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At least 883 records · Page 49Linked to original sources
PubMed1975

The mechanism of action of glycosidases.

The factors that may contribute to the rate enhancement observed with enzymatic versus non-enzymatic hydrolysis of glycosides are discussed. The nature of the active site as deduced from labelling studies with beta-glucosidases is described. A two-step mechanism involving either an enzyme stabilized glycosyl ion or a covalent glycosyl-enzyme intermediate is proposed. Experiments with a beta-glucosidase from almonds show that even with 2-deoxy glucosides with good leaving groups as aglycon which are hydrolyzed 1000 times more slowly than the corresponding glucosides, the deglucosylation step is faster than the cleavage of the glycosidic bond.

Aspergillus↗
PubMed1984

Glycosylation of the epidermal growth factor receptor in A-431 cells. The contribution of carbohydrate to receptor function.

A-431 cells were treated with inhibitors of either N-linked glycosylation (tunicamycin or glucosamine) or of N-linked oligosaccharide processing (swainsonine or monensin) to examine the glycosylation of epidermal growth factor (EGF) receptors and to determine the effect of glycosylation modification on receptor function. The receptor was found to be an Mr = 130,000 polypeptide to which a relatively large amount of carbohydrate is added co-translationally in the form of N-linked oligosaccharides. Processing of these oligosaccharides accounts for the 10,000-dalton difference in electrophoretic migration between the Mr = 160,000 precursor and Mr = 170,000 mature forms of the receptor. No evidence was found for O-linked oligosaccharides on the receptor. Mr = 160,000 receptors resulting from swainsonine or monensin treatment were present on the cell surface and retained full function, as judged by 125I-EGF binding to intact cells and detergent-solubilized extracts and by in vitro phosphorylation in the absence or presence of EGF. On the other hand, when cells were treated with tunicamycin or glucosamine, ligand binding was reduced by more than 50% in either intact cells or solubilized cell extracts. The Mr = 130,000 receptors synthesized in the presence of these inhibitors were not found on the cell surface. In addition, no Mr = 130,000 phosphoprotein was detected in the in vitro phosphorylation of tunicamycin or glucosamine-treated cells. It appears, therefore, that although terminal processing of N-linked oligosaccharides is not necessary for proper translocation or function of the EGF receptor, the addition of N-linked oligosaccharides is required.

Carcinoma, Squamous Cell↗
PubMedDate not supplied

[Transglycosylation reactions catalyzed by Aspergillus niger 15 beta-xylosidase].

The products of transglycosylation formed as a result of action of beta-xylosidase from Aspergillus niger 15 on p-nitrophenyl-beta-D-glucopyranoside and p-nitrophenyl-beta-D-xylopyranoside were studied by means of chromatography on sephadexes. They are formed at the substrate concentration of 10--100 mM with the amount 7--10 times less than that of hydrolysis products. Peculiarities of chromatography of substrates, products of transglycosylation and p-nitrophenol on Sephadexes G-15 and G-25 were analysed.

Aspergillus niger↗
PubMed1983

Structures of the oligosaccharides present at the three asparagine-linked glycosylation sites of human IgD.

The complete amino acid sequence of the human myeloma IgD:WAH has been determined and the sites of asparagine glycosylation identified as residues 354, 445, and 496 (Takahashi, N., Tetaert, D., Debuiere, B., Lin, L.-C., and Putnam, F. W. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2850-2854). We have determined the structures of the oligosaccharides at each of these positions. Asn 354 bears oligosaccharides exclusively of the high mannose type containing from 5 to 9 mannose residues. Twenty per cent of the oligosaccharides at this site contain 1 glucose residue at the terminus of the branch emanating from the alpha 1 leads to 3-linked core mannose which is believed to reflect incomplete processing of the triglucosyl-high mannose oligosaccharide intermediate following transfer from dolichol to nascent peptide. Asn 445 and Asn 496 bear exclusively dibranched complex oligosaccharide structures; 30-40% of these molecules contain a bisecting GlcNAc-linked beta 1 leads to 4 to the innermost core mannose residue. At Asn 445, 40% of both the bisected and nonbisected oligosaccharides contain 1 residue of fucose on the Asn-linked GlcNAc and 50% bear a single N-acetylneuraminic acid residue. The oligosaccharides at Asn 496 are devoid of sialic acid and fucose. Thus, IgD:WAH is notable for the presence of virtually unprocessed oligosaccharide structures (glucosylated high mannose) on the same peptide backbone as extensively processed complex type molecules. The finding that each of the 3 Asn glycosylation sites of IgD:WAH bears either exclusively a complex or a high mannose type oligosaccharide indicates that there is considerable specificity in the glycosylation process. These oligosaccharides, nonetheless, display extensive microheterogeneity at each location.

Amino Acid Sequence↗
PubMed1981

Do carbohydrates promote hypertension?

Several earlier studies indicate an association between plasma insulin level and blood pressure independent of weight. A short review summarizes evidences showing: (a) the association between a high carbohydrate content of diet and hyperinsulinemia; (b) effect of insulin on renal reabsorbation of sodium; and (c) effects of carbohydrate intake on sympathetic activity and blood pressure. A pilot study examined the effect of a glycoside-hydrolase inhibitor (BAY g 5421) on blood pressure, fasting blood glucose, fasting plasma insulin and 100 g peroral sucrose test in six obese middle-aged men with borderline hypertension. The protocol included three periods of four weeks each. During the second and third period placebo and BAY g 5421 (100 mg three times per day) were given in a randomized, double-blind fashion. At the end of each period blood pressure was recorded during 30 minutes of rest in a supine position. The subjects also orally received 100 g sucrose as a 50 per cent solution dissolved in water. BAY g 5421 caused a significant decrease of plasma insulin after a 100 g sucrose tolerance test as compared to placebo. Blood pressure after four weeks of treatment with placebo was 135 +/- 5.7 systolic and 92 +/- 6.6 diastolic (mmHg, mean +/-s.d.) and after four weeks of treatment with BAY g 5421 was not significantly different. There was no change in weight. The question whether carbohydrates promote hypertension has to be further investigated.

Acarbose↗
PubMed1979

Tumor markers for ovarian cancer.

The literature on tumor distinctive markers in ovarian cancer has been reviewed. Various immunological and biochemical approaches have been attempted for the diagnosis and management of patients with ovarian cancer. The complex spectrum of antigens that can be detected in human ovarian cancer consists of several tumor-associated antigens, fetal or carcinoembryonic antigens, carcinoplacental markers, and normal tissue antigens. We have described and partially characterized two ovarian tumor-associated antigens designated as OCAA and OCAA-1, which seem to have potential for the immunodiagnosis of ovarian cancer. Several other investigators have carried out similar studies, but in general their serological characterization of these antigens has been limited. The well-defined embryonic proteins that have been examined in the ovarian cancer include carcinoembryonic antigen (CEA), alpha-fetoprotein (alpha-fp), beta-oncofetal antigen (BOFA), Regan and Nagao isoenzymes and human chorionic gonadotropin (HCG). The presence of pregnancy-zone protein (PZP) has also been reported in ovarian cancer. In addition, several normal tissue components include fibrin-fibrinogen degradation products (FDP), alpha 1-globulin, and urokinase have been found associated with ovarian cancer. Both humoral antibodies and cell-mediated immune responses against tumor-associated antigens can be measured in ovarian cancer patients. In addition, serum factors, which block cellular immune reactions, have been identified. However, progress in this area has been hampered by the complexity of the antigens associated with ovarian tumors and the lack of standardized, well-characterized sources of antigens or target cells. Enzymes, especially those involved in glycoprotein biosynthesis, (eg, glycoprotein:glycosyltransferases and glycosidase) have been explored as possible early biochemical indicators of ovarian neoplasia. A serum specific deficiency of alpha-L-fucosidase has been found in patients with ovarian cancers. Of all the glycoprotein:glycosyltransferases studied, galactosyltransferase has been found to be the best enzyme marker for ovarian adenocarcinoma. The determination of serum levels of this enzyme reflected the clinical status of the patient with respect of tumor progression as well as tumor burden. Recently, assay of a phosphodiesterase, which specifically hydrolyzes cytidine 5'-monophospho-N-acetylneuraminic acid, has been found promising in the detection and management of patients with ovarian cancer.

Antigens, Neoplasm↗
PubMed1997

Sequencing of N-linked oligosaccharides directly from protein gels: in-gel deglycosylation followed by matrix-assisted laser desorption/ionization mass spectrometry and normal-phase high-performance liquid chromatography.

A generally applicable, rapid, and sensitive method for profiling and sequencing of glycoprotein-associated N-linked oligosaccharides from protein gels was developed. The method employed sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) for protein separation and purification and in-gel deglycosylation using PNGase F for glycan release. Profiles of the neutral glycans from bovine ribonuclease B, chicken ovalbumin, and human immunoglobulin G (IgG), as well as sialic acid-containing sugars (following esterification of the acidic groups) of bovine fetuin and bovine alpha1-acid glycoprotein, were obtained by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) and by normal-phase high-performance liquid chromatography following fluorescent labeling. Oligosaccharides were sequenced using specific exoglycosidases, and digestion products were analyzed by MALDI MS. Between 50 and 100 pmol (1.5 to 15 microg) of glycoprotein applied to the gel was sufficient to characterize its oligosaccharide contents. The identity of all glycoproteins investigated could be confirmed after deglycosylation by in-gel trypsin treatment followed by MALDI MS mass mapping and matching the measured molecular weights to a sequence database. The technique was used for the characterization of the glycan moieties of human immunodeficiency virus recombinant gp120 (Chinese hamster ovary cells) and to monitor changes in the glycosylation of this glycoprotein when produced in the presence of a glucosidase I inhibitor. Furthermore, since heavy and light chains of IgG became separated by SDS-PAGE, it could be established that most glycans were associated with the heavy chains.

1-Deoxynojirimycin↗
PubMed1986

6-Epicastanospermine, a novel indolizidine alkaloid that inhibits alpha-glucosidase.

A second indolizidine alkaloid, epimeric with castanospermine, has been isolated from seeds of the Australian tree Castanospermum australe. The structure was established as 6-epicastanospermine by proton and carbon-13 nuclear magnetic resonance spectroscopy and mass spectrometry. 6-Epicastanospermine was found to be a potent inhibitor of amyloglucosidase, (an exo-1,4-alpha-glucosidase), a weak inhibitor of beta-galactosidase, and not to inhibit beta-glucosidase and alpha-mannosidase. These results indicate that glycosidase inhibitory activity cannot be predicted by comparison of the structure and stereochemistry with the appropriate sugars, since 6-epicastanospermine is an analog of mannose and not of glucose. The inhibition of amyloglucosidase was found to be competitive and to be more effective at higher pH values. Castanospermine and 6-epicastanospermine differed in their effect upon the mung bean processing enzymes, glucosidase I and II, in that the former is a potent inhibitor whereas the latter is a very poor inhibitor. Subtle alterations in stereochemistry of these alkaloids can therefore produce significant changes in their biological activity.

Alkaloids↗