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At least 307 records · Page 17Linked to original sources
PubMed1977

Evidence for a specific transport of D-hexoses across the human term placenta in vitro.

Isolated cotylidons of human term placentas are perfused artificially on the fetal and maternal side. The relative transfer rates of radioactive labelled D-glucose, L-glucose, D-mannose and D-mannitol across the placenta are measured and the inhibition of these transports by phloretin is studied: 1. Transfer rates of D-glucose and D-mannose exceed that of L-glucose about 1.5-4 times. 2. Phloretin (10(-3) mol/l) decreases the transports of D-hexoses, whereas the transport of L-GLUCOSE REMAINS UNAFFECTED. 3. If the concentration of D-glucose is increased, the transport of D-mannose is inhibited competitively. 4. L-glucose and D-mannitol equal in transfer rates. These results show, that D-hexoses cross the human placenta by a specific carrier and by simple diffusion.

Biological Transport, Active↗
PubMed1989

Inhibition of hexose transport in the human erythrocyte by 5, 5'-dithiobis(2-nitrobenzoic acid): role of an exofacial carrier sulfhydryl group.

The sulfhydryl reagent 5, 5'-dithiobis (2-nitrobenzoic acid) (DTNB) was used to study the functional role of an exofacial sulfhydryl group on the human erythrocyte hexose carrier. Above 1 mM DTNB rapidly inhibited erythrocyte 3-O-methylglucose influx, but only to about half of control rates. Efflux was also inhibited, but to a lesser extent. Uptake inhibition was completely reversed by incubation and washing with 10 mM cysteine, whereas it was only partially reduced by washing in buffer alone, suggesting both covalent and noncovalent interactions. The covalent thiol-reversible reaction of DTNB occurred on the exofacial carrier, since (i) penetration of DTNB into cells was minimal, (ii) blockade of potential uptake via the anion transporter did not affect DTNB-induced hexose transport inhibition, and (iii) DTNB protected from transport inhibition by the impermeant sulfhydryl reagent glutathione-maleimide-I. Maltose at 120 mM accelerated the covalent transport inhibition induced by DTNB, whereas 6.5 microM cytochalasin B had the opposite effect, indicating under the one-site carrier model that the reactive sulfhydryl is on the outward-facing carrier but not in the substrate-binding site. In contrast to glutathione-maleimide-I, however, DTNB did not restrict the ability of the carrier to reorient inwardly, since it did not affect equilibrium cytochalasin B binding. Thus, carrier conformation determines exposure of the exofacial carrier sulfhydryl, but reaction of this group may not always "lock" the carrier in an outward-facing conformation.

3-O-Methylglucose↗
PubMed1987

Insulin-like effect of dichloroacetic acid on hexose transport in Swiss 3T3 cells.

Hexose transport in Swiss 3T3 cells was increased by treatment with dichloroacetic acid as well as by treatment with insulin. Neither extra- nor intracellular Ca2+ was found to be involved in their stimulatory action. On the other hand, the removal of intracellular Mg2+ resulted in a loss of the stimulation. These results suggest that dichloroacetic acid stimulates the hexose transport in Mg2+-dependent manner, similar to that of insulin.

3-O-Methylglucose↗
PubMed1989

Inhibition of hexose transport by glucose in a glucose-6-phosphate isomerase mutant of Saccharomyces cerevisiae.

The rate of hexose transport was approximately 60% lower for both the high- and the low-affinity components of hexose uptake when a glucose-6-phosphate isomerase mutant of Saccharomyces cerevisiae was preincubated with glucose, as compared with preincubation with water. Similarly the Jmax value of the high-affinity system of the mutant was 25-35% of the corresponding Jmax value for normal cells incubated with glucose. Accumulation of glucose 6-phosphate or of some other metabolite, such as fructose 6-phosphate or trehalose, may be responsible for this striking inhibition.

Biological Transport, Active↗
PubMed1981

Phloretinyl-3'-benzylazide: a high affinity probe for the sugar transporter in human erythrocytes. I. Hexose transport inhibition and photolabeling of mutarotase.

A new phloretin derivative, phloretinyl-3'-benzylazide (PBAz), has been synthesized and compared with phloretin for its ability to inhibit the hexose transporter in human erythrocyte membranes in subdued light. Transport measurements were made using the light scattering (Orskov optical) method and a Millipore filtration technique with isotopically labeled sugars. Initial rates of sugar flux were measured under four different conditions to test for inhibition asymmetry. In each experimental condition, PBAz is from 6-20-times more potent than phloretin, making it one of the most effective reversible inhibitors known. Although both agents penetrate the cell membrane, they apparently fail to reach inhibitory levels at the inner surface over the time course of our nonequilibrated experiments, because of extensive binding to hemoglobin. The mechanism by which PBAz and its parent phloretin inhibit transport is pure competition with hexose for the carrier which faces the exterior of the membrane. If given time to equilibrate with the cells, the inhibition by both agents converts to a mixed type, i.e., both competitive and noncompetitive. The noncompetitive component could be due to inhibition of those transporter units oriented internally. Alternatively pre-equilibration with the inhibitors may cause them to attain high levels in the lipid membrane and produce nonspecific effects. PBAz and its precursor amine, phloretinyl-3'-benzylamine (PBA), compete with glucose for the sugar binding site on mutarotase at least as well as phloretin. When exposed to long wavelength ultraviolet radiation, PBAz is converted to a reactive intermediate which becomes covalently bound to the enzyme. Both irreversible ligand attachment and mutarotase inhibition are related to dose of the azide and irradiation time, but inactivation is from 5 to 6-times greater than label incorporation. We conclude that PBAz is a potentially useful photoaffinity labeling agent capable of covalently interacting with the transporter site facing the exterior of the red cell.

3-O-Methylglucose↗
PubMed1981

Hexose transport in Novikoff rat hepatoma cells. A simple carrier with directional symmetry, but variable relative mobilities of loaded and empty carrier.

The kinetics of transport of the non-metabolizable hexose, 3-O-methyl-D-glucose, have been measured in Novikoff rat hepatoma cells by both zero-trans entry and equilibrium exchange procedures. Transport conformed to a simple carrier model which operates symmetrically with respect to direction, but with greater mobility of the loaded than of the empty carrier. Although a complete kinetic description of the transporter can, in theory, be obtained by application of integrated equations describing the time course of substrate equilibrium across the membrane beginning from the zero-trans situation, statistical analysis of hypothetical data indicated that directional asymmetry or differential mobilities of loaded and empty carrier cannot be discerned reliably from such data alone. The difference in mobility of loaded and empty carrier, apparent in a comparison of zero-trans entry and exchange data, ranged from 1.5--7-fold in different batches of cells. It is concluded that the magnitude of the difference is not an inherent property of the transporter, but is determined physiologically, and may be involved in regulation of hexose transport.

Animals↗
PubMed1989

Characteristics of Na+-dependent hexose transport in OK, an established renal epithelial cell line.

The characteristics of Na+-dependent hexose uptake were determined for monolayers of OK, an established renal epithelial cell line derived from an opossum kidney. A comparison is made with other cultured cells, particularly LLC-PK1. The capacity to accumulate alpha-methyl D-glucoside (AMG) in OK cells develops with time, reaching a maximum level of 18 nmol/mg protein per h, 3 days after confluency. In contrast to LLC-PK1, this level is not influenced by the medium D-glucose concentration. AMG uptake in OK cells was characterized by an apparent Km of 2.9 mM and a Vmax of 17.1 nmol/mg protein per min. For Na+-dependent phlorisin binding, a KD of 0.025 microM and a Bmax of 1.5 pmol/mg protein were found. A turnover frequency of 158/s was derived from our data. The hexose carrier of OK shares with the carrier of LLC-PK1 a high level of expression, its substrate specificity and turnover frequency. It differs however with respect to the substrate binding site. The affinity for AMG and D-glucose is 3- and 10-fold lower, whereas the affinity for phlorizin is 3-times higher in OK than in LLC-PK1. The Na+ dependence of AMG uptake was also different for both cell lines and suggested for OK cells a 1:1, Na+:substrate stoichiometry. In OK cells, the phlorizin-sensitive uptake rate of D-glucose is much lower than the one for AMG. Nevertheless, D-glucose interacts with the AMG binding site in a competitive way and with an affinity similar to AMG. This could indicate a malfunction of the carrier with D-glucose as a substrate at the level of the translocation step.

Animals↗
PubMed1985

Stimulation of hexose transport by human polymorphonuclear leukocytes: a possible role for protein kinase C.

The protein C kinase activators 1-O-oleoyl, 2-O-acetylglycerol, 12-O-tetradecanoyl phorbol-13-acetate, and mezerein, stimulated deoxyglucose uptake in human neutrophils. The responses were stimulus specific since no effect was noted with the diether analogues 1-O-hexadecyl-2-O-ethylglycerol, 1-O-palmitoyl-2-O-acetyl or 1-O-palmitoyl-3-O-acetyl diesters of propanediol, or with 1,2-diolein. Stimulation of deoxyglucose uptake had the characteristics of carrier facilitated hexose transport. Stimulated uptake of deoxy-glucose was inhibited by trifluoperazine (10-30 microM). Activation of protein kinase C therefore appears to trigger events involved in hexose transport.

Biological Transport, Active↗
PubMed1985

Sensitivity of adipocyte basal and insulin-stimulated hexose transport to the membrane lipid structure.

A series of anesthetic alcohols inhibited basal and insulin-stimulated 2-deoxy-D-[1-14C]glucose transport in adipocytes over total alcohol concentration ranges that cause local anesthesia of rat sciatic nerve. The relative potencies of the inhibition caused by the alcohols increased in the following order: methanol less than ethanol less than propanol less than butanol less than benzyl alcohol less than hexanol less than octanol. The inhibition was reversible and correlated well with the known partitioning of the alcohols into lipids of biological membranes. Adipocyte membranes were labeled with the 5-nitroxide stearate spin probe to investigate the effects of the alcohols on the dynamic structure of membrane lipids of the adipocyte. The alcohols increased the membrane "fluidity", and the relative concentration dependence of the effects closely paralleled that noted from methanol to octanol in transport studies. Alcohols from methanol to hexanol caused inhibition of hexose transport at molar potencies comparable to that observed for membrane disordering. This suggests that hydrophobic regions of the transporter and its lipid environment are perturbed by a comparable mechanism for each alcohol. The cholesterol-complexing polyene antibiotic filipin inhibited hexose transport and influenced the mobility of lipid domains sampled with the nitroxide cholestane, cholesterol-like spin probe. The data are consistent with the concept that the membrane structural/functional effects are mediated by formation of 1:1 cholesterol:filipin complexes. Alcohols and filipin inhibited inherent transporter activity and perturbed the membrane lipid structure without dramatically diminishing transport stimulation by insulin above basal. The specific organization of membrane lipids (particularly cholesterol) may provide an essential environment for optimal transport system activity.

Adipose Tissue↗
PubMed1986

Membrane structural/functional perturbations induced by gossypol. Effects on membrane order, liposome permeability, and insulin-sensitive hexose transport.

The effects of gossypol on membrane structure and membrane-associated functions were studied to explore possible reasons for the ability of gossypol to disrupt cellular processes, many of which involve intracellular and plasma membranes. The experiments reported here measured the effects of gossypol on membrane order, permeability, and hexose transport. Electron spin resonance (ESR) studies of I(12,3) nitroxide fatty acid spin-labeled unilamellar liposomes showed that exposure to 0.05 to 4 mM gossypol caused a dose-dependent increase in the polarity-corrected order parameter (S), indicating reduced motional freedom of the spin probe after exposure to gossypol. This observation is consistent with the idea that gossypol causes an ordering or "condensing" of the membrane lipid matrix. Gossypol-induced changes in order parameter in phosphatidylcholine:cholesterol liposomes varied depending on the liposome composition. Liposomes exposed to gossypol also showed increasing permeability to glycerol as the gossypol:phospholipid ratio increased up to 10 mole %. Higher concentrations of gossypol were less effective at enhancing permeability. In addition, basal and insulin-stimulated 2-deoxy-D-[3H]glucose transport were inhibited in freshly isolated rat adipocytes incubated with gossypol at 37 degrees. Half-maximal inhibition occurred at approximately 0.2 mM for uptake in both the presence and absence of 40 ng/ml insulin. Microscopic observation of the cells under low power (40 X) confirmed that diminished hexose transport was not simply due to breakage of the adipocyte plasma membrane, resulting in a decrease in intact cell population and decreased accumulation of label in the gossypol-treated cells. Gossypol produced no significant changes in numbers of intact cells or gross morphology at the concentrations tested. We suggest that ordering and increased permeability of the lipid regions of plasma and subcellular membranes may contribute to some of the toxic and pharmacologic properties of gossypol. Our results also support the idea that gossypol may exert more pronounced effects in cells that are most sensitive to variations in availability of glucose substrates for energy metabolism.

Adipose Tissue↗
PubMed1989

Failure of fenfluramine to affect basal and insulin-stimulated hexose transport in rat skeletal muscle.

Fenfluramine is an effective appetite suppressant that mediates its action via serotoninergic neurons. We studied the effect of pure d- and l-fenfluramine on in vitro hexose transport in isolated rat soleus muscles and skeletal muscle cells in culture. We found no evidence to suggest that the fenfluramine enantiomers affect the basal transport activity. Furthermore, the drugs did not interfere with the ability of glucose to regulate its own transport. Muscle responsiveness to insulin was not altered by the enantiomers, nor did insulin unmask any effect of fenfluramine on muscle hexose transport. These conclusions are based on experiments performed with a wide concentration range of drug and insulin, from the therapeutic to suprapharmacological levels. We discuss our results in view of published data on the effects of fenfluramine on peripheral glucose metabolism.

Animals↗
PubMed1988

A micromethod for the estimation of oligosaccharides containing glycosidically linked sialic acid or hexoses, or both, in glycoproteins.

The peeling reaction, the process by which oligosaccharides are degraded in alkali, was used as the basis for an assay to provide structural information about glycosidically linked oligosaccharides in glycoproteins. Glycoproteins were treated with 0.05 M NaOH at 50 degrees to induce release, and subsequent degradation ("peeling"), of glycosidically linked, but not of N-glycosydically linked, oligosaccharides. Among the degradation products generated from O-linked chains were three 3-deoxy sugar acids whose formation was correlated with certain structural features of the oligosaccharides. N-Acetylneuraminic acid was released from terminal positions in the oligosaccharides, and iso- and meta-saccharinic acids were derived from the degradation of 4-O- and 3-O-substituted hexoses, respectively. All of these sugar acids were detected colorimetrically by periodate oxidation and reaction of the product with 2-thiobarbituric acid. The ability of the method to generate 3-deoxy sugar acids was tested in 8 alkali-treated glycoproteins. 3-Deoxy sugar acids were detected only in those glycoproteins whose glycosidically linked carbohydrates contained N-acetylneuraminic acid, or 3-O- or 4-O-substituted hexoses, or both. As little as 0.12 microgram of 3-deoxy sugar acid produced from 5 micrograms of human chorionic gonadotropin was sufficient for detection. This method is novel in its ability to distinguish sialylation of glycosidically linked carbohydrates. Furthermore, it combines the specificity of beta-elimination with the sensitivity of the 2-thiobarbituric acid assay in targeting degradation products of the peeling reaction as candidates for an assay method.

Glycoproteins↗
PubMed1990

A method for the microanalysis of hexoses in glycoproteins.

A procedure has been developed by which less than micrograms quantities of hexoses in glycoproteins can be determined by g.l.c.-m.s. with selected-ion monitoring of the alditol acetates derived from the sugars released by acid hydrolysis. The effectiveness of the method was demonstrated by determination of the hexose and hexosamine composition of 5-micrograms samples of antithrombin III and von Willebrand factor, respectively.

Antithrombin III↗
PubMed1979

Aortic glycopeptide sialic acid, hexose and hexosamine in a genetically selected (WC-2) strain of atherosclerosis-susceptible pigeon.

The aortic content of glycopeptide sialic acid, hexosamine and hexose was studied in a genetically selected strain of White Carneau pigeons (WC-2) with significantly more severe atherosclerosis than randomly bred White Carneau pigeons (RBWC). Pigeons were fed an atherogenic diet for 3 months and examined to determine differences in content of glycopeptide-sugars between WC-2 and RBWC, changes with the progression of atherosclerosis and the relationship of aortic cholesterol to glycopeptide-sugar content. In animals with mainly normal aorta (cholesterol content of 0.2-0.3 mg/cm2-aorta) sialic acid was significantly lower in WC-2 pigeons. The progression of atherosclerosis was associated with increased aortic glycopeptide sialic acid (r = 0.78; p less than 0.05) in WC-2 pigeons whereas an inverse relationship was suggested in RBWC pigeons. In WC-2, but not RBWC pigeons, significant positive relationships were seen for aortic glycopeptide hexosamine and aortic cholesterol and for aortic glycopeptide hexose and aortic cholesterol. The findings implicate a possible role of aortic glycoproteins in either the initiation or modulation of atherosclerosis of the WC-2 pigeon.

Animals↗
PubMed1990

Protein kinase C inhibitors block insulin and PMA-stimulated hexose transport in isolated rat adipocytes and BC3H-1 myocytes.

Effects of protein kinase C (PKC) inhibitors and "down-regulation" on insulin and PMA-stimulated 2-deoxyglucose transport were determined in isolated rat adipocytes or BC3H-1 myocytes. In both model systems, H-7, sangivamycin, and staurosporine, inhibitors of the catalytic domain of PKC, each effectively blocked insulin and PMA-stimulated hexose uptake at similar concentrations. In the myocytes, staurosporine completely blocked the insulin effect retained post-chronic phorbol myristate acetate (PMA)-induced "down-regulation." These findings indicate (1) that chronic pretreatment with PMA may not lead to a complete loss of PKC activity in the myocyte, and (2) that PKC is involved in insulin-stimulated hexose transport in both isolated rat adipocytes and BC3H-1 myocytes.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1986

Effects of ethanol and pentobarbital on neuronal hexose uptake in inbred mice.

The effect of ethanol and pentobarbital narcosis on 2-deoxyglucose uptake into brain synaptosomes prepared from inbred C57BL/6J and DBA/2J mice which exhibit differential central sensitivity to ethanol and heterogeneous ICR mice was examined. A reversible depression of synaptosomal uptake was exhibited in all strains administered ethanol acutely, occurring at 2 min in ICR and C57BL/6J mice and 15 min in DBA/2J. Uptake returned to control values in all strains at 30 min although the mice remained intoxicated. Brain glucose concentration was significantly elevated at this time. Pentobarbital administration was without effect on synaptosomal hexose transport in DBA/2J and C57BL/6J mice but increased it significantly in ICR mice at 30 min. Pentobarbital anesthesia did not alter brain glucose concentration. No correlation was apparent between synaptosomal 2-deoxyglucose uptake and differential CNS sensitivity to ethanol and pentobarbital. The effects of ethanol and pentobarbital on neuronal hexose transport is discussed with respect to reported changes in glycolytic metabolism produced by these agents.

Animals↗
PubMed1982

The 6-hydroxymethyl group of a hexose is essential for the substrate-induced closure of the cleft in hexokinase.

Yeast hexokinase B (ATP:-hexose 6-phosphotransferase, EC 2.7.1.1) was crystallized in the presence of D-xylose and ADP, and its structure was determined at 7 A resolution. The enzyme is in the 'open' conformation which is characteristic of the enzyme crystallized in the absence of glucose, rather than in the 'closed' conformation that is observed with the glucose complex. That is, the binding of xylose into the large cleft that separates the molecule into two lobes does not cause the cleft to close. We conclude, then, that the glucose 6-hydroxymethyl group (which binds to an aspartic acid and a serine) is essential for the hexose-induced conformational change.

Binding Sites↗
PubMed1984

Modulation of hexose transport in cultured skeletal muscle.

The modulation of hexose transport due to insulin and glucose starvation was investigated in cultures derived from the breast musculature of embryonic quail. Fused myotubes at 37 degrees C exhibited a saturable, stereospecific basal uptake of both D-glucose and 3-O-methylglucose which was markedly inhibited by cytochalasin B, a potent inhibitor of hexose transport in other cell systems. In the presence of insulin, 3-O-methylglucose uptake was stimulated relative to untreated controls. Kinetic analysis indicated that insulin increased the Vmax of transport with no significant increase in the apparent Km. Incubation of myotubes in glucose-free medium for 24 h resulted in an increase in D-glucose and 3-O-methylglucose transport activity. Cycloheximide abolished this stimulation effect when it was included during the starvation period, but had no effect on transport in glucose-fed cells. Insulin binding studies on these myotubes indicate that high-affinity insulin receptors are present and continue to increase throughout the life of the culture. This high-affinity binding as well as the capacity to degrade insulin in these cells is characteristically similar to effects observed in other insulin-sensitive cell systems.

3-O-Methylglucose↗