The asparagine-linked sugar chains of subcomponent C1q of the first component of human complement

Evidence for C1q-mediated crosslinking of CD33/LAIR-1 inhibitory immunoreceptors and biological control of CD33/LAIR-1 expression

C1q collagen-like region (CLR) engaging and activating the LAIR-1 inhibitory immunoreceptor represents a non-complement mechanism for maintaining immune quiescence. Given the binding promiscuity of C1q’s globular region (gC1q), we hypothesized that C1q concurrently associates with distinct inhibitory immunoreceptors to produce C1q-mediated modulatory networking. Like LAIR-1, CD33 inhibitory immunoreceptors are highly expressed on monocytes. Binding CD33 restricts cell activation/differentiation; however, natural ligands for CD33 remain elusive. CD33 has IgC2-like domains potentially recognized by gC1q. Thus, we asked whether C1q binds to CD33 and if C1q mediates CD33/LAIR-1 crosslinking. Our findings demonstrate that C1q and gC1q interact with CD33 to activate its inhibitory motifs, while CLR does not. Whole C1q is required to crosslink CD33 and LAIR-1 and concurrently activate CD33/LAIR-1 inhibitory motifs. While C1q binds CD33C2 domains, decreased C1q-CD33 interactions resulting from sialic acid masking of CD33C2 domains suggests a process for regulating C1q-CD33 activity. Consistent with defective self-tolerance, CD33/LAIR-1 expression is reduced in systemic lupus erythematosus (SLE) myelomonocytes. The anti-inflammatory cytokine M-CSF, but not DC growth factors, sustains CD33/LAIR-1 expression on both healthy and SLE cells suggesting further biological control of C1q-CD33/LAIR-1 processes.

Structural and Functional Characterization of a Single-Chain Form of the Recognition Domain of Complement Protein C1q

Complement C1q is a soluble pattern recognition molecule comprising six heterotrimeric subunits assembled from three polypeptide chains (A–C). Each heterotrimer forms a collagen-like stem prolonged by a globular recognition domain. These recognition domains sense a wide variety of ligands, including pathogens and altered-self components. Ligand recognition is either direct or mediated by immunoglobulins or pentraxins. Multivalent binding of C1q to its targets triggers immune effector mechanisms mediated via its collagen-like stems. The induced immune response includes activation of the classical complement pathway and enhancement of the phagocytosis of the recognized target. We report here, the first production of a single-chain recombinant form of human C1q globular region (C1q-scGR). The three monomers have been linked in tandem to generate a single continuous polypeptide, based on a strategy previously used for adiponectin, a protein structurally related to C1q. The resulting C1q-scGR protein was produced at high yield in stably transfected 293-F mammalian cells. Recombinant C1q-scGR was correctly folded, as demonstrated by its X-ray crystal structure solved at a resolution of 1.35 Å. Its interaction properties were assessed by surface plasmon resonance analysis using the following physiological C1q ligands: the receptor for C1q globular heads, the long pentraxin PTX3, calreticulin, and heparin. The 3D structure and the binding properties of C1q-scGR were similar to those of the three-chain fragment generated by collagenase digestion of serum-derived C1q. Comparison of the interaction properties of the fragments with those of native C1q provided insights into the avidity component associated with the hexameric assembly of C1q. The interest of this functional recombinant form of the recognition domains of C1q in basic research and its potential biomedical applications are discussed.

Structural basis for selective recognition of endogenous and microbial polysaccharides by macrophage receptor SIGN-R1

SIGN-R1 is a principal receptor for microbial polysaccharides uptake and is responsible for C3 fixation via an unusual complement activation pathway on splenic marginal zone macrophages. In these macrophages, SIGN-R1 is also involved in anti-inflammatory activity of intravenous immunoglobulin by direct interaction with sialylated Fcs. The high-resolution crystal structures of SIGN-R1 carbohydrate recognition domain and its complexes with dextran sulfate or sialic acid, and of the sialylated Fc antibody provide insights into SIGN-R1’s selective recognition of a-2,6-sialylated glycoproteins. Unexpectedly, an additional binding site has been found in the SIGNR1 carbohydrate recognition domain, structurally separate from the calcium-dependent carbohydrate-binding site. This secondary binding site could bind repetitive molecular patterns, as observed in microbial polysaccharides, in a calcium-independent manner. These two binding sites may allow SIGNR1 to simultaneously bind both immune glycoproteins and microbial polysaccharide components, accommodating SIGN-R1’s ability to relate the recognition of microbes to the activation of the classical complement pathway.

Analysis of human C1q by combined bottom-up and top-down mass spectrometry: detailed mapping of post-translational modifications and insights into the C1r/C1s binding sites

C1q is a subunit of the C1 complex, a key player in innate immunity that triggers activation of the classical complement pathway. Featuring a unique structural organization and comprising a collagen-like domain with a high level of post-translational modifications, C1q represents a challenging protein assembly for structural biology. We report for the first time a comprehensive proteomics study of C1q combining bottom-up and top-down analyses. C1q was submitted to proteolytic digestion by a combination of collagenase and trypsin for bottom-up analyses. In addition to classical LC-MS/MS analyses, which provided reliable identification of hydroxylated proline and lysine residues, sugar loss-triggered MS(3) scans were acquired on an LTQ-Orbitrap (Linear Quadrupole Ion Trap-Orbitrap) instrument to strengthen the localization of glucosyl-galactosyl disaccharide moieties on hydroxylysine residues. Top-down analyses performed on the same instrument allowed high accuracy and high resolution mass measurements of the intact full-length C1q polypeptide chains and the iterative fragmentation of the proteins in the MS(n) mode. This study illustrates the usefulness of combining the two complementary analytical approaches to obtain a detailed characterization of the post-translational modification pattern of the collagen-like domain of C1q and highlights the structural heterogeneity of individual molecules. Most importantly, three lysine residues of the collagen-like domain, namely Lys(59) (A chain), Lys(61) (B chain), and Lys(58) (C chain), were unambiguously shown to be completely unmodified. These lysine residues are located about halfway along the collagen-like fibers. They are thus fully available and in an appropriate position to interact with the C1r and C1s protease partners of C1q and are therefore likely to play an essential role in C1 assembly.

Adhesion properties of Entamoeba histolytica

Trophozoites of Entamoeba histolytica adhere to and phagocytize red blood cells and bacteria. Furthermore, in the initial step of the amoebic infectious process the parasite attaches to intestinal epithelial cells. A lectin (carbohydrate-binding protein) which apparently has a role in the attachment of the parasite to host cells was found in trophozoites of E. histolytica. When amoeba cells were disrupted by freeze-thawing, the lectin activity, as determined by haemagglutination of human erythrocytes, remained associated with the sedimented membrane fraction. This activity was pH dependent and heat and oxidation-sensitive, and was destroyed by proteolysis and on autoincubation. Moreover, the lectin activity was inhibited by a variety of N-acetylglucosamine-containing compounds such as chitin and chitin oligosaccharides, bacterial peptidoglycan, rabbit colonic mucus, bovine and human serum, an IgA fraction isolated from human colostrum, and IgG from sera of amoebiasis patients. These glycoconjugates also interfered with the adherence of intact radiolabelled amoeba trophozoites to human intestinal epithelial cells as well as their attachment to red blood cells. Although the lectin activity and the toxin-like activity previously found in E. histolytica seem to be two separate substances, they share a number of properties which suggest that they are related and may have a function in pathogenicity.

Interactions between Entamoeba histolytica, bacteria and intestinal cells

Axenically grown pathogenic and non-pathogenic isolates of Entamoeba histolytica have been shown to adhere to mammalian epithelial cells and bacteria by virtue of carbohydrate-binding proteins present on their cell surfaces. The interaction of amoeba isolates of low pathogenicity with a variety of gram-negative bacteria, mainly Escherichia coli strains which are readily ingested by the amoebae after relatively short periods, significantly increased the ability of the trophozoites to: (a) destroy and ingest intestinal epithelial cells; (b) secrete a cytopathic substance which morphologically affects a variety of tissue-cultured cells; and (c) cause hepatic abscesses in hamsters. Addition of carbohydrates that inhibit the lectin-mediated attachment of bacteria to amoebae prevented the enhancement of virulence. Interaction of the amoebae with bacteria that were heat-inactivated, glutaraldehyde-fixed or disrupted by sonication, as well as with bacteria precoated with antibodies or concanavalin A, did not lead to an increase in virulence. Moreover, short prior treatments of the bacteria with inhibitors of protein synthesis, but not with cell-wall synthesis inhibitors, also prevented the stimulation. The results indicate that interactions of amoebae with certain bacteria may be responsible for the increase in amoebic virulence.

Mass spectrometry analysis of the oligomeric C1q protein reveals the B chain as the target of trypsin cleavage and interaction with fucoidan

C1q is a subunit of the C1 complex that triggers activation of the complement classical pathway through recognition and binding of immune complexes. C1q also binds to nonimmune ligands such as the sulfated polysaccharide fucoidan, a potent anticomplementary agent. C1q was submitted for the first time to mass spectrometry analysis, yielding insights into its assembly and its interaction with fucoidan. The MALDI-TOF mass spectrometry technique on membrane allowed partial preservation of noncovalent interactions, allowing precise analysis of its substructure and estimation of the C1q molecular weight at 459520-461883, with an average mass of 460793 g x mol(-1). The disulfide-linked A-B and C-C dimers as well as the noncovalent structural unit (A-B:C)-(C:B-A) were detected, providing experimental support to the C1q model based on covalent and noncovalent associations of six heterotrimers. Trypsin treatment of native C1q led to proteolysis of the B chain only, at a single cleavage site (Arg(109)) located in the globular region. Unlike DNA, fucoidan protected C1q from trypsin cleavage, indicating that this polysaccharide binds to the B moiety of the globular head. Given the involvement of the C1q globular heads in the recognition of IgG, this interaction may account for the observed anticomplementary activity of fucoidan.

The crystal structure of the globular head of complement protein C1q provides a basis for its versatile recognition properties

C1q is a versatile recognition protein that binds to an amazing variety of immune and non-immune ligands and triggers activation of the classical pathway of complement. The crystal structure of the C1q globular domain responsible for its recognition properties has now been solved and refined to 1.9 A of resolution. The structure reveals a compact, almost spherical heterotrimeric assembly held together mainly by non-polar interactions, with a Ca2+ ion bound at the top. The heterotrimeric assembly of the C1q globular domain appears to be a key factor of the versatile recognition properties of this protein. Plausible three-dimensional models of the C1q globular domain in complex with two of its physiological ligands, C-reactive protein and IgG, are proposed, highlighting two of the possible recognition modes of C1q. The C1q/human IgG1 model suggests a critical role for the hinge region of IgG and for the relative orientation of its Fab domain in C1q binding.

Structural analysis of glycoconjugates by on-target enzymatic digestion and MALDI-TOF-MS

Exoglycosidase digestion combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been demonstrated to be an effective method for the structural characterization of glycoconjugates and oligosaccharides in picomolar amounts. A sample preparation method is described, in which 6-aza-2-thiothymine (ATT) in water is used as matrix and enzymes are dialyzed before use against a low concentration of volatile buffer such as ammonium acetate. Under these conditions, a series of sequential on-target exoglycosidase treatments was carried out in one single analyte spot in the presence of ATT matrix. Subsequent mass spectrometric analysis of the resulting products yielded information on both the completeness of the reaction and structural features of the glycoconjugates such as monosaccharide sequence, branching pattern, and anomeric configurations of the corresponding glycosidic linkages. The results show that all exoglycosidases used retain their activity in the presence of ATT matrix. Hence, structural analysis of carbohydrates or mixtures thereof can be performed very fast, without intermediate desalting steps or sample splitting. This approach is illustrated by the analysis of underivatized glycans, oligosaccharide derivatives, glycopeptides, and glycolipids. Depending on the analyte, amounts of sample required could be limited to a few picomoles.

Fluorophore-assisted carbohydrate electrophoresis technology and applications

Carbohydrates, in particular the complex carbohydrates conjugated to proteins and lipids, have important functions in a variety of biological systems. Their isolation and structural determination--prerequisites for elucidation of their biological functions--have been technical challenges for many decades. Almost all available chromatographic and electrophoretic methods as well as NMR and MS have been applied to carbohydrate analysis but none has proved satisfactory in terms of simplicity, sensitivity, reproducibility, cost and requirement for materials. Recently, a technique called fluorophore-assisted carbohydrate electrophoresis was developed which is very promising. It separates fluorescently-labeled carbohydrates on polyacrylamide gels and uses a charge-coupled device camera to detect and quantitate the products. This review describes the principles of the method and its applications to several aspects of research on carbohydrate-containing biological biomolecules.

Analysis of protein glycosylation by mass spectrometry

There is a growing pharmaceutical market for protein-based drugs for use in therapy and diagnosis. The rapid developments in molecular and cell biology have resulted in production of expression systems for manufacturing of recombinant proteins and monoclonal antibodies. These proteins are glycosylated when expressed in cell systems with glycosylation ability. For glycoproteins intended for therapeutic administration it is important to have knowledge about the structure of the carbohydrate side chains to avoid cell systems that produce structures, which in humans can cause undesired reactions, e.g., immunological and unfavorable serum clearance rate. Structural analysis of glycoprotein oligosaccharides requires sophisticated instruments like mass spectrometers and nuclear magnetic resonance spectrometers. However, before the structural analysis can be conducted, the carbohydrate chains have to be released from the protein and purified to homogeneity, and this is often the most time-consuming step. Mass spectrometry has played and still plays an important role in analysis of protein glycosylation. The superior sensitivity compared to other spectroscopic methods is its main asset. Structural analysis of carbohydrates faces several problems, however, due to the chemical nature of the constituent monosaccharide residues. For oligosaccharides or glycoconjugates, the structural information from mass spectrometry is essentially limited to monosaccharide sequence, molecular weight, an only in exceptional cases glycosidic linkage positions can be obtained. In order to completely establish an oligosaccharide structure, several other structural parameters have to be determined, e.g., linkage positions, anomeric configuration and identification of the monosaccharide building blocks. One way to address some of these problems is to work on chemical pretreatment of the glycoconjugate, to specifically modify the carbohydrate chain. In order to introduce specific modifications, we have used periodate oxidation and trifluoroacetolysis with the objective of determining glycosidic linkage positions by mass spectrometry.

Structure and localization of O- and N-linked oligosaccharide chains on basement membrane protein nidogen

The carbohydrate content of mouse nidogen predicts the occupation of two N- and about seven O-linked acceptor sites. The corresponding oligosaccharides were examined by sequential exoglycosidase digestions. The data indicate N-linked substitutions by several bi-, tri- and tetraantennary complex types of oligosaccharides which are further modified by additional lactosamines and terminal alpha-galactose and/or sialic acid. Mannose-rich oligosaccharides were of low abundance. O-linked structures included a di- and tetrasaccharide core structure that were in addition sialylated and may be similar to structures found in fetuin. Evidence is provided that the two sequence-predicted asparagine acceptors are almost fully substituted. Sequence analysis of tryptic peptides identified Thr-271, Ser-303, Thr-309, Thr-317, Thr-320, Thr-892 and Thr-905 as the most likely sites for galactosamine substitutions. These residues are located in the flexible link connecting the N-terminal globular domains G1 and G2 of nidogen and at the border between the rod and the C-terminal globe G3. Four of them showed Pro in the -1 or +3 position. All these Ser, Thr and Pro residues but not the N-linked attachment sites are identical in human nidogen.

Structures and functions of the sugar chains of glycoproteins

Most proteins within living organisms contain sugar chains. Recent advancements in cell biology have revealed that many of these sugar chains play important roles as signals for cell-surface recognition phenomena in multi-cellular organisms. In order to elucidate the biological information included in the sugar chains and link them with biology, a novel scientific field called 'glycobiology' has been established. This review will give an outline of the analytical techniques for the structural study of the sugar chains of glycoproteins, the structural characteristics of the sugar chains and the biosynthetic mechanism to produce such characteristics. Based on this knowledge, functional aspects of the sugar chains of glycohormones and of those in the immune system will be described to help others understand this new scientific field.

Structure of N-linked oligosaccharide chains in the triple-helical domains of human type VI and mouse type IV collagen

Asparagine-linked oligosaccharides were liberated from pepsin-treated type VI collagen and the 7S domain of type IV collagen by hydrazinolysis and their structures analysed by exoglycosidase treatment. The major component in both proteins was complex biantennary oligosaccharide being partly modified by the addition of fucose and sialic acid residues. The 7S domain contained in addition distinct amounts of truncated biantennary structures lacking one or two beta-galactose residues and a minor triantennary structure. Carbohydrate analysis indicated that all of the N-linked and 80-90% of the O-linked acceptor sites are occupied. The lack of galactosamine content in both collagens showed the O-linked oligosaccharides were only those attached to hydroxylysine and not to serine or threonine. The high carbohydrate density along both triple helical domains is discussed with regard to their limited ability to form lateral aggregates.

Glycosylation of the human erythrocyte glucose transporter: a minimum structure is required for glucose transport activity

The involvement of the carbohydrate moiety of the human erythrocyte glucose transporter in glucose transport activity was previously demonstrated (Feugeas et al. (1990) Biochim. Biophys. Acta 1030, 60-64): N-glycanase treatment of the transport glycoprotein reconstituted in proteoliposomes resulted in a dramatic decrease of the Vmax. In this study, kinetic measurements of glucose equilibrium influx confirm our previous results. In order to investigate that a minimum glycosidic structure is required to maintain glucose transport activity, proteoliposomes were respectively treated with either sialidase, or sialidase and endo-beta-galactosidase, or a pool of exo-glycosidases which allows the release of all the sugar residues, except the proximal N-acetylglucosamine. Kinetic measurements of zero-trans influx made on sialidase- and (sialidase + endo-beta-galactosidase)-treated proteoliposomes did not reveal any significant changes in the glucose transport activity. On the contrary, treatment of the same proteoliposomes by a pool of exoglycosidases led to a complete abolition of activity, suggesting that a minimum glycosidic structure is required for glucose transport activity.

The structure of the asparagine-linked sugar chains of bovine brain ribonuclease

The asparagine-linked sugar chains of bovine brain ribonuclease were quantitatively released as oligosaccharides from the polypeptide backbone by hydrazinolysis. After N-acetylation, they were converted into radioactively-labeled oligosaccharides by NaB3H4 reduction. The radioactive oligosaccharide mixture was fractionated by ion-exchange chromatography, and the acidic oligosaccharides were converted into neutral oligosaccharides by sialidase digestion. The neutral oligosaccharides were then fractionated by Bio-Gel P-4 column chromatography. Structural studies of each oligosaccharide by sequential exoglycosidase digestion in combination with methylation analysis revealed that bovine brain ribonuclease showed extensive heterogeneity. It contains bi- and tri-antennary, complex-type oligosaccharides having alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp -(1----4)-beta-D- GlcpNAc-(1----4)-[alpha-L-Fucp-(1----6)]-D-GlcNAc as their common core. Four different outside oligosaccharide chains, i.e., beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----, alpha-Neu5Ac-(2----6)-beta-D- Galp-(1----4)-beta-D-GlcpNAc-(1----, alpha-Neu5Ac-(2----3)-beta-D-Galp-(1----4)- beta-D-GlcpNAc-(1----, and alpha-D-Galp-(1----3)-beta-D-Galp-(1----4)-beta-D-GlcpNAc-(1----, were found. The preferential distribution of the alpha-D-Galp-(1----3)-beta-D-Galp-(1----4)-beta-D-GlcpNAc group on the alpha-D-Manp-(1----6) arm is a characteristic feature of the sugar chains of this enzyme.

N-acetylneuraminic acid and N-glycolylneuraminic acid in glycopeptides of colonic tumor and mucosa in rats treated with carrageenan and 1,2-dimethylhydrazine

N-linked glycopeptides were prepared from colonic tumor (adenocarcinoma) and mucosa in rats treated with carrageenan, an indigestible polysaccharide, and 1,2-dimethylhydrazine. Sialic acids, N-acetylneuraminic acid and N-glycolylneuraminic acid, obtained by acid hydrolysis of the glycopeptides were determined by HPLC. The N-acetylneuraminic acid/N-glycolylneuraminic acid ratio in colonic tumor was 25.2, while each treated mucosa had the values between 0.29 and 0.55. Thus, necessity which observes the qualitative change of sialic acid in malignant transformation was suggested.

Fast atom bombardment mass spectrometry (FAB-MS): analysis of complex carbohydrate chains of tissue kallikreins

Molecular ions and fragment ions of underivatized and permethylated oligosaccharides derived from human urinary kallikrein and some model glycoproteins gave information about the sugar composition and arrangement of sugars. These ions were conveniently created by FAB-MS in a JEOL JMS-HX110 high field high resolution mass spectrometer. Glycoprotein samples were digested with N-glycanase and the asparagine(Asn)-linked oligosaccharides were separated from polypeptides by Sephadex G-50 chromatography. The mixture of oligosaccharides was converted to the reduced p-aminobenzoic ethyl ester (ABEE) derivative and the components separated by HPLC on an ion-exchange (AX-10) column. Individual components were analyzed by negative FAB-MS using glycerol as a matrix. Permethylated oligosaccharides were also analyzed by positive FAB-MS.

Two species of dermatan sulfate proteoglycans with different molecular sizes from newborn calf skin

Two species of dermatan sulfate-proteoglycans (DS-PGs) were isolated from calf skin. The first species, PDS-H (high-molecular-weight proteodermatan sulfate), contains the core protein with a molecular weight of either about 55,000 or 53,000. Both the core proteins are capable of binding to concanavalin A (Con A). The second species, PGs-L (low-molecular-weight proteoglycan containing dermatan sulfate and/or chondroitin sulfate), contains a core protein of Mr = 20,000 that did not bind to Con A. Tryptic peptide mappings revealed that Mr = 55,000 core protein and Mr = 53,000 core protein were of the same origin. However, the tryptic peptides and the amino acid composition of PGs-L core protein were completely different from those of PDS-H core proteins. The polyclonal antibodies against Mr = 55,000 core protein reacted with both the core proteins of Mr = 55,000 and Mr = 53,000 but not with the core protein from PGs-L. The DS was found to be the only glycosaminoglycan component of PDS-H. That is, the glycosaminoglycan from PDS-H was composed of 46% iduronosylhexosamine units and 54% glucuronosylhexosamine units, while the glycosaminoglycan of PGs-L was composed of 30% iduronosylhexosamine units and 70% glucuronosylhexosamine units.

Purification, identification and characterization of chicken C1q, a subcomponent of the first component of complement

A component, having the equivalent haemolytic activity to that of human complement subcomponent C1q, was purified by a combination of precipitation with EGTA, gel filtration, ion exchange and adsorption chromatography from chicken serum. Yields ranged from 8 to 15 mg/litre of serum. The finally purified preparation generates full Cl haemolytic activity when assayed with human complement subcomponents C1r and C1s, and have been identified as chicken C1q. The molecular weight of undissociated C1q, as estimated on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate (SDS), is 504,000. Under dissociating but non-reducing conditions, the C1q was shown to consist of 2 subunits having molecular weights of 52,700 and 51,200 in a molar ratio of 2:1. On reduction, the 52,700 molecular weight subunit gave chains with molecular weights of 25,900 and 24,800 in equimolar ratio, and the 51,200 molecular weight subunit decreased to 24,800. The C1q contains hydroxyproline, hydroxylysine, a high percentage of glycine and approximately 7% carbohydrate. Collagenase digestion of C1q caused a rapid loss of haemolytic activity and produced much smaller peptide fragments.

Reversibility of monensin inhibition of oligosaccharide processing of human fibronectin

Monensin impairs oligosaccharide processing in fibronectin primarily by inhibiting the conversion of oligosaccharides from the high mannose type to the complex type. The separate effects of monensin and cations on alpha-mannosidase activity in fibroblasts were examined using an in vitro assay system. The results indicated that monensin did not directly inhibit alpha-mannosidase activity in vitro, although prior treatment of fibroblasts with monensin caused an irreversible suppression of enzyme activity. The reversibility of monensin action on oligosaccharide processing was also examined. Analyses using concanavalin A (ConA) Sepharose affinity chromatography showed that the inhibitory action of monensin on oligosaccharide processing was biologically reversible. A progressive return to complex type oligosaccharides began about 11 h after the removal of the monensin. These composite results indicate that the reversibility of monensin action on oligosaccharide processing in fibronectin may be attributed to the restoration of enzyme activity, although the mechanism by which restoration occurs remains to be deciphered.

A study of the role of the asparagine-linked sugar chains of human complement subcomponent C1q in its biological activities

The sialic acid residues were removed from asparagine-linked sugar chains on the C-terminal non-collagenous globular regions of human C1q by sialidase digestion. Both the haemolytic activity and the binding ability to immunoglobulin G (IgG) (Fc-binding ability) of C1q were unimpaired, even after the complete removal of sialic acid from these sugar chains. On the other hand, the rate of disappearance of C1q from the circulation was greatly accelerated by its desialylation, that is, the radioactivity of the infused intact and desialylated C1q was reduced to half for 200 min and for 140 min in the circulation of rats, respectively. A mixture of entire asparagine-linked sugar chains consisting of neutral, monosialyl and disialyl oligosaccharides was isolated from the intact C1q molecule by hydrazinolysis. The oligosaccharide-mixture isolated, after NaBH4 reduction, was added to assay system of C1q, but neither the haemolytic activity nor the Fc-binding ability was influenced.

The structure of sialyl-glycopeptides of the O-glycosidic type, isolated from sialidosis (mucolipidosis I) urine

Sialyl-glycopeptides containing an O-glycosidically linked tetrasaccharide chain were obtained from the urine of a patient suffering from mucolipidosis I. Isolation of these compounds was achieved by gel filtration, ion-exchange chromatography and preparative paper chromatography. Their structures were determined by a combination of carbohydrate and amino acid analysis, dansylation, periodate oxidation, methylation studies, enzymatic hydrolysis and 1H-NMR spectroscopy, to be as follows: (formula; see text) wherein R = peptide linked through -Thr-, -Ser-Thr- or -Thr-Ser-. The finding of these glycopeptides in urine shows that mucolipidosis I is characterized by a general "glycoprotein-specific" sialidase deficiency. The possibility of the existence of a human endo-alpha-N-acetylgalactosaminidase is discussed.

Comparative studies on biological activities of subcomponents C1q of the first component of human, bovine, mouse and guinea-pig complement

Both the haemolytic activity and the binding ability to immunoglobulin G(IgG) (Fc-binding ability) were comparatively assayed among human, bovine, mouse and guinea-pig C1q. The haemolytic activity was measured by using the sensitized sheep erythrocytes with rabbit immunoglobulin M(IgM)- or IgG-haemolysin. The Fc-binding ability was assayed by using immune complexes made of rabbit IgG-antibody against human serum albumin as well as agglutination of latex particles coated with human, bovine or rabbit IgG (IgG-latex). The specific haemolytic activity was comparable with between bovine and mouse C1q, while those of guinea pig and human C1q were significantly lower than those of the others. Only the human and mouse C1q showed significantly positive agglutinating activity of human or bovine IgG-latex. In the case of the use of rabbit IgG-latex, each of these C1q gave much weaker agglutination. On the other hand, the ability of all these C1q to bind to Fc of immune complexes specifically was almost comparable. The discrepancy in specific activities between the haemolysis and the Fc-binding ability may suggest that these two biological activities are not always correlative and that these are independent biological phenomena.

The use of two-dimensional correlated spectroscopy to obtain new assignments in the high-resolution 1H nuclear magnetic resonance spectrum of the biantennary complex oligosaccharide isolated from human serum transferrin by hydrazinolysis

The asialo biantennary complex type oligosaccharide from human serum transferrin was isolated by hydrazinolysis, a method which results in the quantitative release of the intact oligosaccharide free of all amino acids. The 1H-NMR chemical shifts of the previously assigned anomeric and H-2 protons from the peripheral residues of the glycopeptide are identical to the corresponding values for the reduced oligosaccharide. The chemical shift of GlcNAc-1 H-1 proton in the reduced oligosaccharide was assigned by selective deuteration. Proton J connectivities were determined using two-dimensional 1H-1H correlated high resolution NMR spectroscopy. Twelve new assignments were made within the central envelope of the NMR spectrum and a further six were tentatively proposed. The ability to assign proton resonances in this way should allow further conformational studies of the oligosaccharide using nuclear Overhauser effects between the relevant assigned protons on different saccharide residues (Homans, S.W., Dwek, R.A., Fernandes, D.L. and Rademacher, T.W. (1982) FEBS Lett. 150, 503-506).

Purification and molecular characterization of rat intestinal brush border membrane dipeptidyl aminopeptidase IV

Dipeptidyl aminopeptidase IV (EC 3.4.14.-) was solubilized from a particulate membrane fraction of rat intestinal mucosa with Triton X-100. The solubilized enzyme was purified to homogeneity following ammonium sulfate fractionation, chromatography on DEAE-Sepharose and hydroxyapatite, gel filtration and preparative polyacrylamide gel electrophoresis. The final enzyme preparation had a specific activity of 55 units/mg protein representing a 1373 fold purification over the starting material. Purity was judged by polyacrylamide gel electrophoresis and double immunodiffusion. The molecular weight of the native undenatured enzyme was estimated to be 230000 by gel filtration and polyacrylamide gel electrophoresis. Electrophoresis under denaturing conditions (sodium dodecyl sulfate) indicated that the protein consists of two identical 98 kDa subunits. Dipeptidyl aminopeptidase IV is a glycoprotein containing approx. 8% carbohydrate by weight. A detailed analysis of the individual sugar components demonstrated that fucose, galactose, glucose, mannose, sialic acid and hexosamine sugars were present. The nature of the constituent asparagine linked oligosaccharide side chains was further examined following cleavage from the peptide backbone by hydrazinolysis. Following high voltage paper electrophoresis approx. 80% of the isolated oligosaccharide was found with the neutral fraction while the remaining 20% consisted of a single acidic component. Gel filtration of the neutral oligosaccharide fraction indicated that it contains approx. 19 sugar residues.

Subcomponents C1q of the first component of guinea pig and mouse complement. Comparative study of their asparagine-linked sugar chains

Guinea pig and mouse C1q, subcomponents of the first component of complement, contained six asparagine-linked sugar chains on the C-terminal non-collagenous globular regions of each molecule. After N-acetylation and successive NaB3H4-reduction of asparagine-linked sugar chains liberated by hydrazinolysis, their structure was analysed by sequential exoglycosidase digestion in combination with sugar composition analyses. The sugar chains of C1q molecules of both animals were very similar and composed of the biantennary complex type sugar chains with the following outer chains in various combination is: (+/- NeuNAc alpha leads to)Gal beta 1 leads to GlcNAc beta 1 leads to and Gal beta 1 leads to Gal beta 1 leads to GlcNAc beta 1 leads to. These outer chain moieties were found to be linked to a common core structure of Man alpha 1 leads t o (Man alpha 1 leads to)Man beta 1 leads to GlcNAc beta 1 leads to (Fuc alpha 1 leads to)GlcNAc.

Carbohydrates of influenza virus hemagglutinin: structures of the whole neutral sugar chains

The carbohydrates of BHA, a solubilized hemagglutinin of influenza virus by bromelain digestion, were quantitatively released as oligosaccharides by hydrazinolysis. The oligosaccharide mixture was separated into a neutral and two acidic fractions by paper electrophoresis. Both acidic fractions were resistant to sialidase digestion but were slowly converted to the neutral fraction by incubation with sulfatases. The neutral fraction which comprised about 80% in molar ratio of total oligosaccharides was separated into 13 oligosaccharides by paper chromatography and by Con A-Sepharose column chromatography. Structural studies of these oligosaccharides by sequential exoglycosidase digestion and by methylation analysis revealed that BHA contains a series of high mannose type and bi-, tri-, and tetraantennary complex type sugar chains. Occurrence of Gal beta l leads to 3GlcNAc outer chain in two and bisectional N-acetylglucosamine in one of the biantennary sugar chains is an interesting characteristic of the sugar chains of BHA.

Substrate specificity of cerebral GDP-fucose: glycoprotein fucosyltransferase

Solubilized sheep brain fucosyltransferase was shown to transfer fucose from GDP-fucose onto glycoprotein and glycopeptide acceptors, such as asialofetuin, asialotransferrin, their glycopeptides and glycopeptides from ovalbumin, but not on to monosaccharides and disaccharides such as galactose, N-acetylglucosamine and lactose. Competition studies between asialofetuin and glycopeptide V from ovalbumin provided evidence that both substrates compete for a common enzyme active site. The position of the fucosyl linkage was then investigated. Endo-beta-N-glucosaminidase D digestion of fucosylated and acetylated glycopeptide V showed that fucose is not linked to asparagine-linked N-acetylglucosamine. Hydrazinolysis and nitrous acid deamination performed on asialofetuin and glycopeptide V proved that fucose is not linked to external galactose or N-acetylglucosamine either. Thus we assume that fucose is linked to the oligomannochitobiosyl core of the glycan, and probably to the second N-acetylglucosamine.

Isolation and structural studies of the neutral oligosaccharide units from bovine glycophorin

The O-glycosidically linked carbohydrate units of glycophorin from bovine erythrocyte membranes were released as reduced oligosaccharides by alkaline borohydride treatment. These oligosaccharides were separated by ion-exchange chromatography followed by gel filtration. Three oligosaccharides, a penta- a hepta- and a decasaccharide were obtained as the major components from the neutral fraction, and seven fractions were separated from the acidic fractions. All of the fractions were found to contain galactose and N-acetylglucosamine in variable amounts, as well as N-acetylgalactosaminitol. Studies of the neutral oligosaccharides by methylation analyses, nitrous acid deamination and Smith degradation, indicated the structure of the pentasaccharide to be Gal(1 leads to 3)Gal(1 leads to 4)GlcNAc(1 leads to 3)GalNAcol and that of the heptasaccharide to be Gal(1 leads to 3)Gal(1 leads to 4)GlcNAc(1 leads to 3)Gal(1 leads to 4)GlcNAc(1 leads to 3)Gal(1 leads to 3)GalNAcol. The highest molecular weight fraction, decasaccharide in the neutral fraction had a branching point at C-6 of a galactose residue.

Comparable content of hydroxylysine-linked glycosides in subcomponents C1q of the first component of human, bovine and mouse complement

The hydroxylysine-glycosides in bovine and mouse C1q are directly quantified in parallel with those in human C1q after the alkaline hydrolysis of these molecules. Human, bovine and mouse C1q contain 68.3, 66.3 and 64.0 hydroxylysine-galactosylglucose residues in each of these molecules respectively. Only human C1q contains 2.5 residues of hydroxylysine-galactose per molecule, and both of bovine and mouse C1q contain no detectable hydroxylysine-monosaccharides in their molecules. The percentage of hydroxylysine residues glycosylated to total hydroxylysine residues in each of these molecules is calculated to be 86.4, 92.0 and 95.1% for human, bovine and mouse C1q respectively and is comparable with each other. The percentages of hydroxylysine residues resistant to periodate oxidation to total hydroxylysine residues in these molecules were 61.1, 65.3 and 74.3% for human, bovine and mouse C1q respectively and were significantly lower than those estimated by the direct quantification of hydroxylysine-glycosides after the alkaline hydrolysis of these molecules.

Sugar residues on proteins

Glycoproteins have become increasingly important in the structure and function of many different mammalian systems; for example, membrane glycoproteins and glycoprotein hormones. It is, therefore, important to understand their chemistry, which would include an understanding of both the carbohydrate and protein parts of the molecule. Since the chemical characterization of the protein moiety has been extensively examined and the techniques for its characterization are well worked out, only the carbohydrate portion of glycoproteins will be reviewed in this article. The chemical nature of the carbohydrate moiety of glycoproteins will be examined. First, the types of monosaccharides present in animal systems, especially those in the mammalian systems, will be described. Next, various types of simple and complex carbohydrate chains will be discussed to establish the diversity, size, and number of chains present in the carbohydrate units in different glycoproteins. Then, the type of linkages of the carbohydrate to the protein will be examined to determine if the primary sequence of protein is important in determining the size and type of carbohydrate chains present in glycoproteins. Finally, the current methods of structural elucidation such as monosaccharide sequence, intersugar bonds, and anomeric linkages in the carbohydrate moiety of glycoproteins will be reviewed. These methods include the techniques of periodate oxidation, methylation, partial acid hydrolysis, and specific glycosidase digestion of glycoproteins, as well as the latest techniques using micromethods of carbohydrate quantitation and characterization involving gas chromatography and mass spectrometry. The function of the carbohydrate in glycoproteins will also be considered. First, hormone glycoproteins will be discussed in their relationship to the immunological and biological function of the glycoprotein when the carbohydrate is sequentially removed. Next, the function of the carbohydrate in the turnover of glycoproteins will be discussed. These topics will be considered in order to develop an understanding of a specific function(s) of the carbohydrate in glycoproteins.