Effect of benzimidazole drugs on the uptake of low molecular weight nutrients in Trichuris globulosa.

Thiabendazole (100 microM) and fenbendazole (250 microM) were found to inhibit U-14 C-glucose, (Na)-1-14C-palmitate and (Na)-1-14 C-acetate uptake markedly (P less than 0.001) in adult Trichuris globulosa. The inhibition was more pronounced with thiabendazole than with fenbendazole. 14C-Glucose was found to be Na(+)-dependent and a mediated process. Bile salts, viz. sodium cholate and sodium desoxycholate in the concentration range of 5-10 mM, were found to inhibit the uptake of 14C-glucose and 14C-palmitate (P less than 0.01) by the parasites. The optimum pH for (Na)-1-14C-palmitate and (Na)-1-14C-acetate uptake was 6.7 and 6.2, respectively while the optimum temperature for the uptake of these compounds was 37 degrees C.     

Radiorespirometry evidence for the discrimination between 13C-enriched glucose and unlabelled glucose molecules by Paracoccus denitrificans

Paracoccus denitrificans was grown on either unlabelled glucose, [1-13C]glucose or [6-13C]glucose as the sole carbon source for growth. The cells were then incubated with a range of 14C-glucose substrates to compare the 14CO2-evolution rates between cells grown on the glucose and the 13C-labelled glucose. Cells grown on 13C-glucose had significantly faster rates of 14CO2-evolution than those grown on unlabelled glucose. The % yields of 14CO2, per [1-14C]-, [6-14C]- and [U-14C]glucose supplied were also substantially greater than those measured for cells grown on unlabelled glucose. The data indicated that growth of Paracoccus on 13C-enriched glucose substrates resulted in cells with notably different 14C-glucose oxidation metabolism compared to that observed in cells grown on unlabelled glucose.     

Pancreatic fate of 14C-labelled hexoses

In order to assess the respective contribution of the exocrine and endocrine moieties of the pancreas to the overall net uptake of selected monosaccharides by the pancreatic gland, the apparent distribution space of L-[1-14C]glucose, 3-O-[14C-methyl]-D-glucose, D-[U-14C]glucose, D-[U-14C]mannose and D-[U-14C]fructose was measured in pieces of pancreas obtained from either control rats or animals injected with streptozotocin. Although the time course for the uptake of 3-O-[14C-methyl]-D-glucose, D-[U-14C]glucose, D-[U-14C]mannose and D-[U-14C]fructose was much slower in the pieces of pancreas than that previously documented in isolated pancreatic islets, no significant difference could, as a rule, be detected between the results obtained in pancreatic pieces of control and streptozotocin rats. A comparable situation prevailed in the pancreas of animals examined 3 min after the intravenous injection of 3-O-[14C-methyl]-D-glucose. D-Glucose inhibited the uptake of 3-O-[14C-methyl]-D-glucose and that of D-[U-14C]fructose. Likewise, 3-O-methyl-D-glucose inhibited the uptake of D-[U-14C]glucose. Cytochalasin B (20 microm) also inhibited the uptake of 3-O-[14C-methyl]-D-glucose and D-[U-14C]glucose, but not that of D-[U-14C]fructose. D-Mannoheptulose hexaacetate, but not the unesterified heptose, inhibited the metabolism of tritiated and 14C-labelled D-glucose, as well as the net uptake of D-[U-14C]glucose and D-[U-14C]mannose and, to a lesser extent, that of D-[U-14C]fructose. These findings indicate that despite marked differences between endocrine and exocrine pancreatic cells in terms of both the time course for the uptake of several hexoses and the inhibition of their phosphorylation by D-mannoheptulose, little or no preferential labelling of the endocrine moiety of the pancreas by the 14C-labelled hexoses is observed, at least when judged from their distribution space in pancreatic pieces or the whole pancreatic gland. Nevertheless, the findings made with D-mannoheptulose and its hexaacetate ester raise the view that this heptose could conceivably be used to achieve a sizeable preferential labelling of the endocrine pancreas under the present experimental conditions.     

Inhibition of gamma-glutamyl transpeptidase decreases amino acid uptake in human keratinocytes in culture

Acivicin inhibits gamma-glutamyl transpeptidase activity in human keratinocytes in culture. Treatment of these cells with acivicin produces a decrease in the uptake of L-[U-14C]alanine, 2-amino-[1-14C]-isobutyrate, L-[U-14C]leucine and 1-aminocyclopentane-1-[14C]carboxylate. D-[U-14C]glucose uptake is not affected by the presence of acivicin. These results support, for the first time in vitro, the hypothesis that the gamma-glutamyl cycle may be involved in amino acid uptake by human cells.     

The role of adipose tissue in glucose utilization in irradiated rats

The authors studied the conversion of U-14C-glucose to total lipids, fatty acids and glyceride glycerol in the epididymal adipose tissue of rats X-irradiated with a single whole body dose of 14.4 Gy X-rays. Analyses were carried out 1, 24, 48 and 72 h after irradiation. In the adipose tissue of irradiated rats, the incorporation of 14C-glucose into all the lipid fractions was raised throughout the whole time of observation (300-600% of the control value). Most of the 14C-glucose was incorporated into the glyceride glycerol fraction.     

Effects of thiamine deficiency on the biosynthesis of insulin in rats.

The effect of thiamine deficiency on insulin biosynthesis was studied. In thiamine deficient rats the total pancreatic protein content was not altered when compared to control rats whereas the pancreatic insulin content was decreased. Though the in vivo incorporation of 3H-leucine and the in vivo conversion of U-14C-glucose into proinsulin and insulin were not affected in thiamine deficient rats, the tolbutamide induced increased in vivo incorporation of 3H-leucine and in vivo conversion of U-14C-glucose into proinsulin and insulin was not seen in thiamine deficient rats. These results suggest that the biosynthesis of insulin is impaired in thiamine deficiency. Even tolbutamide could not increase the biosynthesis of insulin in this condition.     

Aerobic and anaerobic metabolism in Entamoeba histolytica

Respiration by Entamoeba histolylica is confirmed. A doubling of the rate of oxygen uptake was observed upon the addition of d-glucose to cells in which the glycogen reserve had been partially depleted. In cells metabolizing endogenous substrates the rate of oxygen uptake was not influenced by sodium cyanide or sodium succinate. It was slightly depressed when d-mannose was the added sugar. The end products, CO₂, ethanol, and acetate accounted for essentially all of the glucose carbon utilized in both aerobic and anaerobic experiments. The radioactivity from uniformly labelled ¹⁴C-glucose was found in these products. Three times as much ethanol as acetate was produced in the anaerobic experiments and in the aerobic experiments this ratio was approximately reversed.     

Active chloride transport in rabbit thick ascending limb of Henle's loop and elasmobranch rectal gland: chloride fluxes in isolated plasma membranes

Summary To investigate directly whether a sodium-potassium-chloride cotransport system is operating in the mammalian thick ascending limb of Henle's loop (TALH) and in the elasmobranch rectal gland, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and from rectal glands ofSqualus acanthias, and chloride uptake was measured by a rapid filtration technique. Chloride uptake into TALH vesicles in the presence of a 25 mM Na₂SO₄, 25 mM K₂SO₄ gradient reached 70% of equilibrium at 2.5 min. In the presence of both sodium and potassium, the 15 s chloride uptake was inhibited 35% by 1 mM bumetanide. When either sodium or potassium was removed from the incubation medium, chloride uptake decreased to the level observed in the presence of 1 mM bumetanide. 0.5 mM SITS had no effect on chloride uptake by the plasma membrane vesicles. This sodium and potassium dependent, bumetanide sensitive chloride uptake was also observed under tracer exchange conditions. Chloride uptake into rectal gland plasma membrane vesicles in the presence of a 50 mM Na₂SO₄, 50 mM K₂SO₄ gradient reached 80% of equilibrium at 2.5 min. 1 mM bumetanide inhibited the 15 s uptake of chloride by 34% and removal of either sodium or potassium from the incubation medium reduced chloride uptake to the level observed in the presence of bumetanide under both gradient and tracer exchange conditions. These studies provide additional support for the hypothesis that a sodium-potassium-chloride cotransport system is operating in these epithelia.Abbreviations SITS 4-acetamido-4-isothiocyanato-stilbene-2,2-disulfonic acid - TALH thick ascending limb of Henle's loop     

Phospholipase D1 Mediates AMP-Activated Protein Kinase Signaling for Glucose Uptake

Background

Glucose homeostasis is maintained by a balance between hepatic glucose production and peripheral glucose utilization. In skeletal muscle cells, glucose utilization is primarily regulated by glucose uptake. Deprivation of cellular energy induces the activation of regulatory proteins and thus glucose uptake. AMP-activated protein kinase (AMPK) is known to play a significant role in the regulation of energy balances. However, the mechanisms related to the AMPK-mediated control of glucose uptake have yet to be elucidated.

Methodology/Principal Findings

Here, we found that AMPK-induced phospholipase D1 (PLD1) activation is required for ¹⁴C-glucose uptake in muscle cells under glucose deprivation conditions. PLD1 activity rather than PLD2 activity is significantly enhanced by glucose deprivation. AMPK-wild type (WT) stimulates PLD activity, while AMPK-dominant negative (DN) inhibits it. AMPK regulates PLD1 activity through phosphorylation of the Ser-505 and this phosphorylation is increased by the presence of AMP. Furthermore, PLD1-S505Q, a phosphorylation-deficient mutant, shows no changes in activity in response to glucose deprivation and does not show a significant increase in ¹⁴C-glucose uptake when compared to PLD1-WT. Taken together, these results suggest that phosphorylation of PLD1 is important for the regulation of ¹⁴C-glucose uptake. In addition, extracellular signal-regulated kinase (ERK) is stimulated by AMPK-induced PLD1 activation through the formation of phosphatidic acid (PA), which is a product of PLD. An ERK pharmacological inhibitor, PD98059, and the PLD inhibitor, 1-BtOH, both attenuate ¹⁴C-glucose uptake in muscle cells. Finally, the extracellular stresses caused by glucose deprivation or aminoimidazole carboxamide ribonucleotide (AICAR; AMPK activator) regulate ¹⁴C-glucose uptake and cell surface glucose transport (GLUT) 4 through ERK stimulation by AMPK-mediated PLD1 activation.

Conclusions/Significance

These results suggest that AMPK-mediated PLD1 activation is required for ¹⁴C-glucose uptake through ERK stimulation. We propose that the AMPK-mediated PLD1 pathway may provide crucial clues to understanding the mechanisms involved in glucose uptake.     

Effect of epinephrine on the synthesis of glyceride glycerol in adipose tissue in vitro.

In order to study the effect of epinephrine on the rate of esterification of fatty acids in adipose tissue, pieces of epididymal fat pad were incubated in KRB in the presence of purified albumin, glucose and either 1-14C-glycerol, 1-14C-glucose or 6-14C-glucose. Epinephrine enhances the production of glycerol but reduces the uptake of 1-14C-glycerol by the tissue and its conversion to 14CO2, 14C-fatty acids and 14C-glyceride glycerol. When the change in specific activity of the tracer is taken into account the effect of epinephrine on the utilization of glycerol by the tissue is only observed in the reduction of glyceride glycerol synthesis. When 14C-labelled glucose was used as tracer, epinephrine enhances both the production of 14CO2 from 6-14C-glucose and the synthesis of 14C-glyceride glycerol from 1-14C and 6-14C-glucose. The contrasting effects of epinephrine on the glyceride glycerol formation from glycerol and from glucose can explain the difficulties found in observing any change in the net rate of esterification of fatty acids by adipose tissue.     

Effect of Sendai Virus Infection on Lipid Metabolism in Chick Embryo Fibroblasts

Lipid metabolism in the "abortive" system, Sendai virus-infected primary chick embryo fibroblasts, was examined by using (32)P-orthophosphate, (14)C-glucose, and (14)C-glycerol as precursors. Incorporation of radioactivity from (32)P-orthophosphate and (14)C-glucose into lipid was increased in infected cells. Synthesis of all individual phospholipids was about equally stimulated. There was also evidence for increased lipid synthesis in more productively infected monkey kidney cells. Incorporation of (14)C-glycerol when at a high level in the medium was also increased. However, when this precursor was supplied in minute quantities of high specific activity, incorporation was inhibited. Even though incorporation of radioactivity from (14)C-glucose was stimulated during long labeling periods, the uptake of this precursor during short pulses was inhibited in infected cells. The phenomenon of increased labeling of triglyceride in infected chick cells under certain conditions is discussed, in conjunction with the other effects, in terms of related changes in other virus-infected systems.     

Induction of cyst coat protein synthesis by starvation in the ciliate Colpoda steinii

1. At 28 degrees C, synthesis of protein cyst coat in ciliates of Colpoda steinii is induced by washing with water and, as judged by glutamic acid assays and incorporation studies with l-[U-(14)C]leucine, starts about 30min after the cells have stopped swimming and is largely complete 90min later. During this time up to 70% of the protein synthesized by the cell is coat protein. 2. When cells were placed in l-[U-(14)C]leucine at low concentrations (0.25-0.76mm) during the period of coat synthesis there was no lag in uptake. Only a small proportion of the leucine incorporated into the coat was from the external substrate, implying that the rate of radioactive isotope incorporation measured the rate of transport of amino acid into the cell. Transport of l-[U-(14)C]leucine into the cell was markedly stimulated by l-glutamic acid and l-lysine. 3. When cells were placed in l-[U-(14)C]leucine at high concentrations (38mm) the rate of incorporation was considered to measure the rate of protein synthesis, but because the latter may have been affected by substrate it is concluded that such measurements are of doubtful value.     

An Hg-sensitive channel mediates the diffusional component of glucose transport in olive cells

In several organisms solute transport is mediated by the simultaneous operation of saturable and non-saturable (diffusion-like) uptake, but often the nature of the diffusive component remains elusive. The present work investigates the nature of the diffusive glucose transport in Olea europaea cell cultures. In this system, glucose uptake is mediated by a glucose-repressible, H(+) -dependent active saturable transport system that is superimposed on a diffusional component. The latter represents the major mode of uptake when high external glucose concentrations are provided. In glucose-sufficient cells, initial velocities of D- and L-[U-(14)C]glucose uptake were equal and obeyed linear concentration dependence up to 100 mM sugar. In sugar starved cells, where glucose transport is mediated by the saturable system, countertransport of the sugar pairs 3-O-methyl-D-glucose/D-[U-(14)C]glucose and 3-O-methyl-D-glucose/3-O-methyl-D-[U-(14)C]glucose was demonstrated. This countertransport was completely absent in glucose-sufficient cells, indicating that linear glucose uptake is not mediated by a typical sugar permease. The endocytic inhibitors wortmannin-A and NH(4)Cl inhibited neither the linear component of D- and L-glucose uptake nor the absorption of the nonmetabolizable glucose analog 3-O-methyl-D-[U-(14)C]glucose, thus excluding the involvement of endocytic mediated glucose uptake. Furthermore, the formation of endocytic vesicles assessed with the marker FM1-43 proceeded at a very slow rate. Activation energies for glucose transport in glucose sufficient cells and plasma membrane vesicles were 7 and 4 kcal mol(-1), respectively, lower than the value estimated for diffusion of glucose through the lipid bilayer of phosphatidylethanolamine liposomes (12 kcal mol(-1)). Mercury chloride inhibited both the linear component of sugar uptake in sugar sufficient cells and plasma membrane vesicles, and the incorporation of the fluorescent glucose analog 2-NBDG, suggesting protein-mediated transport. Diffusive uptake of glucose was inhibited by a drop in cytosolic pH and stimulated by the protein kinase inhibitor staurosporine. The data demonstrate that the low-affinity, high-capacity, diffusional component of glucose uptake occurs through a channel-like structure whose transport capacity may be regulated by intracellular protonation and phosphorylation/dephosphorylation.     

Cellobiose uptake and metabolism by Ruminococcus flavefaciens

The cellulolytic ruminal bacterium Ruminococcus flavefaciens FD-1 utilizes cellobiose but not glucose as a substrate for growth. Cellobiose uptake by R. flavefaciens FD-1 was measured under anaerobic conditions (N2), using [G-3H]cellobiose. The rate of cellobiose uptake for early- or late-log-phase cellobiose-grown cells was 9 nmol/min per mg of whole-cell protein. Cellobiose uptake was inhibited by electron transport inhibitors, iron-reactive compounds, proton ionophores, sulfhydryl inhibitors, N,N-dicyclohexylcarbodiimide, and NaF, as well as lasalocid and monensin. The results support the existence of an active transport system for cellobiose. Transport of [U-14C]glucose was not detected with this system. Phosphorylation of cellobiose was not by a phosphoenolpyruvate-dependent system. Cellobiose phosphorylase activity was detected by both a coupled spectrophotometric assay and a discontinuous assay. The enzyme was produced constitutively in cellobiose-grown cells at a specific activity of 329 nmol/min per mg of cell-free extract protein.     

Cellobiose uptake and metabolism by Ruminococcus flavefaciens.

The cellulolytic ruminal bacterium Ruminococcus flavefaciens FD-1 utilizes cellobiose but not glucose as a substrate for growth. Cellobiose uptake by R. flavefaciens FD-1 was measured under anaerobic conditions (N2), using [G-3H]cellobiose. The rate of cellobiose uptake for early- or late-log-phase cellobiose-grown cells was 9 nmol/min per mg of whole-cell protein. Cellobiose uptake was inhibited by electron transport inhibitors, iron-reactive compounds, proton ionophores, sulfhydryl inhibitors, N,N-dicyclohexylcarbodiimide, and NaF, as well as lasalocid and monensin. The results support the existence of an active transport system for cellobiose. Transport of [U-14C]glucose was not detected with this system. Phosphorylation of cellobiose was not by a phosphoenolpyruvate-dependent system. Cellobiose phosphorylase activity was detected by both a coupled spectrophotometric assay and a discontinuous assay. The enzyme was produced constitutively in cellobiose-grown cells at a specific activity of 329 nmol/min per mg of cell-free extract protein.     

An improved procedure for root surface disinfection suitable for observations of proliferation of intraradical hyphae of arbuscular mycorrhizal fungusGlomus fistulosum

A disinfection procedure for mycorrhizal root segment surface was improved using neomycin and polymyxin B besides sodium hypochlorite, streptomycin and penicillin G, to obtain clean material for observation of proliferation of hyphae of arbuscular mycorrhizal (AM) fungi. The procedure of surface disinfection is more efficient, in terms of incidence of contamination, compared to the original procedure involving only sodium hypochlorite, streptomycin and penicillin G. A further decrease of visually detectable contamination can be achieved using bacteriostatic concentrations of rolitetracycline which cause a decrease of hyphal growth of the AM fungusGlomus fistulosum. The technique was used in experiment studying the uptake ofU-¹⁴C-glucose by proliferating hyphae. An active uptake of glucose occurred even in the presence of all four antibiotics at concentrations of 500 mg/L.     

The metabolism of C-glucose by neurons and astrocytes in brain subregions following focal cerebral ischemia in rats

To provide insights into the effects of temporary focal ischemia on the function of neurons and astrocytes in vivo, we measured the incorporation of radiolabel from [U-14C]glucose into both glutamate and glutamine in brain subregions at 1 h of reperfusion following occlusion of the middle cerebral artery for 2 or 3 h. Under the experimental conditions used, 14C-glutamate is mainly produced in neurons whereas 14C-glutamine is generated in astrocytes from 14C-glutamate of both neuronal and astrocytic origin. Radiolabel incorporation into both amino acids was greatly decreased. The change in 14C-glutamate accumulation provides strong evidence for substantial reductions in neuronal glucose metabolism. The resulting decrease in delivery of 14C-glutamate from the neurons to astrocytes was probably also the major contributor to the change in 14C-glutamine content. These alterations probably result in part from a marked depression of glycolytic activity in the neurons, as suggested by previous studies assessing deoxyglucose utilization. Alterations in 14C-glucose metabolism were not restricted to tissue that would subsequently become infarcted. Thus, these changes did not inevitably lead to death of the affected cells. The ATP : ADP ratio and phosphocreatine content were essentially preserved during recirculation following 2 h of ischemia and showed at most only moderate losses in some subregions following 3 h of ischemia. This retention of energy reserves despite the decreases in 14C-glucose metabolism in neurons suggests that energy needs were substantially reduced in the post-ischemic brain. Marked increases in tissue lactate accumulation during recirculation, particularly following 3 h of ischemia, provided evidence that impaired pyruvate oxidation probably also contributed to the altered 14C-glucose metabolism. These findings indicate the presence of complex changes in energy metabolism that are likely to greatly influence the responses of neurons and astrocytes to temporary focal ischemia.     

Metabolic and secretory responses of parotid cells to cationic amino acids. Oxidation of the amino acids and interference with the oxidation of D-glucose or endogenous nutrients.

Cationic amino acids were recently found to stimulate amylase release from rat parotid cells. The possible relevance of their oxidative catabolism to such a secretory stimulation was investigated. D-Glucose, which was efficiently metabolized in parotid cells and which augmented O2 uptake above basal value, failed to affect basal or stimulated amylase release. L-Arginine, L-lysine and L-histidine failed to stimulate the oxidation of either exogenous D-[6-14C]glucose or endogenous nutrients in cells pre-labelled with [U-14C]palmitate or L-[U-14C]glutamine. The oxidation of L-[U-14C]arginine, L-[U-14C]ornithine, L-[U-14C]lysine and L-[U-14C]histidine, all tested at a 10 mM concentration, was much lower than that of D-[U-14C]glucose (5.6 mM). These findings argue against the view that the stimulation of amylase release by cationic amino acids would be related to their role as a source of energy in the parotid cells.     

Characterization of metformin transport system in NIH 3T3 cells.

The biochemical properties of the metformin transport system were studied in NIH 3T3 cells. 14C-metformin uptake appeared to be a sodium dependent process. Iso-osmotical replacement of Na+ by choline chloride in the assay medium resulted in a decrease of metformin uptake. Amiloride (200 microM) inhibited the metformin transport by 35% in these cells. Gramicidin, a channel ionophore, was the most effective in inhibiting the metformin transport as compared to valinomycin, a mobile ion carrier, and Ca2+ ionophore (A 23187). Loading of cells with asparagine, ornithine, or polylysine did not influence the uptake process. However, the addition of lysine or arginine significantly stimulated the metformin uptake by NIH 3T3 cells. Similarly, the addition of metformin stimulated the arginine uptake by these cells, suggesting that metformin shares the y+ transport system. Metformin inhibited competitively the uptake of 14C-spermidine, a molecule of the polyamine family, by NIH 3T3 cells, whereas the latter failed to influence the uptake of the former significantly by these cells. Incubation of NIH 3T3 cells in the presence of difluoromethyl-ornithine (a suicidal inhibitor of polyamine biosynthesis) stimulated the spermidine, but not the metformin, uptake by these cells. Interestingly, a prolonged incubation of these cells in the presence of metformin failed to down-regulate the spermidine transport process. The spermidine- and methylglyoxal-bis(guanylhydrazone), MGBG-transport deficient (3T3MG) cells which do not accumulate exogeneous spermidine or MGBG, took up 14C-metformin. However, 14C-metformin uptake by 3T3MG cells was lower than that by normal NIH 3T3 cells.     

Transport of glucose by Bifidobacterium animalis subsp. lactis occurs via facilitated diffusion

Two strains of Bifidobacterium animalis subsp. lactis were indistinguishable by several nucleic acid-based techniques; however, the type strain DSMZ 10140 was glucose utilization positive, while RB 4825, an industrially employed strain, was unable to grow rapidly on glucose as the principal carbon source. This difference was attributed to the presence of a low-affinity facilitated-diffusion glucose transporter identified in DSMZ 10140 but lacking in RB 4825. Uptake of D-[U-(14)C]glucose in DSMZ 10140 was stimulated by monovalent cations (ammonium, sodium, potassium, and lithium) and inhibited by divalent cations (calcium and magnesium). When competitor carbohydrates were included in the uptake assays, stereospecific inhibition was exhibited, with greater competition by methyl-beta-glucoside than methyl-alpha-glucoside. Significant inhibition (>30%) was observed with phloretin, an inhibitor of facilitated diffusion of glucose, whereas there was no inhibition by sodium fluoride, iodoacetate, sodium arsenate, sodium azide, 2,4-dinitrophenol, monensin, or valinomycin, which typically reduce energy-driven transport. Based on kinetic analyses, the mean values for K(t) and V(max) were 14.8 +/- 3.4 mM D-glucose and 0.13 +/- 0.03 micromol glucose/min/mg cell protein, respectively. Glucose uptake by several glucose-utilizing commercial strains of B. animalis subsp. lactis was also inhibited by phloretin, indicating the presence of facilitated diffusion glucose transporters in those strains. Since DSMZ 10140 has been previously reported to lack a functional glucose phosphoenolpyruvate phosphotransferase system, the glucose transporter identified here is responsible for much of the organism's glucose uptake.