Isolation and partial characterization of rat gastric mucous glycoprotein

Effectiveness of electroacupuncture at Zusanli (ST36) on the immunohistochemical density of enteroendocrine cells related to gastrointestinal function

The purpose of this study was to examine the effects of electroacupuncture at Zusanli on the immunohistochemical density of enteroendocrine cells related to gastrointestinal function. The authors investigated the histochemical changes of mucous substances and immunohistochemical density of gastrin, serotonin, calcitonin gene-related peptide (CGRP), insulin, and pancreatic polypeptide (PP) secreting cells in rats. Staining density of mucous substances and the enteroendocrine cells of the gastrointestinal tract was observed with histochemical and immunohistochemical methods. Stainless steel needles with a diameter of 0.25 mm were inserted into Zusanli (St36, 5mm below the head of the fibula under the knee joint, and 2mm lateral to the anterior tubercle of the tibia) and connected to an electrical stimulator. The electroacupuncture (EA) stimulation was delivered for 30 minutes at 10 mA, 2 Hz in EA stimulation (2EA group) or 4 Hz in EA stimulation (4EA group) in each experimental group. In 4EA stimulation at the Zusanli, staining density of Alcian blue-periodic acid-Schiff on mucous substances of the stomach body was stronger than those of the 2EA and control groups. Periodic acid-Schiff staining density of pyloric mucosa in the 4EA group was stronger than that of the 2EA and control groups. The immunohistochemical staining density of gastrin, serotonin, and CGRP-secreting cells of pylorus in the 2EA and 4EA groups was stronger than that of the control group. Immunohistochemical staining density of insulin and PP secreting cells of islets of the pancreas in the 2EA and 4EA groups was stronger than that of the control group. These results suggest that EA stimulus at St36 has the potential to influence gastric mucous substances and enteroendocrine cells (gastrin, serotonin, CGRP, insulin, and PP) that subsequently modulate digestive functions.

Nitric oxide synthase activity in rat gastric mucosa contributes to mucin synthesis elicited by calcitonin gene-related peptide

The majority of research for the calcitonin gene-related peptide (CGRP) in the stomach has been devoted to the submucosal blood flow, and only slight attention has been paid to its involvement in the gastric epithelial function. In this study, we examined the age-related change in the CGRP-containing nerves and its effects on the mucus metabolism. We compared the immunoreactivity for CGRP in the gastric mucosa of 7-week-old rats (young) to that of 52-week-old animals (middle-aged). The effects of CGRP on the mucin biosynthesis were compared using the stomachs from both young and middle-aged rats. The nitric oxide synthase (NOS) activity was measured in the surface and deep mucosa of the gastric corpus. The density of the CGRP nerve fibers was reduced in both the lamina propria and submucosa of the middle-aged rats compared to the young rats. CGRP stimulated the mucin biosynthesis in the cultured corpus mucosa from the 7-week-old rats, but not from the 52-week-old rats. The total NOS activity of the surface layer in the corpus mucosa was markedly reduced in the middle-aged rats compared to the young rats. These findings demonstrate the age-dependent reduction in the CGRP-induced mucin biosynthesis, as well as in the density of the CGRP fibers in the rat stomach. The decreased NOS activity in the surface layer of the oxyntic mucosa in the aged rats may also be a principal cause for the lack of regulation of the mucin biosynthesis by CGRP.

Effects of histamine on mucin biosynthesis in rat gastric mucosa

Mucin biosynthesis is stimulated by gastrin during the process of glycosylation in the corpus mucosa of the rat stomach. The purpose of this study was to clarify, using an organ culture technique, whether biosynthetic responses to histamine in the rat gastric mucin are the same as that to gastrin. Radiolabeled mucin was obtained from the corpus and antral mucosa of the rat stomach after in vitro incubation for 5 h with [3H]glucosamine (GlcN), [14C]threonine (Thr), and [35S]sulfate. Addition of histamine (10(-7)-10(-5) M) to the culture medium increased [3H]GlcN-labeled mucin in the corpus tissue in a concentration-dependent manner. In the antrum, there was no significant change in the biosynthetic activity of mucin in response to histamine. Histamine at 10(-5) M also increased the incorporation of both [35S]sulfate and [14C]Thr into the corpus mucin. These results indicate that histamine stimulates the biosynthesis of the mucin peptide, as well as the glycosylation step in the corpus, and suggest that the effect of histamine on mucin synthesis is distinct from that of gastrin.

Distinct effects of tetragastrin, histamine, and CCh on rat gastric mucin synthesis and contribution of NO

Although gastrin, histamine, and carbachol (CCh) accelerate gastric mucin metabolism, information about their target cells of mucin production is lacking. To clarify this, we examined the effects of these stimulants, including the possible participation of nitric oxide (NO), on mucin biosynthesis in distinct sites and layers of rat gastric mucosa. Pieces of tissue obtained from the corpus and antrum were incubated in a medium containing radioactive precursors and each stimulant, with or without NO synthase (NOS) inhibitor. Distribution of NOS was compared with that of the specific mucins by immunostaining using specific antiserum and monoclonal antibodies. In the full-thickness corpus mucosa, tetragastrin enhanced [3H]glucosamine incorporation into mucin but had no effect on [14C]threonine incorporation. Both histamine and CCh dose dependently increased3H- and14C-labeled corpus mucin. Only CCh stimulated antral mucin biosynthesis. CCh stimulation was noted in the corpus mucosa after removal of surface mucous cells, but stimulation by tetragastrin or histamine disappeared as a result of this pretreatment. Only tetragastrin-induced activation was completely blocked by the NOS inhibitor. NOS immunoreactivity was limited to surface mucous cells. Mucus-producing cells present in the different sites and layers of the gastric mucosa have distinct mechanisms for regulation of mucin biosynthesis. Gastrin-stimulated mucin biosynthesis mediated by NO is limited to surface mucous cells of rat gastric oxyntic mucosa.

Possible role of hepatic bile mucus glycoprotein in development of intrahepatic gallstones

To study the role played by hepatic bile mucus glycoprotein in the development of hepatolithiasis, mucus glycoprotein, isolated from the bile of patients with intrahepatic gallstones by gel filtration and ultracentrifugation, was examined for precipitability in control hepatic bile obtained postoperatively from patients successfully treated for cholecysto- and/or choledocho-lithiasis. When the mucus glycoprotein was incubated at 38 degrees C for 48 h in the control hepatic bile in the presence of calcium ions, massive precipitation was produced. The precipitation was inhibited by treating the mucus glycoprotein with acid, alkali, a reducing reagent, or protease, the inhibition being most effective with acid, which splits up carbohydrate chains. This suggests that the precipitability of the mucus glycoprotein resides mainly in its carbohydrate chains. These observations imply that the development of intrahepatic gallstones, calcium bilirubinate stones in particular, could be prevented by degrading mucus glycoprotein in hepatic bile.

Co-purification of gastric mucoproteins with DNA: an explanation for the reported 'interaction' of omeprazole with DNA in rat tissues

Recently, Phillips et al. reported that small amounts of radioactivity derived from [14C]omeprazole were 'associated' with DNA purified from gastrointestinal tissues of treated rats (Mutagenesis 7, 277-283, 1992). We hypothesized that this radioactivity arose from omeprazole bound to contaminating protein in the DNA fraction (Mutagenesis 7, 395-396, 1992). Using rats injected with 35S-labeled amino acids, we found significant protein contamination (0.06 microgram of protein per microgram of DNA) in DNA purified from gastrointestinal tissues. Gastric mucous proteins represent likely candidates for binding of omeprazole in the rat model used by Phillips et al. To investigate this, we partially purified proteins from gastric mucus, incubated them with [14C]omeprazole, and then added these radiolabeled mucoproteins to homogenates of rat colon and duodenum before starting the DNA purification. Detectable amounts of the added mucoproteins remained in the DNA fraction, but none of the control protein, bovine serum albumin, remained with the DNA. Further characterization of the mucoproteins by hydroxyapatite chromatography indicated that a certain population of these proteins survived the DNA purification procedures. These data indicate that the association of omeprazole with DNA reported by Phillips et al. most probably is explained by binding of omeprazole to mucous glycoproteins (or other proteins present in the GI tract) that selectively survive DNA purification protocols.

Stimulation of mucus glycoprotein biosynthesis in rat gastric mucosa by gastrin

We examined the effects of the gastrin family of peptides on gastric mucus glycoprotein (mucin) biosynthesis in rat gastric mucosa using an organ culture technique. Radiolabeled mucin was obtained from the tissue and culture medium of the corpus and antrum of rat stomach incubated for 5 hr with [3H]glucosamine (GlcN), [14C]threonine (Thr), and [35S]sulfate in vitro. With the addition of 10(-8) and 10(-7) M tetragastrin to the culture medium, [3H]GlcN labeled mucin in the corpus tissue increased to 120-135% that of the control (P < 0.01). The biosynthetic responses to cholecystokinin (CCK)-8 and the 17-peptide gastrin were essentially the same as that to tetragastrin. Tetragastrin 10(-8) M also increased the incorporation of [35S]sulfate into the corpus mucin but failed to change [14C]Thr incorporation. In the antrum, biosynthetic activity showed no significant change with 10(-9) approximately 10(-5) M tetragastrin. Ranitidine, diphenhydramine and omeprazole at 10(-5) M did not suppress the tetragastrin-induced increase in [3H]GlcN incorporation into mucin, but L-365,260 at a concentration of 10(-6) M completely blocked this effect. These results suggest that gastrin stimulates mucin production via CCK-B/gastrin receptors in the oxyntic region of rat gastric mucosa.

Determination of fermentable carbohydrate from the upper gastrointestinal tract by using colectomized rats

The primary aim of this study was to characterize the carbohydrate that would be supplied to the colon for fermentation under physiological conditions. Colectomized rats were fed fiber-free diets or diets containing 5% (wt/wt) gum arabic. Four (fucose, galactose, glucosamine, and galactosamine) of 11 analyzed sugars accounted for 77% of the total sugar in ileal excreta from colectomized rats fed fiber-free diets. The three sugars in gum arabic, rhamnose, arabinose, and galactose, accounted for 84% of the total sugars in gum arabic ileal excreta. Comparisons of the sugar compositions of the ileal excreta, the water-soluble fractions of the excreta, and three gel filtration fractions of the water-soluble material with those of the water-soluble fraction of rat mucosa, the acetone-soluble fraction of pancreas, and pancreatin suggested that the major source of endogenous carbohydrate is mucin. Gum arabic increased the daily excretion of the four mucin-derived sugars (fucose, galactose, glucosamine, and galactosamine) by the colectomized rats from 473 mumol per day to 634 mumol per day. We conclude that mucin is the major endogenous carbohydrate excreted from the upper gut and that gum arabic increases the amount of this endogenous carbohydrate.

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

Types of oligosaccharide sulphation, depending on mucus glycoprotein source, corpus or antral, in rat stomach

Radiolabelled mucus glycoprotein was obtained from tissue and a culture medium each of the corpus and antrum of rat stomach incubated with [35S]sulphate in vitro. Gel-filtration analysis of oligosaccharides liberated by alkaline-borohydride treatment from glycoproteins indicated that 35S-labelled oligosaccharides from the corpus vary considerably with respect to chain length whereas those from antral mucus glycoprotein are composed of small oligosaccharides. Examination of the reduced radiolabelled products obtained by HNO2 cleavage of the hydrazine-treated oligosaccharides indicated sulphate esters of N-acetylglucosamine to be present at three locations on a carbohydrate unit: [35S]sulphated monosaccharide (2,5-anhydromannitol 6-sulphate), [35S]sulphated disaccharide [galactosyl(beta 1-4)-2,5-anhydromannitol 6-sulphate] and [35S]sulphated trisaccharide [fucosyl(alpha 1-2)-galactosyl(beta 1-4)-2,5-anhydromannitol 6-sulphate]. Sulphated disaccharide and trisaccharide, possibly originating from the N-acetyl-lactosamine and fucosyl-N-acetyl-lactosamine sequences respectively, were detected in the corpus, especially as large oligosaccharides, but were present in the antrum in only very small amounts. The sulphated monosaccharide, however, most probably originating from 6-sulphated N-acetylglucosamine residues at non-reducing termini, was present in all oligosaccharide fractions in both the corpus and antrum.

Isolation and structural analysis of rat gastric mucus glycoprotein suggests a homogeneous protein backbone

We isolated monomeric gastric mucus glycoprotein from the rat stomach by applying three successive CsCl-density-gradient steps in the continuous presence of guanidinium chloride. The rat gastric mucin was pure as compared with mucin isolated without the chaotropic reagent. In addition, the presence of guanidinium chloride resulted in a better preservation of the protein moiety. The purified mucin was fractionated according to buoyant density and chemically radiolabelled on tyrosine or cysteine residues and digested with specific proteinases. Analysis of mucin fractions of various densities gave identical peptide patterns, suggesting that the fractions contain a common protein backbone. Electron-microscopic images of the individual mucin molecules were recorded using rotary shadowing. They showed large filamentous molecules with a mean length of 208 nm that, after proteolytic digestion, yielded glycopeptides with a mean length of 149 nm. Heterogeneity in buoyant density and electrophoretic mobility is located in this large glycopeptide which remains after proteolytic digestion. Metabolic labelling of the mucin with [35 S]sulphate and [3H]galactose, followed by purification and proteolytic digestion, revealed that this glycopeptide accounts for most of the mass and contains relatively little protein, but probably all the oligosaccharides and sulphate. As this protein part is masked by the oligosaccharides, detailed study by the methods described was not possible. The results indicate that rat gastric mucin is homogeneous in a major part of the protein backbone and that the heterogeneity of the molecule originates most likely from differences in sulphate and/or sugar composition.

Sulphation causes heterogeneity of gastric mucins

The synthesis of mucus glycoprotein in rat stomach was studied in stomach segments, which were pulse-labelled with both [3H]galactose and [35S]sulphate and chased for various times. The radioactive glycoproteins were analyzed by CsCl centrifugation and by agarose gel electrophoresis. After a pulse-labelling for 15 min with [3H]galactose, a possible intermediate with an Mr of 200,000 and a buoyant density of 1.60 g/ml could be demonstrated. Following chase periods of 1 and 4 h, [3H]galactose and [35S]sulphate were present in glycoproteins with a mean buoyant density of 1.50 g/ml. This is clearly different from the main density of glycoproteins isolated from mucosal scrapings (1.46 g/ml). Another difference is the high electrophoretic mobility on gel electrophoretic analysis of newly synthesized glycoproteins compared to that of the major portion of the glycoproteins from mucosal scrapings. When sulphation of glycoproteins was inhibited by sodium chlorate, electrophoretic mobility and buoyant density both decreased. Sodium chlorate had no effect on glycoprotein synthesis nor on glycoprotein secretion. We conclude from our data that the heterogeneity in electrophoretic mobility and buoyant density can be attributed to a different degree of sulphation of the same glycoprotein.

Effects of endogenous and exogenous prostaglandins on glycoprotein synthesis and secretion in isolated rabbit gastric mucosa

We studied gastric glycoprotein synthesis and secretion in organ culture before and during cyclooxygenase inhibition and replacement with exogenous prostaglandins (16,16-dimethyl prostaglandin E2 and prostaglandin F2 alpha). Isolated rabbit antral and fundic mucosal explants incorporated [14C]N-acetylglucosamine and [3H]leucine in a linear fashion and steadily secreted labeled proteins and glycoproteins during the 24-h incubation period. On sepharose 4B, greater than 90% of the secreted protein-bound [14C]N-acetylglucosamine was found in the high molecular weight peak. Incorporation of tracer was not influenced by cyclooxygenase inhibition with indomethacin or the addition of exogenous prostaglandins. Secretion of newly formed glycoprotein, however, was significantly inhibited by indomethacin and stimulated by both tested prostaglandins in a concentration-dependent manner. 16,16-Dimethyl prostaglandin E2 caused significant stimulation in concentrations that are well in the physiologic range for endogenous prostaglandin E2, whereas prostaglandin F2 alpha stimulated in 100 times higher concentrations. We conclude that in the isolated gastric mucosa both endogenous and exogenous prostaglandins stimulate mucus secretion. For prostaglandin E2, but not prostaglandin F2 alpha, a role in the physiologic regulation of gastric mucus secretion is probable.

Effect of biosynthetic human epidermal growth factor on the synthesis and secretion of mucin glycoprotein from primary culture of rabbit fundal mucosa cells

Biosynthetic human epidermal growth factor (Bh-EGF) induced dose-dependent synthesis and secretion of neutral mucin glycoprotein when the fundal cells isolated from rabbit stomach were cultured in serum-free medium containing Bh-EGF at concentrations as high as 10 to 100 ng/ml. At these high concentrations, Bh-EGF had no effect on the cell growth. In marked contrast, much lower concentrations from 0.1 to 1.0 ng/ml of Bh-EGF failed to stimulate mucin synthesis, but enhanced proliferation of the cells. Electrophoretic pattern of the mucin secreted from the cultured mucosal cells was very similar to that of the authentic mucin obtained from rabbit stomach. Maximal secretion of the mucin from the cells was observed at Hour 96 of the culture. Although fetal bovine serum (5%) and insulin (0.5 microgram/ml) also stimulated the mucosal cells, both in growth and in mucin synthesis and release, the enhancing activity of the mucin synthesized and released by Bh-EGF at a concentration of 100 ng/ml per microgram DNA of cultured cells was far superior to that of 5% fetal bovine serum and 0.5 microgram/ml insulin.

Effect of 16,16-dimethyl prostaglandin E2 on mucus glycoprotein biosynthesis in rat stomach and duodenal glands

The present study describes the effect of 16,16-dimethyl prostaglandin E2 (16,16-dmPGE2) on mucus glycoprotein biosynthesis in rat stomach and duodenal glands. After in vivo treatment with 16,16-dmPGE2 (10 micrograms/kg subcutaneously) for 1 h, the incorporation rate of [3H]galactose, [3H]glucosamine, and [3H]serine in the ex vivo vascularly perfused stomach was determined by light microscopic autoradiography. As was previously found by us for the surface mucous cells in the fundus of 16,16-dmPGE2-treated rats, the incorporation of [3H]galactose and [3H]glucosamine (indicative of mucus glycoprotein synthesis) in the isthmus was increased two- to fourfold. Small if any increases were detected in the mucous cells near the base of the glands of the fundus (neck cells), the mucous cells in the antrum and the mucous cells in the duodenal glands. Total protein synthesis as measured by [3H]serine incorporation was not increased in any of these cells. We conclude that 16,16-dmPGE2 has different effects on mucus glycoprotein biosynthesis in various regions of the rat stomach. Increased biosynthesis in the fundus points to a role for mucus in the prostaglandin-induced protection of the gastric mucosa against injury.

Molecular weight of gastric mucus glycoprotein is a determinant of the degree of subsequent aspirin induced chronic gastric ulceration in the rat

Mucus was sampled from the gastric mucosal surface of anaesthetised rats. Three weeks later these rats were orally dosed each day with aspirin (375 mg/kg) for six months. Then the number and size of the aspirin induced chronic gastric ulcers were assessed. Gel filtration chromatography of the mucus samples showed that mucus glycoprotein was present in both high and low molecular weight forms. There was a natural variation between individual rats in the percentage of glycoprotein in the high molecular weight form (mean = 58.9%; SD = 9.6%; n = 23). This variation correlated strongly with the degree of subsequent aspirin induced chronic gastric ulceration (r = -0.85, p less than 0.001). This is the first time that a pre-existent variability in a mucosal defence factor has been shown to predict susceptibility of the stomach to chronic ulceration.

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.

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.

Comparative study on mucus glycoproteins in rat stomach and duodenum

The density of mucus glycoprotein compared to that of the corpus, antrum and duodenum was; 1.52, 1.49 and 1.57 g/ml respectively. Carbohydrate composition of gastrointestinal mucus glycoprotein consisted of N-acetylgalactosamine, N-acetylglucosamine, galactose, fucose and sialic acid. Ratios of carbohydrate composition among corpus, antral and duodenal mucus glycoproteins differed. The average length of an oligosaccharide was found to be about 12-13, 14 and 10 sugars in the corpus, antrum and duodenum, respectively. In the corpus, the amino acid content was found to have the following quantitative order: Thr greater than Ser greater than Glx = Pro; in the antrum: Thr greater than Ser greater than Glx; and in the duodenum: Thr greater than Ser greater than Pro. Corpus, antral and duodenal mucus glycoproteins have the blood-group A antigen; antral mucus glycoprotein in particular exhibited strong blood-group A activity.

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.

Human duodenal gland (Brunner's gland) mucus glycoprotein analysis

A mucus glycoprotein of the duodenal gland is characterized. The glycoprotein was isolated from a water-soluble homogenate fraction of the submucosal tissue of the most proximal part of the small intestine, containing the duodenal gland, and was purified from contaminating protein by two sequential equilibrium-centrifugation steps in CsCl density gradients. Structural analysis of the purified glycoprotein showed two regions in the protein core: one part characterized by the presence of essentially all of the cysteine residues and another by the presence of most of the serine and threonine. Carbohydrate was found linked to the latter part. Rat (H. L. Smits, P. J. M. van Kerkhof, and M. F. Kramer (1982) Biochem. J. 203, 779-785.) and human duodenal gland mucus glycoprotein show homology in chemical composition. Both glycoproteins have a relatively high protein content and contain little sulfate and no neuraminic acid. In man the mucus glycoprotein, however, has a higher content of serine plus threonine, a lower content of N-acetylglucosamine, a slightly higher content of fucose, and a lower molar ratio of N-acetylgalactosamine relative to serine plus threonine.

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.

Terminal alpha (1 leads to 4)-linked N-acetylglucosamine: a characteristic constituent of duodenal-gland mucous glycoproteins in rat and pig. A high-resolution 1H-NMR study

The structure of the carbohydrate chains of mucous glycoproteins from the gastro-intestinal tract was examined for species- and tissue-specificity. To this purpose, oligosaccharides were released from purified glycoprotein preparations of rat and pig gastric, duodenal-gland and small-intestinal mucus, by alkaline borohydride reductive cleavage. Based on the results of 500-MHz 1H-NMR spectroscopy and of sugar analysis of the total oligosaccharide fractions, terminal GlcNAc, alpha (1 leads to 4)-linked to galactose, appears to be a characteristic constituent of duodenal-gland oligosaccharides. Similarly, NeuAc in alpha (2 leads to 3)-linkage to galactose turns out to be a typical constituent of small-intestinal mucous glycoproteins. In general, glycoproteins from gastric mucus possess larger and more-branched carbohydrate chains than those from duodenal-gland and small-intestinal mucus. Comparing rat and pig, oligosaccharide structures for corresponding tissues are less complex for the former. After fractionation, the rat duodenal-gland oligosaccharides could be characterized by application of 1H-NMR spectroscopy as being branched tetra- up to hexa-saccharide chains, all sharing the italicized trisaccharide element. The chains exhibit microheterogeneity as to the termination by fucose in alpha (1 leads to 2)- or by GlcNAc in alpha (1 leads to 4)-linkage to galactose. The following structures can be proposed for the most abundant rat duodenal-gland oligosaccharides: (table; see text).

The site of sulfated glycoprotein biosynthesis in rat gastric mucosa

The site of the synthesis of the sulfated glycoproteins in rat gastric mucosa using organ culture system has been studied. About 98% of the 35S radioactivity associated with the glycoproteins was found in corpus region. The sulfated glycoproteins were synthesized mainly in corpus, poorly in antrum and none in forestomach. The synthesized sulfated glycoproteins were heterogeneous by equilibrium centrifugation in CsCl density gradients. The 35S-labeled oligosaccharides released from the glycoproteins under alkaline condition were composed of 4-8 sugar residues.

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.

Distribution of mucosal macromolecular glycoproteins in rat stomach

1. The mucosal macromolecular glycoproteins were extracted from forestomach, corpus and antrum region of rat whose weight ratio was 2:5:1, respectively. 2. The glycoproteins were fractionated on Bio-Gel A-1.5 m. 96% of the glycoproteins was localized in glandular stomach. 3. The carbohydrate of the glycoproteins composed of N-acetylgalactosamine, N-acetylglucosamine, galactose, fucose and sialic acid in the proportions 1.0:2.7:3.2:1.0:0.14 for corpus, while 1.0:1.4:1.5:0.7:0.04 for antrum.

Ultrastructural visualization of galactose in the glycoprotein of gastric surface cells with a peanut lectin conjugate

The ultrastructural localization of peanut lectin-binding sites in the gastric surface epithelial cell has been studied using a horseradish peroxidase-labelled peanut lectin (PL-HRP) conjugate and other cytochemical techniques. The PL-HRP procedure has visualized glycoprotein with presumed terminal galactose residues in the apical plasmalemma and secreted mucins and has localized such glycoprotein selectively in the 'intermediate Golgi cisternae' situated between the saccules of the maturing face and those of the forming face of the Golgi stacks. Other cytoplasmic organelles, including the forming and stored secretory granules, did not reveal glycoprotein with terminal galactosyl residues. These results demonstrate the applicability of the PL-HRP labelling technique at the electron microscopical level to localize not only extra but also intracellular peanut lectin-reactive sites. The observation afford information concerning the possible site in the Golgi apparatus where galactose residues are added to the growing oligosaccharide side-chain of mucous glycoprotein.

Glycoprotein synthesis in the mucous cells of the vascularly perfused rat stomach. III. Mucous cells of the antrum and the duodenal glands

Labeled leucine, serine, galactose, glucosamine, fucose, and sulfate were administered to rat stomachs in a vascular perfusion system. Sections of gastric fundus and antrum, and of the duodenal glands, were studied by light-microscopic autoradiography. The rate of incorporation of radioactive label in the various mucous cell types of the antrum and of the duodenal glands was measured by counting the silver grains over each cell type and comparing this rate with that of the surface mucous cells of the fundus. The following major observations were made: The patterns of incorporation in the mucous cells on the surface and in the pits of the antrum did not differ essentially from those of the surface mucous cells of the fundus. On the contrary, the incorporation patterns of the surface mucous cells of the fundus differed considerably from those of the mucous cells of the antral glands and from the mucous cells of the duodenal glands. The mucous cells of the antral glands showed a lower incorporation rate of amino acids and of (amino) sugars, especially of galactose and glucosamine, but a higher incorporation rate of sulfate. The mucous cells of the duodenal glands showed a considerably higher rate of incorporation for serine, but a lower rate for glucosamine. No differences were seen for leucine, galactose, and sulfate. The relatively low synthetic activity of the mucous cells of antral glands correlates well with the various amounts of RER, Golgi system, and secretory granules, found in these cells, and with the histochemical staining characteristics of their mucous granules. The mucous cells of the duodenal glands are characterized by typical parallel arrays of their numerous RER cisternae and by a low number of small secretory granules.

Glycoprotein synthesis in the mucous cells of the vascularly perfused rat stomach. II. Differentiating mucous cells

Labeled leucine, serine, galactose, glucosamine and sulphate were administered to rat stomachs in a perfusion system. Sections of the gastric fundus were studied by light microscopic autoradiography. Five categories of mucous cells were distinguished and their glycoprotein synthetic activity was measured in autoradiographs by counting silver grains over each category. During their differentiation, while migrating from the isthmus of the fundic glands to the free luminal surface, the surface mucous cells (SMC) showed an increase in incorporation of all precursors used. Differences between the incorporation patterns of the various precursors, in cells of different ages, suggest that structural development runs ahead of functional activity, and that the latter continues up to the very moment the cell is shed from the surface. Sulphate was incorporated at a considerably lower rate by the SMC of the free surface than by the foveolar SMC, in which by cytochemical staining strongly acidic glycoproteins were shown. Since the mucous neck cells incorporated all precursors at a low rate, these cells apparently do not play an important role in gastric mucus synthesis. They did not incorporate sulphate, which is consistent with histochemical observations.