The macromolecular properties of blood-group-specific glycoproteins. Characterization of a series of fractions obtained by density-gradient ultracentrifugation

Reduction of colonic mucus by repeated short-term stress enhances experimental colitis in rats

The role of stress in inflammatory bowel disease remains debated and few studies have tested the role of stress in conjunction with experimental animal models of colitis. In this investigation we tested the hypothesis that cold-restraint stress would adversely effect the severity of 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats, and examined mechanisms for the response. Results indicated that increasing intermittent prior exposures to stress significantly enhanced TNBS-induced colitis severity. An associated stress-induced decrease in colonic mucin glycoprotein content, reduction in goblet cells, and histochemical mucin suggested reduced mucin was a pathogenetic factor. Myeloperoxidase content increased and mast cell counts in the colon decreased but colonic permeability only temporarily increased with increasing stress exposure. Prior adrenalectomy or administration of an adrenergic blocking agent did not prevent the colonic changes to stress, but mast cell stabilization or inhibition of cholinergic pathways reduced the stress-induced colonic changes.

A neurotensin antagonist, SR 48692, inhibits colonic responses to immobilization stress in rats

We previously reported that short-term immobilization stress of rats causes increased colonic mucin release, goblet cell depletion, prostaglandin E2 secretion, and colonic mast cell activation, as well as increased colonic motility. The purpose of this study was to investigate whether neurotensin (NT), a peptide expressed in both brain and digestive tract, participates in these responses. Rats were pretreated with SR 48692 (1 mg/kg, i.p.), an NT antagonist, 15 min before immobilization (30 min). The administration of the antagonist significantly inhibited stress-mediated secretion of colonic mucin, prostaglandin E2, and a product of rat mast cells, rat mast cell protease II (P < 0.05), but did not alter the increase in fecal pellet output caused by immobilization stress. Immobilization stress also resulted in a quantifiable decrease in the abundance of NT receptor mRNA in rat colon compared with that in colonic tissues from nonimmobilized rats as measured by densitometric analysis of in situ hybridization studies (P < 0.03). We conclude that the peptide NT is involved in colonic goblet cell release and mucosal mast cell activation after immobilization stress.

Structural analysis of purified human tracheobronchial mucins

Light scattering has been used to investigate the structure of human tracheobronchial mucin glycoproteins (HTBM) from the sputum of cystic fibrosis patients. The specimen was extracted using 6M guanidinium hydrochloride solution and fractionated by gel exclusion chromatography on Sephacryl S-1000. The fractionated HTBM was purified by density gradient ultracentrifugation. Purity of the resulting material was confirmed by SDS polyacrylamide gel electrophoresis and uv spectroscopy. Light scattering measurements on the fractionated mucins yield weight-average molecular weights Mw, and z-average radii of gyration Rg,z. The native cystic fibrosis HTBM consisted of a high molecular weight fraction with Mw = 9.3 X 10(6) daltons and a lower molecular weight fraction containing partly degraded mucins. After reduction and carboxymethylation of the high molecular weight native fraction, the resulting material was separated into three pools with Mw values of 5.1 X 10(6), 1.6 X 10(6), and 400,000. The derived molecular weights for the protein cores Mp,w, and the experimental radii of gyration are found to be consistent with the Mp,w -Rg relation established previously for submaxillary, cervical, and gastric mucins. These results imply that HTBM has the same extended-coil conformation reported for other mucins and has a molecular structure consisting of subunits, linked into linear chains via covalent (disulfide) bonds.

Biosynthesis of proteins by human gastric mucosa in vitro

Incorporation of L-[U-14C]leucine and of D[U-14C]glucose into proteins of fresh human gastric mucosa in vitro was studied after incubation of homogenized tissue and of intact mucosal pieces. CsCl centrifugation was used to separate high-density mucus glycoproteins from other mucosal proteins, and the macromolecular nature of radioactive mucosal glycoprotein fractions was confirmed by SDS/polyacrylamide-gel electrophoresis and autoradiography of the polyacrylamide gels. In all experiments a substantial proportion of total incorporated radioactivity was associated with gastric-mucosal glycoprotein fractions (CsCl fraction L3), indicating their biosynthesis. Radioactivity of these fractions was shown to co-chromatograph with carbohydrates when fractionated either directly or after reduction and alkylation (1) Sephadex G-200 chromatography in the excluded fractions and (2) by DEAE-cellulose ion-exchange chromatography. On incubation of intact mucosa, the major portion of radioactivity associated with the glycoprotein fractions of both leucine- and glucose-labelled specimens was secreted into the mucosal media during the course of the experiment. It is suggested that biosynthesis of mucus in vivo by gastric mucosa may be associated with rapid secretion of the synthesized macromolecules into the lumen of the stomach and that investigations of the metabolic processes within the mucosa should consider the products of secretion of the tissue. Incorporation of L-[U-14C]leucine implies biosynthesis of the polypeptide components of the macromolecules.

Ethanol, its effect on the synthesis of proteins by guinea-pig gastric mucosa

Incorporation of L-[U-14C]leucine into proteins is taken to indicate the synthesis of proteins by guinea pig gastric mucosa. Ethanol reduced the synthesis of proteins in vitro by homogenized mucosa, by isolated gastric epithelial cell preparations and by intact tissue. Intact stomach wall incubated without ethanol in phosphate buffered saline showed progressively increasing incorporation of the precursor into tissue proteins and into proteins which were secreted into the mucosal incubation media. On isopycnic CsCl gradient fractionation radioactive tissue proteins were found at the top of the gradient (fraction L1, sp.gr.1.11-1.20) while radioactive secreted proteins sedimented to the bottom of the gradient as the carbohydrate rich high density gastric mucosal glycoprotein fraction L3 (sp.gr.1.29-1.33). Ethanol significantly but reversibly reduced the incorporation of radioactive leucine by intact mucosa into both the tissue proteins and the secreted proteins. Uptake of the precursor into the intracellular acid soluble pool was not impaired by ethanol and no significant differences were detected in the specific activities of free intracellular leucine between the ethanol treated samples and the corresponding controls. It is suggested that the ulcerogenic nature of ethanol may be associated with inhibition of the synthesis of proteins within mucosal epithelium leading to reduction in the output of mucosal secretory glycoproteins with subsequent impairment of the cytoprotective properties of the dynamic mucous barrier.

Density gradient analysis of secretions produced in vitro by human and canine airway mucosa: identification of lipids and proteoglycans in such secretions

Human and canine airway mucosal explants synthesize and secrete high molecular weight glycoconjugates, incorporating 14C-glucosamine, a radioactive precursor to epithelial glycoprotein. Our examination of secretions produced by several individual specimens, however, did not reveal epithelial glycoprotein of typical buoyant density (1.5 g/ml in CsBr); only a high-density component with features of glycoprotein and proteoglycan. To provide sufficient material for characterization, secretions from several specimens of human and canine explants were separately pooled and subjected to DGU in CsBr. After removal of lipids and proteins, the glycoconjugates were recovered into five fractions of different density. 14C-glucosamine had been incorporated in all five fractions. Fractions 1-4 together accounted for 88% of the radiolabel but gas chromatography indicated that none of these contained epithelial glycoprotein. Their amino acid compositions were similar to those of proteoglycans and electrophoresis confirmed the presence of chondroitin sulfates A, B, C, heparan sulfate and hyaluronic acid. Sugars typical of epithelial glycoprotein were identified only in the glycoconjugate subfraction 5 of lowest density (and also lowest in yield) in which glycosaminoglycans were also identified. By addition of radioactive precursors, 14C acetate, 14C palmitate and 14C mevalonic acid to the culture medium and autoradiography of the secreted lipids we have shown that the tracheal explants actively synthesize lipids. Lipids accounted for a high proportion, almost half by weight, of the explant secretion. While neutral and phospholipids predominate, glycolipids were also identified.

Density gradient study of bronchial mucus aspirates from healthy volunteers (smokers and nonsmokers) and from patients with tracheostomy

Because it is difficult to obtain, little is known of bronchial mucus from the normal human airway; it has been mainly studied as sputum expectorated in chronic bronchitis with particular attention to epithelial glycoprotein. We have now applied density gradient methods to study this and other macromolecules and lipids in normal airway mucus. After lavage at bronchoscopy, mucus was aspirated from six normal volunteers, that include one light and two heavy smokers. This normal mucus has been compared with that obtained from four patients with tracheostomy because of respiratory muscle paralysis due to neurological disease. The normal aspirates contained small threads of mucus, the tracheostomy aspirates viscous blobs of jelly, a difference in physical appearance reflected in macromolecular yields, 0.3-1 mg/ml and 6-24 mg/ml respectively. On analytical ultracentrifugation normal mucus showed no discernible material in the buoyant density region typical of epithelial glycoprotein (1.5 g/ml): Virtually all the material migrated to the miniscus and was predominantly lipids and proteins. A trace amount of material recovered from a higher density region (greater than or equal to 1.6 g/ml) was found to contain both glycoprotein and proteoglycan. Aspirates from the heavy smokers contained appreciable amounts of material with typical buoyant density (approximately 1.5 g/ml) but still with features of proteoglycan. In contrast in tracheostomy aspirates epithelial glycoprotein of typical buoyant density and chemical composition accounted for up to 25% of nondialyzable material. We conclude that under normal conditions typical epithelial glycoprotein is virtually absent from airway mucus and that the glycoconjugate present has features of glycoprotein and proteoglycan.

Polyelectrolyte behaviour in mucus glycoproteins

Mucus glycoproteins isolated from a human ovarian cyst and the sputum of a cystic fibrotic exhibit a significant decrease in reduced viscosity with increase in ionic strength, I. The molecular weights of the glycoproteins showed little variation with I, implying that the change is conformational rather than a dissociation. This change is ascribed to a polyelectrolyte-type contraction rather than to a reduction in particle asymmetry. Guanidine hydrochloride acts as a classical electrolyte in the reversible suppression of charge effects, and not as a denaturing or dissociation agent. These observations help to resolve some discrepancies in earlier studies. The occurrence of polyelectrolyte effects in these glycoproteins is ascribed to flexibility of structure and to their content of N-acetylneuraminic acid. The ionic strength values necessary for different types of physical measurement are discussed.

Studies of the limited degradation of mucus glycoproteins. The effect of dilute hydrogen peroxide

1. The action of dilute H2O2 on a series of ovarian-cyst glycoproteins and glycopolypeptides was investigated. 2. Both native glycoproteins and the glycopolypeptides were carbohydrate-rich, of relatively low molecular weight and of simple structure. 3. At pH 5.6 and 37 degrees C, exposure to H2O2 for a limited time brought about a partial degradation, the molecular weight being decreased by 2-4-fold. 4. Carbohydrate analysis showed very little change in the oligosaccharide moiety, apart from a small decrease in sialic acid in some samples. 5. Amino acid analysis showed minor changes in serine, threonine and proline contents, but almost total loss of histidine. Concomitantly, there was a small gain in aspartic acid. 6. Myosin, examined at both pH 5.7 and 6.7, exhibited generally similar behaviour, there being losses of other amino acid residues as well as histidine: the viscosity was decreased to a low value, and a range of peptides of widely varying size was produced. 7. It is suggested that attack on the histidine residue, with partial conversion into aspartic acid, is accompanied by scission of the histidyl peptide bond.

Structure of intestinal-mucus glycoprotein from human post-mortem or surgical tissue: inferences from correlation analyses of sugar and sulfate composition of individual mucins

The carbohydrate composition of 14 human, small-intestine mucins, obtained at surgery or post-mortem, varied greatly from specimen to specimen with respect to individual sugars and average chain-length (ratio of total carbohydrate to N-acetylgalactosamine). Three monosaccharides, galactose, N-acetylglucosamine, and fucose gave good correlations with each other, and to total carbohydrate content, when expressed as a ratio to the chain-terminal N-acetylgalactosamine residue. In contrast, sialic acid gave a good correlation only with N-acetylgalactosamine. In eight specimens the molar sulfate to N-acetylgalactosamine ratios gave good correlation with the ratios of galactose to N-acetylgalactosamine, N-acetylglucosamine to N-acetylgalactosamine, and total carbohydrate to N-acetylgalactosamine. These results indicate that the intraspecies variability of intestinal-mucin carbohydrates arises from the interdependent addition of galactose, N-acetylglucosamine, fucose, and sulfate residues. Partial correlation-analysis indicated that proportions of N-acetylglucosamine and fucose were correlated only through a mutual dependence on galactose, suggesting that the key elongating-factors involve the addition of galactose residues. The number of sialic acid residues per oligosaccharide chain remained relatively unchanged from mucin to mucin, and this, coupled with the close correlation between the proportions of sialic acid and N-acetylgalactosamine, suggests that almost all sialic acid residues are bound to the core N-acetylgalactosamine residues in intestinal mucin. High fucose-to-sialic acid and high sulfate-to-sialic acid ratios reported in some disease states are explained as the consequence of chain elongation.

Isolation and characterization of human cervical-mucus glycoproteins

Mucus glycoproteins (mucins) were extracted from human cervical pregnancy mucus by 6 M-guanidinium chloride in the presence of proteinase inhibitors. Purification was subsequently achieved by isopycnic density-gradient centrifugation in CsCl/ guanidinium chloride gradients. The purified macromolecules represented approx. 85% of the total and were devoid of nucleic acids and proteins, as judged by analytical density-gradient centrifugation, disc electrophoresis and u.v. spectroscopy. Sedimentation-velocity centrifugation revealed a single unimodal peak with S20,W 50.1S in 0.2M-NaCl and 37.0S in 6 M-guanidinium chloride. Molecular weights obtained by light-scattering were 9.7 X 10(6) and 5.9 X 10(6) in 0.2M-NaCl and 6 M-guanidinium chloride respectively. The chemical analyses were typical of those of epithelial mucins. The macromolecules contained approx. 20% (w/w) of protein, and 65% (w/w) was accounted for as carbohydrate. Serine and threonine constituted 32 mol/100 mol and proline 10 mol/100 mol of the amino acids. The major sugars found were N-acetylglucosamine (12.8%), N-acetylgalactosamine (9.7%), galactose (18.7%), sialic acid (15.0%) and fucose (7.5%).

In vitro studies on the effect of salicylates on the synthesis of proteins by guinea pig gastric mucosal tissue

In the work reported in this paper we have studied the effect of salicylates on protein synthesis by (A) intact guinea pig gastric mucosa, (B) isolated gastric epithelial cells of guinea pig stomach and (C) cell-free homogenates of the isolated cells. In experiments on intact gastric mucosa, (A), secretion of newly-synthesised proteins into the mucosal media was also investigated and the nature of the effected, secreted proteins examined by isopycnic CsCl gradient fractionation. Results indicate that synthesis of proteins, as assessed by incorporation of L-[U-14C]leucine into trichloroacetic acid-insoluble proteins, is significantly impaired in the presence of salicylates in all three systems investigated. In experiments using isolated epithelial cells and cell-free homogenates of these cells, the effect was found to be dose-dependent and not associated with a corresponding reduction of the uptake of the precursor from the medium into the acid-soluble intracellular pool. The inhibitory effect of salicylates on protein synthesis was found to be reversible in experiments using both the intact gastric mucosa, as also the isolated epithelial cell preparations. In all experiments it was found to last for the duration of exposure of the tissue to the action of the drug. In experiments using intact gastric mucosa secretion of the newly-synthesised radioactive proteins into the mucosal medium was also impaired by O-acetyl salicylic acid. Isopycnic CsCl gradient fractionation of the secreted proteins did not reveal any qualitative differences between the salicylate-treated samples and the control samples, indicating non-specific inhibition of protein synthesis by the drug.

Radioimmunoassay of human intestinal goblet cell mucin. Investigation of mucus from different organs and species

We have developed a double-antibody radioimmunoassay for the quantitative measurement of human goblet cell mucin (GCM) in order to study intestinal mucus in human and other species. The assay used 3H-labeled mucin as the antigen, rabbit antisera, and sheep anti-rabbit IgG antisera as the second antibody. A number of applications of the assay were investigated. A survey of human tissues revealed that mucins of the rectum, colon, and small intestine had identical affinity for the rabbit antibody, whereas lung eyelid conjunctiva, esophagus, and stomach reacted less strongly. GCM concentration ranged from 1.9 to 14 microgram mucin protein/mg tissue protein in the small and large intestine, respectively. The radioimmunoassay was also found to be useful as a marker during the isolation of GCM from human ileal extracts, where it indicated that a 10,000-fold purification had been achieved. Antigenic determinants of the mucin did not rely upon ABH blood group-specific terminal sugars in oligosaccharide chains. A comparison of mucins among various species revealed a partial species specificity of the GCM antibody. Human GCM cross-reacted with dog, monkey, and rabbit mucins, but not with mucins of rat, pig, toad, and oyster. Organ distributions of cross-reactive mucins in rabbit tissues indicated a pattern that was qualitatively similar to that seen in human tissues. Possible implications of these findings for autoimmune diseases are briefly discussed.

The role of disulphide bonds in human intestinal mucin

Goblet-cell mucin (mucin 1) was isolated and purified from human small-intestinal scrapings. After application of mucin 1 to DEAE-Bio-Gel (A) columns, most of the glycoprotein (76-94% of hexoses) was eluted in the first peak (designated mucin 2). Minor amounts of acidic glycoproteins were eluted with 0.2m- and 0.4m-NaCl in later peaks. Analyses of mucin 1 and mucin 2 revealed mucin 2 to be a monodisperse highly glycosylated glycoprotein containing 6.3% by wt. of protein, N-acetylgalactosamine, N-acetylglucosamine, galactose and fucose. Mucin 1 was similar in composition, but was polydisperse and contained more protein (12.3% by wt.) as well as N-acetylneuraminic acid. Analytical CsCl-gradient ultracentrifugation showed both mucin 1 and mucin 2 to have a major component with an average buoyant density of 1.47000g/ml. Mucin 1 also contained a slightly less-dense minor glycoprotein component. After exhaustive reduction and alkylation mucin 1 retained its major component, but partly dissociated into two lighter glycoprotein components. Mucin 2, in contrast, did not change its density distribution after reduction. Band ultracentrifugation in (2)H(2)O-containing iso-osmotic buffers showed that mucin 1 contained a major fast-sedimenting component (s(o)=37+/-2S), and a minor amount of a slower-sedimenting component. After reduction there was an increased quantity of the latter component, for which an s(o) value of 14.5S was calculated. In contrast, mucin 2 was unaltered by reduction (s(o)=33+/-2S). These findings indicate that the major component of goblet-cell mucin (mucin 2) does not dissociate after S-S-bond reduction, and thus does not apparently rely for its polymeric structure on the association of subunits through covalent disulphide bonds. However, the effects of reduction on mucin 1 suggest that in the native mucin intramolecular disulphide bonds in the minor glycoproteins may stabilize their structure, permitting secondary non-covalent interactions to develop with the major dense mucin (mucin 2) protein.

The use of equilibrium density-gradient ultracentrifugation in the isolation and characterisation of glycoproteins with blood group P1 activity from sheep hydatid-cyst fluid

Equilibrium density-gradient ultracentrifugation in caesium choride and caesium sulphate has been used in the isolation and fractionation of the glycoproteins specific for blood-group P1 from hydatid cyst fluids. The fractions obtained have distinct and systematic differences specifically related to their buoyant densities, chemical compositions and specific-activities for group P1. High levels of specific-activity were maintained over a large range of chemical compositions. The peptide content varied systematically from 2.5% for the densest fraction to 37% for the least dense fraction. The amino acid composition was essentially constant over all fractions. The proportion of glucosamine decreased and the proportions of galactosamine, mannose and glucose increased with increasing peptide content of the fractions. The data presented suggest the present of oligosaccharide side-chains of various lengths and compositions and/or the presence of oligosaccharide side-chains with very different chemical compositions, of which only some are associated with the specificity for group P1. The properties of the glycoproteins from hydatid cyst fluids have been compared with those of the glycoproteins from human ovarian cysts. Although some similarities have been demonstrated there are significant differences.

The action of proteolytic enzymes on the glycoprotein from pig gastric mucus

A glycoprotein of mol.wt. 2x10(6) was isolated in homogeneous form from pig gastric mucus by isopycnic centrifugation in CsCl but without enzymic digestion or reductive cleavage of disulphide bonds. Digestion of the purified glycoprotein with trypsin, pepsin or Pronase resulted in the formation of glycoprotein subunits, of mol.wt. 5.2x10(5)-5.8x10(5), one-quarter that of the undigested glycoprotein. The glycoprotein subunits were isolated by gel filtration and shown to contain all the carbohydrate present in the undigested glycoprotein, but 18.6-25.6% of the total amino acids originally present were lost on digestion. The relative amount of threonine, serine and proline had increased from 41% (w/w) in the undigested glycoprotein to 61-67% of the total amino acids in the glycoprotein subunits after digestion. The results support the previously proposed structure for the glycoprotein, namely that of four subunits joined by disulphide bridges. These results show the presence of two distinct regions in the glycoprotein molecule, one rich in threonine, serine and proline, which is glycosylated and resistant to proteolyis, whereas the other, with an amino acid composition more characteristic of a globular protein, is not glycosylated and is susceptible to proteolysis. In addition, the region that is susceptible to proteolysis contains the disulphide bridges which join the glycoprotein subunits together to form the gastric glycoprotein.

Macromolecular distribution near the limits of density-gradient columns. Some applications to the separation and fractionation of glycoproteins

1. Expressions are derived for the distribution at density-gradient equilibrium of macromolecules whose densities are (a) close to the values characterizing the solution limits or (b) outside the span of the gradient. 2. Density-distribution predicted by the expressions agree with those obtained by rigorous methods. 3. The distribution equations are applied to hypothetical mixtures of proteins and glycoproteins in commonly used density-gradient media to simulate separation and fractionation conditions. 4. It is shown that CsBr, although less efficient than CsCl for fractionation, is nevertheless adequate for most purposes; in analytical experiments it may often have advantages over CsCl. Limitations on the use of LiBr are explored. 5. An expression is derived which allows the variance of the partial specific volume of the macromolecular component to be determined from the variance of the buoyant density. It is shown that the relative resolving powers of different salts is expressed by their values of the quantity (formula: see text). 6. The equations are applied to a well-characterized glycoprotein preparation at equilibrium in CsCl and in Cs2SO4:it is shown that the much wider distribution in CsCl than in Cs2SO4 is explicable in terms of the variance in buoyant density and the solvation properties of the salts. 7. Limitations of the expressions arise when dispersity in density is represented by a low apparent molecular weight; realistic simulations can then only be obtained when the component is fully banded.

Isolation, composition and reactivity of the neutral glycoproteins from human meconiums with specificities of the ABO and Lewis systems

Blood-group-specific A1, B, AB, H and Lea neutral glycoproteins were isolated from suitable pools of human normal meconiums by a preliminary fractionation with a cationic detergent at pH5 and 9 (borate), followed by ion-exchange and gel chromatography. The ABH materials have sedimentation coefficients of about 10S-11S, whereas the Lea preparation, not strictly homogeneous, shows a coefficient of 7S. From the detailed analytical data collected, the following relations are deduced between these various substances; they all possess a common peptide core; there are stable ratios of N-acetylglucosamine/N-acetylgalactosamine in the B, H and Lea materials and of N-acetylglucosamine/galactose in A, H and Lea materials, from which the numbers of A and B determinants are estimated. In the ABH substances, the ratio of glucosamine to the sum of threonine and serine is stable. Presumably because of genetic factors, the amount of fucose varies among the different glycoproteins, but it is always definitely lower than in the average cyst substances. Various serological tests and precipitin methods were used to measure the potency, purity and integrity of the preparations, including comparisons between A1 and A2 substances from this source. The Leb activity did not appear as high as it is in glycoproteins from adults and a possible interpretation would be the immature Lewis system as observed on erythrocytes; this could explain their very strong inhibiting power towards iso-agglutinins. This family of substances with various specificities has common features with that prepared from ovarian cysts, but differs clearly on some points.