Characterization of gastric mucoproteins isolated by equilibrium density-gradient centrifugation in caesium chloride

Intestinal mucus protects Giardia lamblia from killing by human milk

We have previously shown that nonimmune human milk kills Giardia lamblia trophozoites in vitro. Killing requires a bile salt and the activity of the milk bile salt-stimulated lipase. We now show that human small-intestinal mucus protects trophozoites from killing by milk. Parasite survival increased with mucus concentration, but protection was overcome during longer incubation times or with greater milk concentrations. Trophozoites preincubated with mucus and then washed were not protected. Protective activity was associated with non-mucin CsCl density gradient fractions. Moreover, it was heat-stable, non-dialyzable, and non-lipid. Whereas whole mucus inhibited milk lipolytic activity, protective mucus fractions did not inhibit the enzyme. Furthermore, mucus partially protected G. lamblia trophozoites against the toxicity of oleic acid, a fatty acid which is released from milk triglycerides by lipase. These studies show that mucus protects G. lamblia both by inhibiting lipase activity and by decreasing the toxicity of products of lipolysis. The ability of mucus to protect G. lamblia from toxic lipolytic products may help to promote intestinal colonization by this parasite.

Peptic erosion of gastric mucus in the rat

1. The effect of pepsin on the loss of mucus glycoprotein from the gastric epithelial mucus layer was studied in the rat. 2. Pepsin was instilled into the gastric lumen, and luminal contents were subsequently assayed. 3. Glycoprotein loss increased with luminal pepsin, up to a concentration of 1 mg pepsin/ml. 4. Luminal glycoprotein had a molecular size distribution intermediate between subunit, and native mucus glycoprotein of the epithelial mucus layer. 5. Incubation of gastric epithelial scrapings with pepsin demonstrated that insoluble, native mucus glycoprotein was rapidly degraded to soluble glycoprotein of similar molecular size distribution to that found in vivo in the lumen.

Changes in mucus glycoprotein synthesized in rat gastric mucosa exposed to ethanol

The resistance to proteolysis by pepsin of gastric mucus glycoprotein synthesized by tissue culture in the presence and absence of 0.1 M ethanol was investigated. The glycoprotein product of ethanol-supplemented culture was found to contain 68% less associated lipids and 81% less covalently bound fatty acids, but exhibited unaltered content of carbohydrate and protein. The lipid and fatty acyl deficient glycoprotein was 5-times more rapidly and 2-3-times more extensively degraded by pepsin than the glycoprotein synthesized in the absence of ethanol. Following delipidation with organic solvents and deacylation with hydroxylamine both glycoproteins were digested at the same rate and degraded to the same extent. The lower content of fatty acyl residues markedly affected the overall pattern of the proteolytic fragments identified by SDS gel electrophoresis. The peptides corresponding to the acylated fragments of control were degraded and an increase in the amount of smaller peptides was observed. The in vitro assays of the fatty acyltransferase activity towards the substrates obtained from control and alcohol-containing cultures revealed that the enzyme activity was similar and increased proportionally with increased concentration of both glycoprotein substrates and enzyme. However, addition of 0.1 M ethanol to the assay tubes containing complete incubation mixture decreased the acylation of either glycoprotein by 40%. Based on the results presented here, and on previous studies of mucus glycoprotein synthesis in the presence of ethanol, we conclude that ethanol interferes with the process of acylation of mucus glycoprotein with fatty acids.

Biosynthesis, processing and secretion of mucus glycoprotein in the rat stomach

For the study of the biosynthesis, processing and secretion of mucus glycoproteins in rat gastric mucous cells, antibodies were raised against purified gastric mucus glycoproteins and against deglycosylated gastric mucus glycoproteins. Indirect immunofluorescence analysis of gastric mucosa sections revealed that both antibodies specifically labelled the mucus glycoprotein-synthesizing cells in the gastric mucosa. Stomach segments were pulse-labelled with [35S]cysteine and chased for various times. The radioactively labelled (glyco)proteins were quantitatively immunoprecipitated and analyzed by SDS-polyacrylamide gel electrophoresis. Less than 3% of the total radioactivity incorporated in protein was found to be present in mucus glycoproteins. Antibodies raised against native mucus glycoproteins recognized only high-molecular-weight mucus glycoproteins, while the antibodies against deglycosylated glycoproteins also bound to probable precursor forms. The synthesis of mature mucus glycoproteins (Mr greater than 300 000) required about 90 min. After 3 h of chase, only a small portion of the pulse-labelled mucus glycoproteins had been secreted; the majority of the radioactive glycoproteins at that time was still associated with the tissue. Immature (glyco)proteins were not secreted into the medium.

Quantitative and qualitative comparison of gall bladder mucus glycoprotein from patients with and without gall stones

Human gall bladder mucus glycoprotein was isolated by Sepharose 4B gel filtration followed by caesium chloride density gradient ultracentrifugation from four groups: patients with cholesterol gall stones, patients with pigmented stones, patients with complete obstruction of the cystic duct and patients with no biliary tract abnormalities (controls). Mucus glycoprotein concentrations in cholesterol gall stone bile (203 micrograms/ml +/- 199 SD, n = 17), pigment gall stone bile (110 micrograms/ml +/- 77 SD, n = 6) and control gall bladder bile (96 micrograms/ml +/- 98 SD, n = 11) were not significantly different. While bile from patients with complete obstruction of the cystic duct contained significantly higher concentrations of mucus glycoprotein (6220 micrograms/ml +/- 4130, n = 4). In vitro cholesterol nucleation time was not correlated to gall bladder mucus glycoprotein concentrations. Qualitative analysis of the carbohydrate and amino acid composition showed a basic structure typical of mucus glycoproteins in general. It is unlikely that either quantitative or qualitative differences in mucus glycoproteins are responsible for the rapid in vitro nucleation time characteristic of cholesterol gall stone patients.

Interactions of Giardia lamblia with human intestinal mucus: enhancement of trophozoite attachment to glass

Giardia lamblia trophozoites frequently are associated with mucus in vivo. We investigated the effects of human intestinal mucus on parasite attachment and survival in vitro. All samples of mucus from the duodenum and ileum (from four humans and two rabbits) enhanced attachment at 100 micrograms/ml. Attachment increased with mucus concentrations from 1 to 1000 micrograms/ml but declined toward the unstimulated level at concentrations above 1000 micrograms/ml. Mucus from the small intestine also promoted the survival of the parasites during the 2-h incubation. In contrast, colonic mucus promoted survival, but inhibited attachment. Fractionation of mucus from the human small intestine by cesium chloride equilibrium density gradient ultracentrifugation revealed that both attachment- and survival-promoting activities were in the low density, protein-rich fraction. The high density fractions containing the mucins were devoid of activity. Thus, a non-mucin fraction of mucus from the human small intestine may promote colonization by G. lamblia.

Identification of gallbladder mucin-bilirubin complex in human cholesterol gallstone matrix. Effects of reducing agents on in vitro dissolution of matrix and intact gallstones

The goals of this study were to isolate and characterize the nonlipid matrix of human cholesterol gallstones. The lipid portion of gallstones was dissolved in ethanol/ether, leaving an insoluble, granular, brown-black matrix that constituted 12.5% of solitary large stones and 3.5% of multiple small stones. The matrix was partially solubilized by sonication and studied by exclusion gel chromatography and density gradient ultracentrifugation. On Sepharose 2B column chromatography, bile pigment eluted with glycoprotein in the void volume, suggesting the presence of a high molecular weight complex (Mr greater than 2 X 10(6)). The identity of mucin in this complex was confirmed by its typical buoyant density during ultracentrifugation. The major bile pigments in the matrix were identified as bilirubin (84%) and bilirubin monoglucuronide (15%) by thin-layer chromatography. Because of their ability to solubilize mucin-type glycoproteins, we tested the ability of the reducing agents 2-mercaptoethanol (2ME) and N-acetylcysteine (NAcCys) to solubilize gallstone matrix. Both reducing agents caused a two- to threefold enhancement of matrix dissolution after 4 d compared to aqueous buffer alone (P less than 0.01). Sepharose 2B chromatography revealed that 2ME released a high molecular weight mucin-bilirubin complex as well as unbound pigment from the insoluble matrix. We also tested the effect of reducing agents on dissolution of matched cholesterol gallstones by monooctanoin, a cholesterol solvent. Both 2ME and NAcCys significantly accelerated gallstone dissolution in monooctanoin. Matched human cholesterol stones (n = 10) incubated for 4 d in monooctanoin plus either 2ME or NAcCys (1 M final concentration) weighed approximately half as much (P less than 0.01 for each) as stones incubated in monooctanoin alone. This study describes, for the first time, the isolation of a bilirubin-mucin complex in the insoluble matrix of human cholesterol gallstones. The ability of reducing agents to dissolve the matrix and thereby accelerate gallstone dissolution by monooctanoin in vitro may be relevant to gallstone dissolution in humans.

Purification and characterization of goblet-cell mucin of high Mr from the small intestine of sheep

Crude soluble mucus from sheep small intestine was freed of nearly all the nucleic acid contaminants by precipitation with protamine sulphate and treatment with nucleases. After removal of non-covalently bound proteins by equilibrium density-gradient centrifugation in CsCl, a high-Mr glycoprotein was isolated by repeated h.p.l.c. from the partially purified mucin. The high degree of purity of the high-Mr mucin was borne out by (a) the observation of a single boundary on analytical ultracentrifugation in the presence of 5M-guanidinium chloride and (b) the observation of apparent monodispersity on sedimentation-equilibrium analysis. The Mr of the highly purified mucin, determined by sedimentation equilibrium, was 5.0 (+/- 0.1) X 10(6) and was concentration-independent. Finally, only goblet cells and the mucus blanket lining the intestinal epithelial cells were immunofluorescent when guinea-pig anti-(highly purified mucin) serum was used in an indirect immunofluorescence assay. The above antiserum reacted with apparently equal strength with goblet cells and with free mucin in abomasum, caecum and colon. The chemical composition of the glycoprotein was 66% carbohydrate and 34% protein, 45% of the latter being composed of valine and threonine. The glycoprotein migrated anodally on immunoelectrophoresis and contained 7.1% (w/w) sulphate. Neutral hexoses accounted for nearly half of the total carbohydrate content, followed by galactosamine and glucosamine. Whereas fucose and sialic acid were present in only small amounts, uronic acid was not detectable in the highly purified mucus glycoprotein.

Quantitative aspects of mucus glycoprotein biosynthesis in rat gastric mucosa

The synthesis of the polypeptide backbone of mucus glycoproteins in rat stomach was studied. CsCl centrifugation of the homogenate of [3H]serine pulse-chase labelled stomach or mucosal scrapings showed that [3H]serine was mainly incorporated into molecules having a density identical to that of proteins and that only 8-12% was incorporated into macromolecules with the density of mucus glycoproteins. [3H]-Galactose, however, was almost exclusively incorporated into macromolecules with a density identical to that of mucus glycoproteins. Electrophoretic analysis of the CsCl fraction containing the mucus glycoprotein revealed that 78% of the [3H]serine-labelled macromolecules had an electrophoretic behaviour identical to that of mucus glycoproteins. Thus, only a small portion (about 6-10%) of incorporated [3H]serine was present in the backbone of the mucus glycoprotein. Translation in a wheat germ cell-free system of total RNA derived from both whole stomach and superficial mucosal scrapings, using either [35S]methionine or [35S]cysteine as radioactive amino acid, yielded a wide range of proteins. On sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, one major translation product of whole stomach RNA had an apparent Mr (43000) identical to that of rat pepsinogen. As this polypeptide could not be found amongst the translation products of RNA from scrapings it probably was pepsinogen. The present data provide strong evidence that the backbone polypeptide of mucus glycoproteins only accounts for a small part of the proteins synthesized by mucus-producing cells.

Distribution of mucus glycoprotein in four compartments of lamb and sheep stomach

The regional distribution of mucus glycoprotein in lamb before weaning and sheep was studied. Mucus glycoprotein was present in the forestomach region of lamb and glandular stomach (abomasum) of lamb and sheep. Ester sulfates were found in all mucus glycoproteins obtained.

Properties of gastric and duodenal mucus: effect of proteolysis, disulfide reduction, bile, acid, ethanol, and hypertonicity on mucus gel structure

Small deformation oscillatory rheologic measurements have been used to investigate the structure of human and pig gastric mucus and pig duodenal mucus. All three secretions had viscoelastic properties characteristic of water-insoluble, viscoelastic gels. Mucus will flow and anneal if damaged, due to the making and breaking of its elastic structure, the measured lifetime of which was 10-120 min. Mucus reconstituted by concentration of the purified glycoprotein (pig gastric and duodenal mucus) had the same viscoelastic properties as the fresh mucus, giving evidence that the glycoprotein alone will reproduce the rheologic characteristics of the mucus. The structure of fresh mucus gel was unaffected by prolonged exposure to the following mucosal damaging agents: undiluted pig bile, 20 mM sodium taurocholate or 20 mM sodium glycocholate (all at pH 2, 6, and 8), HCl at pH 1, 2 M NaCl, and ethanol less than 40% (vol/vol). Higher concentrations of ethanol greater than 40% (vol/vol), caused dehydration and denaturation of mucus. Proteolysis by pepsin and other enzymes resulted in solubilization of the mucus gel with a complete change in the properties from an "elastic" gel to those of a "viscous" liquid. A similar collapse of mucus gel structure was observed after reduction of disulfide bonds in 0.2 M mercaptoethanol, but only after incubation for at least 50 min. This study demonstrates the stability of mucus to several mucosal damaging agents. It is proposed in vivo that although adherent gastroduodenal mucus allows penetration of these agents to the underlying mucosa, it can remain in situ and continue to protect against acid (with HCO3-) and pepsin, thus minimizing mucosal damage and maximizing repair.

Age-related changes in chemical composition and physical properties of mucus glycoproteins from rat small intestine

Mucus glycoproteins from newborn and adult rat small intestine were radiolabelled in vivo with Na2 35SO4 and isolated from mucosal homogenates by using Sepharose 4B column chromatography followed by CsCl-density-gradient centrifugation. Non-covalently bound proteins, lipids and nucleic acids were not detected in the purified glycoproteins. Amino acid, carbohydrate and sulphate compositions were similar to chemical compositions reported for other intestinal mucus glycoproteins, as were sedimentation properties. There were, however, important differences in the chemical and physical characteristics of the mucus glycoproteins from newborn and adult animals. The buoyant density in CsCl was higher for the glycoproteins from newborn rats (1.55 g/ml versus 1.47 g/ml). On sodium dodecyl sulphate/polyacrylamide/agarose-gel electrophoresis, the glycoprotein from newborn rats had a greater mobility than the adult-rat sample. Although both preparations had similar general amino acid compositions, variations were observed for individual amino acids. The total protein content was greater in the glycoprotein from newborn animals (27%, w/w, versus 18%, w/w). The molar ratio of carbohydrate to protein was less in the newborn, primarily owing to a decreased fucose and N-acetylgalactosamine content. Comparison of the molar ratio of fucose and sialic acid to galactose for both glycoproteins demonstrated a reciprocal relationship similar to that described by Dische [(1963) Ann. N.Y. Acad. Sci. 106, 259-270]. The sulphate content was greater in the glycoprotein from newborn rats (5.5%, w/w, versus 0.9%, w/w). Both had similar sedimentation coefficients in a dissociative solvent. These results suggest an age-related difference in the types of mucus glycoproteins synthesized by small intestine.

Heterogeneity in gastrointestinal mucins

Pig digestive tract mucins have often been used as model mucins for studying mucin structure, function and metabolism. In the present study pig gastric mucin and pig colonic mucin in the subunit form have been characterised and compared. Following Sepharose 4B or 2B-CL gel chromatography, the mucin eluant fractions were assayed colorimetrically by both the periodic acid-Schiff and the Alcian blue binding assays. Subunit colonic mucin eluted as a single unimodel peak that was easily detected by both assays. In contrast, subunit gastric mucin gave a peak primarily detected by periodic acid-Schiff that was overlapped by, but partially separated from, another peak primarily detected by Alcian blue. Subunit gastric mucin was separated into two periodic acid-Schiff staining spots when electrophoresed on cellulose acetate. Cetylpyridinium chloride (CPC) was able to precipitate only about half the subunit gastric mucin. The CPC-precipitable subunit gastric mucin corresponded to the faster running spot on electrophoresis, and the subunit gastric mucin in the CPC supernatant (which may have more than one subunit mucin type) to the slower spot(s). The former had a higher sulphate content and stained with Alcian blue. The latter had a lower sulphate content and showed very little Alcian blue reactivity. These results indicate that subunit pig gastric mucin is heterogeneous with respect to both size and charge. The differences between the types may be important in biological and physiochemical behaviour of gastric mucin. It seems likely that different laboratories may have worked on one or other of the pig gastric mucin types or a mixture, depending on the preparation method.

An ovarian tumour specific mucin antigen--immunohistological and biochemical studies

A new mucin antigen was detected in a mucinous cystadenoma of human ovary. An antiserum produced in rabbits against the crude cyst fluid, following appropriate absorptions, showed immunoreactivity exclusively with tumour epithelium, particularly endocervical type epithelium. The antigen responsible for the immunoreactivity was isolated in the fraction with density 1.45g/ml following density gradient ultracentrifugation in cesium chloride, which indicated that it was indeed a glycoprotein. The native glycoprotein was very large since it was excluded from Sepharose CL-2B. The complex could be dissociated following reduction with dithiothreitol. The subunits were included in Sepharose CL-2B and the position of elution would indicate a molecular weight in the order of 1-5 x 10(6). This suggests the native antigen was a complex made up of subunits held together by disulphide bonds. Amino acid analysis of the new antigen showed a resemblance with intestinal mucins, as two-thirds of the protein consists of threonine, serine, proline, alanine and glycine. This similarity in peptide core may explain the potential of the epithelium in ovarian mucinous cystadenoma to produce inappropriate intestinal mucins during the process of malignant transformation.

Mucin glycoprotein content of human pigment gallstones

Mucin glycoproteins, a secretory product of the gallbladder, are thought to contribute to the matrix or nucleus of gallstones. Human black pigment stones originate in the gallbladder and have as their major constituent calcium bilirubinate, as well as inorganic salts and small amounts of cholesterol. The object of this study was to estimate the amount of glycoprotein in black pigment stones and to isolate gallbladder mucin from dissolved stones. Black pigment stones containing 18 to 65% calcium bilirubinate were first dissolved in 12.5 mM EDTA/0.1 N NaOH and decolorized, then subjected to glycoprotein assay. The mean glycoprotein content of eight stones was 12.4%. In separate experiments, pigment stones were partially dissolved by brief exposure to EDTA/NaOH to minimize glycoprotein breakdown, and the glycoproteins isolated by gel filtration and ultracentrifugation. Pigment stones contained two glycoprotein fractions on Sepharose 4B; a high molecular weight mucin glycoprotein in the void volume and a lower molecular fraction in the included volume. Mucin was further purified by density gradient ultracentrifugation in cesium chloride. Three separate mucin fractions had an average buoyant density of 1.48 gm per ml which is typical for these glycoproteins. Bile pigment was associated with high molecular weight mucin even after extensive dialysis, gel filtration, and density gradient ultracentrifugation. The identity of mucin was further established by beta-elimination of glycoproteins in alkaline borohydride which yielded galactosaminitol from cleavage of O-glycosidic bonds. Our results indicate that mucin glycoproteins are present in significant concentrations in human black pigment stones and can be purified from stones solubilized in EDTA/NaOH. The association of bile pigment with gallbladder mucin, even after extensive purification, is consistent with the hypothesis that mucin contributes to the matrix of pigment gallstones.

Isolation and partial characterization of rat duodenal-gland (Brunner's-gland) mucus glycoprotein

A mucus glycoprotein was isolated from the duodenal glands of the rat and purified by repeated density-gradient centrifugation. The characterized glycoprotein is unique to the mucous cells of the duodenal glands and is not present in parts of the small intestine devoid of these glands. The chemical composition of the purified glycoprotein is characteristic for glycoproteins of the mucin-type. Its protein content is relatively high and amount to 35% by weight. No neuraminic acid and little sulphate (2%) is present. Evidence is presented that the native glycoprotein is built up from subunits held together via disulphide bridges in a non-glycosylated region of the protein core.

Role of salivary mucins in the protection of the oral cavity

Mucins are the principal organic constituents of mucus, the slimy visco-elastic material that coats all mucosal surfaces. Compelling evidence suggests that they play an integral role in non-immune protection of the oral cavity. Specific protective functions include: 1) protection against desiccation and environmental insult, 2) lubrication, and 3) antimicrobial effects against potential pathogens. Biosynthesis of mucin is regulated by both intrinsic ("cooperative sequential specificity") and extrinsic ("structural modulation") controls. These controls form the basis by which mucin's structure can be modified to meet a dynamically changing biological need.

A 70000-molecular-weight protein isolated from purified pig gastric mucus glycoprotein by reduction of disulphide bridges and its implication in the polymeric structure

The glycoprotein of pig gastric mucus has been isolated free of non-covalently bound protein as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and equilibrium density-gradient centrifugation. After reduction with 0.2 M-mercaptoethanol, protein was released from the glycoprotein, which consisted of a major 70000-mol.wt. component and a minor 60000-mol.wt. component. The 70000-mol.wt. protein fraction was separated from the reduced glycoprotein by either density-gradient centrifugation in CsCl or by gel filtration. Analysis of the 70000-mol.wt. protein fraction showed that, within the limits of the analysis, it was non-glycosylated, and its amino acid analysis was quite different from that of the reduced glycoprotein, which is high in serine, threonine and proline. There was a ratio of one 70000-mol.wt. protein per native glycoprotein molecule of 2 X 10(6) mol.wt. Dissociation of the native glycoprotein into glycoprotein subunits (5 X 10(5) mol.wt.) by reduction or proteolysis results in the release or hydrolysis respectively of the 70000-mol.wt. protein. A similar 70000-mol.wt. protein is demonstrated in human gastric mucus glycoprotein. A structural role for the proteins in these mucus glycoproteins is proposed.

Isolation and characterization of the native glycoprotein from pig small-intestinal mucus

Glycoprotein from pig small-intestinal mucus was isolated free of non-covalently bound protein and nucleic acid with a yield of over 60%. No non-covalently bound protein could be detected by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis or by equilibrium centrifugation in a density gradient of CsCl with 4 M-guanidinium chloride. The intrinsic viscosity and reduced viscosity of the glycoprotein preparations rose with the removal of non-covalently bound protein and nucleic acid from the glycoprotein, evidence that non-covalently bound protein does not contribute to the rheological properties of the glycoprotein in the mucus. The pure glycoprotein, in contrast with impure preparations, gelled at the same concentration of glycoprotein as that present in the gel in vivo. The glycoprotein was a single component, as judged by gel filtration and analytical ultracentrifugation. The distribution of sedimentation coefficients was polydisperse but unimodal with an s025,w of 14.5S and a molecular weight of 1.72 X 10(6). The chemical composition of the glycoprotein was 77% carbohydrate and 21% protein, 52% of which was serine, threonine and proline. The glycoprotein had a strong negative charge and contained 3.1% and 18.3% by weight ester sulphate and sialic acid respectively. The molar proportion of N-acetylgalactosamine was nearly twice that of any of the other sugars present, the glycoprotein had A and H blood-group activity and the average maximum length of the carbohydrate chains was deduced to be six to eight sugar residues.

Purification and composition of colonic epithelial mucin

Colonic mucin was purified from homogenized scrapings of rat colonic epithelial cells using gel filtration and ion-exchange chromatography. High molecular weight water-soluble mucin was separated from low molecular weight proteins by gel exclusion chromatography on Sepharose 4B, and was further separated into two major mucin fractions and several non-mucin fractions on DEAE-cellulose. Fraction IV, the major mucin, was a sulphated glycoprotein with 62% carbohydrate by weight, and high concentrations of serine and threonine. A more acidic mucin, fraction V, had similar composition. Approx. 85% of the sialic acid of fractions IV and V were removed after incubation with Clostridium perfringens neuraminidase. Blood group A but not group H activity was present in fractions III, IV, and V. Ultracentrifugation experiments showed that fraction IV migrated as a single peak, whereas fraction V contained two components. Our study indicates that colonic mucin consists of at least two closely related acidic high molecular weight glycoproteins which can be separated from non-mucin contaminants by ion-exchange chromatography.

Isolation and partial characterization of rat gastric mucous glycoprotein

Mucus glycoproteins from the rat stomach were characterized after their isolation from homogenates of the superficial gastric mucosa by equilibrium centrifugation in CsCl density gradients. Water-soluble as well as water-insoluble glycoproteins were studied. The latter were solubilized by 2-mercaptoethanol reduction of the homogenate. From both homogenate fractions the sames two glycoproteins 1 and 2 were purified, glycoprotein 1 being present in considerably higher amount than glycoprotein 2. Their respective buoyant densities in a CsCl gradient were 1.47--1.50 g/ml and 1.56--1.58 g/ml. The two glycoproteins expressed slight differences in gel electrophoresis and gel filtration. The results from column chromatographic comparisons between reduced and unreduced glycoproteins indicated strongly that both glycoproteins 1 and 2 were built from subunits kept together by S-S bonds. The s20,w values of the reduced glycoproteins 1 and 2 were 15.7 S and 11.6 S. Glycoprotein 1 contained 5% protein, 70% carbohydrate and 1--2% sulphate, whereas these percentages for glycoprotein 2 were 10% protein, 65% carbohydrate and 10% sulphate. The molar proportions of the main sugar components galactose, fucose, glucosamine and galactosamine were 4 :2 : 4 : 1 (glycoprotein 1) and 3 : 2 : 3 : 1 (glycoprotein 2). Blood-group activity A was expressed by glycoprotein 1, whereas glycoprotein 2 showed mainly blood-group activity Leb, some B activity and also some A activity, but to a lesser extent than glycoprotein 1.

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.

Initial glycosylation of proteins with acetylgalactosaminylserine linkages

Epithelial glycoprotein like that produced by the gastric surface consists of a polypeptide chain rich in serine and threonine; to these amino acid residues oligosaccharide chains of variable length are linked. The linking sugar is acetylgalactosamine. To find out whether the initial glycosylation takes place at the ribosomal level. I treated purified peptidyl-tRNA, derived from rat gastric membrane-bound polysomes, with alkali in the presence of boro[3H]hydride. Alkali specifically splits glycosidic bonds between serine or threonine and oligosaccharide side chains (beta-elimination reaction). The linking sugar is converted to an alditol and simultaneously labeled. GalNAc was identified as the linking sugar by paper chromatography. Furthermore, nascent peptides with lengths between 40 and 60 amino acid residues already contained this linking sugar. Gel filtration on Bio-Gel P-2 of 3H-labeled saccharides revealed that nascent chains contained mainly monosaccharides, but some di- or trisaccharides were found with GalNAc as the linkage sugar. These findings demonstrate that initial glycosylation of epithelial glycoprotein occurs at the ribosomal level rather shortly after the nascent peptide chain has reached the cisternal lumen of the endoplasmic reticulum.

Studies on human gastric mucosal immunoglobulin A

Immunoglobulin A (IgA) was found in mucus scraped from the surface of the human antrum. Fresh human gastric mucosa removed at operation was washed free of loosely adhering material and the gelatinous mucus lining the tissue scraped. The scrapings were separated by gel filtration on Sephadex G-200 and on Sepharose 4B into two carbohydrate-containing fractions. One of these fractions was shown by immunodiffusion to contain IgA which differs from human colostral secretory IgA by being devoid of secretory component activity. Moreover, secretory component was not detected in our unfractionated gastric mucosal scrapings. It is concluded that, contrary to the general belief, the predominant immunoglobulin A of human gastric mucus is not associated with the secretory component. Our results do not exclude the possibility that, as in serum, small amounts of secretory IgA and of the secretory component may be present in gastric secretions, however if so, the levels of these compounds would fall below the level of sensitivity of our methods.

The isolation and characterization of the high-molecular-weight glycoprotein from pig colonic mucus

1. A high-molecular-weight glycoprotein constitutes over 80% by weight of the total glycoprotein from water-soluble pig colonic mucus. 2. It was isolated from from nucleic acid and non-covalently bound protein by nuclease digestion followed by equilibrium centrifugation in a CsCl gradient. 3. The glycoprotein has the following composition by weight: fucose 10.4%; glucosamine 23.9%; galactosamine 8.3%; sialic acid 9.9%; galactose 20.8%; sulphate 3.0%; protein 13.3%; moisture about 10%. 4. The native glycoprotein has the high mol.wt. of 15 X 10(6). 5. Reduction of the native glycoprotein with 2-mercaptoethanol results in a glycoprotein of mol.wt. 6 X 10(6). 6. Pronase digestion removes 29% of the protein (3% of the glycoprotein) but none of the carbohydrate. 7. The molecular weight of the Pronase-digested glycoprotein is 1.5 X 10(6), which is halved to 0.76 X 10(6) on reduction with 2-mercaptoethanol. 8. The contribution of non-covalent interactions, disulphide bridges and the non-glycosylated peptide core to the quaternary structure of the glycoprotein are discussed and compared with the known structure of pig gastric glycoportein.

The separation and characterization of bronchial glycoproteins by density-gradient methods

1. Sputum samples from a total of 18 asthmatic and chronic bronchitic patients were examined by analytical density-gradient ultracentrifugation. CsBr was used as the dispersal agent and dense electrolyte. 2. The patterns show two main groups of components, banding at about 1.3g/ml and 1.5g/ml; in addition, a few samples showed a further zone at approx. 1.65g/ml. These components were identified as protein, secretory glycoprotein and DNA respectively. The glycoprotein zone was frequently hypersharp, and usually contained two or more partially resolved bands; it was always well resolved from the protein. 3. The glycoprotein components were isolated from nine representative sputum samples by density-gradient ultracentrifugation on a preparative scale. Analytical density-gradient ultracentrifugation was used to monitor the efficiency of the separations. 4. Some sputum samples separated cleanly under these conditions, the glycoprotein being essentially devoid of free protein; in others, separation was apparently incomplete, although computer simulation indicated that the conditions were adequate to ensure separation. Further density-gradient separations in CsCl were necessary with several samples before satisfactory products were obtained; mixtures of CsCl with guanidinium chloride were no more effective than CsCl alone. The reluctance to separate indicates a very strong, but non-covalent, interaction between protein and glycoprotein, probably associated with the gelatinous character of the secretion. 5. The purified glycoprotein components were characterized analytically and physicochemically. They contained N-acetylgalactosamine, N-acetylglucosamine, galactose, fucose and N-acetylneuraminic acid, and had an amino acid composition in which serine, threonine and proline predominated; however, aspartic acid, glutamic acid and cystine were also appreciable. The glycoproteins were of very high molecular weight, and usually showed more than one component in sedimentation velocity; their distribution in a density gradient indicated a substantial, but largely monotonic, density heterogeneity. 6. Thiol reduction decreased the molecular weight very substantially, but the products were relatively more homogeneous than the native materials. The amino acid composition was changed significantly and a small and variable proportion of protein or peptide was liberated. It is concluded that the native materials are disulphide-linked aggregates, probably through a cross-linking peptide, in confirmation of earlier studies.

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.

Glycoprotein synthesis in gastric epithelial cells of the rat. Properties of microsomal glycoprotein glycosyltransferases

1. Optimal assay conditions were determined for a microsomal glycoprotein galactosyl- and fucosyltransferase derived from gastric epithelial scrapings with both exogenous and endogenous acceptor glycoprotein. 2. Subcellular fractionation of the homogenate yielded microsomal fractions enriched in glycosyltransferases. 3. The effect of feeding on galactosyltransferase activity per cell was examined. 4. Endogenous acceptor molecules were identified as glycoproteins after labeling by means of UDP-[3H]galactose in the cell-free system.

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

1. Equilibrium density-gradient ultracentrifugation in caesium salts was used in two stages in the isolation and subfractionation of the glycoprotein component from a human ovarian-cyst fluid. The eight main subfractions thus obtained were the subject of detailed physicochemical characterization. 2. The fractions were unimodal in buoyant-density distribution, but had discrete rho(0) values ranging from 1.31 to 1.35. 3. Weight-average molecular weights and sedimentation coefficients decreased regularly with decreasing density of the fraction, whereas the partial specific volumes and selective solvation parameters increased. The latter behaviour correlates well with the increasing peptide content of the lighter fractions. 4. The fractions exhibited a range of analytical composition, although all were within the limits previously observed for blood-group substances of Le(a) specificity. All fractions had approximately equal Le(a) activity. The peptide content varied systematically from 7% for the densest fraction to 15% for the lightest, but the relative distributions of the amino acids remained essentially constant throughout the series. In particular, serine plus threonine plus proline made up about 50% of the peptide content of all the fractions. Fucose, galactose and N-acetylglucosamine contents decreased with increasing peptide content of the fractions, but N-acetylgalactosamine and sialic acid exhibited the opposite trend. Molar ratios of N-acetylgalactosamine to the sum of serine and threonine remained essentially constant at 0.8-0.9, implying a high degree of glycosylation of all the molecules, but the ratio of N-acetylglucosamine to N-acetylgalactosamine decreased steadily with increasing peptide content, suggesting the presence of oligosaccharide side chains of various lengths. The results are discussed in terms of the accepted structure of glycoprotein molecules. 5. Experiments on the glycoproteins extracted with phenol from the same cyst fluid have confirmed that equilibrium centrifugation in caesium salts does not remove any non-covalently bound protein nor cause any changes in the tertiary structures of these glycoprotein molecules.