Recovery of intact 2-aminobenzamide-labeled O-glycans released from glycoproteins by hydrazinolysis

The initial step in quantitative analysis of O-linked glycans of glycoproteins is to release them in high yield, nonselectively, unmodified, and with a free reducing terminus. In contrast to other techniques, hydrazinolysis can meet these criteria. However, when analyzing pools of O-linked glycans as described in the accompanying article by L. Royle et al. (2002, Anal. Biochem. 304), some peeling of the glycans was observed. Critical steps in the sample preparation and glycan recovery were therefore evaluated by analyzing and identifying both intact O-glycans and degraded products. Synthetic O-glycopeptides were characterized by mass spectrometry. Released glycans were identical to those on the glycopeptide. O-Linked glycans from a range of glycoproteins of increasing complexity, namely, bovine serum fetuin, glycophorin A, and previously uncharacterized glycopeptides isolated from human salivary mucin Muc5B, were also analyzed. Quantitative analysis of the glycan profile confirmed that there was <2% peeling of O-glycans released by hydrazinolysis conditions of 60 degrees C for 6 h, and recovered using the optimised procedure now described. This demonstrated that O-glycans can be prepared by hydrazinolysis without degradation and, as part of an analytical strategy, makes the analysis of O-glycans attached to low-microgram levels of naturally occurring glycoproteins feasible.     

Annotation of a serum N-glycan library for rapid identification of structures

Glycosylation is one of the most common post-translational modifications of proteins and has been shown to change with various pathological states including cancer. Global glycan profiling of human serum based on mass spectrometry has already led to several promising markers for diseases. The changes in glycan structure can result in altered monosaccharide composition as well as in the linkages between the monosaccharides. High-throughput glycan structural elucidation is not possible because of the lack of a glycan template to expedite identification. In an effort toward rapid profiling and identification of glycans, we have constructed a library of structures for the serum glycome to aid in the rapid identification of serum glycans. N-Glycans from human serum glycoproteins are used as a standard and compiled into a library with exact structure (composition and linkage), liquid chromatography retention time, and accurate mass. Development of the library relies on highly reproducible nanoLC-MS retention times. Tandem MS and exoglycosidase digestions were used for structural elucidation. The library currently contains over 300 entries with 50 structures completely elucidated and over 60 partially elucidated structures. This database is steadily growing and will be used to rapidly identify glycans in unknown biological samples.     

Integrated mass spectrometric strategy for characterizing the glycans from glycosphingolipids and glycoproteins: direct identification of sialyl Le(x) in mice

The current interest in applying systems biology approaches to studying an organism's form or function promises to reveal further insights into the role of glycosylation in cells and whole organisms. This has prompted the development of a rapid, sensitive method of profiling the glycan component of both glycosphingolipids and glycoproteins from a single sample. Here we report a new mass spectrometric screening strategy for characterizing glycosphingolipid-derived oligosaccharides, which can be integrated into an existing highly sensitive glycoprotein glycomics strategy. Using ceramide glycanase to release the glycans from glycosphingolipids, this method provides a reliable profile of the glycosphingolipid-derived glycans present in a sample and has revealed new glycan structures. Glycoproteins are also efficiently recovered using this method, allowing the subsequent analysis of glycoprotein-derived glycans by mass spectrometry. The high sensitivity of this glycomic screening method allowed us to directly characterize the sialyl Le(x) epitope from mouse brain for the first time, where it was observed on an O-mannose structure. Thus, we present a mass spectrometric method that allows glycomic screening of N- and O-glycans as well as glycosphingolipid-derived glycans from a single tissue.     

Abnormal glycosylation with hypersialylated O-glycans in patients with Sialuria

Sialuria is an inborn error of metabolism characterized by coarse face, hepatomegaly and recurrent respiratory tract infections. The genetic defect in this disorder results in a loss of feedback control of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine-kinase by CMP-N-acetylneuraminic acid (CMP-NeuAc) resulting in a substantial overproduction of cytoplasmic free sialic acid. This study addresses fibroblast CMP-NeuAc levels and N- and O-glycan sialylation of serum proteins from Sialuria patients. CMP-NeuAc levels were measured with HPLC in fibroblasts. Isoelectric focusing (IEF) of serum transferrin and of apolipoprotein C-III (apoC-III) was performed on serum of three Sialuria patients. Isoforms of these proteins can be used as specific markers for the biosynthesis of N- and core 1 O-glycans. Furthermore, total N- and O-linked glycans from serum proteins were analyzed by HPLC. HPLC showed a clear overproduction of CMP-NeuAc in fibroblasts of a Sialuria patient. Minor changes were found for serum N-glycans and hypersialylation was found for core 1 O-glycans on serum apoC-III and on total serum O-glycans in Sialuria patients. HPLC showed an increased ratio of disialylated over monosialylated core 1 O-glycans. The hypersialylation of core 1 O-glycans is due to the increase of NeuAcalpha2,6-containing structures (mainly NeuAcalpha2-3Galbeta1-3[NeuAcalpha2-6]GalNAc). This may relate to KM differences between GalNAc-alpha2,6-sialyltransferase and alpha2,3-sialyltransferases. This is the first study demonstrating that the genetic defect in Sialuria results in a CMP-NeuAc overproduction. Subsequently, increased amounts of alpha2,6-linked NeuAc were found on serum core 1 O-glycans from Sialuria patients. N-glycosylation of serum proteins seems largely unaffected. Sialuria is the first metabolic disorder presenting with hypersialylated O-glycans.     

The sperm agglutination antigen-1 (SAGA-1) glycoforms of CD52 are O-glycosylated

CD52 is composed of a 12 amino acid peptide with N-linked glycans bound to the single potential glycosylation site at position 3, and a glycosylphosphatidylinositol-anchor attached at the C-terminus. Some glycoforms of this molecule expressed in the male reproductive tract are recognized by complement-dependent sperm-immobilizing antibodies in infertile patients making this antigen an important target for immunocontraception and fertility studies. Although the amount of posttranslational modification is already remarkable for such a small polypeptide, O-glycosylation of CD52 has additionally been implicated by several studies, but never rigorously characterized. In this report, we show clear evidence for the presence of O-glycans in CD52 preparations immunopurified using the murine S19 monoclonal antibody generated against sperm agglutination antigen-1 (SAGA-1), a male reproductive tract specific form of CD52. The O-glycans have been characterized by MALDI-TOF and tandem mass spectrometry after reductive elimination and permethylation. The data indicate that the major SAGA-1 O-glycans are core 1 and 2 mucin-type structures, with and without sialic acid (NeuAc(0-2)Hex(1-3)HexNAc(1-2)HexNAcitol). Minor fucosy- lated O-glycans are also present including some struc- tures with putative Le(y) epitopes (NeuAc(0-1)Fuc(1-3)Hex(1-2) HexNAc(0-1)HexNAcitol). Analysis of O-glycopeptides by tandem mass spectrometry provided an additional level of support for the O-glycosylation of SAGA-1. Elucidation of the O-glycosylation of SAGA-1 adds to the complexity of this molecule and may help to explain its biological activity.     

A rapid mass spectrometric strategy for the characterization of N- and O-glycan chains in the diagnosis of defects in glycan biosynthesis

Glycosylation of proteins is a very complex process which involves numerous factors such as enzymes or transporters. A defect in one of these factors in glycan biosynthetic pathways leads to dramatic disorders named congenital disorders of glycosylation (CDG). CDG can affect the biosynthesis of not only protein N-glycans but also O-glycans. The structural analysis of glycans on serum glycoproteins is essential to solving the defect. For this reason, we propose in this paper a strategy for the simultaneous characterization of both N- and O-glycan chains isolated from the serum glycoproteins. The serum (20 microL) is used for the characterization of N-glycans which are released by enzymatic digestion with PNGase F. O-glycans are chemically released by reductive elimination from whole serum glycoproteins using 10 microL of the serum. Using strategies based on mass spectrometric analysis, the structures of N- and O-glycan chains are defined. These strategies were applied on the sera from one patient with CDG type IIa, and one patient with a mild form of congenital disorder of glycosylation type II (CDG-II) that is caused by a deficiency in the Cog1 subunit of the complex.     

Exploring the glycan repertoire of genetically modified mice by isolation and profiling of the major glycan classes and nano-NMR analysis of glycan mixtures

The production of mice with genetic alterations in glycosyltransferases has highlighted the need to isolate and study complex mixtures of the major classes of oligosaccharides (glycans) from intact tissues. We have found that nano-NMR spectroscopy of whole mixtures of N- and O-glycans can complement HPLC profiling methods for elucidating structural details. Working toward obtaining such glycan mixtures from mouse tissues, we decided to develop an approach to isolate not only N- and O-glycans, but also to separate out glycosphingolipids, glycosaminoglycans and glycosylphosphatidylinositol anchors. We describe here a comprehensive Glycan Isolation Protocol that is based primarily upon the physicochemical characteristics of the molecules, and requires only commonly available reagents and equipment. Using radiolabeled internal tracers, we show that recovery of each major class of glycans is as good or better than with conventional approaches for isolating individual classes, and that cross-contamination is minimal. The recovered glycans are of sufficient purity to provide a "glycoprofile" of a cell type or tissue. We applied this approach to compare the N- and O-glycans from wild type mouse tissues with those from mice genetically deficient in glycosyltransferases. N- and O-glycan mixtures from organs of mice deficient in ST6Gal-I (CMP-Sia:Galbeta1-4GlcNAc alpha2-6 sialyltransferase) were studied by the nano-NMR spectroscopy approach, showing no detectable alpha2-6-linked sialic acids. Thus, ST6Gal-I is likely responsible for generating most or all of these residues in normal mice. Similar studies indicate that this linkage is very rare in ganglioside glycans, even in wild-type tissues. In mice deficient in GalNAcT-8 (UDP-GalNAc:polypeptide O-Ser/Thr GalNAc transferase 8), HPLC profiling indicates that O-glycans persist in the thymus in large amounts, without a major change in overall profile, suggesting that other enzymes can synthesize the GalNAc-O-Ser/Thr linkage in this tissue. These results demonstrate the applicability of nano-NMR spectroscopy to complex glycan mixtures, as well as the versatility of the Glycan Isolation Protocol, which makes possible the concurrent examination of multiple glycan classes from intact vertebrate tissues.     

Glycan structures of ocular surface mucins in man,rabbit and dog display species differences

The composition of the mucus gel of the tear film reflects the competing needs for transparency, stability, hydration, and protection of the ocular surface. Mucins form the macromolecular scaffolding of this hydrated gel, and glycans decorating these glycoproteins represent a rich source of binding ligands that may both modulate microbial binding and regulate the physicochemical characteristics of the gel. This study compares the structure of O-linked glycans derived from the ocular mucins of three species, to determine whether the ocular surface microenvironment dictates the need for a common pattern of O-linked carbohydrate structures. Ocular mucus aspirates were collected from healthy humans, rabbits and dogs. Mucins were purified using standard protocols. O-glycans were released by hydrazinoloysis and subsequently analysed by a combination of HPLC, exoglycosidase digestions and LC–MS/MS. A total of 12 different O-glycans were identified. In human ocular mucin, the majority were negatively charged and terminated in sialic acid, whilst those from rabbit or dog were mainly neutral and terminated in α 1-2 fucose and/or α 1-3 N-acetylgalactosamine. The glycans were short: the most common structures being tetra-, tri- or disaccharides. Less elaborate glycan structures are encountered at the ocular surface than at many other mucosal surfaces. Species-specific glycan expression is a feature of ocular surface mucins, and has implications for their defensive properties where different microbial and environmental challenges are encountered. Louise Royle and Elizabeth Matthews contributed equally to this work.     

Altered O-glycosylation and sulfation of airway mucins associated with cystic fibrosis

Cystic fibrosis (CF) is the most lethal genetic disorder in Caucasians and is characterized by the production of excessive amounts of viscous mucus secretions in the airways of patients, leading to airway obstruction, chronic bacterial infections, and respiratory failure. Previous studies indicate that CF-derived airway mucins are glycosylated and sulfated differently compared with mucins from nondiseased (ND) individuals. To address unresolved questions about mucin glycosylation and sulfation, we examined O-glycan structures in mucins purified from mucus secretions of two CF donors versus two ND donors. All mucins contained galactose (Gal), N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc), fucose (Fuc), and sialic acid (Neu5Ac). However, CF mucins had higher sugar content and more O-glycans compared with ND mucins. Both ND and CF mucins contained GlcNAc-6-sulfate (GlcNAc-6-Sul), Gal-6-Sul, and Gal-3-Sul, but CF mucins had higher amounts of the 6-sulfated species. O-glycans were released from CF and ND mucins and derivatized with 2-aminobenzamide (2-AB), separated by ion exchange chromatography, and quantified by fluorescence. There was nearly a two-fold increase in sulfation and sialylation in CF compared with ND mucin. High performance liquid chromatography (HPLC) profiles of glycans showed differences between the two CF samples compared with the two ND samples. Glycan compositions were defined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Unexpectedly, 260 compositional types of O-glycans were identified, and CF mucins contained a higher proportion of sialylated and sulfated O-glycans compared with ND mucins. These profound structural differences in mucin glycosylation in CF patients may contribute to inflammatory responses and increased pathogenesis by Pseudomonas aeruginosa.     

Glycosylation of serum ribonuclease 1 indicates a major endothelial origin and reveals an increase in core fucosylation in pancreatic cancer

Human pancreatic ribonuclease 1 (RNase 1) is a glycoprotein expressed mainly by the pancreas and also found in endothelial cells. The diagnosis of pancreatic cancer (PaC) remains difficult and therefore the search for sensitive and specific markers is required. Previous studies showed that RNase 1 from human healthy pancreas contained only neutral glycans, whereas RNase 1 from PaC cell lines contained sialylated structures. To determine whether these glycan tumor cell-associated changes were also characteristic of serum RNase 1 and could be used as a marker of PaC, we have analyzed the glycosylation of serum RNase 1. The origin of serum RNase 1 was also investigated. Serum RNase 1 from two PaC patients and two controls was purified and the glycans analyzed by high-performance liquid chromatography (HPLC)-based sequencing and mass spectrometry. Although normal and tumor serum RNase 1 contained the same glycan structures, there was an increase of 40% in core fucosylation in the main sialylated biantennary glycans in the PaC serum RNase 1. This change in proportion would be indicative of a subset of tumor-associated glycoforms of RNase 1, which may provide a biomarker for PaC. Two-dimensional electrophoresis of the RNase 1 from several endothelial cell lines, EA.hy926, human umbilical vein endothelial cells (HUVEC), human mammary microvessel endothelial cells (HuMMEC), and human lung microvessel endothelial cells (HuLEC), showed basically the same pattern and was also very similar to that of serum RNase 1. RNase 1 from EA.hy926 was then purified and presented a glycosylation profile very similar to that from serum RNase 1, suggesting that endothelial cells are the main source of this enzyme.     

Structure determination by MALDI-IRMPD mass spectrometry and exoglycosidase digestions of O-linked oligosaccharides from Xenopus borealis egg jelly

Differences in the fertilization behavior of Xenopus borealis from X. laevis and X. tropicalis suggest differences in the glycosylation of the egg jellies. To test this assumption, O-linked glycans were chemically released from the egg jelly coat glycoproteins of X. borealis. Over 50 major neutral glycans were observed, and no anionic glycans were detected from the released O-glycan pool. Preliminary structures of ～30 neutral oligosaccharides were determined using matrix-assisted laser desorption/ionization (MALDI) infrared multiphoton dissociation tandem mass spectrometry (MS). The mass fingerprint of a group of peaks for the core-2 structure of O-glycans was conserved in the tandem mass spectra and was instrumental in rapid and efficient structure determination. Among the 29 O-glycans, 22 glycans contain the typical core-2 structure, 3 glycans have the core-1 structure and 2 glycans contained a previously unobserved core structure with hexose at the reducing end. There were seven pairs of structural isomers observed in the major O-linked oligosaccharides. To further elucidate the structures of a dozen O-linked glycans, specific and targeted exoglycosidase digestions were carried out and the products were monitored with MALDI-MS. Reported here are the elucidated structures of O-linked oligosaccharides from glycoproteins of X. borealis egg jelly coats. The structural differences in O-glycans from jelly coats of X. borealis and its close relatives may provide a better understanding of the structure-function relationships and the role of glycans in the fertilization process within Xenopodinae.     

Analysis of folate binding protein N-linked glycans by mass spectrometry

The folate binding protein (FBP), also known as the folate receptor (FR), is a glycoprotein which binds the vitamin folic acid and its analogues. FBP contains multiple N-glycosilation sites, is selectively expressed in tissues and body fluids, and mediates targeted therapies in cancer and inflammatory diseases. Much remains to be understood about the structure, composition, and the tissue specificities of N-glycans bound to FBP. Here, we performed structural characterization of N-linked glycans originating from bovine and human milk FBPs. The N-linked glycans were enzymatically released from FBPs, purified, and permethylated. Native and permethylated glycans were further analyzed by matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectrometry (MS), while tandem MS (MS/MS) was used for their structural characterization. The assignment of putative glycan structures from MS and MS/MS data was achieved using Functional Glycomics glycan database and SimGlycan software, respectively. It was found that FBP from human milk contains putative structures that have composition consistent with high-mannose (Hex(5-6)HexNAc(2)) as well as hybrid and complex N-linked glycans (NeuAc(0-1)Fuc(0-3)Hex(3-6)HexNAc(3-5)). The FBP from bovine milk contains putative structures corresponding to high-mannose (Hex(4-9)HexNAc(2)) as well as hybrid and complex N-linked glycans (Hex(3-6)HexNAc(3-6)), but these glycans mostly do not contain fucose and sialic acid. Glycomic characterization of FBP provides valuable insight into the structure of this pharmacologically important glycoprotein and may have utility in tissue-selective drug targeting and as a biomarker.     

The development of retrosynthetic glycan libraries to profile and classify the human serum N‐linked glycome

Annotation of the human serum N‐linked glycome is a formidable challenge but is necessary for disease marker discovery. A new theoretical glycan library was constructed and proposed to provide all possible glycan compositions in serum. It was developed based on established glycobiology and retrosynthetic state‐transition networks. We find that at least 331 compositions are possible in the serum N‐linked glycome. By pairing the theoretical glycan mass library with a high mass accuracy and high‐resolution MS, human serum glycans were effectively profiled. Correct isotopic envelope deconvolution to monoisotopic masses and the high mass accuracy instruments drastically reduced the amount of false composition assignments. The high throughput capacity enabled by this library permitted the rapid glycan profiling of large control populations. With the use of the library, a human serum glycan mass profile was developed from 46 healthy individuals. This paper presents a theoretical N‐linked glycan mass library that was used for accurate high‐throughput human serum glycan profiling. Rapid methods for evaluating a patient's glycome are instrumental for studying glycan‐based markers.     

Role of N-linked glycans in antigenicity, processing, and cell surface expression of bovine herpesvirus 1 glycoprotein gIV.

Glycoprotein gIV, a structural component of bovine herpesvirus type 1, stimulates high titers of virus-neutralizing antibody. The protein contains three potential sites for the addition of N-linked carbohydrates. Three mutants were constructed by oligonucleotide-directed mutagenesis, in each case changing one N-linked glycosylation site from Asn-X-Thr/Ser to Ser-X-Thr/Ser. A fourth mutant was altered at two sites. The altered forms of the gIV gene were cloned into a vaccinia virus transfer vector to generate recombinant vaccinia viruses expressing mutant proteins. Analysis of these mutants revealed that only two (residues 41 and 102) of the three (residues 41, 102, and 411) potential sites for the addition of N-linked glycans are actually utilized. Absence of glycans at residue 41 (gN1) showed no significant effect on the conformation of the protein or induction of a serum neutralizing antibody response. However, mutant proteins lacking glycans at residue 102 (gN2) or residues 41 and 102 (gN1N2) showed altered reactivity with conformation-dependent gIV-specific monoclonal antibodies. These mutants also induced significantly lower serum neutralizing antibody responses than wild-type gIV. Nonetheless, each of the mutant proteins were modified by the addition of O-glycans and transported to the cell surface. Our results demonstrate that absence of N-linked glycans at one (residue 102) or both (residues 41 and 102) utilized N-linked glycosylation sites alters the conformation but does not prevent processing and transport of gIV to the cell surface.     

Resistance to deglycosylation by ammonia of IgA1 O-glycopeptides: implications for the beta-elimination of O-glycans linked to serine and threonine

Pools of O-glycopeptides (and their deglycosylated analogues) derived from trypsin-digested normal human serum IgA1 have been treated with ammonia under conditions reported to result in complete liberation of O-glycans linked to serine and threonine residues in glycopeptides and glycoproteins. MALDI-TOF MS analysis has revealed that only one of the six glycosylated sites is susceptible to beta-elimination under these conditions. It is likely that resistance to beta-elimination is due to very close proximity of proline to the glycosylated serine or threonine residues. Preliminary results using 0.1M NaOH (instead of ammonia) to perform beta-elimination indicated that there was also selective de-O-glycosylation with this reagent, however, these results were complicated by the concomitant hydrolysis of the peptide bonds. These findings may have implications for similarly O-glycosylated peptides and proteins and possibly for other chemical methods that are used to carry out beta-eliminations of O-glycans.     

Differences in N-glycan structures foundon recombinant IgA1 and IgA2 producedin murine myeloma and CHO cell lines

The development and production of recombinant monoclonal antibodies is well established. Although most of these are IgGs, there is also great interest in producing recombinant IgAs since this isotype plays a critical role in providing immunologic protection at mucosal surfaces. the choice of expression system for production of recombinant antibodies is crucial because they are glycoproteins containing at least one N-linked carbohydrate. these glycans have been shown to contribute to the stability, pharmacokinetics and biologic function of antibodies. We have produced recombinant human IgA1 and all three allotypes of IgA2 in murine myeloma and CHo cell lines to systematically characterize and compare the N-linked glycans. Recombinant IgAs produced in murine myelomas differ significantly from IgA found in humans in that they contain the highly immunogenic Galα(1,3)Gal epitope and N-glycolylneuraminic acid residues, indicating that murine myeloma is not the optimal expression system for the production of human IgA. In contrast, IgAs produced in CHo cells contained glycans that were more similar to those found on human IgA. expression of IgA1 and IgA2 in Lec2 and Lec8 cell lines that are defective in glycan processing resulted in a less complex pool of N-glycans. In addition, the level of sialylation of rIgAs produced in murine and CHo cells was significantly lower than that previously reported for serum IgA1. these data underscore the importance of choosing the appropriate cell line for the production of glycoproteins with therapeutic potential.Key words: recombinant antibody, IgA, glycosylation, expression system, mass spectrometry     

Analysis of protein glycosylation by mass spectrometry

We present a detailed protocol for the structural analysis of protein-linked glycans. In this approach, appropriate for glycomics studies, N-linked glycans are released using peptide N-glycosidase F and O-linked glycans are released by reductive alkaline beta-elimination. Using strategies based on mass spectrometry (matrix-assisted laser desorption/ionization-time of flight mass spectrometry and nano-electrospray ionization mass spectrometry/mass spectrometry (nano-ESI-MS-MS)), chemical derivatization, sequential exoglycosidase digestions and linkage analysis, the structures of the N- and/or O-glycans are defined. This approach can be used to study the glycosylation of isolated complex glycoproteins or of numerous glycoproteins encountered in a complex biological medium (cells, tissues and physiological fluids).     

Extensive determination of glycan heterogeneity reveals an unusual abundance of high mannose glycans in enriched plasma membranes of human embryonic stem cells

Most cell membrane proteins are known or predicted to be glycosylated in eukaryotic organisms, where surface glycans are essential in many biological processes including cell development and differentiation. Nonetheless, the glycosylation on cell membranes remains not well characterized because of the lack of sensitive analytical methods. This study introduces a technique for the rapid profiling and quantitation of N- and O-glycans on cell membranes using membrane enrichment and nanoflow liquid chromatography/mass spectrometry of native structures. Using this new method, the glycome analysis of cell membranes isolated from human embryonic stem cells and somatic cell lines was performed. Human embryonic stem cells were found to have high levels of high mannose glycans, which contrasts with IMR-90 fibroblasts and a human normal breast cell line, where complex glycans are by far the most abundant and high mannose glycans are minor components. O-Glycosylation affects relatively minor components of cell surfaces. To verify the quantitation and localization of glycans on the human embryonic stem cell membranes, flow cytometry and immunocytochemistry were performed. Proteomics analyses were also performed and confirmed enrichment of plasma membrane proteins with some contamination from endoplasmic reticulum and other membranes. These findings suggest that high mannose glycans are the major component of cell surface glycosylation with even terminal glucoses. High mannose glycans are not commonly presented on the surfaces of mammalian cells or in serum yet may play important roles in stem cell biology. The results also mean that distinguishing stem cells from other mammalian cells may be facilitated by the major difference in the glycosylation of the cell membrane. The deep structural analysis enabled by this new method will enable future mechanistic studies on the biological significance of high mannose glycans on stem cell membranes and provide a general tool to examine cell surface glycosylation.     

Structural Characterization by Multistage Mass Spectrometry (MSn) of Human Milk Glycans Recognized by Human Rotaviruses

We have shown that recombinant forms of VP8* domains of the human rotavirus outer capsid spike protein VP4 from human neonatal strains (N155(G10P[11]) and RV3(G3P[6]) and a bovine strain (B223) recognize unique glycans within the repertoire of human milk glycans. The accompanying study by Yu et al.², describes a human milk glycan shotgun glycan microarray that led to the identification of 32 specific glycans in the human milk tagged glycan library that were recognized by these human rotaviruses. These microarray analyses also provided a variety of metadata about the recognized glycan structures compiled from anti-glycan antibody and lectin binding before and after specific glycosidase digestions, along with compositional information from mass analysis by matrix-assisted laser desorption ionization-mass spectrometry. To deduce glycan sequence and utilize information predicted by analyses of metadata from each glycan, 28 of the glycan targets were retrieved from the tagged glycan library for detailed sequencing using sequential disassembly of glycans by ion-trap mass spectrometry. Our aim is to obtain a deeper structural understanding of these key glycans using an orthogonal approach for structural confirmation in a single ion trap mass spectrometer. This sequential ion disassembly strategy details the complexities of linkage and branching in multiple compositions, several of which contained isomeric mixtures including several novel structures. The application of this approach exploits both library matching with standard materials and de novo approaches. This combination together with the metadata generated from lectin and antibody-binding data before and after glycosidase digestions provide a heretofore-unavailable level of analytical detail to glycan structure analysis. The results of these studies showed that, among the 28 glycan targets analyzed, 27 unique structures were identified, and 23 of the human milk glycans recognized by human rotaviruses represent novel structures not previously described as glycans in human milk. The functional glycomics analysis of human milk glycans provides significant insight into the repertoire of glycans comprising the human milk metaglycome.     

牛血清白蛋白胰蛋白酶酶解产物的色谱-质谱联用分析及其三种数据库搜寻鉴定方法的比较

利用反相高效液相色谱 (RP HPLC)和电喷雾串联质谱 (ESI MS MS)联用技术直接对模式蛋白分子 (牛血清白蛋白 ,BSA)的胰蛋白酶酶解产物进行分离和测定 .获得的一系列BSA酶解片段的一级 (MS)和二级 (MS MS)质谱数据经分析软件处理后 ,分别在不同处理和不同参数条件下 ,用 3种不同的方法通过网上蛋白质数据库进行蛋白质搜寻鉴定 .结果显示 ,3种搜寻法都能正确地鉴定该蛋白质 ,其中以利用MS数据的肽质量指纹谱搜寻法 (PMF法 )较为快捷方便 ,但鉴定结果易受数据处理和数据库搜寻鉴定时参数设置等因素的影响 ;利用未解析MS MS数据 (rawMS MSdata)的搜寻法可在较宽的搜寻参数变化范围内获得明确的鉴定结果 ;而借助从头测序 (denovosequencing)结果的序列搜寻法 (sequencequery)则显示出更高的专一性 ,利用较少酶解片段数据就能得到稳定和明确的鉴定结果 ,搜寻参数变化的影响很小 .就酶解条件、数据处理和搜寻参数设置对蛋白质鉴定结果的影响展开详细的讨论 ,为蛋白质组学研究中的数据处理和库搜寻鉴定积累了可借鉴的资料