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At least 973 records · Page 54Linked to original sources
PubMed1967

Imine-bonding in membrane transport of monosaccharides: invalidity of kinetic evidence.

The proposition that carrier mediation of sugar transport may involve formation of imine complexes with specific cell membrane proteins has recently been advanced. However, the primary data presented accord more quantitatively with the presumption of a nonspecific reaction and furnish no evidence for the existence of the high-affinity glucose-binding sites which are essential to the proposed interpretation.

Binding Sites↗
PubMed1982

Monosaccharide sequence of protein-bound glycans of Uukuniemi virus.

Uukuniemi virus, a member of the Bunyaviridae family, was grown in BHK-21 cells in the presence of [(3)H]mannose. The purified virions were disrupted with sodium dodecyl sulfate and digested with pronase. The [(3)H]mannose-labeled glycopeptides of the mixture of the two envelope glycoproteins G1 and G2 were characterized by degrading the glycans with specific exo-and endoglycosidases, by chemical methods, and by analyzing the products with lectin affinity and gel chromatography. The glycopeptides of Uukuniemi virus fell into three categories: complex, high-mannose type, and intermediate. The complex glycopeptides probably contained mainly two NeuNAc-Gal-GlcNAc branches attached to a core (Man)(3)(GlcNAc)(2) peptide. The high-mannose-type glycans were estimated to contain at least five mannose units attached to two N-acetylglucosamine residues. Both glycan species appeared to be similar to the asparagine-linked oligosaccharides found in many soluble and membrane glycoproteins. The results suggested that the intermediate glycopeptides contained a mannosyl core. In about half of the molecules, one branch appeared to be terminated in mannose, and one appeared to be terminated in N-acetylglucosamine. Such glycans are a novel finding in viral membrane proteins. They may represent intermediate species in the biosynthetic pathway from high-mannose-type to complex glycans. Their accumulation could be connected with the site of maturation of the members of the Bunyaviridae family. Electron microscopic data suggest that the virions bud into smooth-surfaced cisternae in the Golgi region. The relative amounts of [(3)H]mannose in the complex, high-mannose-type, and intermediate glycans were 25, 62, and 13%, respectively, which corresponded to the approximate relative number of oligosaccharide chains of 2:2.8:1, respectively, in the roughly equimolar mixture of G1 and G2. Endoglycosidase H digestion of isolated [(35)S]methionine-labeled G1 and G2 proteins suggested that most of the complex and intermediate chains were attached to G1 and that most of the high-mannose-type chains were attached to G2.

Bunyaviridae↗
PubMed1978

Jejunal monosaccharide, water, and electrolyte transport in patients with chronic pancreatitis.

Jejunal perfusion studies were performed to assess water, electrolyte, d-xylose, and d-glucose transport in 16 patients with chronic calcific pancreatitis (eight with and eight without steatorrhoea) and in 10 control subjects. The patients with steatorrhoea demonstrated significantly less xylose, water, and electrolyte absorption than patients without steatorrhoea and control subjects, when an isosmotic slaine-xylose solution was perfused. On the other hand, when an isosmotic saline-glucose solution was perfused, the patients with steatorrhoea absorbed significantly more glucose, water, and electrolytes than control subjects. Significant correlation was demonstrated between the absorption of xylose as measured by the segmental perfusion technique and the peak serum xylose level during perfusion as well as the five-hour urinary xylose excretion after a 25 g oral dose of xylose. The xylose absorption measured by small bowel perfusion also correlated significantly with pancreatic juice amylase and trypsin concentrations obtained during a standard pancreatic function test.

Adult↗
PubMed1986

Absorption capacity of fructose in healthy adults. Comparison with sucrose and its constituent monosaccharides.

The capacity to absorb fructose in 10 healthy adults was investigated by means of hydrogen breath analysis. Fructose absorption was quantified with lactulose standards. Significant hydrogen production (greater than or equal to 20 ppm rise of breath hydrogen) was found after challenge with 10% solutions of 50, 37.5, 25, 20, and 15 g fructose in eight, seven, five, four and one subjects, respectively. One subject showed malabsorption after a 10 g dose and possibly also 5 g fructose. In contrast, no malabsorption could be detected in any of the 10 subjects after ingestion of 100 g, 75 g, or 50 g sucrose or a mixture of 50 g glucose and 50 g fructose. After ingestion of mixtures of 50 g fructose +25 g glucose and 50 g fructose +12.5 g glucose malabsorption was present in three and seven subjects, respectively. Symptoms during all challenges were mild, or absent. It is concluded that in the healthy state the absorption capacity of fructose given alone ranges from less than 5 g to more than 50 g. The absorption capacity of fructose given as sucrose is much higher. Glucose stimulates fructose uptake in a dose dependent fashion. The possible existence of more than one intestinal transport system for fructose is considered. The elucidation of the clinical relevance of the findings is important.

Adult↗
PubMed1979

Defect in regulation of membrane transport of monosaccharides in dystrophic muscle.

The penetration of a nonmetabolized glucose analogue, 3--O-methyl-D-glucose, across the plasma membranes of tissues from dystrophic mice and cardiomyopathic (dystrophic) hamsters has been compared with that of normal controls. Under basal conditions the penetration of test sugar was similar in lens and diaphragm of normal and dystrophic 129/ReJ mice. Stimulation of sugar transport by 2,4-dinitrophenol did occur in normal but not in dystrophic diaphragm. A submaximal concentration of insulin had a more variable effect in dystrophic than in normal muscle while a supramaximal concentration of the hormone increased the uptake of the glucose analogue to an equal extent in the two tissues. In the BIO 14.6 strain of cardiomyopathic hamsters, uncoupling of oxidative phosphorylation did not increase sugar transport in extensor digitorum longus muscles, while the normal effect was observed in dystrophic soleus and in both these muscles of the random bred controls. The absence of an effect by a condition simulating anoxia suggests that in dystrophy, certain muscles are unable to accelerate the entry of glucose when this is required.

Animals↗
PubMed1980

Monosaccharide transport across microvillous membrane of human placenta.

Transport through the microvillous membrane of the syncytium is the first step in placental transfer of nutrients. We have therefore studied glucose transport by isolated microvillous membrane vesicles. Transport occurred by selective and rapid facilitated diffusion, which was inhibited by phloridzin, phloretin, cytochalasin B, and HgCl2. Nonmediated diffusion of the substrate was found by three independent methods to be very slow. Competition studies showed that aldohexoses in the C-1 chair conformation were the preferred substrates. Transport was independent of sodium gradients and was not modulated by insulin. However, several steroids inhibited transport including estriol and progesterone, which are abundant in utero. Kinetic analysis by equilibrium exchange demonstrated a Km of 31 mM and a Vmax of 120 nmol . s-1 . mg protein-1. The Km and Vmax suggest a large capacity in relation to calculated fetal needs. In consequence of this capacity, intrasyncytial concentrations of glucose are probably maintained near those of maternal blood. Augmentation of transport at this membrane by hormones or other agents is unlikely to increase fetal supply of glucose, but down regulation by steroids may serve a regulatory function.

Biological Transport↗
PubMed1975

Effect of dietary substitution of sucrose and its constituent monosaccharides on the activity of aromatic hydroxylase and the level of cytochrome P-450 in hepatic microsomes of growing rats.

The effect of dietary substitution of starch by sucrose, glucose, fructose or an equimolar mixture of glucose and fructose on the activity of biphenyl hydroxylase and the level of the major terminal mixed function oxygenase, cytochrome P-450, has been studied in hepatic microsomal preparations from growing rats. The substitution of sucrose for starch depressed the contration and total activity per liver of biphenyl 4-hydroxylase and the concentration, but not the total amount, of cytochrome P-450 in weanling rats. The absolute amount of these enzymes in whole liver was not, however, similarly depressed by the constituents of sucrose, namely glucose and fructose, either when given alone or in equimolar mixture. All these sugars, however, depressed the total activity of biphenyl 2-hydroxylase. The activity of biphenyl 4-hydroxylase in the liver of weanling rats, but not of adults, was reduced when the level of sucrose in the diet was reduced from 60 to 10%.

Animals↗
PubMed1975

Effect of secretin on intestinal monosaccharide absorption and new water movement in the rat.

Repeated intravenous injections of 2.0 clinical units (CU) secretin per kg body weight showed no effect on jejunal glucose absorption and net water movement in perfusion studies in vivo. The in vitro uptake of galactose and 3-O-methylglucose was not altered by a secretin load given 12 min before sacrifice. The values for Km and Vmax were identical after secretin and in a control group. The conflicting results of different authors concerning the effect of secretin on net water movement and solute absorption may be due to differences in experimental techniques, different, mostly pharmacological doses of the hormone, and a diverse response of heterogeneous species.

Animals↗
PubMed1973

Transport of monosaccharides. I. Asymmetry in the human erythrocyte mechanism.

Transport of D-glucose across human erythrocyte membranes occurs via a facilitated diffusion process which demonstrates influx-efflux asymmetry. The mechanism of the asymmetry has been studied by estimating unidirectional fluxes in the presence or absence of trans equilibrium hexose. In the absence of transhexose, the half-saturation constant for efflux at 15 degrees C was approximately 10 mM as compared with 27 mM for influx; the corresponding values for maximal transfer rates (mumol/min per ml cell H(2)O) were approximately 51 vs. 18. The estimation of kinetic parameters, including the constant F(s), which is the ratio of maximal transfer rate/half-saturation constant, indicates a unique effect of intracellular hexose on the transfer system. Further evidence to support this conclusion was obtained by studying the effects of noncompetitive inhibitors on efflux vs. influx. N-ethylmaleimide, p-chloromercuribenzenesulfonate, and dichloroallyldiethylstilbestrol all inhibited efflux much more than influx. Glucose rendered the transport system more reactive to N-ethylmaleimide as assayed by efflux, whereas influx was much less affected. The results support the hypothesis that the transport system exists in two states. Transition from one state to the other is dependent on the presence of intracellular hexose.

Biological Transport↗
PubMed2005

Kinetic characterization of glycosidase activity from disaccharide conjugate to monosaccharide conjugate in Caco-2 cells.

Glycosidase activity influences the intestinal absorption of glycosides. Our previous study in rats suggested that disaccharide conjugates might be prototypes for pre-prodrugs aiming at the Na(+)/glucose co-transporter-mediated transport of prodrugs (drug glucoside) as a novel absorption pathway. One of the crucial factors is the formation of a glucoside drug from the disaccharide conjugate. Since there is a large species difference in metabolism, it is necessary to examine the cells and/or enzymes derived from human tissue to confirm this concept. In this paper, we kinetically characterized the glycosidase activity of disaccharide conjugates in Caco-2 cells. Disaccharide conjugates of p-nitrophenol (p-NP) (p-NP beta-cellobioside, p-NP beta-lactoside and p-NP beta-maltoside) were hydrolysed to p-NP beta-glucoside. beta-glucosidase or beta-galactosidase (lactase/phloridzin hydrolase, LPH) and alpha-glucosidase (sucrase-isomaltase) had different pH-dependent activities for disaccharide conjugates. At neutral pH, LPH has low affinity and low capacity, and sucrase-isomaltase has high affinity and high capacity, whereas at acid pH, LPH has high affinity and low capacity, and sucrase-isomaltase has low affinity and high capacity. The hydrolysis clearance calculated with Vmax/Km indicated that sucrase-isomaltase activity is much higher than LPH activity at either neutral or acid pH in Caco-2 cells. Since the hydrolysis rate of the disaccharide conjugate was highly dependent on the pH value and type of glycoside linkage, the appropriate selection of a glycoside form after consideration of these differences is the key to designing a sugar-conjugate prodrug.

Caco-2 Cells↗