Glycoprotein transport in the surface mucous cells of the rat stomach

Site of synthesis, intracellular transport and secretion of glycoprotein in exocrine cells

The site of attachment of the first sugar, N-acetylgalactosamine, to the seryl and threonyl residues of the protein chain is unknown in exocrine cells. The subsequent sugars of the carbohydrate side chains, galactose and N-acetylglucosamine alternately, and the end-group sugars, galactose, N-acetylgalactosamine and fucose, are attached in the Golgi complex. Sulphate too is attached in that structure. In the stomach, sulphate is probably transferred in the most mature cisterna of the Golgi stacks, galactose and fucose in other cisternae, suggesting a gradient in transferase activities along the stack. The possibilities of regulating the amount and relative sugar composition of the glycoproteins are discussed. The secretory product is stored in granules. Their polygonal, large and swollen appearance and complex formation by loss of bordering membranes, as observed in many kinds of glycoprotein-secreting cells ('mucous cells') might be caused by ineffective fixation techniques. Direct vascular perfusion results in a picture no different from what is found in non-mucous cells. Whether secretion is merely exocytotic, as in non-mucous cells, or whether it is accompanied by a loss of membrane and even cytoplasm needs thorough investigation, with the effects of various fixation techniques being compared.

Histological effect of (R)-alpha-methylhistamine on ethanol damage in rat gastric mucosa: influence on mucus production

(R)-alpha-Methylhistamine, a selective agonist of histamine H3 receptors, prevents macroscopically visible gastric lesions by absolute ethanol in the rat. A further insight into its activity was the aim of our study. Rats were given saline or (R)-alpha-methylhistamine (100 mg/kg) intragastrically. After 30 min, absolute ethanol was given and gastric mucosa was sampled 60 min later. Histologic damage and intracellular and adherent mucus were quantified. Luminal surface and mucous cells were examined by scanning and transmission electron microscopy. (R)-alpha-Methylhistamine reduced the extent of lesions by ethanol from 96 to 18%. Surface mucous cells and mucous neck cells were increased in volume and number, packaging of intracellular mucus was modified, and the secretory processes were promoted by (R)-alpha-methylhistamine itself, although these modifications were mostly evident in stomachs subsequently exposed to ethanol. Adherent mucus layer thickness was increased by (R)-alpha-methylhistamine only after ethanol exposure. It is concluded that (R)-alpha-methylhistamine predisposes mucous cells to react to ethanol.

Sulphated macromolecules produced by in vivo labelling in the rat gastric mucosa

The aim of this study was to investigate the nature and distribution of sulphated macromolecules of the extracellular matrix in rat gastric mucosa. This was achieved by developing an in vivo labelling system. An intraperitoneal injection of 1 mCi [35S]-sulphate was given for either 4 h (0.01% incorporation into macromolecular fraction) or 8 h (0.13% incorporation). At the end of the labelling period the stomach was removed and the mucosa and submucosa was either taken as a single combined sample or separated into four layers by blunt dissection. Each sample was papain digested and analysed by ion-exchange chromatography. This analysis revealed sulphated species of differing charge existing in differing proportions throughout the mucosa. These sulphated species eluted at NaCl concentrations of approximately 0 (A), 0.19 (B), 0.34 (C) and 0.78 mol/L (D) from a Q-Sepharose ion exchange column. Further analysis by size exclusion chromatography and chemical and enzymatic digestion showed that peaks B and C had molecular weights of 2.4 x 10(5) and 2.8 x 10(5), respectively and were resistant to chondroitinase ABC, heparitinase and nitrous acid digestion. Peak D was found to contain a polydisperse population of molecules with a molecular weight range of approximately 1 x 10(4) to 6 x 10(4). This sample was susceptible to nitrous acid and chondroitinase ABC digestion and was found predominantly in the sample isolated from deeper in the tissue. We have thus developed an in vivo labelling technique for sulphated macromolecules that can be used in the further study of injury to the gastric mucosa.

Dynamics of epithelial cells in the corpus of the mouse stomach. II. Outward migration of pit cells

The pit cells (or surface mucous cells) present along pit walls and gastric surface have been investigated by electron microscopy and radioautography after a pulse or continuous infusion of 3H-thymidine. For these studies, the pit region has been subdivided into four segments: three of equal length along the pit wall, respectively named low pit, mid pit and high pit, and a last one at the surface named pit top. The pit region includes an average of 37 pit cells, characterized by dense mucous granules accumulated along the apical membrane in an organelle-free zone referred to as ectoplasm. Continuous 3H-thymidine infusion reveals that pit cells come from pre-pit cells, which are believed to arise in the isthmus region from the undifferentiated granule-free cells through a pre-pit cell precursor stage. The pre-pit cells, characterized by the presence of a few mucous secretory granules scattered in the cytoplasm, migrate outward (i.e., in the direction of the gastric lumen). When the secretory granules line up along the apical membrane in the ectoplasm, the pre-pit cell becomes pit cell. It is estimated that 87% of pit cells differentiate from pre-pit cells, while the remaining 13% come from their own mitoses. Observations at successive times after a 3H-thymidine pulse demonstrate that pit cells, like pre-pit cells, migrate toward the gastric surface where they are eventually lost. The continuous 3H-thymidine infusion results indicate that this migration takes 3.1 days on the average. Cells spend almost a day in each pit wall segment. In the low pit segment, cells produce more and larger mucous secretory granules than do pre-pit cells. In the mid and high pit segments, the number and size of the granules generally keeps on increasing, thus indicating that mucous differentiation is progressing. The secretory granules arising in the Golgi apparatus of pit wall cells are mostly spherical; they retain this shape during the few minutes taken to cross the cytoplasm and enter the apical ectoplasm. They spend about an hour in the ectoplasm, where they change to an ovoid shape as they approach the apical membrane to finally release their content by exocytosis. The mucous differentiation along the pit wall is associated with a progressive decline in the organelles: nucleoli and mitochondria decrease in size while the amount of free ribosomes diminishes. When pit cells reach the free surface, they produce fewer, smaller secretory granules and at a lower rate than in mid and high pit.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Subcompartment sugar residues of gastric surface mucous cells studied with labeled lectins

We examined the intracellular localization of sugar residues of the rat gastric surface mucous cells in relation to the functional polarity of the cell organellae using preembedding method with several lectins. In the surface mucous cells, the nuclear envelope and rough endoplasmic reticulum (rER) and cis cisternae of the Golgi stacks were intensely stained with Maclura pomifera (MPA), which is specific to alpha-Gal and GalNAc residues. In the Golgi apparatus, one or two cis side cisternae were stained with MPA and Dolichos biflorus (DBA) which is specific to terminal alpha-N-acetylgalactosamine residues, while the intermediate lamellae were intensely labeled with Arachis hypogaea (PNA) which is specific to Gal beta 1,3 GalNAc. Cisternae of the trans Golgi region were also stained with MPA, Ricinus communis I (RCA I) which is specific to beta-Gal and Limax flavus (LFA) which is specific to alpha-NeuAc. Immature mucous granules which are contiguous with the trans Golgi lamellae were weakly stained with RCA I, while LFA stained both immature and mature granules. The differences between each lectin's reactivity in the rough endoplasmic reticulum, in each compartment of the Golgi lamellae and in the secretory granules suggest that there are compositional and structural differences between the glycoconjugates in the respective cell organellae, reflecting the various processes of glycosylation in the gastric surface mucous cells.

Traffic through the Golgi apparatus as studied by radioautography

The ability to radiolabel biological molecules, in conjunction with radioautographic or cell fractionation techniques, has brought about a revolution in our knowledge of dynamic cellular processes. This has been particularly true since the 1940's, when isotopes such as 35S and 14C became available, since these isotopes could be incorporated into a great variety of biologically important compounds. The first dynamic evidence for Golgi apparatus involvement in biosynthesis came from light microscope radioautographic studies by Jennings and Florey in the 1950's, in which label was localized to the supranuclear Golgi region of goblet cells soon after injection of 35S-sulfate. When the low energy isotope tritium became available, and when radioautography could be extended to the electron microscope level, a great improvement in spatial resolution was achieved. Studies using 3H-amino acids revealed that proteins were synthesized in the rough endoplasmic reticulum, migrated to the Golgi apparatus, and thence to secretion granules, lysosomes, or the plasma membrane. The work of Neutra and Leblond in the 1960's using 3H-glucose provided dramatic evidence that the Golgi apparatus was involved in glycosylation. Work with 3H-mannose (a core sugar in N-linked side chains), showed that this sugar was incorporated into glycoproteins in the rough endoplasmic reticulum, providing the first radioautographic evidence that glycosylation of proteins did not occur solely in the Golgi apparatus. Studies with the tritiated precursors of fucose, galactose, and sialic acid, on the other hand, showed that these terminal sugars are mainly added in the Golgi apparatus. With its limited spatial resolution, radioautography cannot discriminate between label in adjacent Golgi saccules. Nonetheless, in some cell types, radioautographic evidence (along with cytochemical and cell fractionation data) has indicated that the Golgi is subcompartmentalized in terms of glycosylation, with galactose and sialic acid being added to glycoproteins only within the trans-Golgi compartment. In the last ten years, radioautographic tracing of radioiodinated plasma membrane molecules has indicated a substantial recycling of such molecules to the Golgi apparatus.

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.

Cotranslational fatty acylation of mucus glycoprotein. Addition of palmitic acid to peptidyl-tRNA occurs prior to peptide chain completion and its release

1. The fatty acylation of mucus glycoprotein nascent peptides was investigated using [3H]palmitic acid and [35S]methionine-labeled peptidyl-tRNA of rat gastric mucous cells. 2. The mucus glycoprotein peptidyl-tRNA fraction was found to contain covalently bound palmitic acid in its complexes. 3. RNase digestion of the mucus glycoprotein peptidyl-tRNA released [3H]palmitic acid labeled peptides which, on SDS-polyacrylamide gel, separated into a multitude of bands ranging in size from 2000 to 60,000 Da. 4. The analyses of low molecular weight peptides revealed that palmitic acid was present in methionine-labeled peptides containing 30-43 amino acids and those of 18-25 amino acids or larger devoid of methionine, but was not identified in methionine-labeled peptides containing 10-15 amino acids. 5. The results indicate that the N-terminal fatty acylation of mucus glycoprotein nascent peptides is a cotranslational process which is occurring in an immediate vicinity of the signal peptide fragment.

Subnuclear vacuolated mucous cells: a novel abnormality of simple mucin-secreting cells of non-specialized gastric mucosa and Brunner's glands

We report a novel abnormality affecting simple mucin-secreting cells of the glands of pyloric gastric mucosa and of Brunner's glands. Subnuclear vacuolated mucous cells show a distinctive appearance on haematoxylin and eosin staining. They are columnar cells of similar size to simple mucin-secreting cells, but have a central nucleus beneath which the cytoplasm has a uniform glassy eosinophilic appearance or contains a clear area. Subnuclear vacuolated mucous cells are found focally lining the lower third of pyloric gastric glands or in Brunner's glands, and may be mistaken for a form of metaplasia. Histochemically, the apical portion of the cell stains for neutral mucin, but the basal portion stains only weakly and variably for protein. Electron microscopy reveals that the basal portion consists of a large, single, membrane-bound vacuole, variably indented by the nucleus, and probably derived from either endoplasmic reticulum or the Golgi apparatus. The vacuole contains granular material which varies in electron density from cell to cell. The apical portion of the cell contains organelles similar to simple mucin-secreting cells. These appearances are highly suggestive of an abnormal accumulation of non-glycoconjugated mucus core protein. Although the cause of this is unknown, it was associated with the histological changes of chronic gastritis in 10 of our 12 cases.

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.

Morphological and biochemical changes in mucous cells of the murine sublingual salivary gland during the carbamylcholine-induced secretory cycle

Changes in these cells have been evaluated over 6 h following cholinergic stimulation. Carbamylcholine administration resulted in the release of almost 50 per cent of secretory material within 15 min, which caused a reduction of 33 per cent in cell size. After 2 h the cells were depleted of secretory material. However, in the second hour the release of secretory material was accompanied by an enlargement of the nucleus, Golgi complexes and rough endoplasmic reticulum (RER), which suggests an elevation of biosynthetic activity. The enlargement of the RER was not the result of an increase in RNA, i.e. in the number of ribosomes, but of dilatation of its cisternal spaces. Before release took place, there was a continuous coalescence of secretory granules. After this extensive fusion, which is probably the result of an altered physiological state of the granule membrane and subsequent water uptake caused by cholinergic stimulation, the viscous mucins could be squeezed out, water transport is likely to assist in this ejection. Refilling of the mucous cells was almost complete within 6 h after stimulation.

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.

Changes in volume of stored mucus in pit and surface cells during their maturation and migration in the antral mucosa of the mouse

In antral mucosa of the mouse stomach, the volume of mucus in mucous cells was measured morphometrically to determine whether this value changes during cell migration from the base of the pit to the surface. Both the volume density of mucous granules (the fraction of cell volume occupied by the granules) and the volume of intracellular mucus were reduced to about half in surface cells compared with those of upper pit cells. This indicates that mucus secretion is substantial during the later part of the lifespan of these cells, and is not due simply to the shedding of senescent cells.

Ultrastructural visualization of galactosyl residues in various alimentary epithelial cells with the peanut lectin-horseradish peroxidase procedure

A conjugate of peanut lectin with horseradish peroxidase (PL-HRP) has been employed for ultrastructural localization of glycoprotein with presumed terminal galactose residues in mouse alimentary epithelial cells. The PL-HRP conjugate imparted electron opacity in sites that stain at the light microscopic level, as for example, Golgi cisternae in surface epithelial cells of the stomach and in superficial and deep crypt cells and goblet cells of the large intestine. Ultrastructural staining revealed that Golgi cisternae intermediate between the trans and cis faces stained selectively in these sites. Secretion stored in secretory granules or Golgi vesicles in the cells lacked affinity for PL-HRP conjugate, however. Selective staining of intermediate Golgi cisternae in cells with unreactive secretory product is interpreted as indicating the site of galactosyl transferase activity and a location where galactose occurs transitorily as the terminal sugar in the glycoprotein side chains. The luminal aspect of the surface epithelial cells in the stomach and columnar cells in the colon also stained, but with some variability. Staining of these surfaces was considered possibly attributable to PL affinity of some of the secretory glycoprotein which, after absorbing to the cell surface, lost terminal sialic acid through action of luminal enzyme. PL-HRP conjugate stained granules in pancreatic zymogen cells near the block surface but not in other cells, presumably because of limited penetration of reagent. Secretion on the surface of pancreatic acinar cells or in the lumen also exhibited affinity for PL-HRP complex as did the luminal surface of gastric chief cells. Staining of secretion in the pancreatic zymogen cells and gastric chief cells for galactose appeared inconsistent with lack of evidence for presence of glycoprotein in these sites which failed to stain with the periodic acid-Schiff or periodic acid-thiocarbohydrazide-silver proteinate methods for demonstrating glycoprotein at the light and electron microscopic levels. This discrepancy points to possible selective binding of PL-HRP conjugate to a moiety other than terminal galactose of glycoprotein in a few histologic sites. These results demonstrate the applicability of the PL-HRP technique at the ultrastructural level and provide information concerning the chemical structure of epithelial cell glycoproteins and their biosynthesis.

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.

Effect of carbenoxolone on the synthesis of glycoproteins and DNA in rat gastric epithelial cells

The influence of carbenoxolone on the synthesis of glycoproteins in the surface mucous cells and the production of new cells in the rat gastric mucosa was studied by means of a vascular perfusion system. The rate of incorporation of tritiated galactose, glucosamine, serine, and sulphate in surface mucous cells, studied by autoradiography, was not affected by the addition of carbenoxolone to the drinking water. The sugar composition (determined by gas-liquid chromatography) of the gastric glycoproteins (isolated by centrifugation in CsCl), was not changed in carbenoxolone-treated rats. Compared with untreated animals, the number of [3H]-thymidine labelled nuclei per fundic pit increased by 38% to 76% in carbenoxolone-treated rats, implying a higher number of mitotically active cells. This results in an increased supply of young mucous cells; if this also proves to be true in human gastric mucosa, it may be relevant to the therapeutic effect of carbenoxolone.

Vascular perfusion of the isolated rat stomach with a fluorocarbon emulsion

A constant-pressure system using a fluorocarbon (FC-75) containing artificial medium was used for vascular perfusion of the isolated rat stomach. A 10% emulsion of FC-75 in a modified Tyrode solution, containing 3.5% bovine serum albumin, amino acids, nucleosides, and beta-hydroxybutyrate appeared to allow an adequate perfusion for at least seven hours. The following criteria were used to estimate for functional state of the perfused stomach: (1) venous flow, (2) oxygen consumption, (3) leakage from the blood vessels to the gastric lumen, (4) ultrastructure, (5) secretion of H+ and pepsinogen after stimulation, and (6) transport of labelled macromolecules. During perfusion oxygen consumption and venous flow remained, after an initial high value, constant for at least seven hours. Leakage from the vascular bed was absent. Ultrastructure was preserved for at least seven hours. Administration of pilocarpine led to secretion of HCl ad pepsinogen. Pentagastrin and histamine stimulated HCl secretion in only some of the preparations. [3H]-galactose was incorporated into macromolecules and transported from the Golgi-region towards the apex of the cell. These observations led to the conclusion that the described vascular perfusion system in at least an appropriate model for studying glycoprotein synthesis.

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.

Role of galactosyl-transferases in rat gastric epithelial glycoprotein synthesis

Two galactosyl-transferases have been found in the Golgi-enriched subcellular fractions derived from rat gastric mucosa. One incorporates galactose into ovomucoid at optimal pH 6.8. The reaction can be completely inhibited by acetylglucosamine. The apparent Km for UDPgalactose is 0.024 mM. The other galactosyl-transferase incorporates galactose into desialated ovine submaxillary mucin at optimal pH 7.5 and the transfer cannot be inhibited by acetylglucosamine. The apparent Km for UDPgalactose is 0.191 mM. Both enzymes require Mn2+ and Triton X-100 for optimal galactose incorporation. The enzymes could be separated by polyacrylamide gel electrophoresis. Incorporation into endogenous glycoprotein was studied in conditions optimal for the two galactosyl-transferases: (1) at pH 6.8, using Mes as buffer system, and (2) at pH 7.5, using Tris-HCl in the presence of an inhibitory excess of acetylglucosamine. In both cases, most radioactive galactose is incorporated into macromolecules, which could be identified as epithelial glycoprotein. Endogenous incorporation in the presence of excess acetylglucosamine results in the formation of a substantial amount of a disaccharide (probably galactose-beta-(1-3)acetylgalactosamine), whereas upon incorporation at pH 6.8 almost no disaccharide is formed. Quantitative immunoprecipitation experiments with specific antibodies to the endogenous product, labelled by [3H]galactose in the presence of varying amounts of desialated ovine submaxillary mucin and/or acetylglucosamine, indicated that other galactosyl-transferases are involved in the biosynthesis of epithelial glycoprotein.

Synthesis and secretion of glycoproteins by mucosa of rat gastric antrum in organ culture

Measurement of gastric mucus production in vivo poses several problems. We therefore used organ culture of rat antrum to measure mucosal synthesis and secretion of glycoprotein. Tissue maintained good viability for 20 hr. Glycoprotein synthesis rate, assessed by incorporation of [3H]fucose and [3H]glucosamine, was constant during 20 hr. [3H]Fucose incorporation increased in proportion to increasing [fucose] in medium until the latter reached 10(-3) M, then the slope decreased. Cycloheximide abolished [3H]fucose incorporation into glycoprotein. Two methods for measuring secretion gave parallel results: prelabeled glycoprotein was secreted linearly, with 60% of pulse-labeled tissue glycoprotein appearing in medium in 20 hr. The data suggest that (1) antral mucous cells contain a high-concentration pool of endogenous fucose, but possibly only a small-capacity pool of preformed apoglycoprotein, (2) synthesis and secretion of glycoprotein can be successfully examined with organ culture, and (3) secretion in vitro occurs at a steady, slow rate.

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.

Glycoprotein synthesis in the mucous cells of the vascularly perfused rat stomach. I. Surface mucous cells

We developed a constant-pressure vascular perfusion system of the isolated rat stomach, utilizing an artificial, fluorocarbon (FC-75)-containing medium. Perfusion could be maintained for at least six hours, as demonstrated by the ultrastructure of the mucosal cells and by the constant incorporation of [3H]-galactose in the surface mucous cells. Moreover all mucous cell types in tissue fixed after six hours of perfusion showed the same histochemical reactions for glycoproteins as in tissue fixed shortly after decapitation of the animal. The surface mucous cells of the antrum incorporated 30% less [3H]-galactose, [3H]-serine and [35S]-sulphate than those of the fundus. The amount of radioactivity incorporated per cell did not decrease during a subsequent 2 hour chase.

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.

Phenidone-ascorbic acid development in electronmicroscopic autoradiography

Phenidone-ascorbic acid development in electronmicroscopic autoradiography, using Ilford L4 as photographic emulsion and microdol-x as reference developer. Grain yield and efficiency were studied on pale gold section of uniformly labeled tritium methacrylate. For determination of the resolution, a radioactive line source was prepared by cross-sectioning of an epon-embedded film of tritium labeled albumin. The spatial relationship between silver grains and silver bromide crystals was investigated by shadowing the emulsion with platinumcarbon before development. In shadowed autoradiographs both, silver grains and silver bromide crystal were visible. Phenidone was about twice as sensitive as microdol-x and had a half distance value (Salpeter et al., 1969) of 175 mm. Most of the silver grains of both developers were located within the perimeters of their parent silver bromide crystals. In the case of phenidone more than 80% of the excited crystals gave rise to just one silver deposit. These parameters, together with grain size and shape, and counting feasibility make phenidone a useful developer for quantitative EM-autoradiography.