Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.2.1.108 (lactase)
 2,133 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Structural changes have been studied during the life cycles of three glycosidases: sucrase-isomaltase (EC 3.2.48-10), lactase-phlorizin hydrolase (EC 3.2.1.23-62), maltase-glucoamylase (EC 3.2.1.20); and three peptidases: aminopeptidase A (EC 3.4.11.7), aminopeptidase N (EC 3.4.11.2) and dipeptidyl peptidase IV (EC 3.4.14.5). The final forms of the enzymes can be divided into at least two groups: the sucrase-isomaltase type, characterized as dimers, which are asymmetric in their hydrophilic parts, have two types of active site and anchor only on one subunit; and the aminopeptidase N type, characterized as dimers, which are symmetric in their hydrophilic part, have only one type of active site and anchor on both subunits. These enzymes are likely to be synthesized on rough endoplasmic reticulum and simultaneously glycosylated into endoglycosidase H-sensitive forms. They are later reglycosylated to endoglycosidase H-resistant forms, which have relative molecular masses similar to the final forms. Enzymes of the sucrase-isomaltase type seem to be synthesized with a polypeptide-chain length corresponding to the sum of both subunits, whereas enzymes of the aminopeptidase N type seem to be synthesized with a polypeptide-chain length corresponding to the constituent subunits themselves. Not much is known about the catabolism of these enzymes. The enzyme activities and the amounts of enzyme protein decrease at the top of the villi, probably due to release into the lumen. The subunits of aminopeptidase N are cleaved by pancreatic proteases to smaller peptides, and sucrase-isomaltase may lose its sucrase polypeptide, while both enzymes remain bound to the membrane.
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PMID:Structure of microvillar enzymes in different phases of their life cycles. 613 6

Precursor forms of lactase-phlorizin hydrolase, sucrase-isomaltase and aminopeptidase N were studied by pulse-labelling of organ-cultured human intestinal biopsies. After labelling the biopsies were fractionated by the Ca2+-precipitation method and the enzymes isolated by immunoprecipitation. The results indicate that the lactase-phlorizin hydrolase is synthesized as a Mr 245 000 polypeptide, which is intracellularly cleaved into its mature Mr 160 000 form. Sucrase-isomaltase is shown to be synthesized as a single chain precursor (Mr 245 000 and 265 000) while the precursor of aminopeptidase N is shown to be of apparently the same size as the mature enzyme (Mr 140 000 and 160 000).
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PMID:Evidence for biosynthesis of lactase-phlorizin hydrolase as a single-chain high-molecular weight precursor. 614 71

The biosynthesis of pig small intestinal lactase-phlorizin hydrolase (EC 3.2.1.23-62) was studied by labelling of organ cultured mucosal explants with [35S]methionine. The earliest detactable form of the enzyme was an intracellular, membrane-bound polypeptide of Mr 225 000, sensitive to endo H as judged by its increased electrophoretic mobility (Mr 210 000 after treatment). The labelling of this form decreased during a chase of 120 min and instead two polypeptides of Mr 245 000 and 160 000 occurred, which both barely had their electrophoretic mobility changed by treatment with endo H. The Mr 160 000 polypeptide is of the same size as the mature lactase-phlorizin hydrolase and was the only form expressed in the microvillar membrane. Together, these data are indicative of an intracellular proteolytic cleavage during transport. The presence of leupeptin during labelling prevented the appearance of the Mr 160 000 form but not that of the Mr 245 000 polypeptide, suggesting that the proteolytic cleavage takes place after trimming and complex glycosylation. The proteolytic cleavage was not essential for the transport since the precursor was expressed in the microvillar membrane in the presence of leupeptin.
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PMID:Biosynthesis of intestinal microvillar proteins. Intracellular processing of lactase-phlorizin hydrolase. 643 Feb 96

Microscopical studies showed that initial differentiation of the guinea-pig small intestine occurs between days 35 and 55 of foetal development. Changes observed at this time include formation of villi (by day 42), elaboration of submucosal duodenal Brunner's glands (by day 49) and the appearance of a well-developed microvillus membrane (by day 56). Different microvillus membrane-associated hydrolases appear at different stages of foetal and postnatal development. The 'early' enzymes such as aminopeptidase, alkaline phosphatase and sucrase show a sharp increase and reach their maximal levels between days 35 and 50, whereas the late enzymes such as dipeptidyl peptidase IV and lactase increase gradually between days 35 and 50, and reach maximal activity between days 50 and 60. A combination of techniques involving precipitation with Mg2+ followed by fractionation on sucrose density gradients has enabled us to prepare, for the first time, a 21-fold enriched microvillus membrane fraction from the foetal intestine. Polypeptide analysis of this membrane fraction by sodium dodecyl sulphate/polyacrylamide gel electrophoresis showed the presence of developmentally specific polypeptides at different stages of foetal and postnatal development. Three polypeptides of molecular weights 205 000, 80 000 and 47 000 are major microvillus membrane components at the 40-day foetal stage. Two other polypeptides of molecular weights 60 000 and 131 000 are major microvillar components at 56-day and older foetal stages as well as at the 3-day neonatal stage. The adult microvillus membrane contained 112 000 and 122 000 Mr polypeptides as major components. The above results were confirmed using two-dimensional isoelectric focussing-sodium dodecyl sulphate/polyacrylamide gel electrophoretic techniques.
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PMID:Structural and biochemical differentiation of the mammalian small intestine during foetal development. 653 51

Explants of pig small intestine were maintained at 37 degrees C in organ culture for periods up to 24 h in a system using Trowell T-8 medium supplemented with 10% foetal-calf serum. The mucosal morphology was well preserved during culture, as judged by light and electron microscopy. The explant contents of protein and two brush-border enzymes, microvillus aminopeptidase (EC 3.4.11.2) and dipeptidyl peptidase IV (EC 3.4.14.5), were not significantly modified during culture compared with controls, but a moderate, continuous release of both protein and enzyme activities into the medium was observed. Continuous labelling with [35S]methionine resulted in an even incorporation of radioactivity in the protein components, and the rate of labelling only moderately decreased over the 24 h period. The polypeptide compositions of sucrase (EC 3.2.1.48)--isomaltase (EC 3.2.1.10), maltase--glucoamylase (EC 3.2.1.20) lactase (EC 3.2.1.23)--phlorizin hydrolase (EC 3.2.1.62), microvillus aminopeptidase and aspartate aminopeptidase (EC 3.4.11.7) synthesized during culture were studied, and some were found to be similar to those of the pro-forms of the enzymes isolated from animals that had had their pancreatic duct disconnected 3 days before being killed. These results confirmed earlier findings of the existence of pro-forms of some of the microvillar enzymes and thus indicate a low activity of pancreatic proteinases in the culture system.
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PMID:Biosynthesis of intestinal microvillar proteins. Characterization of intestinal explants in organ culture and evidence for the existence of pro-forms of the microvillar enzymes. 709 36

Human small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a single-chain polypeptide precursor, prepro-LPH, that undergoes two sequential cleavage steps: the first in the endoplasmic reticulum to pro-LPH (215-kDa) and the second, following terminal glycosylation in the Golgi apparatus, to mature 160-kDa LPH (denoted LPH beta). The LPH beta molecule is subsequently targetted to the brush-border membrane. Characterization of the N-terminal profragment (denoted LPH alpha) of pro-LPH using an epitope-specific, anti-peptide polyclonal antibody reveals that LPH alpha (i) has an apparent molecular weight of approximately 100,000, (ii) is not associated with LPH beta after cleavage of pro-LPH has occurred, and (iii) is not transported to the cell surface or secreted into the extracellular medium. In biosynthetic labeling experiments, a clear precursor/product relationship could be demonstrated between pro-LPH and the LPH alpha and LPH beta polypeptides. Further, LPH alpha has a significantly shorter half-life than LPH beta. LPH alpha is neither N- nor O-glycosylated, despite the presence of 5 potential N-glycosylation sites. LPH alpha, which is rich in cysteine and hydrophobic amino acid residues, may fold rapidly into a tight and rigid globular domain in which carbohydrate attachment sites are no longer accessible to glycosyltransferases. When expressed independently in COS-1 cells, the LPH beta polypeptide forms a misfolded, transport-incompetent molecule. We propose a role for the LPH alpha domain within the pro-LPH molecule as an intramolecular chaperone during folding in the ER.
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PMID:The pro region of human intestinal lactase-phlorizin hydrolase. 752 15

The pro-region of intestinal lactase-phlorizin hydrolase (LPH alpha) has been proposed to be important for the correct folding of pro-LPH and mature LPH (LPH beta). In this communication, analysis of the catalytic function of the LPH alpha pro-region is presented. Expression of a cDNA encoding LPH alpha in COS-1 cells reveals a polypeptide that does not hydrolyse lactose. Likewise, no lactase activity is detected in LPH alpha purified from trypsin-treated pro-LPH. Mixing of LPH alpha and LPH beta does not lead to the activation of the latter. We conclude that LPH alpha does not contribute to the lactase activity despite the strong homologies with mature LPH beta. LPH alpha may play an important role as an intra-molecular chaperone.
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PMID:The pro-region of human intestinal lactase-phlorizin hydrolase is enzymatically inactive towards lactose. 762 35

The aim of this study was to determine the effects of a model of intestinal extrinsic denervation on mucosal structure and function. Six dogs underwent in situ neural isolation of the jejunoileum (Group 2); six other dogs served as operated controls (Group 1), and five nonoperated dogs were naive controls (Group 3). Thirty-centimeter segments of proximal jejunum and distal ileum were excised before (time zero) and at 2 weeks and 8 weeks postoperatively in Groups 1 and 2, while similar regions were removed at time zero in Group 3. Tissues were analyzed for morphology with quantitative morphometry, mucosal disaccharidase activities (sucrase, maltase, and lactase), and tissue content of selected regulatory peptides in transmural, mucosa/submucosa, and muscularis regions. In situ neural isolation had no significant or consistent effects on morphology/morphometry or on mucosal disaccharidase activities. Tissue content of neuropeptide Y decreased markedly (P < 0.002) in all layers of the jejunal and ileal walls, but tissue content of vasoactive inhibitory polypeptide, substance P, cholecystokinin, neurotensin, met-enkephalin, neurokinin A, somatostatin, and calcitonin gene-related peptide demonstrated only minor changes. The physiologic effects of intestinal transplantation (extrinsic denervation and disruption of intrinsic, enteric neural continuity, and lymphatic drainage) have little effect on morphology, mucosal disaccharidase activity, and tissue content of most regulatory peptides. How these minor alterations might affect enteric function, however, needs to be investigated.
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PMID:Neural isolation of the jejunoileum. Effect on tissue morphometry, mucosal disaccharidase activity, and tissue peptide content. 865 18

The Caco-2 cell line is derived from a human colon adenocarcinoma and differentiates in vitro into small-intestinal enterocyte-like cells, expressing the hydrolases lactase and sucrase-isomaltase. We cultured Caco-2 cells on permeable supports from 0 to 37 days after plating to study endogenous lactase and sucrase-isomaltase gene expression in relation to cell differentiation. Profiles of lactase and sucrase-isomaltase mRNA, protein and enzyme activity were analysed on a per-cell basis, using immunocytochemistry, RNase protection assays, metabolic polypeptide labelling and enzyme activity assays. Tight-junction formation was complete 6 days after plating. Immunocytochemistry of Caco-2 cross-sections showed lactase and sucrase-isomaltase predominantly in the microvillar membrane of polarized cells. mRNA, protein and enzyme activity of lactase appeared consecutively, reaching maximum levels 8-11 days after plating. Whereas lactase mRNA and protein biosynthesis showed a sharp decline after peak levels, lactase activity remained high until 37 days after plating. In contrast, mRNA and protein biosynthesis and activity of sucrase-isomaltase peaked successively 11-21 days after plating, and exhibited comparable levels throughout the entire experiment. The following conclusions were reached. (1) In Caco-2 cells, biosynthesis of lactase and sucrase-isomaltase is regulated by the amount of their mRNAs, indicating transcriptional control. (2) Sucrase-isomaltase activity is most probably transcriptionally controlled at all time points. (3) In contrast, lactase activity is initially regulated by its level of biosynthesis. After its peak at 8 days, the slow decline in activity compared with its biosynthesis indicates high stability. (4) Different mRNA profiles for lactase and sucrase-isomaltase indicate different mechanisms of transcriptional regulation of these genes.
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PMID:Lactase and sucrase-isomaltase gene expression during Caco-2 cell differentiation. 894 31

Insulin-like growth factor-I (IGF-I), a polypeptide growth factor found in milk, is hypothesized to play a functional role in the growth and development of neonates, particularly the gastrointestinal tract. Considerable evidence, based on direct tracer studies with 125I-labeled IGF-I and measurements of circulating IGF-I concentrations in neonatal animals fed a range of IGF-I doses, indicates that the intestinal absorption of IGF-I and the possible effect on metabolism and somatic growth are negligible. However, studies in neonatal animals indicate that oral administration of pharmacological doses of IGF-I increases small intestinal mucosal growth, whereas oral IGF-I provided within the physiological range may enhance the development of intestinal lactase. Therefore, clinical trials exploring the therapeutic use of oral IGF-I as an intervention for preterm neonates and those with compromised intestinal function seem warranted. However, milk-borne IGF-I may not be essential for normal healthy infants, perhaps because endogenous IGF-I provides a sufficient stimulus for maintenance of gastrointestinal structure and function. Future studies should explore the significance of endogenous IGF-I and whether milk-borne IGF-I may be important under pathological conditions in which the endogenous IGF-I production may be compromised.
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PMID:Is milk-borne insulin-like growth factor-I essential for neonatal development? 916 77


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