group of the sugar ring was essential for beta4GalT activity, but that the 3-hydroxyl could be replaced by an electronegative group. Compounds having the anomeric beta-configuration were more active than those having the alpha-configuration, and O-, S- and C-glycosyl compounds were all active as substrates. The aglycone was a major determinant for the rate of Gal-transfer. Derivatives containing a 2-naphthyl aglycone were inactive as substrates although quinolinyl groups supported activity. Several compounds having a bicyclic structure as the aglycone were found to bind to the enzyme and inhibited the transfer of Gal to control substrates. The best small hydrophobic GlcNAc-analog inhibitor was found to be 1-thio-N-butyrylGlcNbeta-(2-naphthyl) with a K(i) of 0.01 mM. These studies help to delineate beta4GalT-substrate interactions and will aid in the development of biologically applicable inhibitors of the enzyme.

Oligosaccharide Preferences of β1,4-Galactosyltransferase-I: Crystal Structures of Met340His Mutant of Human β1,4-Galactosyltransferase-I with a Pentasaccharide and Trisaccharides of the N-Glycan Moiety

beta-1,4-Galactosyltransferase-I (beta4Gal-T1) transfers galactose from UDP-galactose to N-acetylglucosamine (GlcNAc) residues of the branched N-linked oligosaccharide chains of glycoproteins. In an N-linked biantennary oligosaccharide chain, one antenna is attached to the 3-hydroxyl-(1,3-arm), and the other to the 6-hydroxyl-(1,6-arm) group of mannose, which is beta-1,4-linked to an N-linked chitobiose, attached to the aspargine residue of a protein. For a better understanding of the branch specificity of beta4Gal-T1 towards the GlcNAc residues of N-glycans, we have carried out kinetic and crystallographic studies with the wild-type human beta4Gal-T1 (h-beta4Gal-T1) and the mutant Met340His-beta4Gal-T1 (h-M340H-beta4Gal-T1) in complex with a GlcNAc-containing pentasaccharide and several GlcNAc-containing trisaccharides present in N-glycans. The oligosaccharides used were: pentasaccharide GlcNAcbeta1,2-Manalpha1,6 (GlcNAcbeta1,2-Manalpha1,3)Man; the 1,6-arm trisaccharide, GlcNAcbeta1,2-Manalpha1,6-Manbeta-OR (1,2-1,6-arm); the 1,3-arm trisaccharides, GlcNAcbeta1,2-Manalpha1,3-Manbeta-OR (1,2-1,3-arm) and GlcNAcbeta1,4-Manalpha1,3-Manbeta-OR (1,4-1,3-arm); and the trisaccharide GlcNAcbeta1,4-GlcNAcbeta1,4-GlcNAc (chitotriose). With the wild-type h-beta4Gal-T1, the K(m) of 1,2-1,6-arm is approximately tenfold lower than for 1,2-1,3-arm and 1,4-1,3-arm, and 22-fold lower than for chitotriose. Crystal structures of h-M340H-beta4Gal-T1 in complex with the pentasaccharide and various trisaccharides at 1.9-2.0A resolution showed that beta4Gal-T1 is in a closed conformation with the oligosaccharide bound to the enzyme, and the 1,2-1,6-arm trisaccharide makes the maximum number of interactions with the enzyme, which is in concurrence with the lowest K(m) for the trisaccharide. Present studies suggest that beta4Gal-T1 interacts preferentially with the 1,2-1,6-arm trisaccharide rather than with the 1,2-1,3-arm or 1,4-1,3-arm of a bi- or tri-antennary oligosaccharide chain of N-glycan.

Biosynthesis and function of beta 1,6 branched mucin-type glycans

The contribution of carbohydrate structure to biomolecular, cellular, and organismal function is well-established, but has not yet received the attention it deserves, perhaps due to the complexity of the structures involved and to a lack of simple experimental methods for relating structure and function. In particular, beta1,6 GlcNAc branching plays a key functional role in processes ranging from inflammation and immune system function to tumor cell metastasis. For instance, synthesis of the core 2 beta1,6 branched structure in the mucin glycan chain by C2GnT enables the expression of functional structures at the termini of polylactosamine chains, such as blood group antigens and sialyl Lewis x. Also, IGnT can create multiple branches on the polylactosamine chain, which may serve as a mechanism for amplifying the functional potency of cell surface glycoproteins and glycolipids. The family of enzymes which creates beta1,6 branched structure in mucin glycans is proving to be quite complex, since multiple isoforms appear to exist for these enzymes, and some of the enzymes are adept at forming more than one type of beta1,6 branched structure, as in the case of C2GnT-M. Furthermore, the enzymes do not appear to be restricted to acting on mucin-type acceptor structures, but are able to act on glycolipid structures as well. Much remains to be learned regarding the specific biological niche filled by each of these enzymes and how their activities complement one another, as well as the manner in which the activities of these enzymes are regulated in the cell.

Characterization of a core alpha1-->3-fucosyltransferase from the snail Lymnaea stagnalis that is involved in the synthesis of complex-type N-glycans

We have identified a core alpha1-->3-fucosyltransferase activity in the albumin and prostate glands of the snail Lymnaea stagnalis. Incubation of albumin gland extracts with GDP-[(14)C]Fuc and asialo/agalacto-glycopeptides from human fibrinogen resulted in a labeled product in 50% yield. Analysis of the product by 400 MHz (1)H-NMR spectroscopy showed the presence of a Fuc residue alpha1-->3-linked to the Asn-linked GlcNAc. Therefore, the enzyme can be identified as a GDP-Fuc:GlcNAc (Asn-linked) alpha1-->3-fucosyltransferase. The enzyme acts efficiently on asialo/agalacto-glycopeptides from both human fibrinogen and core alpha1-->6-fucosylated human IgG, whereas bisected asialo/agalacto-glycopeptide could not serve as an acceptor. We propose that the enzyme functions in the synthesis of core alpha1-->3-fucosylated complex-type glycans in L. stagnalis. Core alpha1-->3-fucosylation of the asparagine-linked GlcNAc of plant- and insect-derived glycoproteins is often associated with the allergenicity of such glycoproteins. Since allergic reactions have been reported after consumption of snails, the demonstration of core alpha1-->3-fucosylation in L. stagnalis may be clinically relevant.

Poly-N-acetyllactosamine synthesis in branched N-glycans is controlled by complemental branch specificity of I-extension enzyme and beta1,4-galactosyltransferase I

Poly-N-acetyllactosamine is a unique carbohydrate that can carry various functional oligosaccharides, such as sialyl Lewis X. It has been shown that the amount of poly-N-acetyllactosamine is increased in N-glycans, when they contain Galbeta1-->4GlcNAcbeta1-->6(Galbeta1-->4GlcNAcbeta1 -->2)Manalpha1-->6 branched structure. To determine how this increased synthesis of poly-N-acetyllactosamines takes place, the branched acceptor was incubated with a mixture of i-extension enzyme (iGnT) and beta1, 4galactosyltransferase I (beta4Gal-TI). First, N-acetyllactosamine repeats were more readily added to the branched acceptor than the summation of poly-N-acetyllactosamines formed individually on each unbranched acceptor. Surprisingly, poly-N-acetyllactosamine was more efficiently formed on Galbeta1-->4GlcNAcbeta1-->2Manalpha-->R side chain than in Galbeta1-->4GlcNAcbeta1-->6Manalpha-->R, due to preferential action of iGnT on Galbeta1-->4GlcNAcbeta1-->2Manalpha-->R side chain. On the other hand, galactosylation was much more efficient on beta1,6-linked GlcNAc than beta1,2-linked GlcNAc, preferentially forming Galbeta1-->4GlcNAcbeta1-->6(GlcNAcbeta1-->2)Manalph a1-->6Manbeta -->R. Starting with this preformed acceptor, N-acetyllactosamine repeats were added almost equally to Galbeta1-->4GlcNAcbeta1-->6Manalpha-->R and Galbeta1-->4GlcNAcbeta1-->2Manalpha-->R side chains. Taken together, these results indicate that the complemental branch specificity of iGnT and beta4Gal-TI leads to efficient and equal addition of N-acetyllactosamine repeats on both side chains of GlcNAcbeta1-->6(GlcNAcbeta1-->2)Manalpha1-->6Manbet a-->R structure, which is consistent with the structures found in nature. The results also suggest that the addition of Galbeta1-->4GlcNAcbeta1-->6 side chain on Galbeta1-->4GlcNAcbeta1-->2Man-->R side chain converts the acceptor to one that is much more favorable for iGnT and beta4Gal-TI.

The acceptor substrate specificity of human beta4-galactosyltransferase V indicates its potential function in O-glycosylation

In order to assess the function of the different human UDP-Gal:GlcNAc beta4-galactosyltransferases, the cDNAs of two of them, beta4-GalT I and beta4-GalT V, were expressed in the baculovirus/insect cell expression system. The soluble recombinant enzymes produced were purified from the medium and used to determine their in vitro substrate specificities. The specific activity of the recombinant beta4-GalT V was more than 15 times lower than that of beta4-GalT I, using GlcNAc beta-S-pNP as an acceptor. Whereas beta4-GalT I efficiently acts on all substrates having a terminal beta-linked GlcNAc, beta4-GalT V appeared to be far more restricted in acceptor usage. Beta4-GalT V acts with high preference on acceptors that contain the GlcNAc beta1-->6GalNAc structural element, as found in O-linked core 2-, 4- and 6-based glycans, but not on substrates related to V-linked or blood group I-active oligosaccharides. These results suggest that beta4-GalT V may function in the synthesis of lacNAc units on O-linked chains, particularly in tissues which do not express beta4-GalT I, such as brain.

Bovine mammary gland UDP-GalNAc:GlcNAcbeta-R beta1-->4-N-acetylgalactosaminyltransferase is glycoprotein hormone nonspecific and shows interaction with alpha-lactalbumin

We have identified a novel N -acetylgalactosaminyltransferase activity in lactating bovine mammary gland membranes. Acceptor specificity studies and analysis of products obtained in vitro by 400 MHz1H-NMR spectroscopy revealed that the enzyme catalyses the transfer of N -acetylgalactosamine (GalNAc) from UDP-GalNAc to acceptor substrates carrying a terminal, beta-linked N -acetylglucosamine (GlcNAc) residue and establishes a beta1-->4-linkage forming a GalNAcbeta1-->4GlcNAc ( N, N '-diacetyllactosediamine, lacdiNAc) unit. Therefore, the enzyme can be identified as a UDP-GalNAc:GlcNAcbeta-R beta1-->4-N-acetylgalactosaminyltransferase (beta4-GalNAcT). This enzyme resembles invertebrate beta4-GalNAcT as well as mammalian beta4-galactosyltransferase (beta4-GalT) in acceptor specificity. It can, however, be clearly distinguished from the pituitary hormone-specific beta4-GalNAcT by its incapability of acting with an elevated activity on a glycoprotein substrate carrying a hormone-specific peptide motif. Furthermore, the GalNAcT activity appeared not to be due to a promiscuous action of a beta4-GalT as could be demonstrated by comparing the beta4-GalNAcT and beta4-GalT activities of the mammary gland, bovine colostrum, and purified beta4-GalT, by competition studies with UDP-GalNAc and UDP-Gal, and by use of an anti-beta4-GalT polyclonal inhibiting antibody. Interestingly, under conditions where mammalian beta4-GalT forms with alpha-lactalbumin (alpha-LA) the lactose synthase complex, the mammary gland beta4-GalNAcT was similarly induced by alpha-LA to act on Glc with an increased efficiency yielding the lactose analog GalNAcbeta1-->4Glc. This enzyme thus forms the second example of a mammalian glycosyltransferase the specificity of which can be modified by this milk protein. It is proposed that the mammary gland beta4-GalNAcT functions in the synthesis of lacdiNAc-based, complex-type glycans frequently occurring on bovine milk glycoproteins. The action of this enzyme is to be considered when aiming at the production of properly glycosylated protein biopharmaceuticals in the milk of transgenic dairy animals.

Enzyme-aided construction of medium-sized alditols of complete O-linked saccharides. The constructed hexasaccharide alditol Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4GlcNAc beta 1-6(Gal beta 1-3)GalNAc-ol resists the action of endo-beta-galactosidase from Bacteroides fragilis

We have constructed by enzyme-aided in vitro synthesis a hexasaccharide alditol Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4GlcNAc beta 1-6(Gal beta 1-3) GalNAc-ol and shown that it resists the action of endo-beta-galactosidase from Bacteroides fragilis under conditions where a related pentasaccharide alditol, GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-6(Gal beta 1-3)GalNAc-ol, was completely cleaved. Together with earlier results from this laboratory, our present data imply that endo-beta-galactosidase from B. fragilis, apparently, can be used to distinguish between GlcNAc beta 1-6Gal and GlcNAc beta 1-3Gal units within linear backbone sequences of all known types of oligo-(N-acetyllactosamino)glycans.

Elongation of both branches of biantennary backbones of oligo-(N-acetyllactosamino)glycans by human serum (1----3)-N-acetyl-beta-D- glucosaminyltransferase

Partial reactions catalyzed by a (1----3)-N-acetyl-beta-D- glucosaminyltransferase (EC2.4.1.149), known to be present in human serum, were studied by use of biantennary "backbone" saccharides of oligo-N-acetyllactosamine-type as acceptors. Incubation of the radiolabeled blood-group I-active hexasaccharide, beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----3)-[beta-D-Galp- (1----4)-beta-D-GlcpNAc-(1----6)]-beta-D-Galp-(1----4)-D-GlcNAc (1) and UDP-GlcNAc with serum gave first a transient 1:1 mixture of two isomeric heptasaccharides, beta-D-GlcpNAc-(1----3)-beta-D-Galp-(1----4)-beta-D- GlcpNAc-(1----3)-[beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----6)]-beta-D- Galp-(1----4)-D-GlcNAc (2) and beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----3)-[beta-D-GlcpNAc-(1----3)- beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----6)]-beta-D-Galp-(1----4)-D-Glc NAc (3), showing that both branches of 1 react equally well. The two heptasaccharides reacted further in the incubation mixture to form the radiolabeled octasaccharide, beta-D-GlcpNAc-(1----3)-beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----3)-[be ta-D- GlcpNAc-(1----3)-beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----6)]-beta-D-Ga lp- (1----4)-D-GlcNAc (4); during this second reaction, the composition of the heptasaccharide mixture remained unchanged, indicating that 2 and 3 reacted at approximately equal rates. The heptasaccharides 2 and 3 could not be separated from each other, but they could be detected, identified, and quantitatively determined by stepwise enzymic degradations. Partial (1----3)-N-acetyl-beta-D-glucosaminylation reactions, carried out with another acceptor, the branched pentasaccharide, beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----3)-[beta-D-Galp-(1----4)-beta- D- GlcpNAc-(1----6)]-beta-D-Gal (11), revealed that it reacted also equally well at both branches.(ABSTRACT TRUNCATED AT 400 WORDS)

Construction of linear GlcNAc beta 1-6Gal beta 1-OR type oligosaccharides by partial cleavage of GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-OR sequences with jack bean beta-N-acetylhexosaminidase

Radiolabelled GlcNAc beta 1-3(GlcNAc beta 1-6)Gal (1), GlcNAc beta 1-3)GlcNAc beta 1-6)Gal beta 1-OCH3 (4), GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4Glc (7), and GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc (10) were cleaved partially with jack bean beta-N-acetylhexosaminidase (EC 3.2.1.30), and the digests were analysed chromatographically. All four oligosaccharides were hydrolysed faster at the (1-6) branch, than at the (1-3) branch, but a high branch specificity was observed only with the glycan 4. The saccharides 1 and 7 resembled each other in the kinetics of the enzyme-catalysed release of their two non-reducing N-acetylglucosamine units, but the glycan 10 was rather different. The partial digestions made it possible to obtain radiolabelled GlcNAc beta 1-6Gal, GlcNAc beta 1-6Gal beta 1-OCH3, GlcNAc beta 1-6Gal beta 1-4Glc, and, in particular, GlcNAc beta 1-6Gal beta 1-4GlcNAc.

Synthetic mucin fragments. Benzyl O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-(1----3)-O-beta-D- galactopyranosyl-(1----3)-O-[(2-acetamido-2-deoxy-beta-D-glucopyran osy l)- (1----6)]-2-acetamido-2-deoxy-alpha-D-galactopyranoside and benzyl O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-(1----3)-O-beta-D- galactopyranosyl-(1----3)-O-[beta-D-galactopyranosyl-(1----6)]-2-ac eta mido- 2-deoxy-alpha-D-galactopyranoside

Treatment of benzyl 2-acetamido-2-deoxy-alpha-D-galactopyranoside with 4-methoxybenzaldehyde dimethyl acetal in N,N-dimethylformamide in the presence of 4-toluenesulfonic acid afforded the 4,6-O-(4-methoxybenzylidene) acetal, which was glycosylated with 2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl bromide (1). Reductive ring-opening of the acetal group provided a 6-O-(4-methoxybenzyl) derivative (4) which was glycosylated with 1, followed by removal of the 4-methoxybenzyl ether group, to give benzyl 2-acetamido-2-deoxy-3,4-di-O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyran osyl)- alpha-D-galactopyranoside (7). The disaccharide diol 5, obtained from 4, and benzyl O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-glucopyranosyl-(1----3) -O- (2,4,6-tri-O-acetyl-beta-D-galactopyranosyl)-(1----3)-2-acetamido-2-deox y- alpha-D-galactopyranoside (11) were similarly glycosylated with 1 to afford a trisaccharide derivative 9 and a tetrasaccharide derivative 14, respectively. Diol 11 was also condensed with 2-methyl-(3,4,6-tri-O-acetyl-1,2-di-deoxy-alpha-D-glucopyrano)-[2, 1-d]-2- oxazoline to give a tetrasaccharide derivative 16. O-Deacetylation of trisaccharides 7 and 9, and tetrasaccharides 14 and 16 furnished trisaccharides 8 and 10, and the title tetrasaccharides 15 and 17, respectively. The structures of compounds 8, 10, 15, and 17 were established by 13C-n.m.r. spectroscopy.

The use of an immobilised cyclic multi-enzyme system to synthesise branched penta- and hexa-saccharides associated with blood-group I epitopes

The branched hexasaccharide beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----3)- [beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----6)]-beta-D-Galp-(1----4)-beta -D- GlcOMe (1) was obtained on a 0.1-mmol scale by enzymic bigalactosylation of the tetrasaccharide beta-D-GlcpNAc-(1----3)-[beta-D-GlcpNAc-(1----6)]-beta-D-Galp-(1----4)-b eta- D-GlcOMe by the use of an immobilised multi-enzyme system which regenerated UDP-alpha-D-galactose in situ. The formation of 1 proceeded in two steps. An intermediate compound was identified on the basis of 1H- and 13C-n.m.r. data as the pentasaccharide beta-D-GlcpNAc-(1----3)-[beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----6)]-b eta- D-Galp-(1----4)-beta-D-GlcOMe.

Branch specificity of beta-D-galactosidase from Escherichia coli

The "branch specificities" of the beta-D-galactosidases from Escherichia coli, jack bean, Aspergillus niger, and human liver were investigated with two branched oligosaccharide substrates, one which forms part of a complex-type biantennary N-linked glycan (compound 1) and a structure having blood group I activity (compound 2), respectively. Both substrates were available as radioactive compounds having a known distribution of 3H and 14C label in each of the terminal galactosyl groups, which allowed accurate estimation of the branch specificity of the enzymes from the ratio of 3H and 14C radioactivity in the galactose released by these hydrolases. It was found that the beta-D-galactosidase from E. coli preferentially released the galactosyl group at the 1----3 branch of compound 1 and that the 1----6 branch of compound 2. By contrast, the other beta-D-galactosidases investigated showed little or no branch specificity. These results suggest that the branch specificity of the beta-D-galactosidase from E. coli has to be explained from a specific recognition of certain parts of the aglycon of the substrates by this enzyme rather than from a better accessibility of the galactose at one particular branch.

Effects of tunicamycin, N-methyl-1-deoxynojirimycin, and manno-1-deoxynojirimycin on the biosynthesis of lactosaminoglycans in F9 teratocarcinoma cells

F9 teratocarcinoma cells were incubated with D-[2-3H]mannose or D-[6-3H]galactose, and the labeled glycopeptides obtained after exhaustive digestion by pronase were fractionated on Bio-Gel P-6 before and after treatment by endo-beta-N-acetylglucosaminidase H. Tunicamycin almost completely inhibited the synthesis of lactosaminoglycans found in excluded glycopeptides of large molecular weight. Manno-1-deoxynojirimycin greatly inhibited the incorporation of labeled mannose into both lactosaminoglycan and complex oligosaccharides, while it greatly increased that into Man8GlcNAc and Man9GlcNAc oligosaccharides. In contrast, N-methyl-1-deoxynojirimycin only partially inhibited the incorporation into lactosaminoglycan and complex oligosaccharides, and caused the accumulation of Glc3Man7-9GlcNAc oligosaccharides. These results demonstrate that, in these cells, lactosaminoglycans are N-linked, and suggest that there is transfer of both glucosylated and nonglucosylated oligosaccharides from lipid to protein.

Golgi and secreted galactosyltransferase

Galactosyltransferase (GT) belongs to the glycosyltransferases. In several tissues and cell lines, the enzyme is localized by immunocytochemistry to the two to three trans cisternae of the Golgi complex and may thus be considered a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies. Study of its biosynthesis, membrane orientation, and turnover in several tissues and cultured cell lines has broadened our knowledge about Golgi function itself. The enzyme is oriented towards the lumen of the cisternal space. In this orientation, it catalyzes the transfer of galactose to glycoprotein-bound acetylglucosamine and, in the presence of alpha-lactalbumin, to glucose, as shown in the Golgi complex of mammary gland epithelial cells. The enzymatic properties of GT are well known. The metabolism of GT has been extensively studied in HeLa and human hepatoma cells. The enzyme is synthesized in the rough endoplasmic reticulum (RER) and provided with one N-linked oligosaccharide and palmitate residues. In the Golgi complex, terminal sugars are attached to the N-linked oligosaccharide and extensive O-glycosylation takes place. The half-life of the enzyme is about 20 hr, after which a soluble form appears in the culture medium. Release of GT into the medium is observed in all cell lines studied. This phenomenon is in accordance with the presence of soluble GT in body fluids such as serum, ascites, milk, and saliva. In patients suffering from ovarian and breast cancer, increased levels of GT enzyme activity have been reported. Whether extracellular GT is of biological significance is still a point of discussion.

High-pressure liquid chromatography of neutral oligosaccharides: effects of structural parameters

Sixty-five neutral oligosaccharides were analyzed by high-pressure liquid chromatography on an amine-modified silica column (Lichrosorb-NH2). By systematic comparison of the retention times, it was possible to attribute chromatographic behavior to specific structural features. It appeared that retention times increase with the number of sugar residues. The presence of a fucose or an N-acetylglucosamine residue results in a decreased retention time, in particular when the latter sugar is at the reducing end. A dramatic increase in retention time is shown by oligosaccharides having a 1----6 linkage, regardless of whether this linkage is involved in a branch. Less important features are the nature of the component sugars other than N-acetylglucosamine and fucose, the anomeric configuration of the sugars, and the presence of a reduced terminal sugar.

Synthesis of methyl 3-O- and 2-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-alpha D-galactopyranoside

Condensation of methyl 2-O-benzoyl-4,6-O-benzylidene-alpha-D-galactopyranoside with 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl bromide (1) in dichloromethane, in the presence of silver trifluoromethanesulfonate, 2,4,6-trimethylpyridine, and molecular sieves, afforded methyl 2-O-benzoyl-4,6-O-benzylidene-3-O-(3,4,6-tri-O-acetyl-2-deoxy-2-phthalim ido- beta-D-glucopyranosyl)-alpha-D-galactopyranoside (4). Deacetalation of 4 in hot, 80% aqueous acetic acid gave methyl 2-O-benzoyl-3-O-(3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido- beta-D-glucopyranosyl)-alpha-D-galactopyranoside (5), which, on deacylation, followed by peracetylation, furnished the peracetylated disaccharide derivative (6). The structures of 5 and 6 were established by 1H-n.m.r. spectroscopy. O-Deacetylation of 6 afforded the title beta-(1 leads to 3)-linked disaccharide 7. For the synthesis of the beta-(1 leads to 2)-linked isomer, methyl 3-O-benzoyl-4,6-O-benzylidene-alpha-D-galactopyranoside was similarly condensed with bromide 1 to give the fully protected disaccharide derivative (8). Cleavage of the benzylidene group of 8 gave methyl 3-O-benzoyl-2-O-(3,4,6- tri-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl)-alpha-D- galactopyranoside (9). Deacetylation of 9, followed by peracetylation, afforded the peracetate (10). O-Deacetylation of 10 gave the beta-(1 leads to 2)-linked disaccharide (11). The structures of the disaccharides 7 and 11 were confirmed by 13C-n.m.r. spectroscopy.

Characterization of oligosaccharides from the urine of loco-intoxicated sheep

Two major oligosaccharides were isolated by preparative HPLC from the urine of a locoweed-fed sheep. Analysis by gas-liquid chromatography and mass-spectrometry indicated compositions of (Man)4(GlcNAc)2 and (Man)5(GlcNAc)2, respectively. Structures were determined by digestion with alpha-D-mannosidase and endo-beta-N-acetylglucosaminidases D and H, and comparison of the products by HPLC with synthetic standards, and oligosaccharides isolated from human mannosidosis urine. Incubation with an exo-beta-N-acetylglucosaminidase was without effect.