Carbohydrate analysis

Development of an ion-exchange chromatography method for monitoring the degradation of prebiotic arabinoxylan-oligosaccharides in a complex fermentation medium

Arabinoxylan-oligosaccharides (AXOS) are a new class of prebiotics with promising health-promoting characteristics. However, the mechanism by which bacteria break down these compounds in the colon is still uncharacterized, due to their structural complexity. A new analytical method that offers structural information was developed to characterize AXOS degradation during fermentation. The method was based on the simultaneous determination of arabinose, xylose, xylo-oligosaccharides (XOS), and AXOS by applying high-performance anion-exchange chromatography with pulsed amperometric detection. To study the structural features of AXOS in solution without the use of spectroscopic techniques or standards, enzymatic-based reference degradation chromatograms were generated based on enzymes with known specificity. The new method for fingerprinting showed to be a powerful and fast tool to study AXOS degradation with high repeatability with respect to peak area, peak width at half height, and retention time (respective relative standard deviations of ≤3.1%, 2.8%, and 0.8%). This method was successfully applied to study the degradation kinetics of AXOS in a complex fermentation medium by Bifidobacterium longum LMG 11047. The results showed that this strain could use both the arabinose side chains and xylose backbones up to xylotetraose. The characterization of the degradation abilities of AXOS by colon bacteria will allow a better understanding of the beneficial effects of these prebiotics. Furthermore, if the appropriate enzymes are available to design the reference degradation chromatograms, this new method for the qualitative fingerprinting of AXOS breakdown can also be applied for the breakdown of other complex oligosaccharides and polysaccharides.

Peracetylated 4-fluoro-glucosamine reduces the content and repertoire of N- and O-glycans without direct incorporation

Prior studies have shown that treatment with the peracetylated 4-fluorinated analog of glucosamine (4-F-GlcNAc) elicits anti-skin inflammatory activity by ablating N-acetyllactosamine (LacNAc), sialyl Lewis X (sLe(X)), and related lectin ligands on effector leukocytes. Based on anti-sLe(X) antibody and lectin probing experiments on 4-F-GlcNAc-treated leukocytes, it was hypothesized that 4-F-GlcNAc inhibited sLe(X) formation by incorporating into LacNAc and blocking the addition of galactose or fucose at the carbon 4-position of 4-F-GlcNAc. To test this hypothesis, we determined whether 4-F-GlcNAc is directly incorporated into N- and O-glycans released from 4-F-GlcNAc-treated human sLe(X) (+) T cells and leukemic KG1a cells. At concentrations that abrogated galectin-1 (Gal-1) ligand and E-selectin ligand expression and related LacNAc and sLe(X) structures, MALDI-TOF and MALDI-TOF/TOF mass spectrometry analyses showed that 4-F-GlcNAc 1) reduced content and structural diversity of tri- and tetra-antennary N-glycans and of O-glycans, 2) increased biantennary N-glycans, and 3) reduced LacNAc and sLe(X) on N-glycans and on core 2 O-glycans. Moreover, MALDI-TOF MS did not reveal any m/z ratios relating to the presence of fluorine atoms, indicating that 4-F-GlcNAc did not incorporate into glycans. Further analysis showed that 4-F-GlcNAc treatment had minimal effect on expression of 1200 glycome-related genes and did not alter the activity of LacNAc-synthesizing enzymes. However, 4-F-GlcNAc dramatically reduced intracellular levels of uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc), a key precursor of LacNAc synthesis. These data show that Gal-1 and E-selectin ligand reduction by 4-F-GlcNAc is not caused by direct 4-F-GlcNAc glycan incorporation and consequent chain termination but rather by interference with UDP-GlcNAc synthesis.

Mass Spectrometry-Based Glycomics Strategy for Exploring N-Linked Glycosylation in Eukaryotes and Bacteria

N-Glycosylation of proteins is recognized as one of the most common posttranslational modifications in eukaryotes. To date, most glycomics techniques are limited to examining eukaryotic pathways. Technologies capable of characterizing newly described N-linked glycosylation systems in bacteria from biologically relevant samples in an accurate, rapid, and cost-effective manner are needed. In this paper, a new glycomics strategy, based on the combination of nonspecific proteolytic digestion and permethylation, was devised that can be used for both eukaryotic and bacterial glycoproteins. Eukaryotic glycoproteins were digested with Pronase E with a higher ratio (enzyme/protein, 2:1-3:1) and a longer reaction time (48-72 h). The Asn-glycans were then purified using porous graphitic carbon cartridges followed by permethylation. The mass spectrometric data indicated that hydroxyl groups were methylated, while the amino group in asparagine underwent beta-elimination. Both modifications were evident from an increase of 111 Da in the molecular masses of permethylated Asn-glycans compared to the corresponding free oligosaccharides. NMR spectroscopy corroborated these results by showing that the mass difference resulted from beta-elimination of the free amino group in the asparagine residue. The method was validated with the characterization of the N-linked glycoproteins in total protein extracts from the bacterium Campylobacter jejuni 11168H. In addition to detecting the Asn-linked bacterial heptasaccharide, we also observed an unexpected free heptasaccharide intermediate that required a functional glycosylation pathway. These results demonstrate the usefulness of this method for screening N-linked glycans expressed by eukaryotes and bacteria and for detecting novel intermediates of N-linked glycosylation pathways.

2JC,H Index: A Nondestructive NMR Method for Differentiation of Aldohexopyranosyl Residues

A new 2J(C,H) index method is described for identification of aldohexopyranose. This method is based on a fact that 2J(C,H) values reflect the stereochemistry for glycol connectivity. Based on the observed 2J(C,H) values for galactose, glucose, and mannose, 2J(C,H) profiles for other aldohexopyranoses are proposed. A combination of 2J(C,H) values was found to be useful for identification of aldohexopyranosyl residues in glycans. [structure: see text]

A comparative study on structural elucidation of sialyl oligosaccharides by mass spectrometry with fast atom bombardment, electrospray ionization, and matrix assisted laser desorption/ionization

To establish a new protocol for sensitive detection and structural characterization of sialyl oligosaccharides, their sensitivities and structural information from mass spectrometry and tandem mass spectrometry with FAB-, ESI-, and MALDI were evaluated in detail. Among these ionization methods, FAB-MS and FAB-MS/MS gave reproducible and predictable spectra carrying information on sequence and branching of sialyl oligosaccharides after derivatization with 2-aminopyridine (PA). With both positive and negative ion modes, their structural elucidation promises to be straightforward, MS/MS spectra being measurable at as low as 200 pmol. Thus, this method constitutes a powerful tool for sensitive detection and structural characterization of limited quantities of sialyl oligosaccharides by FAB-MS and FAB-MS/MS.

High-sensitivity detection and postsource decay of 2-aminopyridine-derivatized oligosaccharides with matrix-assisted laser desorption/ionization mass spectrometry

The sensitivities of oligosaccharide derivatives in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) were compared using two matrixes, 2,5-dihydroxybenzoic acid (DHBA) and alpha-cyano-4-hydroxycinnamic acid (CHCA). For this purpose, maltopentaose was tagged with 2-aminopyridine (PA), 4-aminobenzoic acid ethyl ester (ABEE), and trimethyl-(p-aminophenyl)ammonium chloride (TMAPA). DHBA was more advantageous for enhancement than CHCA. Among the derivatives, the sensitivity with the PA-tagged maltopentaose showed a 100-fold improvement over the native one with DHBA as a matrix, while the oligesaccharide derivatized with ABEE and TMAPA gave 30- and 10-fold increases in sensitivity over the underivatized one. To obtain structural information from these derivatized oligosaccharides, postsource decay (PSD) during flight in the field-free drift in MALDI-TOFMS was measured. Predictable and reproducible fragmentation patterns could be obtained in all cases. Furthermore, we found matrix-dependence fragmentation with the PA-labeled oligosaccharide. With CHCA, a simple spectrum ascribable to Y series ions was obtained. On the other hand, both B and Y series ions were clearly observed in the DHBA case. The results demonstrate the usefulness of derivatives for sensitive analysis of oligosaccharides with MALDI.

Sensitive detection and structural characterization of trimethyl(p-aminophenyl)-ammonium-derivatized oligosaccharides by electrospray ionization-mass spectrometry and tandem mass spectrometry

Mass spectrometric methodologies which use electrospray ionization (ESI) for oligosaccharide structural determination and which are linked to classical procedures, such as reductive amination, have been established. Several derivatives of a model oligosaccharide were prepared and their ionization efficiencies in electrospray ionization (ESI) and fast-atom bombardment (FAB) mass spectrometry were investigated. The trimethyl-(p-aminophenyl)ammonium (TMAPA) derivatives showed extremely high sensitivity in positive-ion ESI-MS and gave Y- and Z-series sequence ions by ESI tandem mass spectromety (MS/MS). This study suggests that TMAPA-derivatized oligosaccharides used in ESI-MS and ESI-MS/MS would be effective for structural characterization of oligosaccharides that are available only in limited quantities.