Inhibition of IgE binding to cross-reactive carbohydrate determinants enhances diagnostic selectivity.

BACKGROUND: Allergy diagnosis by determination of allergen-specific IgE is complicated by clinically irrelevant IgE, of which the most prominent example is IgE against cross-reactive carbohydrate determinants (CCDs) that occur on allergens from plants and insects. Therefore, CCDs cause numerous false-positive results. Inhibition of CCDs has been proposed as a remedy, but has not yet found its way into the routine diagnostic laboratory. We sought to provide a simple and affordable procedure to overcome the CCD problem. METHODS: Serum samples from allergic patients were analysed for allergen-specific IgEs by different commercial tests (from Mediwiss, Phadia and Siemens) with and without a semisynthetic CCD blocker with minimized potential for nonspecific interactions that was prepared from purified bromelain glycopeptides and human serum albumin. RESULTS: Twenty two per cent of about 6000 serum samples reacted with CCD reporter proteins. The incidence of anti-CCD IgE reached 35% in the teenage group. In patients with anti-CCD IgE, application of the CCD blocker led to a clear reduction in read-out values, often below the threshold level. A much better correlation between laboratory results and anamnesis and skin tests was achieved in many cases. The CCD blocker did not affect test results where CCDs were not involved. CONCLUSION: Eliminating the effect of IgEs directed against CCDs by inhibition leads to a significant reduction in false-positive in vitro test results without lowering sensitivity towards relevant sensitizations. Application of the CCD blocker may be worthwhile wherever natural allergen extracts or components are used.

Antibody binding to venom carbohydrates is a frequent cause for double positivity to honeybee and yellow jacket venom in patients with stinging-insect allergy.

BACKGROUND: Up to 50% of patients with stinging-insect allergy have double-positive RAST results to honeybee and yellow jacket (YJ) venom. True double sensitization and crossreactivity through venom hyaluronidases are considered main reasons for this multiple reactivity. OBJECTIVE: We investigated the role of antibodies against cross-reactive carbohydrate determinants in venom double positivity. METHODS: CAP inhibition experiments were performed with crude oilseed rape (OSR) and timothy grass pollen extracts and a neoglycoprotein construct displaying a MUXF glycan, as present in pineapple-stem bromelain (MUXF-BSA). CAP to OSR was used as a rough measure for carbohydrate-specific IgE in individual sera. RESULTS: CAP results to OSR pollen were positive in 2 of 14 single-positive honeybee venom sera, 2 of 16 single-positive YJ venom sera, and 33 (80.5%) of 41 double-positive sera (P < .00001, chi(2) test). CAP inhibition was performed in 16 selected patients with a CAP class of 3 or higher to both venoms. In 9 of 11 patients with a highly positive CAP result to OSR (CAP score to OSR > CAP score to second venom), pollen extracts, MUXF-BSA, or both were able to completely inhibit IgE binding to one of the venoms, whereas this was not the case in 5 patients with a negative or weakly positive CAP result to OSR (CAP score to OSR < CAP score to second venom). CONCLUSIONS: The data suggest that carbohydrate-specific IgE is a major cause for the double positivity to honeybee and YJ venom seen in patients with Hymenoptera allergy. Because these antibodies may have low clinical relevance, they may severely impede the correct diagnosis of Hymenoptera venom allergy.

Carrot allergy: double-blinded, placebo-controlled food challenge and identification of allergens.

BACKGROUND: Allergic reactions to carrot affect up to 25% of food-allergic subjects. Clinical manifestations of carrot allergy and IgE responses to carrot proteins, however, have never been studied in subjects with carrot allergy confirmed by means of double-blinded, placebo-controlled food challenge (DBPCFC). OBJECTIVE: The purposes of this investigation were to confirm clinically relevant sensitizations to carrot by means of DBPCFC, to validate current diagnostic methods, and to identify IgE-reactive carrot proteins in patients with true allergy. METHODS: DBPCFCs were performed in 26 subjects with histories of allergic reactions to carrot. Patients underwent skin prick tests with carrot extract, fresh carrot, and various pollen extracts. Specific IgE to carrot, celery, birch, and mugwort pollen and to rBet v 1, rBet v 2, and rBet v 6 were measured through use of the CAP method. Carrot allergens were identified by means of immunoblotting and blotting inhibition. RESULTS: Twenty of 26 patients had positive DBPCFC results. The sensitivity of the determination of carrot-specific IgE antibodies through use of the CAP method (> or =0.7 kU/L) was 90%, the sensitivity for skin prick testing with commercial extracts was 26%, and the sensitivity for prick-to-prick tests with raw carrot was 100%. The Bet v 1--related major carrot allergen Dau c 1 was recognized by IgE from 85% of patients; 45% were sensitized to cross-reactive carbohydrate determinants and 20% to carrot profilin. In 1 subject, a Bet v 6--related carrot allergen was recognized. In 4 patients, IgE binding to Dau c 1 was not inhibited or was weakly inhibited by rBet v 1 or birch pollen extract. CONCLUSION: This study confirmed the allergenicity of carrot by means of DBPCFC. DBPCFC-positive patients had exclusively specific IgE antibodies to birch pollen--related carrot allergens, Dau c 1 being the major allergen. The lack of inhibition of IgE binding to Dau c 1 by birch allergens in a subgroup of patients might indicate an secondary immune response to new epitopes on the food allergen that are not cross-reactive with Bet v 1.

N-Glycan analysis by matrix-assisted laser desorption/ionization mass spectrometry of electrophoretically separated nonmammalian proteins: application to peanut allergen Ara h 1 and olive pollen allergen Ole e 1.

A method has been developed which allows the analysis of glycoproteins separated by SDS-PAGE. The procedure, though applicable to N-glycosylated glycoproteins of any origin, is particularly devised for glycoproteins potentially containing fucose in alpha1,3-linkage to the reducing GlcNAc as may be found in plants and invertebrates, e.g., insects and parasitic helminths. Starting with an established procedure for mass spectrometric peptide mapping, the analysis of N-glycans by matrix-assisted laser desorption/ionization mass spectrometry involved the use of peptide:N-glycosidase A, a triphasic microcolumn for sample cleanup, and a new matrix mixture consisting of 2,5-dihyhydroxybenzoic acid, 1-hydroxyisoquinoline, and arabinosazone. The method was tested on proteins with N-glycans of known structure, i.e., as horseradish peroxidase, zucchini ascorbate oxidase, soybean agglutinin, honeybee venom hyaluronidase, bovine ribonuclease B, and bovine fetuin. An electrophoretic band corresponding to 4 microg of glycoprotein was generally sufficient to allow detection of the major N-glycan species. As an additional benefit, a peptide mass map is generated which serves to identify the analyzed protein. The method was applied to glycoprotein allergens whose glycan structures were unknown. Ara h 1 and Ole e 1, major allergens from peanut and olive pollen, respectively, contained mainly xylosylated N-glycans with the composition Man(3(-4))XylGlcNAc(2) in the case of Ara h 1 and GlcNAc(1-2)Man(3)XylGlcNAc(2) in the case of Ole e 1 where also some GlcNAc(0-2)Man(3)XylFucGlcNAc(2) was found.

Involvement of carbohydrate epitopes in the IgE response of celery-allergic patients.

BACKGROUND: This study was performed to get further insights into antibody responses to cross-reactive carbohydrate determinants (CCD), including initial experiments to prove the biological activity of anti-CCD IgE. Earlier studies have shown that IgE specific for CCD occurs in about 25% of celery-allergic patients. The clinical significance of these antibody specificities is doubtful. METHODS: Patient sera were selected on the basis of a positive case history of celery allergy and multiple binding to high molecular weight celery allergens on immunoblots. Specific IgE to native and heated celery tuber was determined by the enzyme allergosorbent test (EAST). N-glycans were purified after extensive digestion of specific glycoproteins, such as pineapple stem bromelain, bovine fibrin, and human IgG, and used as antigens in an IgE ELISA as well as in EAST and immunoblotting inhibition experiments. Dose-related histamine release was performed with BSA neoglycoproteins containing 3-4 units of the purified glycopeptides. RESULTS: Seven celery-allergic patients were identified who clearly presented IgE against the N-glycan purified from bromelain which is a common structure within the plant kingdom. Chemical defucosylation showed that alpha1, 3-fucose is a key structure for IgE binding. In patients with anti-CCD IgE, the maximal inhibition of celery EAST by the bromelain glycan ranged from 22 to 100%. Inhibition of celery immunoblots by preincubation of patient serum with this glycan led to a quenching of multiple bands at masses >40 kD. After linking the bromelain glycopeptide to BSA, a strong dose-related histamine release was obtained in a celery-allergic patient, occurring at lower concentrations than with the recombinant major protein allergen from celery, Api g 1. CONCLUSIONS: Our results demonstrate that IgE specific for CCD is common in celery-allergic patients, and can represent the major proportion of IgE against this food. alpha1, 3-fucose was confirmed to be an essential part of the IgE epitope. Immunoblotting inhibition indicated the presence of this carbohydrate determinant on multiple glycoproteins in celery extract. Although histamine release was only performed in 1 patient, our data show that proteins carrying multiple glycan units can be biologically active in patients sensitized to CCD.

Purification, cDNA cloning, and expression of GDP-L-Fuc:Asn-linked GlcNAc alpha1,3-fucosyltransferase from mung beans.

Substitution of the asparagine-linked GlcNAc by alpha1,3-linked fucose is a widespread feature of plant as well as of insect glycoproteins, which renders the N-glycan immunogenic. We have purified from mung bean seedlings the GDP-L-Fuc:Asn-linked GlcNAc alpha1,3-fucosyltransferase (core alpha1,3-fucosyltransferase) that is responsible for the synthesis of this linkage. The major isoform had an apparent mass of 54 kDa and isoelectric points ranging from 6. 8 to 8.2. From that protein, four tryptic peptides were isolated and sequenced. Based on an approach involving reverse transcriptase-polymerase chain reaction with degenerate primers and rapid amplification of cDNA ends, core alpha1,3-fucosyltransferase cDNA was cloned from mung bean mRNA. The 2200-base pair cDNA contained an open reading frame of 1530 base pairs that encoded a 510-amino acid protein with a predicted molecular mass of 56.8 kDa. Analysis of cDNA derived from genomic DNA revealed the presence of three introns within the open reading frame. Remarkably, from the four exons, only exon II exhibited significant homology to animal and bacterial alpha1,3/4-fucosyltransferases which, though, are responsible for the biosynthesis of Lewis determinants. The recombinant fucosyltransferase was expressed in Sf21 insect cells using a baculovirus vector. The enzyme acted on glycopeptides having the glycan structures GlcNAcbeta1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1- 6)Manbeta1-4GlcNAcbet a1-4GlcNAcbeta1-Asn, GlcNAcbeta1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1- 6)Manbeta1-4GlcNAcbet a1-4(Fucalpha1-6)GlcNAcbeta1-Asn, and GlcNAcbeta1-2Manalpha1-3[Manalpha1-3(Manalpha1-6 )Manalpha1-6]Manbeta1 -4GlcNAcbeta1-4GlcNAcbeta1-Asn but not on, e.g. N-acetyllactosamine. The structure of the core alpha1,3-fucosylated product was verified by high performance liquid chromatography of the pyridylaminated glycan and by its insensitivity to N-glycosidase F as revealed by matrix-assisted laser desorption/ionization time of flight mass spectrometry.

Structural characterization of the N-linked oligosaccharides from tomato fruit.

The primary structures of the N-linked oligosaccharides from tomato fruit (Lycopersicon esculentum) have been elucidated. For the isolation of the protein fraction, two procedures were employed alternatively: a low temperature acetone powder method and ammonium sulfate precipitation of the tomato extract. After peptic digestion, the glycopeptides were purified by cation-exchange chromatography; the oligosaccharides were released by N-glycosidase A and fluorescently labelled with 2-aminopyridine. Structural characterization was accomplished by means of two-dimensional HPLC in combination with exoglycosidase digestions and MALDI-TOF mass spectrometry. Two varieties as well as two stages of ripening were investigated. In all the samples, the same sixteen N-glycosidic structures were detected; the two most abundant glycans showed identical properties to those of the major N-linked oligosaccharides of horseradish peroxidase and pineapple stem bromelain, respectively and accounted for about 65-78% of the total glycan amount; oligomannosidic glycans occurred only in small quantities (3-9%). The majority of the N-glycans were beta 1,2-xylosylated and carried an alpha 1,3-fucose residue linked to the terminal N-acetylglucosamine. This structural element contributes to cross-reactions among non-related glycoproteins and has been shown to be an IgE-reactive determinant (Tretter, Altmann, Kubelka, März, & Becker, 1993). The presented study gives a possible structural explanation for reported immunological cross-reactivities between tomato and grass pollen extracts due to carbohydrate IgE epitopes (Petersen, Vieths, Aulepp, Schlaak, & Becker, 1996), thereby demonstrating the importance of the structural characterization of plant N-glycans for a more reliable interpretation of immunological data.

Implications of the grass group I allergens on the sensitization and provocation process.

Grass pollen allergens of group I are particularly important because of their high IgE prevalence and occurrence in all grass species. Four independent IgE-binding regions and one continous epitope were identified. The posttranslational modifications on the molecule increased allergenicity. Phl p 1 is a cysteine protease, as determined by specific substrates, inhibitors and consensus sequence motifs. In analogy to other allergens and/or proteases, we deduce that Phl p 1 might enhance the permeability of the epithelium, influence T helper cells to bias Th2, and increase the IgE production of plasma cells. Thus, the group I allergens seem to be the crucial components in a pollen extract which can mediate sensitization and enhance the triggering of symptoms leading to the persistence of a grass pollen allergy.

IgE antibodies specific for carbohydrates in a patient allergic to gum arabic (Acacia senegal).

The present study deals with the detailed investigation of the IgE antibody response of a gum arabic-allergic patient. The patient showed multiple serologic and skin test sensitizations to a range of pollen, other inhalants and foods, and bee venom, and to the recombinant allergens Bet v 1 and Bet v 2. Moreover, the patient's serum reacted strongly to gum-arabic extract. The NaIO4-treated and thus deglycosylated extract showed no binding to IgE. In contrast, removal of the protein backbone by basic hydrolysis did not deplete the IgE reactivity. Therefore, it is concluded that the gum arabic-specific IgE antibodies of this patient were mainly directed against the carbohydrate fraction of this material. In IgE-inhibition assays, cross-reactions occurred in the range of 60% between gum arabic and known immunogenic N-glycans containing alpha1-3-linked fucose. Since the inhibition graphs were not parallel and the inhibition was not complete with heterologue antigens, the cross-reacting epitopes of gum arabic appeared to be different from the latter well-known cross-reactive carbohydrate determinants (CCD). Inhibition may have been caused by a partial immunologic identity of the investigated carbohydrate moieties. A strong IgE response to the fucose-containing glycan from bromelain was measured in a glycan ELISA that utilizes purified glycopeptides at the solid phase. This response, which may explain the multiple sensitizations without clinical significance diagnosed in the patient, could originate from inhalation of pollen, which is known to contain similar glycans, or from occupational sensitization during work as a baker and confectioner. Since the gum-arabic protein showed only very weak participation in the IgE reactivity, the clinical symptoms of the patient caused by gum arabic may be attributed to carbohydrate epitopes. Due to the repetitive polysaccharide sequence of gum arabic, several epitopes for the cross-linking of IgE should exist.

Core alpha1,3-fucose is a key part of the epitope recognized by antibodies reacting against plant N-linked oligosaccharides and is present in a wide variety of plant extracts.

Carbohydrates have been suggested to account for some IgE cross-reactions between various plant, insect, and mollusk extracts, while some IgG antibodies have been successfully raised against plant glycoproteins. A rat monoclonal antibody raised against elderberry abscission tissue (YZ1/2.23) and rabbit polyclonal antiserum against horseradish peroxidase were screened for reactivity in enzyme-linked immunosorbent assay against a range of plant glycoproteins and extracts as well as neoglycoproteins, bee venom phospholipase, and several animal glycoproteins. Of the oligosaccharides tested, Man3XylFucGlcNAc2(MMXF3) derived from horseradish peroxidase was the most potent inhibitor of the reactivity of both YZ1/2.23 and anti-horseradish peroxidase to native horseradish peroxidase glycoprotein. The reactivity of YZ1/2. 23 and anti-horseradish peroxidase against Sophora japonica lectin was most inhibited by a neoglycoconjugate of bromelain glycopeptide cross-linked to bovine serum albumin, while the defucosylated form of this conjugate was inactive as an inhibitor. A wide range of plant extracts was found to react against YZ1/2.23 and anti-horseradish peroxidase, with particularly high reactivities recorded for grass pollen and nut extracts. All these reactivities were inhibitable with the bromelain glycopeptide/bovine serum albumin conjugate. Bee venom phospholipase and whole bee venom reacted weakly with YZ1/2.23 but more strongly with anti-horseradish peroxidase in a manner inhibitable with the bromelain glycopeptide/bovine serum albumin conjugate, while hemocyanin from Helix pomatia reacted poorly with YZ1/2.23 but did react with anti-horseradish peroxidase. It is concluded that the alpha1, 3-fucose residue linked to the chitobiose core of plant glycoproteins is the most important residue in the epitope recognized by the two antibodies studied, but that the polyclonal anti-horseradish peroxidase antiserum also contains antibody populations that recognize the xylose linked to the core mannose of many plant and gastropod N-linked oligosaccharides.

HPLC method for the determination of Fuc to Asn-linked GlcNAc fucosyltransferases.

Mammalian cells often contain an enzyme which transfers fucose onto the reducing terminal GlcNAc (GlcNAc-1) of N-glycans with an alpha1,6-linkage. In plants, on the other hand, the fucose is transferred to GlcNAc-1 with an alpha1,3-inkage. Insect cells can exhibit both enzymatic activities. Hitherto, the activity of these fucosyltransferases has been determined by the incorporation of radioactively labelled fucose into an acceptor glycopeptide. This assay, however, cannot discriminate these two activities. Here we report on the use of dansylated glycoasparagine for the specific determination of 1,3- and 1,6-fucosyltransferases. The two possible products and the substrate are separated on a reversed phase column and detected by fluorescence.

Concanavalin A binding and endoglycosidase D resistance of beta1,2-xylosylated and alpha1,3-fucosylated plant and insect oligosaccharides.

The binding to concanavalin A (Con A) by pyridylaminated oligosaccharides derived from bromelain (Manalpha1,6(Xylbeta1,2) Manbeta1, 4GlcNAcbeta1,4(Fucalpha1,3)GlcNAc), horseradish peroxidase (Manalpha1,6(Manalpha1,3) (Xylbeta1,2)Manbeta1, 4GlcNAcbeta1,4(Fucalpha1,3) GlcNAc), bee venom phospholipase A2 (Manalpha1,6Manbeta1,4GlcNAcbeta1,4GlcNAc and Manalpha1,6(Manalpha1,3)Manbeta1,4GlcNAcbeta1,4 (Fucalpha1,3)GlcNAc) and zucchini ascorbate oxidase (Manalpha1,6(Manalpha1,3) (Xylbeta1,2)Manbeta1,4 GlcNAcbeta1,4GlcNAc) was compared to the binding by Man3GlcNAc2, Man5GlcNAc2 and the asialo-triantennary complex oligosaccharide from bovine fetuin. While the fetuin oligosaccharide did not bind, bromelain, zucchini, Man2GlcNAc2 and horseradish peroxidase were retarded (in that order). The alpha1,3-fucosylated phospholipase, Man3GlcNAc2 and Man5GlcNAc2 structures were eluted with 15 mM alpha-methylmannoside. It is concluded that core alpha1,3-fucosylation has little or no effect on ConA binding while xylosylation decreases affinity for ConA. In a parallel study comparing the endoglycosidase D (Endo D) sensitivities of Man3GlcNAc2, IgG-derived GlcNAcbeta1, 2Manalpha1,6(GlcNAcbeta1,2Manalpha1,3)Manbeta1,+ ++4GlcNAcbeta1,4(Fucalpha1,6)GlcNAc, the phospholipase Manalpha1,6(Manalpha1,3) Manbeta1, 4GlcNAcbeta1,4(Fucalpha1,3)GlcNAc, and horseradish and zucchini pyridylaminated N-linked oligosaccharides, it was found that only the Man3GlcNAc2 structure was cleaved. The IgG structure was sensitive only when beta-hexosaminidase was also present. Thus, in contrast to core alpha1,6-fucosylated structures, such as those present in mammals, the presence of core alpha1,3-fucose, as found in structures from plants and insects, and/or beta1,2-xylose, as found in plants, causes resistance to Endo D.

Structural analysis of N-glycans from allergenic grass, ragweed and tree pollens: core alpha1,3-linked fucose and xylose present in all pollens examined.

The N-glycans from soluble extracts of ten pollens were examined. The pyridylaminated oligosaccharides derived from these sources were subject to gel filtration and reverse-phase HPLC, in conjunction with exoglycosidase digests, and in some cases matrix-assisted laser desorption-ionisation mass spectrometry. In comparison to known structures, it was possible to determine the major structures of the N-glycans derived from Kentucky blue grass (Poa pratensis), rye (Secale cerale), ryegrass (Lolium perenne), short ragweed (Ambrosia elatior), giant ragweed (Ambrosia trifida), birch (Betula alba), hornbeam (Carpinus betulus), horse chestnut (Aesculus hippocastanum), olive (Olea europaea) and snake-skin pine (Pinus leucodermis) pollen extracts. For grass pollens the major glycans detected were identical in properties to: [structure in text] Grass pollens also contained some minor structures with one or two non-reducing terminal N-acetylglucosamine residues. In the ragweed pollens, the major structures carried core alpha1,3-linked fucose with or without the presence of xylose. In tree pollen extracts, the major structures were either xylosylated, with or without fucose and terminal N-acetylglucosamine residues, with also significant amounts of oligomannose structures. These results are compatible with the hypothesis that the carbohydrate structures are another potential source of immunological cross-reaction between different plant allergens.

A capillary electrophoretic study on the specificity of beta-galactosidases from Aspergillus oryzae, Escherichia coli, Streptococcus pneumoniae, and Canavalia ensiformis (jack bean).

The specificities of the beta-galactosidases from Aspergillus oryzae, Escherichia coli, Streptococcus pneumoniae, and Canavalia ensiformis (jack bean) have been studied by capillary zone electrophoresis. Various di- and oligosaccharides as well as a biantennary asialo N-glycan were used as substrates. Following enzymatic hydrolysis, the mixtures of substrates and products were derivatized with ethyl 4-aminobenzoate and separated by high-performance capillary electrophoresis in a borate buffer system using uv detection. Baseline separation of the respective peaks was obtained in 4 min, allowing the analysis of a large number of samples. Therefore, initial rates of hydrolysis could be determined. The beta-galactosidase from A. oryzae exhibited minimal activity toward Galbeta1-3GlcNAc. In contrast to the enzyme from S. pneumoniae which is almost specific for beta1-4 linkages, the Aspergillus galactosidase readily hydrolyzed Galbeta1-4GlcNAc and Galbeta1-6GlcNAc. Neither of the four beta-galactosidases acted upon Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Gl c (lacto-N-fucopentaose III) even though the corresponding nonfucosylated oligosaccharides were good substrates. With the exception of the enzyme from E. coli, the beta-galactosidases degalactosylated a biantennary N-linked oligosaccharide.

Functional purification and characterization of a GDP-fucose: beta-N-acetylglucosamine (Fuc to Asn linked GlcNAc) alpha 1,3-fucosyltransferase from mung beans.

An alpha 1,3-fucosyltransferase was purified 3000-fold from mung bean seedlings by chromatography on DE 52 cellulose and Affigel Blue, by chromatofocusing, gelfiltration and affinity chromatography resulting in an apparently homogenous protein of about 65 kDa on SDS-PAGE. The enzyme transferred fucose from GDP-fucose to the Asn-linked N-acetylglucosaminyl residue of an N-glycan, forming an alpha 1,3-linkage. The enzyme acted upon N-glycopeptides and related oligosaccharides with the glycan structure GlcNAc2Man3 GlcNAc2. Fucose in alpha 1,6-linkage to the asparagine-linked GlcNAc had no effect on the activity. No transfer to N-glycans was observed when the terminal GlcNAc residues were either absent or substituted with galactose. N-acetyllactosamine, lacto-N-biose and N-acetylchito-oligosaccharides did not function as acceptors for the alpha 1,3-fucosyltransferase. The transferase exhibited maximal activity at pH 7.0 and a strict requirement for Mn2+ or Zn2+ ions. The enzyme's activity was moderately increased in the presence of Triton X-100. It was not affected by N-ethylmaleimide.

Kinetic comparison of peptide: N-glycosidases F and A reveals several differences in substrate specificity.

The initial velocities of hydrolysis of nineteen glycopeptides by peptide: N-glycosidase F and A were determined. Substrates were prepared from bovine fetuin, hen ovalbumin, pineapple stem bromelain, bovine fibrin and taka-amylase. From these glycopeptides, several variants with regard to peptide and carbohydrate structure were prepared and derivatized with dabsyl chloride, dansyl chloride or activated resorufin. Tyrosine containing glycopeptides were also used without an additional chromophore. Enzymatic hydrolysis of glycopeptides was quantified by narrow bore, reversed phase HPLC with turnaround cycle times of down to 6 min, but usually 15 min. KM values ranging from 30 to 64 microM and from 4 to 36 microM were found for N-glycosidase F and A, respectively. Relative velocities of hydrolysis of the different substrates by each enzyme varied considerably. Little, if any, similarity of the performance of N-glycosidase F and A with the different substrates was observed. The minimal carbohydrate structure released by peptide: N-glycosidase F was a di-N-acetylchitobiose. N-glycosidase A could release even a single N-acetylglucosamine, albeit 3000 times slower than a di-N-acetylchitobiose or larger glycans. In general the structure of the intact glycan had little effect on activity, and with both enzymes the rate of hydrolysis appeared to be primarily governed by peptide structure and length. However, N-glycosidase F did not release glycans alpha 1,3-fucosylated at the asparagine linked N-acetylglucosamine irrespective of the presence of xylose in the substrate.

Fucose alpha 1,3-linked to the core region of glycoprotein N-glycans creates an important epitope for IgE from honeybee venom allergic individuals.

The reactivity of sera from honeybee venom allergic patients with the N-glycan of phospholipase A2 was investigated using neoglycoproteins with an enzyme-linked immunosorbent assay. Of 122 sera with appreciable levels of IgE antibodies directed against bee venom as measured by radioallergosorbent test, 34 sera exhibited significant amounts of glycan-reactive IgE. These sera cross-reacted with the N-glycan from the plant glycoprotein bromelain. The interaction of IgE with the N-glycan from phospholipase could be inhibited with glycopeptides from bromelain which shares the alpha 1,3-fucosylation of the asparagine-bound N-acetylglucosamine with bee venom phospholipase. Since defucosylated bromelain glycopeptides or glycopeptides containing a Man3GlcNAc2 oligosaccharide were not recognized by most of these sera, we conclude that alpha 1,3-fucosylation of the innermost N-acetylglucosamine residue of N-glycoproteins forms an IgE-reactive determinant. This structural element is frequent in glycoproteins from plants, and it occurs also in insects. It is suspected to be one of the major causes of the broad allergenic cross-reactivity among various allergens from insects and plants.

Determination of amino sugars and amino acids in glycoconjugates using precolumn derivatization with o-phthalaldehyde.

This report examines the RP-HPLC separation of o-phthalaldehyde derivatives of amino acids, amino sugars, and amino sugar alcohols using either 2-mercaptoethanol or 3-mercaptopropionic acid. A method with pmol sensitivity for the analysis of N-acetylamino sugars of glycoconjugates was elaborated. Upon hydrolysis, amino sugars are reduced with borohydride. Automated precolumn derivatization and chromatographic conditions for the resulting hexosaminitols are the same as those used for the analysis of amino acids. The method has been tested with as little as 2 micrograms of bovine fetuin, with a glycopeptide from bromelain and with an oligosaccharide after periodate oxidation.

Peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F cannot release glycans with fucose attached alpha 1----3 to the asparagine-linked N-acetylglucosamine residue.

The ability of peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F (PNGase F) from Flavobacterium meningosepticum and PNGase A from sweet almonds to deglycosylate N-glycopeptides and N-glycoproteins from plants was compared. Bromelain glycopeptide and horseradish peroxidase-C glycoprotein, which contain xylose linked beta 1----2 to beta-mannose and fucose linked alpha 1----3 to the innermost N-acetylglucosamine, were used as substrates. In contrast to PNGase A, the enzyme from F. meningosepticum did not act upon these substrates even at concentrations 100-fold higher than required for complete deglycosylation of commonly used standard substrates. After removal of alpha 1----3-linked fucose from the plant glycopeptide and glycoprotein by mild acid hydrolysis, they were readily degraded by PNGase F at moderate enzyme concentrations. Hence we conclude that alpha 1----3 fucosylation of the inner N-acetylglucosamine impedes the enzymatic action of PNGase F. Knowledge of this limitation of the deglycosylation potential of PNGase F may turn it from a pitfall into a useful experimental tool.