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At least 1,639 records · Page 91Linked to original sources
PubMed1989

Dependence on pH of substrate binding to a mutant lactose carrier, lacYun, in Escherichia coli. A model for H+/lactose symport.

The lacYun gene, which encodes a lactose carrier showing the uncoupled phenotype of substrate transport in Escherichia coli [Wilson, Kusch & Kashket (1970) Biochem. Biophys. Res. Commun. 40, 1409-1414], was cloned on a plasmid vector, pBR322. The binding of a substrate, p-nitrophenyl alpha-galactoside, to the lacYun carrier in membranes from the strain harbouring the lacYun clone showed a pH-dependence different from its binding to the wild-type lactose carrier. This finding indicated that the lacYun mutation confers higher affinity for H+ on the carrier, exerting its effect on the less efficient dissociation of substrate inside cells. The result coincides with the proposal [Yamato & Rosenbusch (1983) FEBS Lett. 151, 102-104] that the proton affecting the substrate binding is the coupling proton of the proton/lactose symport reaction, which allows only the ordered mechanism of binding of substrate to an H+-carrier binary complex. From the simplest model of the symport reaction, constructed on the basis of these results, the coupling site of energy in the carrier cycle of the transport reaction can be identified at the substrate-dissociation step inside cells.

Binding Sites↗
PubMed1999

Cell death by overload of the elastin-laminin receptor on human activated lymphocytes: protection by lactose and melibiose.

BACKGROUND: Activated human lymphocytes were shown to express the elastin-laminin receptor in vitro and also in vivo in atherosclerotic plaques. In the presence of the agonist, elastin peptides, this receptor was shown to mediate an increased cell proliferation and an increased synthesis and excretion of an elastase-type serine endopeptidase. In this study, we investigated the variation of the above reaction as a function of agonist concentration. MATERIALS AND METHODS: Human lymphocytes were obtained by tonsillectomy and cultured in the presence of phytohaemagglutinin and elastin peptides. Cell viability was evaluated by vital dye exclusion. Elastase and cathepsin G activities were determined in culture supernates and cell lysates using synthetic substrates. Apoptotic cells were identified by the TUNEL method and by electron microscopy. RESULTS: At increasing concentrations of elastin peptides, a dose-dependent increase in cell death was observed. Up to 100 micrograms mL-1 elastin peptides and an increasing fraction of lymphocytes were found permeable to trypan blue, and a large proportion was in apoptosis. Elastin peptide-induced cell death was inhibited by 1 microgram mL-1 lactose and melibiose. CONCLUSION: We describe here cell death of human activated lymphocytes expressing the elastin-laminin receptor in the presence of increasing concentrations of elastin peptides, agonists of the receptor. The mechanism of cell death appears to be related to the triggering of the release of elastase and free radicals mediated by the elastin-laminin receptor. Antagonists of this receptor, lactose and melibiose, protected the lymphocytes from the receptor-mediated cell death.

Apoptosis↗
PubMed1997

Lactose-induced expression of Bacillus sp. TS-23 amylase gene in Escherichia coli regulated by a T7 promoter.

Bacillus sp. TS-23 amylase gene was amplified by polymerase chain reaction and cloned into the pET23(+) transcription vector. Lactose induction of Escherichia coli BL21(DE3) cells harbouring this recombinant plasmid resulted in the extracellular production of gene products. By the addition of 20 mmol l-1 lactose, the amylase activity of fermentation broth had a specific activity of 21 U mg-1 of protein.

Bacillus↗
PubMed1988

Effect of lactose hydrolysis on calcium absorption during duodenal milk perfusion.

A multi-lumen intubation system was used to study the absorption of calcium, glucose and galactose in 13 human subjects. The intubation was placed between the duodenum abdomen and proximal jejunum and the subjects were perfused with milk and lactase-supplemented milk. Lactose disappearance over a 20 cm length of intestine was used as the index of lactase activity. The subjects were assigned to one of two groups, lactase-normal and lactase-deficient. There was linear correlation between the absorption of calcium and lactose: lactase-deficient subjects absorbed less calcium than lactase-normal subjects. Perfusion with lactase-supplemented milk enhanced calcium absorption in lactase-deficient subjects but had no effect on that of normal lactase subjects. All subjects absorbed approximately the same percentage of perfused calcium (24%) when perfused with hydrolysed milk. These data indicate that the enhancement of calcium absorption is not a function of lactase per se, but of its hydrolytic products, glucose and galactose.

Adult↗
PubMed1990

In vitro transport of beta-lactoglobulin across the jejunum of lactose-fed rats.

Changes in protein absorption through the intestinal mucosa occur with aging and might reflect modifications in enterocyte membrane characteristics. We have observed that bovine beta-lactoglobulin (beta-LG) was efficiently transported across the small intestine of adult rats in vitro and that 12-16% of the absorbed protein was recovered intact or as large hydrophobic peptides. Ten percent lactose-feeding resulted in decreased tissue conductance and significantly reduced (-58%, P less than 0.05) beta-LG transport across rat small intestinal mucosa. The amounts of beta-LG absorbed, either as amino acids and peptides or as intact protein, were reduced to the same extent. Therefore, the effect of lactose feeding might be related to a decrease in protein endocytosis at the brush-border level, rather than to reduced protein transport across tight junctions.

Animals↗
PubMed1995

Effect of quercitrin on lactose-induced chronic diarrhoea in rats.

Quercitrin (3-rhamnosylquercetin) is a bioflavonoid contained in several crude drugs traditionally used for its antidiarrhoeal activity. The antidiarrhoeic effect of quercitrin on experimental chronic diarrhoea in rats was studied. Adult rats were fed for 14 days with a synthetic diet in which all soluble carbohydrates were substituted by lactose, resulting in chronic diarrhoea with body weight loss, colonic hyperplasia, reduced average cell size, increased alkaline phosphatase activity, increased mucus production and cytopathological alterations of the enterocyte. The rest of the animals were allowed to recover from chronic diarrhoea for 3 or 7 days, by feeding them with a standard diet, and half of them were also given quercitrin orally (50 mg/kg day). Diarrhoea ceased 48 h after lactose withdrawal, and body weight recovery was apparent after 3 days. Nevertheless, most of the alterations of the colonic mucosa persisted at that time. Quercitrin-treated rats had less diarrhoeal output and did not show mucosal hyperplasia after three days of treatment. All animals had greatly recovered by the seventh day, but histological alterations were still present, although to a lesser extent in quercitrin-treated rats. Quercitrin and related flavonoids may play a role in intestinal repair following chronic mucosal injury.

Animals↗
PubMed1990

Influence of hydrocortisone acetate administered to the lactating rat on protein and lactose content in milk and serum protein, glucose and insulin levels in dams and pups.

The influence of i.m. administration to the mother of hydrocortisone acetate (doses of 0.4, 0.8 or 2.0 mg/100 g body weight/day) during the first 15 days of lactation on milk protein and lactose composition and serum levels of protein, glucose and insulin in dams and pups is studied. Total serum proteins and albumin/globulin ratio in dams were unchanged by treatment. The daily injection of 0.4 or 0.8 mg/100 g body weight failed to alter serum levels of glucose or insulin in dams, whereas a dose of 2.0 mg/100 g body weight led to a rise in glucemia (from 118 +/- 3.2 to 133 +/- 5.3) which was accompanied by a sharp change in insulinemia (from 40.7 +/- 4.1 to 83.6 +/- 6.9). All three doses raised protein levels in milk. The smallest increase was recorded with 2.0 mg/100 g body weight; this dose also reduced milk lactose content. Total serum proteins in pups rose slightly but nonsignificantly, and no significant effects were noted on albumin/globulin ratio or serum glucose and insulin levels.

Animals↗
PubMed2003

Intermolecular thiol cross-linking via loops in the lactose permease of Escherichia coli.

Previous experiments using intermolecular thiol cross-linking to determine surface-exposed positions in the transmembrane helices of the lactose permease suggest that only positions accessible from the aqueous phase are susceptible to cross-linking. This approach is now extended to most of the remaining positions in the molecule. Of an additional 143 single-Cys mutants studied, homodimer formation is observed with both a 5-A- and a 21-A-long crosslinking agent containing bis-methane thiosulfonate reactive groups in 33 mutants and exclusively with the 21-A-long reagent in 43 mutants. Furthermore, intermolecular cross-linking has little or no effect on transport activity, thereby providing further support for the argument that lactose permease is functionally, as well as structurally, a monomer in the membrane. In addition, evidence is presented indicating that reentrance loops are unlikely in this polytopic membrane transport protein.

Amino Acid Sequence↗
PubMed1969

A lipid requirement for induction of lactose transport in Escherichia coli.

The rate of derepressed synthesis of a membrane protein required for lactose transport (M protein) by Escherichia coli is increased in response to increased gene dosage to the same extent as the rates of synthesis of beta-galactosidase and galactoside acetylase. However, elevated gene dosage does not increase beta-galactoside transport to the same extent that it increases synthesis of M protein and of the soluble proteins of the lac operon. Though the factor or factors other than M protein which limit induction of the transport system at high levels of lac operon expression have not been identified, studies with Escherichia coli mutants blocked in the synthesis of unsaturated fatty acids indicate that unsaturated fatty acids must be supplied during the course of induction of the lac operon to permit synthesis of a functional lactose transport system, but not of beta-galactosidase or galactoside acetylase.

Biological Transport↗
PubMed1991

Organization and stability of a polytopic membrane protein: deletion analysis of the lactose permease of Escherichia coli.

The overall topology of polytopic membrane proteins is thought to result from either the oriented insertion of the N-terminal alpha-helical domain followed by passive insertion of subsequent helices or from the function of independent topogenic determinants dispersed throughout the molecules. By using the lactose permease of Escherichia coli, a well-characterized membrane protein with 12 transmembrane domains and the N and C termini on the cytoplasmic surface of the membrane, we have studied the insertion and stability of in-frame deletion mutants. So long as the first N-terminal and the last four C-terminal putative alpha-helical domains are retained, stable polypeptides are inserted into the membrane, even when an odd number of helical domains is deleted. Moreover, even when an odd number of helices is deleted, the C terminus remains on the cytoplasmic surface of the membrane, as judged by lacY-phoA fusion analysis. In addition, permease molecules devoid of even or odd numbers of putative transmembrane helices retain a specific pathway for downhill lactose translocation. The findings imply that relatively short C-terminal domains of the permease contain topological information sufficient for insertion in the native orientation regardless of the orientation of the N terminus.

Alkaline Phosphatase↗
PubMed1992

Functional complementation of internal deletion mutants in the lactose permease of Escherichia coli.

Using the lactose permease of Escherichia coli, a well-characterized membrane protein with 12 transmembrane domains, we demonstrated that certain paired in-frame deletion constructs complement each other functionally. Although cells expressing the deletion mutants individually are unable to catalyze active lactose accumulation, cells simultaneously expressing specific pairs of deletions catalyze transport up to 60% as do cells expressing wild-type permease. Moreover, complementation clearly does not occur at the level of DNA but probably occurs at the protein level. Remarkably, functional complementation is observed only with pairs of permease molecules containing large deletions and is not observed with missense mutations or point deletions. Although the mechanism of functional complementation is obscure, the findings indicate that certain pairs of permease molecules containing specific internal deletions can interact to form a functional complex in the same way phenomenologically as do independently expressed polypeptides corresponding to different N- and C-terminal portions of the permease.

Amino Acid Sequence↗
PubMed1993

Properties and purification of an active biotinylated lactose permease from Escherichia coli.

A simplified approach for purification of functional lactose permease from Escherichia coli is described that is based on the construction of chimeras between the permease and a 100-amino acid residue polypeptide containing the biotin acceptor domain from the oxaloacetate decarboxylase of Klebsiella pneumoniae [Cronan, J. E., Jr. (1990) J. Biol. Chem. 265, 10327-10333]. Chimeras were constructed with a factor Xa protease site and the biotin acceptor domain in the middle cytoplasmic loop (loop 6) or at the C terminus of the permease. Each construct catalyzes active lactose transport in cells and right-side-out membrane vesicles. Moreover, the constructs are biotinylated in vivo, and in both chimeras, the factor Xa protease site is accessible from the cytoplasmic surface of the membrane. Both biotinylated permeases bind selectively to immobilized monomeric avidin and are eluted with free biotin in a high state of purity, and the loop 6 chimera catalyzes active transport after reconstitution into proteoliposomes. The methodology described should be applicable to other membrane proteins.

Bacterial Proteins↗
PubMed1998

The increased level of beta1,4-galactosyltransferase required for lactose biosynthesis is achieved in part by translational control.

beta1,4-Galactosyltransferase (beta4GalT-I) participates in both glycoconjugate biosynthesis (ubiquitous activity) and lactose biosynthesis (mammary gland-specific activity). In somatic tissues, transcription of the mammalian beta4GalT-I gene results in a 4.1-kb mRNA and a 3.9-kb mRNA as a consequence of initiation at two start sites separated by approximately 200 bp. In the mammary gland, coincident with the increased beta4GalT-I enzyme level ( approximately 50-fold) required for lactose biosynthesis, there is a switch from the 4.1-kb start site to the preferential use of the 3.9-kb start site, which is governed by a stronger tissue-restricted promoter. The use of the 3.9-kb start site results in a beta4GalT-I transcript in which the 5'- untranslated region (UTR) has been truncated from approximately 175 nt to approximately 28 nt. The 5'-UTR of the 4.1-kb transcript [UTR(4.1)] is predicted to contain extensive secondary structure, a feature previously shown to reduce translational efficiency of an mRNA. In contrast, the 5'-UTR of the 3.9-kb mRNA [UTR(3.9)] lacks extensive secondary structure; thus, this transcript is predicted to be more efficiently translated relative to the 4.1-kb mRNA. To test this prediction, constructs were assembled in which the respective 5'-UTRs were fused to the luciferase-coding sequence and enzyme levels were determined after translation in vitro and in vivo. The luciferase mRNA containing the truncated UTR(3.9) was translated more efficiently both in vitro (approximately 14-fold) and in vivo (3- to 5-fold) relative to the luciferase mRNA containing the UTR(4.1). Consequently, in addition to control at the transcriptional level, beta4GalT-I enzyme levels are further augmented in the lactating mammary gland as a result of translational control.

Animals↗
PubMed1999

Substrate recognition at the cytoplasmic and extracellular binding site of the lactose transport protein of Streptococcus thermophilus.

The lactose transport protein (LacS) of Streptococcus thermophilus catalyzes the uptake of lactose in an exchange reaction with intracellularly formed galactose. The interactions between the substrate and the cytoplasmic and extracellular binding site of LacS have been characterized by assaying binding and transport of a range of sugars in proteoliposomes, in which the purified protein was reconstituted with a unidirectional orientation. Specificity for galactoside binding is given by the spatial configuration of the C-2, C-3, C-4, and C-6 hydroxyl groups of the galactose moiety. Except for a C-4 methoxy substitution, replacement of the hydroxyl groups for bulkier groups is not tolerated at these positions. Large hydrophobic or hydrophilic substitutions on the galactose C-1 alpha or beta position did not impair transport. In fact, the hydrophobic groups increased the binding affinity but decreased transport rates compared with galactose. Binding and transport characteristics of deoxygalactosides from either side of the membrane showed that the cytoplasmic and extracellular binding site interact differently with galactose. Compared with galactose, the IC(50) values for 2-deoxy- and 6-deoxygalactose at the cytoplasmic binding site were increased 150- and 20-fold, respectively, whereas they were the same at the extracellular binding site. From these and other experiments, we conclude that the binding sites and translocation pathway of LacS are spacious along the C-1 to C-4 axis of the galactose moiety and are restricted along the C-2 to C-6 axis. The differences in affinity at the cytoplasmic and extracellular binding site ensure that the transport via LacS is highly asymmetrical for the two opposing directions of translocation.

Binding Sites↗
PubMed2004

Different effects of whole milk and a fermented milk with the same fat and lactose content on gastric emptying and postprandial lipaemia, but not on glycaemic response and appetite.

Longitudinal studies indicate that milk and fermented milk products lower basal plasma cholesterol concentrations, despite their high content of saturated fat, and therefore have favourable health effects. However, there have been few studies on the postprandial effects of milk products. The present study compared the effect of whole milk with a fermented milk, A-38, on postprandial carbohydrate and lipid metabolism, gastric emptying and appetite. Eight healthy young men participated. On the two test days, they arrived fasting for collection of baseline values before consuming the meals, which for a 75 kg subject consisted of 1.4 litre milk or fermented milk, plus 165 mg [13C]acetate (for later determination of gastric emptying by a [13C]acetate breath test). Lactose (15 g) was added to the A-38 meal to equalize the lactose content. Postprandially the A-38 meal resulted in a slower gastric emptying rate than milk (P<0.001). Furthermore, the A-38 meal resulted in a greater increase and a quicker decrease of the triacylglycerol content in all lipoprotein fractions (LDL-fraction, P<0.05; other fractions, P<0.001) and of the gastrointestinal hormones (cholecystokinin and peptide YY, P<0.05; gastric inhibitory polypeptide and glucagon-like polypeptide-1, P<0.001). There were no significant differences in appetite sensations (measured by visual analogue scale) or in the glucose and insulin response (P>0.10). The slower emptying rate of the liquid phase after the A-38 meal is probably due to the higher viscosity of A-38. The lower and more prolonged triacylglycerol response after the milk meal might be caused by coagulation of milk in the stomach.

Adult↗
PubMed2006

Formula-feeding reduces lactose digestive capacity in neonatal pigs.

The intestine of newborn pigs develops rapidly during the first days postpartum. We investigated if feeding milk replacer (infant formula) as an alternative to colostrum has compromising effects on nutrient digestive function in the neonatal period. Nineteen piglets born at term were assigned to one of four treatments: (1) newborn controls; (2) natural suckling for 24 h; (3) tube-fed formula for 24 h; (4) tube-fed porcine colostrum for 24 h. All three fed groups showed significant increases in small-intestinal and colonic weights, villous heights and widths, maltase and aminopeptidase A activities, and decreases in dipeptidylpeptidase IV activity, relative to newborn pigs. Following oral boluses of mannitol, lactose or galactose, formula-fed pigs showed significantly reduced plasma levels of mannitol and galactose compared with colostrum-fed pigs. Activity of intestinal inducible NO synthase and plasma levels of cortisol were significantly increased, whereas intestinal constitutive NO synthase and alpha-tocopherol were decreased in formula-fed pigs compared with colostrum-fed pigs. Although formula-fed pigs only showed minor clinical signs of intestinal dysfunction and showed similar intestinal trophic responses just after birth, as those fed colostrum, lactose digestive capacity was markedly reduced. We conclude that formula-feeding may exert detrimental effects on intestinal function in neonates. Formula-induced subclinical malfunction of the gut in pigs born at term was associated with altered NO synthase activity and antioxidative capacity.

Animals↗
PubMed2005

Effects of Lactobacilli and lactose on Salmonella typhimurium colonisation and microbial fermentation in the crop of the young turkey.

1. Three experiments were performed to examine the effects of Lactobacilli and lactose on microbial fermentation and Salmonella enterica serovar Typhimurium colonisation in the crop of the young turkey. 2. The following carboxylic acids were detected in the crop ingesta: formic, acetic, butyric, lactic, valeric, caproic, oxalic, phenyl acetic, succinic and fumaric; propionic, isobutyric and isovaleric acids were not detectable. 3. At the beginning of the night, there were considerable quantities of ingesta in the crop of young turkeys. During the scotophase, there were progressive reductions in the contents and pH. Moreover, there were linear increases in the concentration of lactic, valeric and caproic acids (by approximately 7-fold over 8 h). Much smaller changes in crop pH were observed in the study where dietary treatments of Lactobacilli were not included. 4. Chronic addition of lactose or Lactobacilli to the diet exerted modest effects on the carboxylic acid concentration in the crop contents but did not consistently influence colonisation of the crop by Salmonella enterica serovar Typhimurium. 5. Young turkeys confine eating to the hours of illumination (photophase) with a peak in consumption prior to the subjective dusk.

Aging↗
PubMed1991

Lactose intolerance. Pinpointing the source of nonspecific gastrointestinal symptoms.

Lactose intolerance is a common condition that can cause nonspecific gastrointestinal symptoms. A reliable diagnosis cannot be made on the basis of the patient's history. The breath hydrogen test is simple, noninvasive, accurate, and inexpensive and is the diagnostic method of choice. In addition to traditional dietary restriction of lactose, treatment may consist of alterations in dietary fat content or caloric density to reduce symptoms and use of dairy products or additives that provide lactase activity.

Abdominal Pain↗