Low Concentrations Of Fructose Research Articles

FRUCTOSE INSENSITIVE1 regulates stem cell function in Arabidopsis in response to fructose signalling.

Stem cell function in different meristems of Arabidopsis is mainly defined by WUSCHEL (WUS)/WUSCHEL-RELATED HOMEOBOX (WOX) family of proteins. Sugars have also been demonstrated to play pivotal roles in stem cell function and development of plants. As a cytosolic fructose 1,6-bisphosphatase, FRUCTOSE INSENSITIVE1 (FINS1) has been demonstrated to regulate plant growth in response to fructose signalling. However, it remains to be elucidated how stem cell function is regulated in response to fructose signalling. Our work showed that FINS1 interacts with WUS/WOX protein as a complex, which further binds to the promoter of WUS/WOX and regulates its expression in response to fructose signalling. FINS1 might act as a bifunctional factor that promotes WUS/WOX expression in the presence of low concentrations of fructose, and represses WUS/WOX expression in the presence of high concentrations of fructose. Therefore, FINS1 regulates stem cell function in response to fructose signalling during the growth and development of Arabidopsis.

A fructose/H+ symporter controlled by a LacI-type regulator promotes survival of pandemic Vibrio cholerae in seawater

The bacterium Vibrio cholerae can colonize the human intestine and cause cholera, but spends much of its life cycle in seawater. The pathogen must adapt to substantial environmental changes when moving between seawater and the human intestine, including different availability of carbon sources such as fructose. Here, we use in vitro experiments as well as mouse intestinal colonization assays to study the mechanisms used by pandemic V. cholerae to adapt to these environmental changes. We show that a LacI-type regulator (FruI) and a fructose/H+ symporter (FruT) are important for fructose uptake at low fructose concentrations, as those found in seawater. FruT is downregulated by FruI, which is upregulated when O2 concentrations are low (as in the intestine) by ArcAB, a two-component system known to respond to changes in oxygen levels. As a result, the bacteria predominantly use FruT for fructose uptake under seawater conditions (low fructose, high O2), and use a known fructose phosphotransferase system (PTS, Fpr) for fructose uptake under conditions found in the intestine. PTS activity leads to reduced levels of intracellular cAMP, which in turn upregulate virulence genes. Our results indicate that the FruT/FruI system may be important for survival of pandemic V. cholerae in seawater.

DNA methylation during human adipogenesis and the impact of fructose

BackgroundIncreased adipogenesis and altered adipocyte function contribute to the development of obesity and associated comorbidities. Fructose modified adipocyte metabolism compared to glucose, but the regulatory mechanisms and consequences for obesity are unknown. Genome-wide methylation and global transcriptomics in SGBS pre-adipocytes exposed to 0, 2.5, 5, and 10 mM fructose, added to a 5-mM glucose-containing medium, were analyzed at 0, 24, 48, 96, 192, and 384 h following the induction of adipogenesis.ResultsTime-dependent changes in DNA methylation compared to baseline (0 h) occurred during the final maturation of adipocytes, between 192 and 384 h. Larger percentages (0.1% at 192 h, 3.2% at 384 h) of differentially methylated regions (DMRs) were found in adipocytes differentiated in the glucose-containing control media compared to adipocytes differentiated in fructose-supplemented media (0.0006% for 10 mM, 0.001% for 5 mM, and 0.005% for 2.5 mM at 384 h). A total of 1437 DMRs were identified in 5237 differentially expressed genes at 384 h post-induction in glucose-containing (5 mM) control media. The majority of them inversely correlated with the gene expression, but 666 regions were positively correlated to the gene expression.ConclusionsOur studies demonstrate that DNA methylation regulates or marks the transformation of morphologically differentiating adipocytes (seen at 192 h), to the more mature and metabolically robust adipocytes (as seen at 384 h) in a genome-wide manner. Lower (2.5 mM) concentrations of fructose have the most robust effects on methylation compared to higher concentrations (5 and 10 mM), suggesting that fructose may be playing a signaling/regulatory role at lower concentrations of fructose and as a substrate at higher concentrations.

Repercussions of low fructose-drinking water in male rats.

Fructose consumption has increased worldwide, and it has been associated with the development of metabolic diseases such as insulin resistance (IR) and steatosis. The aim was to evaluate if lower fructose concentrations may cause pancreatic structural abnormalities, leading to a glucose intolerance without steatosis in male rats. Young male rats orally received 7% fructose solution for 12 weeks. Body weight, food, water, and energy intake were measured. An oral glucose tolerance test (OGTT) was performed. After final experimental period, all rats were anaesthetized and killed. Blood samples were collected for biochemical analyses and organs (liver and pancreas) were processed for morphological analyses. Fructose consumption was not associated with lipid accumulation in liver. However, fructose administration was associated with an increased area under curve from OGTT and an increased percentage of insulin-positive cells, high beta cell mass and reduced pancreatic islet area. Fructose supplementation (7%) did not cause steatosis, but it led to abnormal morphology and function of pancreatic islet cells, contributing for glucose intolerance development. Our findings demonstrate that even low fructose concentrations may cause deleterious effects in animals.

Surface Engineering of Phenylboronic Acid-Functionalized Poly(3,4-ethylenedioxythiophene) for Fast Responsive and Sensitive Glucose Monitoring

In this study, we have successfully demonstrated a nanostructured phenylboronic acid-grafted poly(3,4-ethylenedioxythiophene), poly(EDOT-PBA), platform for fast and sensitive glucose monitoring. The poly(EDOT-PBA) films of well-organized tubular nanostructures can be fabricated by direct electropolymerization without templates. Compared to the smooth poly(EDOT-PBA), the nanotubular poly(EDOT-PBA) shows enhanced glucose sensitivity and a different adsorption process of bovine serum albumin (BSA). Besides, the BSA blocking and low concentration of fructose and galactose do not affect the sensitivity of this platform. Both quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS) methods are used and compared for glucose monitoring by applying nanotubular poly(EDOT-PBA) as conductive substrates. Compared to QCM analysis, EIS has a higher sensitivity to glucose and the detection limit is about 50 μM. Besides, the binding with glucose on poly(EDOT-PBA) is highly reversibly. On the basi...

A Newly Isolated Penicillium oxalicum 16 Cellulase with High Efficient Synergism and High Tolerance of Monosaccharide

Compared to Trichoderma reesei RUT-C30 cellulase (Trcel), Penicillium oxalicum 16 cellulase (P16cel) from the fermentation supernatant produced a 2-fold higher glucose yield when degrading microcrystalline cellulose (MCC), possessed a 10-fold higher β-glucosidase (BGL) activity, but obtained somewhat lower other cellulase component activities. The optimal temperature and pH of β-1,4-endoglucanase, cellobiohydrolase, and filter paperase from P16cel were 50-60 °C and 4-5, respectively, but those of BGL reached 70 °C and 5. The cellulase cocktail of P16cel and Trcel had a high synergism when solubilizing MCC and generated 1.7-fold and 6.2-fold higher glucose yields than P16cel and Trcel at the same filter paperase loading, respectively. Additional low concentration of fructose enhanced the glucose yield during enzymatic hydrolysis of MCC; however, additional high concentration of monosaccharide (especially glucose) reduced cellulase activities and gave a stronger monosaccharide inhibition on Trcel. These results indicate that P16cel is a more excellent cellulase than Trcel.

56. Poly-vitrification – A new approach to organ preservation

Vitrification has long promised to revolutionize organ cryopreservation and banking, but successful vitrification of complex tissues remains elusive. The two pillars of vitrification – high CPA concentrations coupled with rapid cooling/warming rates – are incompatible for large volume cryopreservation. Challenges include (1) delivery of sufficiently high CPA concentrations homogeneously throughout the entire organ prior to onset of CPA-induced toxicity and (2) uneven cooling and warming rates. To overcome these challenges in whole organ cryopreservation, we propose the concept of a CPA cocktail whose constituents undergo reversible polymerization-like reactions at low temperatures. The cocktail would remain fluid and easily perfusable at high-subzero temperatures, but rapidly increases in viscosity at lower temperatures, above and beyond that seen with individual CPAs. To our knowledge, this approach to organ cryopreservation has not been discussed in the literature. We call this method poly-vitrification, for polymerization-enhanced vitrification. Systems showing polymerization-like effects have been observed in nature, namely with the Goldenrod Gall Fly, which relies on interactions between fructose, glycerol and trehalose. Here, a relatively low concentration of fructose (0.011 M) drives extensive interactions with glycerol and trehalose, forming a partial glassy state at approximately −25 °C, well above the sublimation point of CO2. The benefits of poly-vitrification are two-fold: first, the temperature of the glass transition could perhaps be raised to sub-zero temperatures well above the sublimation point of CO2. This would hypothetically reduce the temperature range for which cooling is required, minimizing toxic exposure to cryoprotectants during cooling and warming. With a higher vitrification temperature, organ transport teams could use dry ice to transport organs to and from areas without LN2 infrastructure. Second, this polymerization-like process would reduce the total concentration of CPAs to perhaps only 1-2 Molar, greatly reducing the effect of CPA toxicity. For poly-vitrification to be successful, extra- and intra-cellular CPAs would have to be equilibrated to ensure vitrification extra- and intra-cellularly occurs at the same time. Methods to load cells with impermeable or slowly permeating molecules, such as fructose, would have to be further developed, perhaps through sugar loaded liposomes, micelles, or an enzymatically triggered CPA-delivery platform built on existing technologies. Alternatively or in conjunction with these methods, synthetic molecules analogous to the natural poly-vitrification systems could be developed. Molecular screening is a widespread approach to developing such synthetic compounds, and warrants further attention in cryobiology. We hypothesize that poly-vitrification could be used to both reduce CPA concentration and toxicity and allow vitrification at much higher temperatures. This will enable reliable and robust organ and tissue preservation to become widespread.

Preparation of Ag-coated hollow microspheres via electroless plating for application in lightweight microwave absorbers

Highly conductive Ag film is coated on hollow silica microspheres via electroless plating for application in lightweight microwave absorbers. The Ag plating is conducted using a two-step process of sensitizing and subsequent plating. The complex permeability and permittivity are determined using the reflection/transmission technique in the composite specimens of Ag-coated microspheres and silicone rubber matrix. Due to the large surface area of the microspheres, a relatively high concentration of AgNO3 is required in order to achieve a uniform Ag coating. In addition, a low concentration of fructose reducing agent is recommended for slow plating. The apparent electrical resistance of the Ag-coated microspheres is strongly dependent on the grain morphology. The thin and uniform Ag-coated particles are characterized by their low electrical resistance, which is as low as 0.1Ω. The lower the electrical resistance of the microspheres, the higher the dielectric constant of the composite specimens, which results from the enhanced space-charge polarization between the conductive microspheres. The microwave absorbance is enhanced with decreases in the electrical resistance of microspheres due to the increased dielectric loss.

Anoxia-induced changes in optimal substrate for peripheral nerve

Hyperglycemia accentuates the injury produced by anoxia both in the central and peripheral nervous system. To understand whether this is a consequence of changes in metabolic pathways produced by anoxia, the effect of the metabolic substrate used by the rat peripheral nerve on the nerve action potential (NAP) was studied in the presence and absence of anoxia. In the continuously oxygenated state, the NAP was well preserved with glucose, lactate, as well as with high concentrations of sorbitol and fructose but not β-hydroxybutyrate, acetate or galactose. With intermittent anoxia, the pattern of substrate effects on the NAP changed markedly so that low concentrations of fructose became able to support neurophysiologic activity but not high concentrations of glucose. These alterations occurred gradually with repeated episodes of anoxia as reflected by the progressive increase in the time needed for the NAP to disappear during anoxia when using glucose as substrate. This “preconditioning” effect was not seen with other substrates and an opposite effect was seen with lactate. In fact, the rate at which the NAP disappeared during anoxia was not simply related to degree of recovery after anoxia. These are distinct phenomena. For example, the NAP persisted longest during anoxia in the setting of hyperglycemia but this was the state in which the anoxic damage was most severe.Correlating the results with existing literature on the metabolic functions of Schwann cells and axons generates testable hypotheses for the mechanism of hyperglycemic damage during anoxia and lead to discussions of the role for a metabolic shuttle between Schwann cells and axons as well as a potential important role of glycogen.

ZrFsy1, a High-Affinity Fructose/H+ Symporter from Fructophilic Yeast Zygosaccharomyces rouxii

Zygosaccharomyces rouxii is a fructophilic yeast than can grow at very high sugar concentrations. We have identified an ORF encoding a putative fructose/H+ symporter in the Z. rouxii CBS 732 genome database. Heterologous expression of this ORF in a S. cerevisiae strain lacking its own hexose transporters (hxt-null) and subsequent kinetic characterization of its sugar transport activity showed it is a high-affinity low-capacity fructose/H+ symporter, with Km 0.45±0.07 mM and Vmax 0.57±0.02 mmol h−1 (gdw) −1. We named it ZrFsy1. This protein also weakly transports xylitol and sorbose, but not glucose or other hexoses. The expression of ZrFSY1 in Z. rouxii is higher when the cells are cultivated at extremely low fructose concentrations (<0.2%) and on non-fermentable carbon sources such as mannitol and xylitol, where the cells have a prolonged lag phase, longer duplication times and change their microscopic morphology. A clear phenotype was determined for the first time for the deletion of a fructose/H+ symporter in the genome where it occurs naturally. The effect of the deletion of ZrFSY1 in Z. rouxii cells is only evident when the cells are cultivated at very low fructose concentrations, when the ZrFsy1 fructose symporter is the main active fructose transporter system.

Transcriptional Regulation and Signal-Peptide-Dependent Secretion of Exolevanase (LsdB) in the Endophyte Gluconacetobacter diazotrophicus

Gluconacetobacter diazotrophicus utilizes plant sucrose with a constitutively expressed levansucrase (LsdA), producing extracellular levan, which may be degraded under energetically unfavored conditions. Reverse transcriptase-PCR analysis revealed that lsdA and the downstream exolevanase gene (lsdB) form an operon. lsdB transcription was induced during growth with low fructose concentrations (0.44 to 33 mM) and repressed by glucose. Transport of LsdB to the periplasm involved N-terminal signal peptide cleavage. Type II secretion mutants failed to transfer LsdB across the outer membrane, impeding levan hydrolysis.

The body composition and metabolic status of twin- and triplet-bearing ewes and their fetuses in late pregnancy

Abstract This study investigated the body composition and metabolic status of twin- and triplet-bearing ewes and their fetuses in late pregnancy. The aim was to identify factors that could explain the lower survival rates of triplet lambs compared with twin lambs. Ten twin- and 10 triplet-bearing ewes and their fetuses were euthanased on either day 139 or 140 of pregnancy and fetal and placental measurements recorded. The weights of the gravid uterus ( P P P P = 0.05) individual cotyledon weight, resulting in no differences in either total fetal membrane plus cotyledon or total cotyledon weights. The absolute weight of many organs were significantly ( P P P The mean weight of the lightest triplet fetus within each triplet set was 31% lighter than that of the heaviest triplet ( P P P P = 0.05) greater kidney weights and tended to have heavier heart ( P = 0.07) and lungs ( P = 0.07). It is unknown why the relative organ weight relationships between the heaviest and lightest differ between twin and triplet fetuses. In conclusion this study has identified three factors which may contribute to the lower survival rates observed in triplet-compared to twin-born lambs; low fructose concentrations, lighter fetal weights, especially in one fetus within each triplet set, and maternal mammary gland increases that are not proportional to the increase in numbers of fetuses. While these findings are not novel, there were no other factors identified that could account the lower survival rates reported in triplet-born lambs compared to twins. The other only other significant differences identified were the proportionally lighter livers and hearts in triplets although the possible effects of these on survival is unknown.

Semen Analysis in an Infertile Man with Seminal Vesicles Cysts Associated with Ipsilateral Renal Agenesis

A case of an infertile man with bilateral seminal vesicles cysts with ipsilateral renal agenesis is presented with emphasis on the semen analysis. Ejaculates showed very low concentration of fructose, leukocytospermia, teratozoospermia, asthenozoospermia, hyperviscosity, marked sperm agglutination and a poor hypoosmotic swelling test. Clinical value of these findings is discussed.

GLASS TRANSITION TEMPERATURE BEHAVIOR OF A MODEL BLEND OF CARBOHYDRATESCOMPORTAMIENTO DE LA TEMPERATURA DE TRANSICIÓN VÍTREA DE UNA MEZCLA MODELO DE CARBOHIDRATOSCOMPORTAMENTO DA TEMPERATURA DE TRANSICIÓN VÍTREA DUNHA MEZCLA MODELO DE CARBOHIDRATOS

The glass transition of amorphous food materials in processing and storage of foods is important. The effect of a quaternary model mixture from glucose, fructose, sucrose and water on the glass transition temperature was determined. State and phase diagrams were elaborated as well solubility curves of these mixtures. The measurements of glass transition temperature of mixtures and pure components were carried out on a differential scanning calorimeter. An empirical model was created to involve the mixture behavior. The carbohydrates present in the mixture had influence between them in the glass transition temperature determination. The solubility of glucose and sucrose showed a linear behavior with a change of first order. The solubility of fructose pointed out the presence of eutectics to high and low concentration of fructose, having a high change in the molecular arrangement between 140 and 150°C, as a result the fructose had molecular rejection to glucose and sucrose. © 2005 Altaga. All rights reserved. Resumen La transición vítrea de materiales alimentarios heterogéneos en el procesado y almacenaje de alimentos es importante. En este estudio se determinó el efecto de una mezcla modelo cuaternaria de glucosa, fructosa, sacarosa y agua sobre la temperatura de transición vítrea. Se elaboraron diagramas de fase y estado, así como curvas de solubilidad de las mezclas. Las medidas de temperatura de transición vítrea de las mezclas y componentes puros se realizaron con un calorímetro diferencial de barrido. Se desarrolló un modelo empírico para definir el comportamiento de las mezclas. Los carbohidratos presentes en las mezclas tuvieron influencia sobre la temperatura de transición vítrea. La solubilidad de glucosa y sacarosa mostró un comportamiento lineal con cambios de primer orden. La solubilidad de fructosa apuntó la presencia de eutécnicos de concentración alta y baja de fructosa, teniendo un gran cambio en su conformación molecular entre 140°C y 150°C, con el resultado de que la fructosa mostró un rechazo hacia la glucosa y sacarosa. © 2005 Altaga. Todos los derechos reservados. Palabras clave: Palabras chave, Carbohidratos, transición vítrea, solubilidade. Resumo A transición vítrea de materiais alimentarios heteroxéneos no procesado e almacenaxe de alimentos é importante. Neste estudio determinóu-se o efecto dunha mezcla modelo cuaternaria de glucosa, fructosa, sacarosa e auga sobre a temperatura de transición vítrea. Elaboráron-se diagramas de fase e estado, así como curvas de solubilidade das mezclas. As medidas de temperatura de transición vítrea das mezclas e compoñentes puros realizáron-se cun calorímetro diferencial de barrido. Desenrolou-se un modelo empírico para defini-lo comportamento das mezclas. Os carbohidratos presentes nas mezclas tiveron influencia sobre a temperatura de transición vítrea. A solubilidade de glucosa e sacarosa amosou un comportamento lineal con cambios de primeiro orde. A solubilidade de fructosa apuntou a presencia de eutécnicos de concentración alta e baixa de fructosa, tendo un gran cambio na súa conformación molecular entre 140°C e 150°C, co resultado de que a fructosa amosou un rechazo cara a glucosa e sacarosa. © 2005 Altaga. Tódolos dereitos reservados. Palabras clave: Carbohidratos, transición vítrea, solubilidad.

Effects of shoot girdling on bud abscission, carbohydrate and nutrient concentrations in pistachio (Pistacia vera L.)

SummaryThe effects of shoot girdling and fruit removal on bud abscission, and of shoot girdling on nutrient and carbohydrate concentrations in leaves, inflorescent buds and current wood of pistachio shoots were investigated. Girdling individual shoots at the base of the current year’s shoot (girdle I), thus separating inflorescent buds at the terminus of the current year’s shoot from the developing fruits, reduced inflorescent bud abscission by 62–66% compared to untreated controls. In contrast, girdling at the base of 1 year-old shoots (girdle II) or fruit removal treatments reduced bud abscission by 20% and 16%, respectively, in 1994; but had no effect on bud abscission in 1995. Girdle I significantly decreased N concentrations in leaves and Ca, Mg and Mn concentrations in all parts. In contrast, concentrations of P and K in inflorescent buds were significantly higher in girdled shoots. Girdle I increased the dry weight of inflorescent buds and non-structural carbohydrate concentrations in inflorescent buds and current year’s wood. Current wood and leaves on girdled shoots had higher starch accumulation, but leaves had lower fructose concentrations than control leaves. Inflorescent buds on girdled shoots had higher concentrations of starch and sugars, especially sucrose. The possibility that a reduction in sucrose movement from leaves to inflorescent buds inhibits bud growth and triggers inflorescent bud abscission is discussed.

Regulation of hepatic glucose metabolism by translocation of glucokinase between the nucleus and the cytoplasm in hepatocytes.

We studied the role of glucokinase translocation between the nucleus and the cytoplasm in hepatocytes. In cultured hepatocytes, both the translocation of glucokinase from the nucleus to the cytoplasm and the rate of glucose phosphorylation were increased when cells were incubated with high concentrations of glucose. The addition of low concentrations of fructose, which is known to stimulate glucose phosphorylation, stimulated both glucokinase translocation and glucose phosphorylation. There was a good correlation between the increase in cytoplasmic glucokinase induced by fructose and that in the glucose phosphorylation rate induced by fructose. Furthermore, we observed a linear relationship between cytoplasmic glucokinase activity and rate of glucose phosphorylation over various glucose concentrations in the absence or presence of fructose. These results indicate that glucose phosphorylation in hepatocytes depended on glucokinase in the cytoplasmic compartment--that is, the increase in the rate of glucose phosphorylation was due to the increase in translocation of glucokinase out of the nucleus. Also, oral administration of glucose, fructose, or glucose plus fructose to 24-h fasted rats induced translocation of glucokinase in the liver. All of these results indicate that hepatic glucose metabolism is regulated by the translocation of glucokinase.

Long-term maintenance of low concentrations of fructose for the study of hepatic glucose phosphorylation

The stimulation of glucose phosphorylation in isolated hepatocytes by low fructose concentrations is transient due to the rapid metabolism of fructose. To prolong this stimulatory effect fructose was enzymically generated in the incubation medium from either sucrose with invertase or inulin with inulinase. A maximal rate of glucose phosphorylation was achieved when fructose was formed at at least 0.01 micromol/min, which maintained a concentration of 70 microM fructose in the medium. In the presence of a fructose concentration of 70 microM, the rate of phosphorylation with 5 mM glucose was doubled and remained constant over a 2.5 h period. Under these conditions the rate of glycolysis was increased more than 3-fold. The stimulation of flux through glucokinase by low concentrations of fructose decreased the proportion of glucose phosphorylated, which was cycled between glucose and glucose 6-phosphate, and increased the proportion that was glycolysed. The method described for maintaining the stimulation of glucose phosphorylation by isolated hepatocytes over prolonged incubation periods is especially suited to the further study of the control of glucokinase activity, in particular how the variation of flux through glucokinase affects the flux through all the pathways that utilize the product, glucose 6-phosphate.

Long-term maintenance of low concentrations of fructose for the study of hepatic glucose phosphorylation

The stimulation of glucose phosphorylation in isolated hepatocytes by low fructose concentrations is transient due to the rapid metabolism of fructose. To prolong this stimulatory effect fructose was enzymically generated in the incubation medium from either sucrose with invertase or inulin with inulinase. A maximal rate of glucose phosphorylation was achieved when fructose was formed at at least 0.01 µmol/min, which maintained a concentration of 70 µM fructose in the medium. In the presence of a fructose concentration of 70 µM, the rate of phosphorylation with 5 mM glucose was doubled and remained constant over a 2.5 h period. Under these conditions the rate of glycolysis was increased more than 3-fold. The stimulation of flux through glucokinase by low concentrations of fructose decreased the proportion of glucose phosphorylated, which was cycled between glucose and glucose 6-phosphate, and increased the proportion that was glycolysed. The method described for maintaining the stimulation of glucose phosphorylation by isolated hepatocytes over prolonged incubation periods is especially suited to the further study of the control of glucokinase activity, in particular how the variation of flux through glucokinase affects the flux through all the pathways that utilize the product, glucose 6-phosphate.

The physiological role of glucokinase binding and translocation in hepatocytes

The compartmentation of glucokinase in the hepatocyte is regulated by the extracellular glucose concentration and by substrates that alter the concentration of fructose 1-phosphate in the hepatocyte. At low glucose concentrations, that mimic the fasted state, glucokinase is sequestered in an inactive state bound to the 68 kDa regulatory protein in the nucleus. In these conditions the rate of glucose phosphorylation is less than 15% of the total glucokinase activity. An increase in extracellular glucose concentration, within the range occurring in the portal vein in the absorptive state, or low concentrations of fructose or sorbitol (precursors of fructose 1-phosphate), cause the translocation of glucokinase from the nucleus to the cytoplasm and this is associated with a corresponding increase in glucose phosphorylation. The effect of glucose on translocation is mimicked by mannose which is also phosphorylated by glucokinase as well as by competitive inhibitors of glucokinase (mannoheptulose and 5-thioglucose) which are not phosphorylated. Various lines of evidence suggest that the action of these analogues is most likely due to binding to an allosteric or non-catalytic site. The saturation curve of glucose phosphorylation in intact hepatocytes is sigmoidal with an S0.5 of approximately 20 mM and a Hill coefficient approximately 2. This saturation curve can be explained by the activity of glucokinase in the cytoplasmic compartment. Translocation of glucokinase from the nucleus to the cytoplasm in response to precursors of fructose 1-phosphate (which cause dissociation of glucokinase from the regulatory protein) is associated with stimulation of glucose phosphorylation, glycolysis and glycogen synthesis. Using Metabolic Control Analysis to determine the Control Coefficient (Control Strength) of cytoplasmic (free) glucokinase on glucose metabolism it can be shown that the free glucokinase activity has a very high control strength on glycogen synthesis (CFGKJ > 1), indicating a major role of translocation of glucokinase in the control of hepatic glycogen synthesis. Overexpression of glucokinase in hepatocytes by adenovirus-mediated glucokinase overexpression is associated with a marked increase in glycogen synthesis. The relation between glycogen synthesis and enzyme overexpression is sigmoidal with an enzyme concentration causing half-saturation (S0.5) in the physiological range. The high Control Coefficient of glucokinase on hepatic glycogen synthesis explains the abnormalities of hepatic glycogen synthesis in patients with a single mutant allele of the glucokinase gene (Maturity Onset Diabetes of the Young, type 2).

Human Erythrocytes Express GLUT5 and Transport Fructose

AbstractAlthough erythrocytes readily metabolize fructose, it has not been known how this sugar gains entry to the red blood cell. We present evidence indicating that human erythrocytes express the fructose transporter GLUT5, which is the major means for transporting fructose into the cell. Immunoblotting and immunolocalization experiments identified the presence of GLUT1 and GLUT5 as the main facilitative hexose transporters expressed in human erythrocytes, with GLUT2 present in lower amounts. Functional studies allowed the identification of two transporters with different kinetic properties involved in the transport of fructose in human erythrocytes. The predominant transporter (GLUT5) showed an apparent Km for fructose of approximately 10 mmol/L. Transport of low concentrations of fructose was not affected by 2-deoxy–D-glucose, a glucose analog that is transported by GLUT1 and GLUT2. Similarly, cytochalasin B, a potent inhibitor of the functional activity of GLUT1 and GLUT2, did not affect the transport of fructose in human erythrocytes. The functional properties of the fructose transporter present in human erythrocytes are consistent with a central role for GLUT5 as the physiological transporter of fructose in these cells.