Structural elucidation of the N- and O-glycans of human apolipoprotein(a): role of o-glycans in conferring protease resistance

Apolipoprotein(a) (apo(a)) is a multikringle domain glycoprotein that exists covalently linked to apolipoprotein B100 of low density lipoprotein, to form the lipoprotein(a) (Lp(a)) particle, or as proteolytic fragments. Elevated plasma concentrations of apo(a) and its fragments may promote atherosclerosis, but the underlying mechanisms are incompletely understood. The factors influencing apo(a) proteolysis are also uncertain. Here we have used exoglycosidase digestion and mass spectrometry to sequence the Asn (N)-linked and Ser/Thr (O)-linked oligosaccharides of human apo(a). We also assessed the potential role of apo(a) O-glycans in protecting thermolysin-sensitive regions of the polypeptide. Apo(a) contained two major N-glycans that accounted for 17% of the total oligosaccharide structures. The N-glycans were complex biantennary structures present in either a mono- or disialylated state. The O-glycans were mostly (80%) represented by the monosialylated core type 1 structure, NeuNAcalpha2-3Galbeta1-3GalNAc, with smaller amounts of disialylated and non-sialylated O-glycans also detected. Removal of apo(a) O-glycans by sialidase and O-glycosidase treatment dramatically increased the sensitivity of the polypeptide to thermolysin digestion. These studies provide the first direct sequencing data for apo(a) glycans and indicate a novel function for apo(a) O-glycans that is potentially related to the atherogenicity of Lp(a).

Identification of protein-bound carbohydrates by mass spectrometry

This review outlines the use of modern methods of mass spectrometry, mainly based on electrospray ionization and matrix-assisted lased desorption/ionization, for the identification of protein-bound carbohydrates. The techniques are briefly described together with methods for glycan release and purification prior to mass spectrometry. Fragmentation of glycans, produced under various conditions, is described in the context of obtaining structural information at the sensitivity required for proteomic work.

Glycosylation analysis of gel-separated proteins

Beyond the identification of proteins involved in a particular physiological situation, many aspects of proteomics require more detailed characterization of the proteins involved. Post-translational modifications (PTMs) of proteins are a common means to target proteins, regulate their activities and to mediate communication between proteins and cells. Owing to the much higher analytical complexity of glycan analysis compared to e.g. protein identification, PTM analysis in general and glycosylation analysis in particular is largely neglected in proteomics. In this review, the current technological status of global and site-specific glycosylation analysis of gel-separated proteins is described and the way in which the available technology can be employed in proteomics is critically discussed.

O-glycan analysis of natural human neutrophil gelatinase B using a combination of normal phase-HPLC and online tandem mass spectrometry: implications for the domain organization of the enzyme

Gelatinase B is a matrix metalloproteinase (MMP-9) expressed under strict control by many cell types including neutrophils, monocytes, macrophages, and tumor cells. MMP-9 is a key mediator in the physiological maintenance of the extracellular matrix both in tissue remodeling and development, while uncontrolled enzyme activity contributes to pathologies such as cancer and inflammation. Neutrophils release MMP-9 from granules in response to IL-8 stimulation. Human MMP-9 has three potential N-linked glycosylation sites and contains a Ser/Pro/Thr rich domain, known as the type V collagen-like domain, which is expected to be heavily O-glycosylated. Indeed, approximately 85% of the total sugars on human neutrophil MMP-9 are O-linked. This paper presents the detailed analysis of picomole amounts of these O-glycans using a novel HPLC-based strategy for O-glycan analysis that provides linkage and arm specific information in addition to monosaccharide sequence. The initial structural assignments were confirmed using HPLC with online MS/MS fragmentation analysis. Twelve sugars were identified that contained from two to nine monosaccharide residues. Most of these contained type 2 core structures with Galbeta1-4GlcNAc (N-acetyl lactosamine) extensions, with or without sialic acid or fucose. The O-glycans were modeled using the oligosaccharide structural database. On the basis of the structure of gelatinase A (MMP-2), a model of MMP-9 suggests that the type V collagen-like domain in gelatinase B is located on a loop remote from the active site. Fourteen potential O-glycosylation sites are multiply presented on this loop of 52 amino acids. Many of the O-glycans identified contain terminal galactose residues that may provide recognition epitopes. Importantly, heavy glycosylation of this loop region, absent in gelatinase A, has considerable implications for the domain organization of MMP-9.

Negative ion mass spectrometry of sialylated carbohydrates: discrimination of N-acetylneuraminic acid linkages by MALDI-TOF and ESI-TOF mass spectrometry

Negative ion MALDI and electrospray fragmentation spectra were recorded from 12 sialylated carbohydrates ranging from trisaccharides to biantennary N-linked glycans. D-Arabinosazone was found to be the most satisfactory MALDI matrix for these compounds. Fragmentation mechanisms were investigated with the aid of several synthesized analogues of the sugars labeled with 13C and 2H. The substitution position of the sialic acid (alpha2-->3 or alpha2-->6) was found to have a dramatic effect on the overall fragmentation pattern of these compounds, and several features of the spectra were identified that allowed the substitution pattern to be determined. In particular, the appearance of an ion at m/z 306 appeared to be diagnostic of the presence of an alpha2-->6-linked sialic acid. Selection and further fragmentation of the in-source (conevoltage) fragment ion corresponding to the trisaccharide Neu5Acalpha2-->3(or 6)Galbeta1-->4GlcNAc from larger, N-linked glycans, ionized by electrospray, gave fragmentation patterns identical to those of the reference trisaccharides, thus providing a method for confirming the sialic acid linkage.

Electrospray mass spectrometry and fragmentation of N-linked carbohydrates derivatized at the reducing terminus

Derivatives were prepared from N-linked glycans by reductive amination from 2-aminobenzamide, 2-aminopyridine, 3-aminoquinoline, 2-aminoacridone, 4-amino-N-(2-diethylaminoethyl)benzamide, and the methyl, ethyl, and butyl esters of 4-aminobenzoic acid. Their electrospray and collision-induced dissociation (CID) fragmentation spectra were examined with a Q-TOF mass spectrometer. The strongest signals were obtained from the [M + Na]+ ions for all derivatives except sugars derivatized with 4-amino-N-(2-diethylaminoethyl)benzamide which gave very strong doubly charged [M + H + Na]2+ ions. The strongest [M + Na]+ ion signals were obtained from the butyl ester of 4-aminobenzoic acid and the weakest from 2-aminopyridine. The most informative spectra were recorded from the [M + Li]+ or [M + Na]+ ions. These spectra were dominated by ions produced by sequence-revealing glycosidic cleavages and "internal" fragments. Linkage-revealing cross-ring cleavage ions were reasonably abundant, particularly from high-mannose glycans. Although the nature of the derivative was found to have little effect upon the fragmentation pattern, 3-aminoquinoline derivatives gave marginally more abundant cross-ring fragments than the other derivatives. [M + H]+ ions formed only glycosidic fragments with few, if any, cross-ring cleavage ions. Doubly charged molecular ions gave less informative spectra; singly charged fragments were weak, and molecular ions containing hydrogen ([M + 2H]2+ and [M + H + Na]2+) fragmented as the [M + H]+ singly charged ions with no significant cross-ring cleavages.

Collision-induced fragmentation of underivatized N-linked carbohydrates ionized by electrospray

The electrospray mass spectra and collision-induced fragmentation of neutral N-linked glycans obtained from glycoproteins were examined with a Q-TOF mass spectrometer. The glycans were ionized most effectively as adducts of alkali metals, with lithium providing the most abundant signal and caesium the least. Singly charged ions generally gave higher ion currents than doubly charged ions. Addition of formic acid could be used to produce [M + H]+ ions, but these ions were always accompanied by abundant cone-voltage fragments. The energy required for collision-induced fragmentation was found to increase in a linear manner as a function of mass with the [M + Na]+ ions requiring about four times as much energy as the [M + H]+ ions for complete fragmentation of the molecular ions. Fragmentation of the [M + H]+ ions gave predominantly B- and Y-type glycosidic fragments whereas the [M + Na]+ and [M + Li]+ ions produced a number of additional fragments including those derived from cross-ring cleavages. Little fragmentation was observed from the [M + K]+ and [M + Rb]+ ions and the only fragment to be observed from the [M + Cs]+ ion was Cs+. The [M + Na]+ and [M + Li]+ ions from all the N-linked glycans gave abundant fragments resulting from loss of the terminal GlcNAc moiety and prominent, though weaker, ions as the result of 0,2A and 2,4A cross-ring cleavages of this residue. Most other ions were the result of successive additional losses of residues from the non-reducing terminus. This pattern was particularly prominent with glycans containing several non-reducing GlcNAc residues where successive losses of 203 u were observed. Many of the ions in the low-mass range were products of several different fragmentation routes but still provided structural information. Possibly of most diagnostic importance was an ion formed by loss of 221 u (GlcNAc molecule) from an ion that had lost the 3-antenna and the chitobiose core. This latter ion, although coincident in mass with some other 'internal' fragments, often provided additional information on the composition of the antennae. Other ions defining antenna composition were weak cross-ring fragments produced from the core branching mannose residue. Glycans containing Gal-GlcNAc residues showed successive losses of this moiety, particularly from the B-type fragments resulting from loss of the reducing-terminal GlcNAc residue. The [M + Na]+ and [M + Li]+ ions from high-mannose and hybrid glycans gave a series of ions of composition (Man)nNa/Li+ where n = 1 to the total number of glycans in the molecule, allowing these sugars to be distinguished from the more highly processed complex glycans. Other ions in the spectra of the high-mannose glycans were diagnostic of chain branching but insufficient information was available to determine their mode of formation.

Assay for the transbilayer distribution of glycolipids. Selective oxidation of glucosylceramide to glucuronylceramide by TEMPO nitroxyl radicals

In the present study, 2,2,6,6-tetramethylpiperidinooxy nitroxide (TEMPO) has been applied successfully to discriminate between glucosylceramide in the outer and inner leaflets of closed membrane bilayers. The nitroxyl radicals TEMPO and carboxy-TEMPO, once oxidized to nitrosonium ions, are capable of oxidizing residues that contain primary hydroxyl and amino groups. When applied to radiolabeled glucosylceramide in liposomes, oxidation with TEMPO led to an oxidized product that was easily separated from the original lipid by thin-layer chromatography, and that was identified by mass spectrometric analysis as the corresponding acid glucuronylceramide. To test whether oxidation was confined to the external leaflet, TEMPO was applied to large unilamellar vesicles (LUVs) consisting of egg phosphatidylcholine- egg phosphatidylethanolamine;-cholesterol 55:5:40 (mol/mol). TEMPO oxidized most radiolabeled phosphatidylethanolamine, whereas carboxy-TEMPO oxidized only half. Hydrolysis by phospholipase A(2) confirmed that 50% of the phosphatidylethanolamine was accessible in the external bilayer leaflet, suggesting that TEMPO penetrated the lipid bilayer and carboxy-TEMPO did not. When applied to LUVs containing <1 mol% radiolabeled glucosylceramide or short-chain C(6)-glucosylceramide, carboxy-TEMPO oxidized half the glucosylceramide. However, if surface C(6)-glucosylceramide was first depleted by bovine serum albumin (BSA) (extracting 49 +/- 1%), 94% of the remaining C(6)-glucosylceramide was resistant to oxidation. Carboxy-TEMPO oxidized glucosylceramide on the surface of LUVs without affecting inner leaflet glucosylceramide. At pH 9.5 and at 0 degrees C, the reaction reached completion by 20 min.

Composition of N-linked carbohydrates from ovalbumin and co-purified glycoproteins

Analysis of commercial samples of chicken ovalbumin by reversed-phase high performance liquid chromatography and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) showed the presence of several other co-purifying glycoproteins. These were isolated, subjected to tryptic digestion, and two of them were identified as ovomucoid and chicken riboflavin binding-protein following database matching of the peptide masses obtained by MALDI. The N-linked glycans were released from the glycoproteins and their structures were examined by MALDI-MS in combination with exoglycosidase digestion. Ovalbumin was found to be glycosylated mainly with high-mannose and hybrid structures, consistent with profiles obtained on the intact glycoprotein by electrospray. The other glycoproteins contained mainly larger, complex glycans with up to five antennae, many of which had earlier been associated with ovalbumin.

Postsource decay fragmentation of N-linked carbohydrates from ovalbumin and related glycoproteins

N-linked glycans were released from chicken ovalbumin by hydrazinolysis and examined by matrix-assisted laser desorption/ionization mass spectrometry. Postsource decay analysis showed that most fragment ions arose as the result of internal glycosidic cleavages involving loss of nonreducing terminal residues from ions that had lost one or both GlcNAc residues from the chitobiose core [GlcNAcbeta(1 --> 4)GlcNAc]. Cross-ring fragments were abundant from the reducing-terminal GlcNAc but other cross-ring fragments were weak. The ion found to be most useful for determining the composition of the antennae attached to the 3- or 6-linked core mannose residues was an internal cleavage ion formed by loss of both the chitobiose core and the antenna linked to the 3-position of the core branching mannose. This ion was observed to lose water in the absence of a "bisecting" GlcNAc residue (beta1 --> 4 linked to the core mannose) and to lose a GlcNAc molecule (221 mass units) when a bisecting GlcNAc residue was present.

Hybrid and complex glycans are linked to the conserved N-glycosylation site of the third eight-cysteine domain of LTBP-1 in insect cells

Covalent association of LTBP-1 (latent TGF-beta binding protein-1) to latent TGF-beta is mediated by the third eight-cysteine (also referred to as TB) module of LTBP-1, a domain designated as CR3. Spodoptera frugiperda (Sf9) cells have proved a suitable cell system in which to study this association and to produce recombinant CR3, and we show here that another lepidopteran cell line, Trichoplusia niTN-5B1-4 (High-Five) cells, allows the recovery of large amounts of functional recombinant CR3. CR3 contains an N-glycosylation site, which is conserved in all forms of LTBP known to date. When we examined the status of this N-glycosylation using MALDI-TOF mass spectrometry and enzymatic analysis, we found that CR3 is one of the rare recombinant peptides modified with complex glycans in insect cells. Sf9 cells mainly processed the fucosylated paucomannosidic structure (GlcNAc)(2)(Mannose)(3)Fucose, although hybrid and complex N-glycosylations were also detected. In High-Five cells, the peptide was found to be modified with a wide variety of hybrid and complex sugars in addition to paucomanosidic oligosaccharides. Most glycans had one or two fucose residues bound through alpha1,3 and alpha1,6 linkages to the innermost GlcNAc. On the basis of these results and on the structure of an eight-cysteine domain from fibrillin-1, we present a model of glycosylated CR3 and discuss the role of glycosylation in eight-cysteine domain protein-protein interactions.

Ionisation and fragmentation of complex glycans with a quadrupole time-of-flight mass spectrometer fitted with a matrix-assisted laser desorption/ionisation ion source

This paper reports the use of an experimental matrix-assisted laser desorption/ionisation (MALDI) ion source fitted to a quadrupole time-of-flight (Q-Tof) mass spectrometer for the analysis of carbohydrates, particularly the N-linked glycans from glycoproteins. Earlier work on the Q-Tof instrument, using electrospray ionisation, gave excellent MS/MS spectra, particularly from the [M + Na]+ ions, but suffered from the major disadvantages that the signal was often split between singly and multiply charged ions and that sensitivity fell dramatically as the molecular weight of the carbohydrate rose. The MALDI ion source did not suffer from these problems and the instrument produced excellent MS and MS/MS spectra from small amounts of complex, underivatised glycans as well as those derivatised at the reducing terminus. Positive ion MS spectra of sialylated glycans recorded on the new instrument were much less complex than those recorded with a conventional MALDI-TOF instrument because of the absence of ions resulting from metastable (post-source decay, (PSD)) fragmentations occurring in the flight tube. However, considerable fragmentation by loss of sialic acid still occurred. MS/MS spectra of the [M + Na]+ ions from all compounds were almost identical to those recorded earlier with the electrospray-Q-Tof combination and far superior to MALDI-PSD spectra recorded with reflectron-TOF instruments. Spectra are shown for neutral and sialylated N-linked glycans from chicken ovalbumin, riboflavin binding protein, alpha1-acid glycoprotein, bovine fetuin and ribonuclease B, both as free glycans and as those derivatised at their reducing termini. The technique was applied to the structural determination of N-linked glycans from human secretory IgA and Apo-B 100 from human low-density lipoprotein.

N-(2-diethylamino)ethyl-4-aminobenzamide derivative for high sensitivity mass spectrometric detection and structure determination of N-linked carbohydrates

N-Linked glycans were derivatised by reductive amination using N-(2-diethylamino)ethyl-4-aminobenzamide (DEAEAB, procainamide) and examined by electrospray mass spectrometry. This derivative ionised primarily by protonation of the tertiary amine group and attachment of an alkali metal to give [M + H + X](2+) ions which were much more abundant that doubly charged ions from glycans derivatised with other aromatic amines. Fragmentation of these ions depended on the nature of the alkali metal (X). Lithium and sodium adducts fragmented to give prominent ions produced by cleavages within the DEAEAB derivative whereas the other adducts produced more abundant ions from fragmentation of the carbohydrate. Elimination of a sugar fragment, usually by cleavage adjacent to GlcNAc or sialic acid, together with a hydrogen atom, produced the most abundant singly charged fragment ions. These ions then formally fragmented by glycosidic cleavages. Potassium, rubidium and caesium adducts produced abundant losses of the alkali metal, but the resulting ions appeared not to undergo extensive fragmentation. Most fragment ions from all of the adducts were singly charged, the remainder being doubly charged. Although the spectra of the [M + X + H](2+) ions were not as informative as those from the singly charged ions from other derivatives, they, nevertheless, provided much valuable information on the structure of these glycans.

Glycosylation differences between the normal and pathogenic prion protein isoforms

Prion protein consists of an ensemble of glycosylated variants or glycoforms. The enzymes that direct oligosaccharide processing, and hence control the glycan profile for any given glycoprotein, are often exquisitely sensitive to other events taking place within the cell in which the glycoprotein is expressed. Alterations in the populations of sugars attached to proteins can reflect changes caused, for example, by developmental processes or by disease. Here we report that normal (PrP(C)) and pathogenic (PrP(Sc)) prion proteins (PrP) from Syrian hamsters contain the same set of at least 52 bi-, tri-, and tetraantennary N-linked oligosaccharides, although the relative proportions of individual glycans differ. This conservation of structure suggests that the conversion of PrP(C) into PrP(Sc) is not confined to a subset of PrPs that contain specific sugars. Compared with PrP(C), PrP(Sc) contains decreased levels of glycans with bisecting GlcNAc residues and increased levels of tri- and tetraantennary sugars. This change is consistent with a decrease in the activity of N-acetylglucosaminyltransferase III (GnTIII) toward PrP(C) in cells where PrP(Sc) is formed and argues that, in at least some cells forming PrP(Sc), the glycosylation machinery has been perturbed. The reduction in GnTIII activity is intriguing both with respect to the pathogenesis of the prion disease and the replication pathway for prions.

Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates

This review describes the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to carbohydrate analysis and covers the period 1991-1998. The technique is particularly valuable for carbohydrates because it enables underivatised, as well as derivatised compounds to be examined. The various MALDI matrices that have been used for carbohydrate analysis are described, and the use of derivatization for improving mass spectral detection limits is also discussed. Methods for sample preparation and for extracting carbohydrates from biological media prior to mass spectrometric analysis are compared with emphasis on highly sensitive mass spectrometric methods. Quantitative aspects of MALDI are covered with respect to the relationship between signal strength and both mass and compound structure. The value of mass measurements by MALDI to provide a carbohydrate composition is stressed, together with the ability of the technique to provide fragmentation spectra. The use of in-source and post-source decay and collision-induced fragmentation in this context is described with emphasis on ions that provide information on the linkage and branching patterns of carbohydrates. The use of MALDI mass spectrometry, linked with exoglycosidase sequencing, is described for N-linked glycans derived from glycoproteins, and methods for the analysis of O-linked glycans are also covered. The review ends with a description of various applications of the technique to carbohydrates found as constituents of glycoproteins, bacterial glycolipids, sphingolipids, and glycolipid anchors.

Glycosylation of natural human neutrophil gelatinase B and neutrophil gelatinase B-associated lipocalin

Gelatinase B is a matrix metalloproteinase (MMP-9) involved in tissue remodeling, development, cancer, and inflammation. Neutrophils produce three major forms of (pro)gelatinase B: 92 kDa monomers, homodimers, and complexes of gelatinase B covalently bound to neutrophil gelatinase B-associated lipocalin (NGAL). In contrast to the case for other proteinases, little information about the glycosylation of any natural human MMP is available. Here, both gelatinase B and NGAL were purified from human peripheral blood neutrophils, and the entire contents of the released N- and O-glycan pools were analyzed simultaneously using recently developed high-performance liquid chromatography-based technology. The results are discussed within the context of the domain structure of gelatinase B and a molecular model of NGAL based on data from this study and the three-dimensional nuclear magnetic resonance (NMR) structure of the protein. More than 95% of the N-linked glycans attached to both gelatinase B and NGAL were partially sialylated, core-fucosylated biantennary structures with and without outer arm fucose. The O-linked glycans, which were estimated to comprise approximately 85% of the total sugars on gelatinase B, mainly consisted of type 2 cores with Galbeta1,4GlcNAc (lactosamine) extensions, with or without sialic acid or outer arm fucose. This paper also contains the first report of O-linked glycans attached to NGAL. Although both proteins were isolated from neutrophils and contained O-linked glycans mainly with type 2 cores, the glycans attached to individual serine/threonine residue(s) in NGAL were significantly smaller than those on gelatinase B. In contrast to NGAL, gelatinase B contains a region rich in Ser, Thr, and Pro typical of O-glycosylated mucin-like domains.

CD31 (PECAM-1) exists as a dimer and is heavily N-glycosylated

CD31 (PECAM-1) is a highly abundant cell surface glycoprotein expressed on hemopoietic and endothelial cells where it functions as a homophilic adhesion and signaling receptor. Since dimerization and appropriate glycosylation are important features in the regulation of cell surface interactions and signal transduction, we studied the pattern of glycosylation as well as the ability of CD31 to undergo dimerization, both in solution and when expressed on cell membranes. CD31 is heavily glycosylated, with an approximate carbohydrate content of 21%. Nineteen neutral and thirteen sialylated glycans were identified. Ultracentrifugation analysis showed that soluble recombinant CD31 exists in equilibrium between a monomer and a dimer with an approximate dissociation constant of 12.5 microM. Chemical cross-linking studies of both soluble and membrane-expressed CD31 confirmed that CD31 exists as a dimer. These studies suggest that, like E-cadherin, PECAM-dimerization is likely to play a role in CD31 adhesion and signaling.

Glycosylation of a CNS-specific extracellular matrix glycoprotein, tenascin-R, is dominated by O-linked sialylated glycans and "brain-type" neutral N-glycans

As a member of the tenascin family of extracellular matrix glycoproteins, tenascin-R is located exclusively in the CNS. It is believed to play a role in myelination and axonal stabilization and, through repulsive properties, may contribute to the lack of regeneration of CNS axons following damage. The contrary functions of the tenascins have been localized to the different structural domains of the protein. However, little is known concerning the influence of the carbohydrate conjugated to the many potential sites for N - and O -glycosylation (10-20% by weight). As a first analytical requirement, we show that >80% of the N -glycans in tenascin-R are neutral and dominated by complex biantennary structures. These display the "brain-type" characteristics of outer-arm- and core-fucosylation, a bisecting N -acetylglucosamine and, significantly, an abundance of antennae truncation. In some structures, truncation resulted in only a single mannose residue remaining on the 3-arm, a particularly unusual consequence of the N -glycan processing pathway. In contrast to brain tissue, hybrid and oligomannosidic N -glycans were either absent or in low abundance. A high relative abundance of O -linked sialylated glycans was found. This was associated with a significant potential for O -linked glycosylation sites and multivalent display of the sialic acid residues. These O -glycans were dominated by the disialylated structure, NeuAcalpha2-3Galbeta1-3(NeuAcalpha2-6)GalNAc. The possibility that these O -glycans enable tenascin-R to interact in the CNS either with the myelin associated glycoprotein or with sialoadhesin on activated microglia is discussed.

Sugars including erythronic and threonic acids in human aqueous humour

PURPOSE

Sugars in the aqueous humour of the eye serve both as a source of nutrients to the lens and other anterior ocular tissues, and potentially as an indicator of waste products from these tissues. In this work we intended to measure the levels of sugars in human blood and aqueous humour from cataract patients with and without diabetes. After initial results we decided to identify an unknown sugar component.

METHODS

Sugars were measured by hplc. The unknown sugar peak was identified by gas chromatography/mass spectrometry

RESULTS

Very little fructose and sorbitol were found. Glucose levels were higher in both blood and aqueous from diabetic patients. During these analyses we found a major component that did not correspond to any sugar reported previously in aqueous humour. This was identified as a mixture of threonic and erythronic acids.

CONCLUSIONS

Glucose levels increase in human aqueous humour in diabetes without markedly raised levels of sorbitol or fructose. Erythronic and threonic acids are normal components of aqueous humour and blood. They may be derived from glycated proteins or from degradation of ascorbic acid.

Characterisation of tissue-specific oligosaccharides from rat brain and kidney membrane preparations enriched in Na+,K+-ATPase

The organ-specific nature of the glycosylation of Na+,K+-ATPase-enriched preparations from kidney and brain tissues has earlier been indicated by the use of lectin-staining techniques. Na+,K+-ATPase is ubiquitous and abundant, and subject to upregulation during cell-division and in certain pathological conditions. Lectins specific for the different carbohydrates displayed by the Na+,K+-ATPases may, therefore, be useful carriers/mediators in tissue-specific targeting. N-linked oligosaccharides purified from Na+,K+-ATPase-enriched preparations from rat brain and kidney were consequently characterised in detail in this study using weak anion exchange and normal phase HPLC (combined with serial glycosidase digestions) and matrix-assisted laser desorption/ionisation mass spectrometry. The oligomannose series of glycans were most abundant in the brain tissue preparation and this contrasted with the renal-associated oligosaccharides that were dominated by families of tetra-antennary glycans (with/without a core fucose) with up to four lactosaminylglycan residues in either branched or linear formation.

Biochemical analysis of the Saccharomyces cerevisiae SEC18 gene product: implications for the molecular mechanism of membrane fusion

The SEC18 gene product is 48% identical to mammalian NSF (N-ethylmaleimide-sensitive fusion protein), and both proteins encode cytoplasmic ATPases which are essential for membrane traffic in yeast and mammalian cells, respectively. A wealth of biochemical analysis has led to the description of a model for the action of NSF; through its interaction with SNAPs (soluble NSF attachment proteins), NSF can associate with SNAP receptors (SNAREs) on intracellular membranes, forming 20S complexes. SNAPs then stimulate the intrinsic ATPase activity of NSF, leading to the disassembly of the 20S complex, which is essential for subsequent membrane fusion. Although this model is based almost entirely on in vitro studies of the original clones of NSF and alpha-SNAP, it is nevertheless widely assumed that this mechanism of membrane fusion is conserved in all eukaryotic cells. If so, the crucial biochemical properties of NSF and SNAPs should be shared by their yeast homologues, Sec18p and Sec17p. Using purified recombinant proteins, we report here that Sec18p can specifically interact not only with Sec17p but also with its mammalian homologue, alpha-SNAP. This interaction leads to a stimulation of Sec18p D1 domain ATPase activity, with kinetics similar to those of alpha-SNAP stimulation of NSF, although differences in temperature and N-ethylmaleimide sensitivity were observed between NSF and Sec18p. Furthermore, Sec18p can interact with synaptic SNARE proteins and can synergize with alpha-SNAP to stimulate regulated exocytosis in mammalian cells. We conclude that the mechanistic properties of NSF and SNAPs are shared by Sec18p and Sec17p, thus demonstrating that the biochemistry of membrane fusion is conserved from yeast to mammals.

Oligosaccharide analysis and molecular modeling of soluble forms of glycoproteins belonging to the Ly-6, scavenger receptor, and immunoglobulin superfamilies expressed in Chinese hamster ovary cells

Most cell surface molecules are glycoproteins consisting of linear arrays of globular domains containing stretches of amino acid sequence with similarities to regions in other proteins. These conserved regions form the basis for the classification of proteins into superfamilies. Recombinant soluble forms of six leukocyte antigens belonging to the Ly-6 (CD59), scavenger receptor (CD5), and immunoglobulin (CD2, CD48, CD4, and Thy-1) superfamilies were expressed in the same Chinese hamster ovary cell line, thus providing an opportunity to examine the extent to which N-linked oligosaccharide processing might vary in a superfamily-, domain-, or protein-dependent manner in a given cell. While we found no evidence for superfamily-specific modifications of the glycans, marked differences were seen in the types of oligosaccharides attached to individual proteins within a given superfamily. The relative importance of local protein surface properties versus the overall tertiary structure of the molecules in directing this protein-specific variation was examined in the context of molecular models. These were constructed using the 3D structures of the proteins, glycan data from this study, and an oligosaccharide structural database. The results indicated that both the overall organization of the domains and the local protein structure can have a large bearing on site-specific glycan modification of cells in stasis. This level of control ensures that the surface of a single cell will display a diverse repertoire of glycans and precludes the presentation of multiple copies of a single oligosaccharide on the cell surface. The glycans invariably shield large regions of the protein surfaces although, for the glycoproteins examined here, these did not hinder the known active sites of the molecules. The models also indicated that sugars are likely to play a role in the packing of the native cell surface glycoproteins and to limit nonspecific protein-protein interactions. In addition, glycans located close to the cell membrane are likely to affect crucially the orientation of the glycoproteins to which they are attached.

Picolinyl esters for the structural determination of fatty acids by GC/MS

The position of unsaturation, chain branching, and other structural features of fatty acids are not often apparent from the mass spectra of common derivatives such as methyl esters because of factors such as charge location at the carboxy termiunus and migration of double bonds. The spectra of picolinyl esters, on the other hand, contain fragment ions that provide this information. The esters are synthesized by reaction of the acids with thionyl chloride to form the acid chloride that is reacted with 3-pyridylcarbinol to give the ester. Under electron impact conditions in the mass spectrometer, an electron is removed from the nitrogen of the pyridine ring and a hydrogen atom is abstracted from the alkyl chain to this electron-deficient site. This process produces a radical site in the chain that initiates chain cleavage. Hydorgen atoms can be removed from any position of the chain with varying probability, depending on the chain structure. Thus, diagnostic ions are produced from each type of fatty acid whose masses and relative abundances reflect the structure of the alkyl chain and any substituents. Patterns of fragmentation for straight-chain, branched-chain, unsaturated and cyclic fatty acids are described together with those containing hydroxy-, epoxy-, keto-, and ether groups.

Identification of highly fucosylated N-linked oligosaccharides from the human parotid gland

The glycosylation of a number of constituents of human saliva is known to modify its biological roles, such as its lubricating properties and binding of microbial flora. Gillece-Castro et al. [Gillece-Castro, B. L., Prakobphol, A., Burlingame, A. L., Leffler, H. & Fisher, S. J. (1991) J. Biol. Chem. 266, 17358-17368] have proposed that the major glycan on the salivary proline-rich glycoproteins is a trifucosylated biantennary sugar with one difucosylated and one unfucosylated antenna. Furthermore, they proposed that the non-fucosylated antenna mediated adherence to a peridontal pathogen, Fusobacterium nucleatum. The detailed structures and roles of other highly fucosylated glycans that co-exist in the parotid gland are not fully known. In view of the influence of outer-arm fucosylation on carbohydrate recognition processes in general, this paper reports the use of a combination of HPLC (normal and reversed phase), matrix-assisted laser-desorption/ionisation (MALDI) mass spectrometry and exoglycosidase digestions to dissect the detailed structures of the most abundant of these polyfucosylated glycans. For measurement of reversed-phase HPLC retention times, new calibration units were used which paralleled the glucose units used for normal-phase HPLC. These differed in that the difference in retention times were compared with those derived from a ladder of 2-aminobenzamide-labelled arabinose oligomers instead of the corresponding oligomers from partially hydrolysed dextran. Over sixty neutral sugars were identified from the parotid gland and many of these were additionally found substituted with sialic acid (both alpha2-3-linked and alpha2-6-linked) and sulphate. These glycans were mainly bi- and tri-antennary sugars with up to five and seven fucose residues respectively, containing fucose alpha1-3-linked to the outer-arm GlcNAc residues and alpha1-2-linked to the galactose. All fucosylated structures contained a core (alpha1-6-linked) fucose. The detailed structure of the trifucosylated biantennary glycan was confirmed, together with the structures of another 12 fucosylated biantennary glycans. Smaller amounts of hybrid and tetraantennary structures were also found and bisected glycans were shown to be constituents of parotid glycoproteins for the first time. Acidic glycans were mainly substituted with sialic acid. Most were monosialylated as the presence of fucose on the antennae was found to suppress the addition of extra sialic acid moieties. The possible functional significance of highly fucosylated N-glycans is discussed in relation to their modification of the availability of other non-reducing terminal monosaccharides for recognition processes.

Sialylated N-glycans in adult rat brain tissue--a widespread distribution of disialylated antennae in complex and hybrid structures

This paper extends our earlier work on the analysis of neutral N-glycans from adult rat brain to glycans carrying NeuAc residues as their sole charged groups. These structures comprised at least 40% of the total (acidic and neutral) N-glycan pool. Compounds were identified by a combination of endoglycosidase and exoglycosidase digestions, anion-exchange chromatography, normal and reverse-phase high-performance liquid chromatography, matrix-assisted laser desorption/ionisation-mass spectrometry and combined gas chromatography/mass spectrometry. Mono-, di- and trisialylated components, together with components substituted with four (or more) NeuAc residues, showed abundances of approximately 12, 10, 7 and 7%, respectively, relative to the total N-glycan pool. In addition, neuraminidase digestion resulted in the neutralisation of a fraction of highly charged species, possibly indicating the presence of N-glycans substituted with short chains of polysialic acid. Sialylated bi-, tri- [mainly the (2,4)-branched isomer], tetraantennary complex, polylactosamine and hybrid structures were detected. Typically, for 'brain-type' N-glycosylation, these sialylated structures were variously modified by the presence of core alpha1-6-linked and outer-arm alpha1-3-linked fucose residues and by a bisecting GlcNAc. Structural groups such as sialyl Lewis(x) and NeuAc alpha2-3 substituted Galbeta1-4GlcNAc antennae were common. In contrast to the neutral glycans, however, a widespread distribution of terminal beta1-3-linked galactose residues was observed. The presence of beta1-3-linked galactose allowed for a high degree of sialylation as afforded by the presence of the NeuAc alpha2-3Galbeta1-3(NeuAc alpha2-6)GlcNAc structural group. This revealed a number of novel structures including the presence of tetraantennary N-glycans with more than one beta1-3galactose residue and (2,4)-branched triantennary oligosaccharides containing three such residues. Disialylated hybrid glycans containing beta1-3-linked galactose and 'polylactosamine' N-glycans with one to three terminal beta1-3galactose residues were additional novel features. The N-glycans modified by polysialylation lacked outer-arm fucose and bisecting GlcNAc residues but all contained one or more terminal beta1-3-linked galactose residues. These may be representative, therefore, of the polysialylated N-glycans expressed mainly on neural cell-adhesion molecules and known to be present in adult rat brain. The diversity of presentation of terminal sialylated groups in rat brain implies potential specificity for possible charge or lectin-mediated interactions. The distinguishing sets of sialylated structures described here are indicative of differences in the natural glycosylation processing pathways in different cell types within the central nervous system, a specificity that may be further magnified on the individual glycoproteins.