The identification of rat intestinal membrane enzymes after electrophoresis on polyacrylamide gels containing sodium dodecyl sulphate

Epithelial cell polarity as reflected in enterocytes

Absorptive cells are the main cells present in the intestinal epithelium. The plasma membrane of these tall columnar cells reflects their high degree of polarization, by dividing into apical and basolateral domains with different compositions. The most characteristic structure of these cells consists of closely packed apical microvilli with the same height, looking like a brush, which is why they were named the brush border. The concentrated pattern of some apical markers observed in a restricted brush border domain shows that mature enterocytes are hyperpolarized epithelial cells: the filamentous brush border glycocalyx is anchored at the top of the microvilli and the annexin XIII is concentrated in the lower three fourths. Many studies have been carried out on the biosynthesis and intracellular pathway of domain markers. The results show clearly that the basolateral markers take a direct pathway from the trans-Golgi network to the basolateral membrane. However, the two apical pathways, one direct and one indirect pathway via the basolateral membrane, are used, depending on the apical protein involved. Efficient protein sorting and addressing are essential to the establishment and maintenance of cell polarity, on which the integrity of the epithelial barrier depends.

Quantitative regional variation in the expression of major histocompatibility class II antigens in enterocytes of the mouse small intestine

The qualitative and quantitative expression of major histocompatibility class II antigens was investigated in the absorptive epithelium of the duodenum, jejunum, and ileum from mice of C3H/He (H-2k haplotype) and C57BL/6 (H-2b haplotype) strains by peroxidase-antiperoxidase labelling and image analysis. Immunohistochemical labelling revealed that the expression of class II antigens was greatest in the ileum and decreased proximally towards the duodenum. The villus epithelium of the duodenum showed a granular staining pattern in the apices of some cells. In the jejunum, an increased expression was demonstrated in the apical and basal cytoplasm of all cells covering the villus. Cells at the tip of the villus, in addition, showed staining of the lateral surfaces. Ileal enterocytes demonstrated the most intense immunostaining appearing in the cytoplasm and along baso-lateral surface membranes. Quantitative analyses confirmed that a highly significant (p less than 0.0001) difference in expression of class II antigens occurred in the three regions of the small intestine, which corroborated the qualitative findings. This regional variation of class II molecules by the absorptive epithelium may influence regional differences in antigen presenting functions and immune responsiveness to ingested antigens.

Expression and intracellular transport of microvillus membrane hydrolases in human intestinal epithelial cells

A panel of monoclonal antibodies was produced against purified microvillus membranes of human small intestinal enterocytes. By means of these probes three disaccharidases (sucrase-isomaltase, lactase-phlorizin hydrolase, and maltase-glucoamylase) and four peptidases (aminopeptidase N, dipeptidylpeptidase IV, angiotension I-converting enzyme, and p-aminobenzoic acid peptide hydrolase) were successfully identified as individual entities by SDS PAGE and localized in the microvillus border of the enterocytes by immunofluorescence microscopy. The antibodies were used to study the expression of small intestinal hydrolases in the colonic adenocarcinoma cell line Caco 2. This cell line was found to express sucrase-isomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV, but not the other three enzymes. Pulse-chase studies with [35S]methionine and analysis by subunit-specific monoclonal antibodies revealed that sucrase-isomaltase was synthesized and persisted as a single-chain protein comprising both subunits. Similarly, lactase-phlorizin hydrolase was synthesized as a large precursor about twice the size of the lactase subunits found in the human intestine. Aminopeptidase N and dipeptidylpeptidase IV, known to be dimeric enzymes in most mammals, were synthesized as monomers. Transport from the rough endoplasmic reticulum to the trans-Golgi apparatus was considerably faster for the peptidases than for the disaccharidases, as probed by endoglycosidase H sensitivity. These results suggest that the major disaccharidases share a common biosynthetic mechanism that differs from that for peptidases. Furthermore, the data indicate that the transport of microvillus membrane proteins to and through the Golgi apparatus is a selective process that may be mediated by transport receptors.

Labelling of intestinal brush border membrane proteins in vivo using diazotised[125I]iodosulfanilic acid

Membrane proteins of the intestinal brush border were labelled in vivo by intraluminal injection of diazotised [125I]iodosulfanilic acid, a highly polar molecule. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of brush border membranes labelled in this manner showed 20 protein bands, 11 of which contained significant radioactivity. The most heavily labelled proteins had molecular weights greater than 150000, indicating that they were the most exposed to the intestinal lumen. Little radioactivity was detected in proteins with molecular weights of less than 94000. The majority of these smaller proteins were likely to have been brush border core proteins. The evidence that diazotised [125I]iodosulfanilic acid bound primarily to brush border membrane proteins when administered in this way, was: (a) the specific activity of brush border proteins was up to 3-fold greater than that of total cell particulate proteins (pelleted by 27000 x g from mucosal homogenates); (b) principal peaks in the gel radioactivity profile of total cell particulate proteins corresponded to the most heavily labelled proteins of the isolated brush border membrane; and (c) brush border core proteins showed minimal radioactivity in vivo, but considerably higher radioactivity when brush border membranes were labelled in vitro. A small amount of label was absorbed across the intestinal mucosa. However, secondary labelling of brush border proteins by this absorbed label was minimal, since the specific activity of brush border proteins in jejunum adjacent to the labelled loop was only 0.22% of the level for those proteins in the labelled segment. Since this technique did not affect the cellular morphology, enzyme activity or biochemical integrity of the membrane, it should prove useful as a means of accurately studying in vivo turnover rates of brush border membrane proteins.

Biogenesis of intestinal plasma membrane: posttranslational route and cleavage of sucrase-isomaltase

The biosynthesis in vivo of rat intestinal sucrase-isomaltase [a complex of sucrose alpha-glucohydrolase, EC 3.2.1.48, and oligo-1,6-glucosidase (dextrin 6-alpha-D-glucanohydrolase), EC 3.2.1.10] has been studied by following the incorporation of L-[6-(3)H]fucose into the enzyme with time. Immunoprecipitation of sucrase-isomaltase from Triton-X-100-solubilized Golgi or basolateral membranes and subsequent polyacrylamide gel electrophoresis revealed the presence of an immunoreactive glycoprotein with an apparent molecular weight approximately twice that of the separated sucrase-isomaltase subunits, but no active subunits were found in these membranes. This glycoprotein was also found in the microvillus membrane in addition to the subunits of sucrase-isomaltase. Kinetic studies showed a maximal labeling of this glycoprotein in Golgi membranes at 15 min, in basolateral membranes at 30 min, and in microvillus membranes at 45 min and a half-life of less than 30 min in each membrane. However, the radioactivity of the sucrase-isomaltase subunits in the microvillus membrane reached a plateau after 60 min. These data suggest that sucrase-isomaltase is synthesized as a one-chain polypeptide precursor that is split into the subunits after its transfer to the microvillus membrane. Elastase (EC 3.4.21.11), but not trypsin (EC 3.4.21.4) or alpha-chymotrypsin (EC 3.4.21.1), split the putative precursor into two polypeptides that had electrophoretic behaviors similar to those of the active enzyme subunits. These studies suggest that pancreatic proteases may play an important role in the late posttranslational processing of sucrase-isomaltase in vivo.