PAPER ELECTROPHORESIS OF CARBOHYDRATES

Boron bridging of rhamnogalacturonan-II in Rosa and arabidopsis cell cultures occurs mainly in the endo-membrane system and continues at a reduced rate after secretion

BACKGROUND AND AIMS

Rhamnogalacturonan-II (RG-II) is a domain of primary cell-wall pectin. Pairs of RG-II domains are covalently cross-linked via borate diester bridges, necessary for normal cell growth. Interpreting the precise mechanism and roles of boron bridging is difficult because there are conflicting hypotheses as to whether bridging occurs mainly within the Golgi system, concurrently with secretion or within the cell wall. We therefore explored the kinetics of RG-II bridging.

METHODS

Cell-suspension cultures of Rosa and arabidopsis were pulse-radiolabelled with [14C]glucose, then the boron bridging status of newly synthesized [14C]RG-II domains was tracked by polyacrylamide gel electrophoresis of endo-polygalacturonase digests.

KEY RESULTS

Optimal culture ages for 14C-labelling were ~5 and ~1 d in Rosa and arabidopsis respectively. De-novo [14C]polysaccharide production occurred for the first ~90 min; thereafter the radiolabelled molecules were tracked as they 'aged' in the wall. Monomeric and (boron-bridged) dimeric [14C]RG-II domains appeared simultaneously, both being detectable within 4 min of [14C]glucose feeding, i.e. well before the secretion of newly synthesized [14C]polysaccharides into the apoplast at ~15-20 min. The [14C]dimer : [14C]monomer ratio of RG-II remained approximately constant from 4 to 120 min, indicating that boron bridging was occurring within the Golgi system during polysaccharide biosynthesis. However, [14C]dimers increased slightly over the following 15 h, indicating that limited boron bridging was continuing after secretion.

CONCLUSIONS

The results show where in the cell (and thus when in the 'career' of an RG-II domain) boron bridging occurs, helping to define the possible biological roles of RG-II dimerization and the probable localization of boron-donating glycoproteins or glycolipids.

Derivatization of Levoglucosan for Compound-Specific δ 13C Analysis by Gas Chromatography/Combustion/Isotope Ratio Mass Spectrometry

Levoglucosan is a thermal decomposition product of cellulose in particulate matter. δ ¹³C value of levoglucosan could be used in studying the combustion mechanisms and chemical pathways. In order to introduce a minimum number of carbon atoms, based on the stereostructure of levoglucosan, a two-step derivatization method with methylboronic acid and MSTFA was developed and carefully optimized. The recommended reaction temperature is 70°C; the reaction time is 60 min for MBA and 120 min for MSTFA derivatization; and the molar ratio of levoglucosan : MBA : MSTFA is 1 : 1: 100 and 1 : 1: 120 and the reagent volume ratio of MSTFA : pyridine is between 1 : 3 and 1 : 4. The developed method achieved excellent reproducibility and high accuracy. The differences in the carbon isotopic compositions of the target boronate trimethysilylated derivative between the measured and calculated ranged from 0.09 to 0.36‰. The standard deviation of measured δ ¹³C value of levoglucosan was between 0.22 and 0.48‰. The method was applied to particle samples collected from the combustion of cellulose at four different temperatures. δ ¹³C values of levoglucosan in particle samples generated from a self-made combustion setup suggesting that combustion temperature play a little role on isotope fractionation of levoglucosan, although ¹³C enriched in levoglucosan during the combustion process.

Boron bridging of rhamnogalacturonan-II is promoted in vitro by cationic chaperones, including polyhistidine and wall glycoproteins

Dimerization of rhamnogalacturonan-II (RG-II) via boron cross-links contributes to the assembly and biophysical properties of the cell wall. Pure RG-II is efficiently dimerized by boric acid (B(OH)3 ) in vitro only if nonbiological agents for example Pb(2+) are added. By contrast, newly synthesized RG-II domains dimerize very rapidly in vivo. We investigated biological agents that might enable this. We tested for three such agents: novel enzymes, borate-transferring ligands and cationic 'chaperones' that facilitate the close approach of two polyanionic RG-II molecules. Dimerization was monitored electrophoretically. Parsley shoot cell-wall enzymes did not affect RG-II dimerization in vitro. Borate-binding ligands (apiose, dehydroascorbic acid, alditols) and small organic cations (including polyamines) also lacked consistent effects. Polylysine bound permanently to RG-II, precluding electrophoretic analysis. However, another polycation, polyhistidine, strongly promoted RG-II dimerization by B(OH)3 without irreversible polyhistidine-RG-II complexation. Likewise, partially purified spinach extensins (histidine/lysine-rich cationic glycoproteins), strongly promoted RG-II dimerization by B(OH)3 in vitro. Thus certain polycations, including polyhistidine and wall glycoproteins, can chaperone RG-II, manoeuvring this polyanionic polysaccharide domain such that boron-bridging is favoured. These chaperones dissociate from RG-II after facilitating its dimerization, indicating that they act catalytically rather than stoichiometrically. We propose a natural role for extensin-RG-II interaction in steering cell-wall assembly.

Trans-α-xylosidase, a widespread enzyme activity in plants, introduces (1→4)-α-d-xylobiose side-chains into xyloglucan structures

Angiosperms possess a retaining trans-α-xylosidase activity that catalyses the inter-molecular transfer of xylose residues between xyloglucan structures. To identify the linkage of the newly transferred α-xylose residue, we used [Xyl-(3)H]XXXG (xyloglucan heptasaccharide) as donor substrate and reductively-aminated xyloglucan oligosaccharides (XGO-NH(2)) as acceptor. Asparagus officinalis enzyme extracts generated cationic radioactive products ([(3)H]Xyl·XGO-NH(2)) that were Driselase-digestible to a neutral trisaccharide containing an α-[(3)H]xylose residue. After borohydride reduction, the trimer exhibited high molybdate-affinity, indicating xylobiosyl-(1→6)-glucitol rather than a di-xylosylated glucitol. Thus the trans-α-xylosidase had grafted an additional α-[(3)H]xylose residue onto the xylose of an isoprimeverose unit. The trisaccharide was rapidly acetolysed to an α-[(3)H]xylobiose, confirming the presence of an acetolysis-labile (1→6)-bond. The α-[(3)H]xylobiitol formed by reduction of this α-[(3)H]xylobiose had low molybdate-affinity, indicating a (1→2) or (1→4) linkage. In NaOH, the α-[(3)H]xylobiose underwent alkaline peeling at the moderate rate characteristic of a (1→4)-disaccharide. Finally, we synthesised eight non-radioactive xylobioses [α and β; (1↔1), (1→2), (1→3) and (1→4)] and found that the [(3)H]xylobiose co-chromatographed only with (1→4)-α-xylobiose. We conclude that Asparagus trans-α-xylosidase activity generates a novel xyloglucan building block, α-d-Xylp-(1→4)-α-d-Xylp-(1→6)-d-Glc (abbreviation: 'V'). Modifying xyloglucan structures in this way may alter oligosaccharin activities, or change their suitability as acceptor substrates for xyloglucan endotransglucosylase (XET) activity.

High-voltage paper electrophoresis (HVPE) of cell-wall building blocks and their metabolic precursors

HVPE is an excellent and often overlooked method for obtaining objective and meaningful information about cell-wall "building blocks" and their metabolic precursors. It provides not only a means of analysis of known compounds but also an insight into the charge and/or mass of any unfamiliar compounds that may be encountered. It can be used preparatively or analytically. It can achieve either "class separations" (e.g. delivering all hexose monophosphates into a single pool) or the resolution of different compounds within a given class (e.g. ADP-Glc from UDP-Glc; or GlcA from GalA). All information from HVPE about charge and mass can be obtained on minute traces of analytes, especially those that have been radiolabelled, e.g. by in-vivo feeding of a (3)H- or (14)C-labelled precursor. HVPE does not usually damage the substance under investigation (unless staining is used), so samples of interest can be eluted intact from the paper ready for further analysis. Although HVPE is a technique that has been available for several decades, recently it has tended to be sidelined, possibly because the apparatus is not widely available. Interested scientists are invited to contact the author about the possibility of accessing the Edinburgh apparatus.

Chiral resolution of monosaccharides as 1-phenyl-3-methyl-5-pyrazolone derivatives by ligand-exchange CE using borate anion as a central ion of the chiral selector

Six reducing monosaccharides (mannose, galactose, fucose, glucose, xylose, and arabinose) were derivatized with 1-phenyl-3-methyl-5-pyrazolone (PMP) and chiral resolution of these racemic PMP-monosaccharides was studied by ligand-exchange CE using borate anion as a central ion of the chiral selector and (S)-3-amino-1,2-propanediol (SAP) as a chiral selector ligand. PMP-mannose, PMP-galactose and PMP-fucose were successfully enantioseparated. Lowering the capillary temperature increased the resolution of PMP-mannose system, but decreased that of PMP-galactose and PMP-fucose systems. Whereas the maximum resolution was obtained at pH 8.9 in the PMP-mannose system, resolution increased gradually with pH in the PMP-galactose and PMP-fucose systems. Expecting the formation of the ternary borate complexes with SAP and PMP-monosaccharide in the CE experiments, the optimized structures of the borate diastereomers were obtained by semiempirical molecular orbital calculations to discuss the structural difference of the diastereomers in connection with the enantioseparation behaviors.

Chiral ligand exchange micellar electrokinetic chromatography using borate anion as a central ion

Three compounds having 1,2-diol structure (1-phenyl-1,2-ethanediol, 3-phenoxy-1,2-propanediol, and 3-benzyloxy-1,2-propanediol) were enantioseparated by ligand exchange MEKC using (5S)-pinanediol (SPD) as a chiral selector and borate anion as a central ion together with SDS. When (S)-1,2-propanediol, (S)-1,2,4-butanetriol, or (S)-3-tert-butylamino-1,2-propanediol were used as the chiral ligand instead of SPD, these three compounds were not enantioseparated. When borate was replaced with 2-aminoethane-1-sulfonate or N-cyclohexyl-3-aminopropanesulfonate, no chiral separation was achieved. Therefore, the hydrophobic interaction between the chiral selector and the chiral analytes within the transient diastereomeric complex may play an important role in the enantioseparation achieved by the proposed method.

Rapid analysis of amino sugars by microchip electrophoresis with laser-induced fluorescence detection

Microchip electrophoresis for the short-time analysis of amino sugars is described. D-Glucosamine, D-galactosamine and their reduced forms were labeled with 4-nitro-2,1,3-benzoxadiazole 7-fluoride (NBD-F) at pH 6.0 and the fluorescent derivatives were purified on an octadecyl silica (ODS) gel plate. The derivatives were analyzed by electrophoresis on a microfabricated chip with a 33 mm long separation channel with argon laser-induced fluorescence detection. Under the established conditions, these amino sugarderivatives were well separated from each other within 60 s. Amino sugars of as small an amount as 0.5 fmol could be detected with a signal-to-noise (S/N) ratio of 3, and peak response showed good linearity between at least 0.8 and 8 fmol of samples with a relative standard deviation (RSD) of ca. 4%. This method was also applied to the analysis of amino sugar composition of O-linked glycans released from bovine submaxillary mucin with alkali in the presence of borohydride. The result of amino sugar composition analysis for individual O-glycans fractionated by high-performance liquid chromatography was quite useful for their identification.

Capillary zone electrophoresis and micellar electrokinetic chromatography of 4-aminobenzonitrile carbohydrate derivatives

Aldoses, ketoses and uronic acids were derivatized successfully within 15 min at a temperature of 90 degrees C by reductive amination with 4-aminobenzonitrile. Subsequently, the derivatives were separated as their borate complexes by capillary zone electrophoresis, using 175 mM borate buffer, pH 10.5, as carrier. The electrophoretic mobilities were determined by the complex stability, which was found to depend on the number of hydroxyl groups on any given carbohydrate derivative, the presence of substituents, and most strongly on the configuration of the vicinal hydroxyl groups at C-3 and C-4 in aldoses and uronic acids, and with regard to ketoses on those at C-4 and C-5. Time of analysis could be reduced considerably by the use of micellar electrokinetic chromatography, which separated 4-aminobenzonitrile sugar derivatives on the basis of their differential partitioning into an electroendosmotically driven aqueous phase and into sodium dodecyl sulfate micelles. Optimum resolution was achieved with a Tris-phosphate buffer, pH 7.5, containing 100 mM of sodium dodecyl sulfate. The method made it possible to resolve several carbohydrates which had not been resolved successfully by means of capillary zone electrophoresis, such as glucose and fructose. Moreover, separation selectivity could be adjusted by varying the capillary temperature. Finally, on-column UV monitoring at 285 nm allowed the detection of glucose with a lower mass detection limit of 1 fmol and a concentration sensitivity of 0.3 microM.

Capillary zone electrophoresis of p-aminobenzoic acid derivatives of aldoses, ketoses and uronic acids

Aldoses, ketoses and uronic acids were derivatized with p-aminobenzoic acid and separated as their borate complexes by capillary zone electrophoresis, using a capillary tube of fused silica containing 150 mM borate buffer, pH 10.0, as carrier. The electrophoretic mobilities of 22 carbohydrates were determined and found to increase with increasing stability of the borate complexes formed. Besides the number of hydroxyl groups and the presence of substituents, complex stability depended most strongly on the configuration of the three vicinal hydroxyl groups at C2, C3 and C4. On-column UV monitoring at 285 nm allowed the detection of glucose with a lower mass detection limit of 15 fmol and a concentration sensitivity of 4 microM. Reproducible quantification of carbohydrates was achieved at least in the concentration range of 0.1-10 mM in reaction solutions by the relative peak area method, using cinnamic acid as internal standard. The method was applied successfully to the determination of the monosaccharide composition of polysaccharides extracted from Radix althaeae.

Thin-layer electrophoretic separation of monosaccharides, oligosaccharides and related compounds on reverse phase silica gel

The electrophoretic mobilities of monosaccharides, oligosaccharides, sugar alcohols and sugar acids were determined in 0.3 M borate buffer, pH 10, using thin-layer electrophoresis on silanized silica gel, pretreated with octanol-1. A rapid separation of a number of sugars, occurring in foods, could be achieved. Using a 0.05-0.1 M neutral solution of barium acetate as electrolyte, thin-layer electrophoresis allowed excellent and rapid separation as well as identification of all common uronic acids which are constituents of many acidic polysaccharides.

Immunochemical analysis of streptococcal group A, B, and C carbohydrates, with emphasis on group A

Streptococcal group A, B, and C carbohydrates were analyzed by counterimmunoelectrophoresis, immunoelectrophoresis, and inhibition of immunoprecipitation. Extracts of streptococci group A or C were shown by counterimmunoelectrophoresis to contain both anodic and cathodic migrating components. In immunoelectrophoresis, group A and C substances formed a continuous precipitation line stretching from the anode to the cathode, suggesting a heterogeneous population of molecules with immunochemical identity. This identity was confirmed by inhibition of immunoprecipitation, in which both anodic and cathodic immunoprecipitates were inhibited by the same constituent sugars: group A-anti-A was inhibited by N-acetylglucosamine, and group C-anti-C was inhibited by N-acetylgalactosamine. Extracts of group B showed only anodic migration in counterimmunoelectrophoresis and a narrow, anodic arc in immunoelectrophoresis. The group B-anti-B reaction was inhibited by rhamnose. Carbohydrates of variant strains of group A streptococci were also analyzed by the same methods. The results suggest that the heterogeneity of group A carbohydrate may have resulted from attachment of various amounts of N-acetylglucosamine to the polyrhamnose backbone.

Phenolic components of the primary cell wall. Feruloylated disaccharides of D-galactose and L-arabinose from spinach polysaccharide

1. Cell walls from rapidly growing cell suspension cultures of Spinacia oleracea L. contained ferulic acid and p-coumaric acid esterified with a water-insoluble polymer. 2. Prolonged treatment with trypsin did not release may feruloyl esters from dearabinofuranosylated cell walls, and the polymer was also insoluble in phenol/acetic acid/water (2:1:1, w/v/v). 3. Treatment of the cell walls with the fungal hydrolase preparation "Driselase' did liberate low-Mr feruloyl esters. The major esters were 4-O-(6-O-feruloyl-beta-D-galactopyranosyl)-D-galactose and 3?-O-feruloyl-alpha-L-arabinopyranosyl)-L-arabinose. These two esters accounted for about 60% of the cell-wall ferulate. 4. It is concluded that the feruloylation of cell-wall polymers is not a random process, but occurs at very specific sites, probably on the arabinogalactan component of pectin. 5. The possible role of such phenolic substituents in cell-wall architecture and growth is discussed.

Heterogeneity and distribution of lipopolysaccharide in the cell wall of a gram-negative marine bacterium

Lipopolysaccharide (LPS) extracted from Alteromonas haloplanktis 214, variants 1 and 3, separated into three fractions when subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The fractions appeared in the gels as bands which stained for carbohydrate with the periodate-Schiff reagent. Variant 1, a smooth variant of the organism, and variant 3, a rough colonial variant, produced identical banding patterns. Under similar conditions, LPS from Neisseria meningitidis SDIC, Escherichia coli O111:B4, and Salmonella typhimurium LT2 gave rise to one, two, and three bands, respectively. LPS from Pseudomonas aeruginosa (ATCC 9027) failed to stain clearly with the reagent used. The banding pattern obtained with A. haloplanktis LPS was found not to be due to artifacts produced by the extraction or solubilization procedures employed or to the amount of protein associated with the LPS. When Triton X-100 replaced sodium dodecyl sulfate in the electrophoresis system, LPS failed to migrate into the gel. The lipid A but not the degraded polysaccharide fraction obtained by mild acid hydrolysis of the LPS migrated into the gel on electrophoresis. The three carbohydrate-staining bands obtained with A. haloplanktis LPS and referred to as LPS I, II, and III, in order of increasing electrophoretic mobility, were detected in each of the three outer layers of the cell wall of the organism. Estimations from densitometer scans indicated that 17% of the total LPS in the cell was present in the outer membrane, with the remainder divided almost equally between the loosely bound outer layer and the periplasmic space. Of the three fractions, LPS II was present in each of the layers in greatest amounts. Less LPS I and more LPS III were present in the outer membrane than in the periplasmic space. Pulse-labeling studies indicated that LPS I and II may be synthesized independently, whereas LPS III, which appeared only in cells in the stationary phase of growth, may be a degradation product of LPS I.

Role of a sugar-lipid intermediate in colanic acid synthesis by Escherichia coli

Membrane fractions from a lon strain of Escherichia coli but not a wild-type strain catalyze the incorporation of fucose from guanosine 5'-diphosphate-fucose into a lipid and into polymeric material. Both incorporation reactions specifically require only uridine 5'-diphosphate (UDP)-glucose. The sugar lipid was shown to be an intermediate in the synthesis of the polymer which was related to colanic acid. The sugar lipid had the structure (fucose3, glucose2)-glucose P-P-lipid. Its behavior on column and thin-layer chromatography, the rates of its hydrolysis in acid and base, and the response of its synthesis to inhibitors are all identical to the other sugar-lipid intermediates which have been shown to contain sugars attached to the C55-polyisoprenol, undecaprenol, by a pyrophosphate linkage. The membrane fractions from both the lon strain and the wild-type strain also catalyzed the incorporation of either glucose from UDP-glucose or galactose from UDP-galactose into a lipid fraction which was shown to contain the free sugar attached by a monophosphate linkage to an undecaprenol-like lipid. This lipid was isolated and its nuclear magnetic resonance spectra was identical to undecaprenol. The membrane fractions from both strains also incorporated glucose from UDP-glucose into glycogen and into a polymer that behaved like Escherichia coli lipopolysaccharide. Conditions were found where the incorporation of glucose could be directed specifically into each compound by adding the appropriate inhibitors.