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PubMed1996

Comparison of protection by fructose against paracetamol injury with protection by glucose and fructose-1,6-diphosphate.

We have compared the protective effect of fructose in normal Ringer solution during the onset and progression of cell injury induced by paracetamol in rat liver slices with the protective effect of glucose and fructose-1,6-diphosphate. Liver slices obtained from phenobarbitone-induced and non-induced rats were used in a model in vitro system. Slices were exposed to 10 mM paracetamol for 120 min and then incubated without paracetamol in the presence or absence of protective agents for a further 240 min. Cell injury was quantified by measuring leakage of lactate dehydrogenase (LDH) and potassium (K+). Adenosinetriphosphate (ATP) levels were measured using the luciferin-luciferase bioluminescence assay. Addition of higher concentrations of glucose (10-50 mM) to Ringer solution were not found to result in protection at the end of incubation in paracetamol-treated slices obtained from phenobarbitone-induced rats. Neither did sucrose nor mannitol protect. However, exclusion of glucose from Ringer solution resulted in cell injury in paracetamol-treated slices obtained from non-induced rats. Methionine, a known antidote for paracetamol poisoning, failed to protect in this instances but fructose did protect. This suggests that the presence of a glycolytic substrate plays a crucial role in cell protection. Further evidence for this is the finding that iodoacetate, an inhibitor of glycolysis, not only increase cell injury in paracetamol-treated slices but also reverses fructose protection. Fructose-1,6-diphosphate was found to protect against the onset and progression of cell injury in paracetamol-treated slices obtained from phenobarbitone induced rats. This protective agent is found to maintain high ATP levels and cell viability in paracetamol-treated slices at a time when paracetamol-treated slices show a profound loss of ATP levels and a significant increase in cell injury as measured by leakage of LDH and K+.

Acetaminophen↗
PubMed1991

Fructose transport by rat intestinal brush border membrane vesicles. Effect of high fructose diet followed by return to standard diet.

1. D-Fructose uptake in isolated rat intestinal brush border membrane comprised a simple diffusional component and a saturable, carrier-mediated, component. The latter did not follow typical Michaelis-Menten kinetics and appeared as a low affinity, high capacity process (Kt = 110 mM, Jmax = 160.5 nmol/min.mg protein). 2. Carrier-mediated D-fructose uptake was highly specific, Na independent and unaffected by phloridzin and metabolic inhibitors. 3. Fructose feeding for 3 days resulted in an activation of D-fructose uptake after a latent period superior to 3 days. The increase was of relatively short duration since it was not further observed 6 days after the return to the standard diet. 4. The activation in D-fructose uptake was due to a slight decrease in Kt and a marked increase in Jmax (from 160.5 to 306 nmol/min.mg protein), suggesting a rise in the number of transporters.

Animals↗
PubMed2004

An integrated bienzyme glucose oxidase-fructose dehydrogenase-tetrathiafulvalene-3-mercaptopropionic acid-gold electrode for the simultaneous determination of glucose and fructose.

A bienzyme biosensor for the simultaneous determination of glucose and fructose was developed by coimmobilising glucose oxidase (GOD), fructose dehydrogenase (FDH), and the mediator, tetrathiafulvalene (TTF), by cross-linking with glutaraldehyde atop a 3-mercaptopropionic acid (MPA) self-assembled monolayer (SAM) on a gold disk electrode (AuE). The performance of this bienzyme electrode under batch and flow injection (FI) conditions, as well as an amperometric detection in high-performance liquid chromatography (HPLC), are reported. The order of enzyme immobilisation atop the MPA-SAM affected the biosensor amperometric response in terms of sensitivity, with the immobilisation order GOD, FDH, TTF being selected. Similar analytical characteristics to those obtained with single GOD or FDH SAM-based biosensors for glucose and fructose were achieved with the bienzyme electrode, indicating that no noticeable changes in the biosensor responses to the analytes occurred as a consequence of the coimmobilisation of both enzymes on the same MPA-AuE. The suitability of the bienzyme biosensor for the analysis of real samples under flow injection conditions was tested by determining glucose in two certified serum samples. The simultaneous determination of glucose and fructose in the same sample cannot be performed without a separation step because at the detection potential used (+0.10 V), both sugars show amperometric response. Consequently, HPLC with amperometric detection at the TTF-FDH-GOD-MPA-AuE was accomplished. Glucose and fructose were simultaneously determined in honey, cola softdrink, and commercial apple juice, and the results were compared with those obtained by using other reference methods.

Biosensing Techniques↗
PubMed1985

The reactive cysteine residue of pig kidney fructose 1,6-bisphosphatase is related to a fructose 2,6-bisphosphate allosteric site.

Modification of a highly reactive cysteine residue of pig kidney fructose 1,6-bisphosphatase with N-ethylmaleimide results in the loss of activation of the enzyme by monovalent cations. Low concentrations of fructose 2,6-bisphosphate or high (inhibitory) levels of fructose 1,6-bisphosphate protect the enzyme against the loss of monovalent cation activation, while non-inhibitory concentrations of the substrate gave partial protection. The allosteric inhibitor AMP markedly increases the reactivity of the cysteine residue. The results indicate that fructose 2,6-bisphosphate can protect the enzyme against the loss of potassium activation by binding to an allosteric site. High levels of fructose 1,6-bisphosphate probably inhibit the enzyme by binding to this allosteric site.

Animals↗
PubMed1990

Crystal structure of fructose-1,6-bisphosphatase complexed with fructose 6-phosphate, AMP, and magnesium.

The crystal structure of fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with fructose 6-phosphate, AMP, and Mg2+ has been solved by the molecular replacement method and refined at 2.5-A resolution to a R factor of 0.215, with root-mean-square deviations of 0.013 A and 3.5 degrees for bond lengths and bond angles, respectively. No solvent molecules have been included in the refinement. This structure shows large quaternary and tertiary conformational changes from the structures of the unligated enzyme or its fructose 2,6-bisphosphate complex, but the secondary structures remain essentially the same. Dimer C3-C4 of the enzyme-fructose 6-phosphate-AMP-Mg2+ complex twists about 19 degrees relative to the same dimer of the enzyme-fructose 2,6-bisphosphate complex if their C1-C2 dimers are superimposed on one another. Nevertheless, many interfacial interactions between dimers of C1-C2 and C3-C4 are conserved after quaternary structure changes occur. Residues of the AMP domain (residues 6-200) show large migrations of C alpha atoms relative to barely significant positional changes of the FBP domain (residues 201-335).

Adenosine Monophosphate↗
PubMed1976

Phosphorylation of intracellular fructose in Bacillus subtilis mediated by phosphoenolpyruvate-1-fructose phosphotransferase.

Intracellular fructose provided by the sorbitol pathway in Bacillus subtilis can be phosphorylated by the phosphenolpyruvate-1-fructose phosphotransferase which is known to mediate a vectorial metabolism. The fate of this intracellular fructose was studied using mutants lacking either the fructose 1-phosphate pathway or the fructose 6-phosphate pathway. It was shown that the phosphoenolpyruvate-dependent phosphorylation needs a prior exit of the sugar into the medium, this exit being probably catalysed by a transport system. A low affinitiy intracellular phosphenolpyruvate phosphotransferase system was found, which seems to be devoid of a physiological role.

Bacillus subtilis↗
PubMed1998

Luminal fructose modulates fructose transport and GLUT-5 expression in small intestine of weaning rats.

In neonatal rats, precocious introduction of dietary fructose significantly enhances brush-border fructose transport rates and GLUT-5 mRNA levels during early weaning. In this study, these rates and levels were more than two times higher in the anastomosed intestine compared with those in the bypassed loop of weaning pups that underwent Thiry-Vella surgery and consumed high-fructose (HF) diets. In Thiry-Vella pups fed fructose-free (NF) diets, uptake rates and mRNA levels in the anastomosed intestine were very low and similar to those in the bypassed loop. In sham-operated littermates, transport rates and mRNA levels were similar between intestinal regions that corresponded to anastomosed and bypassed loops in Thiry-Vella pups and were two to three times greater in pups fed HF than in those fed NF diet. In contrast, rates of brush-border glucose transport and levels of SGLT-1 and of GLUT-2 mRNA were independent of diet and were similar between bypassed and anastomosed regions. Changes in GLUT-5 expression did not follow a distinct diurnal rhythm. When pups were fed HF diet after 12 h of starvation to empty the intestinal lumen, fructose transport rates increased with feeding duration and reached a plateau 12-24 h after feeding; in contrast, GLUT-5 mRNA levels were highest within 4 h after arrival of chyme in the jejunum and then decreased gradually and returned to baseline levels 24 h later. In littermates fed NF diet, mRNA levels and uptake rates were each independent of feeding duration. Luminal, and not endocrine, signals regulate GLUT-5 expression in weaning pups.

Animals↗
PubMed1983

Fructose, fructose 1-phosphate, and glyceraldehyde breakdown in carbohydrate metabolism.

Recent investigations showed that all enzymes that are involved in the catabolism of fructose via fructose 1-phosphate (F-1-p) are present in the bovine lens. The present studies were performed to provide information regarding the extent to which fructose, F-1-p and glyceraldehyde (GA) are actually catabolized via this pathway, and how they increase the lactate, alpha-glycerophosphate and glycerol concentrations. Incubation was performed with a homogenate of bovine lens equator. F-1-p, and particularly fructose + ATP, considerably increased lactate and alpha-glycerophosphate concentrations, while the glycerol concentration remained unchanged compared to controls. Lactate formation through GA was not observed; while small amounts of glycerol and alpha-glycerophosphate are formed, it seems that the major part is transformed to glycerate. The results show that the further breakdown of lenticular fructose occurs via F-1-p and that the phosphorylated C3 fragments (dihydroxyacetone phosphate) are transferred to the energy metabolism, which is not the case with GA.

Animals↗
PubMed1978

[Kinetic properties of the fructose influx across the brush border of the rat jejunum. Effects of a diet rich in fructose].

The unidirectional influx (i.e. initial rate of uptake) of D-fructose across the brush border of rat jejunum is a saturable function of concentration, with a Kt of 125 mM, which implicates a carrier mechanism. This mechanism appears to be very specific for fructose in view of the lack of influx inhibition observed in the presence of large concentrations of the sugars or polyols, D-glucose, D-galactose, D-mannose, D-xylose, L-sorbose, D-tagatose, sorbitol or mannitol. D-Fructose uptake is inhibited by incubation, preceded by a 30-min preincubation in the same inhibitory conditions, in the absence of Na, or in the presence of metabolic poisons, NaF, 2,4-dinitrophenol, monoiodoacetate. Phloridzin (10-3 M), with or without preincubation, has no effect on uptake. D-Fructose influx is stimulated by fructose feeding, mainly because the augmentation of the number of active sites of transfer: Jmax is increased two-fold, Kt is more weakly affected.

Animals↗
PubMed1989

[The mechanism of rat liver fructose-1,6-bisphosphate inhibition by fructose-2,6-bisphosphate].

It was found that a decrease in the activating cation (Mg2+) concentration below [A]0.5 causes the disappearance of cooperativity of the fructose 1.6-bisphosphatase substrate binding sites induced by high fructose 2.6-bisphosphate concentrations without any significant alteration in the extent of the enzyme inhibition. Under these conditions, a competitive type of inhibition (with respect to the substrate) is transformed into a non-competitive type with an increase in the fructose 2.6-bisphosphate concentration. The data obtained confirm the viewpoint that fructose 2.6-bisphosphate binds to the enzyme at two distinct sites, a catalytic and an allosteric ones, differing in their affinity for the inhibitor. It is supposed that the interaction between the allosteric fructose 2.6-bisphosphate binding site and the activator site occupied by Mg2+ is necessary for the cooperative response of the enzyme to the substrate.

Allosteric Site↗
PubMed1987

The mechanism of activation of heart fructose 6-phosphate,2-kinase:fructose-2,6-bisphosphatase.

Partially purified fructose-6-P,2-kinase:fructose-2,6-bisphosphatase from beef heart was phosphorylated by cAMP protein kinase. The phosphorylated fructose-6-P,2-kinase shows lower Km for Fru-6-P (43 versus 105 microM) and for ATP (0.55 versus 1.3 mM) but no change in the Vmax, compared to those for unphosphorylated enzyme. There was no detectable change in Km or Vmax of fructose-2,6-bisphosphatase activity by the phosphorylation. These changes in heart fructose-6-P,2-kinase were in direct contrast to previous results for the liver isozyme in which phosphorylation led to inhibition of the kinase activity and activation of the phosphatase activity.

Animals↗
PubMedDate not supplied

[Allosteric regulation in the open futile cycle fructose-6-P--fructose-1,6-P2].

The effect of fructosebisphosphatase inhibition and phosphofructokinase activation by fructose-1,6-bisphosphate on a parasite recirculation of substrates in the futile (energy-dissipating) cycle fructose-6-P in equilibrium fructose-1,6-P2 has been studied. Both the inhibition and activation have been shown to produce self-oscillations in the cycle under flow-through conditions. The common recirculation with both regulatory mechanisms present is about half as high as with only one of them. The analysis fulfilled resulted in a conclusion of the importance of the concerted action of substrate inhibition and product activation both in the cycle fructose-6-P in equilibrium fructose-1,6-P2 and in the cycles glutamine in equilibrium glutamate and acetyl-CoA in equilibrium fatty acids playing an important part in the amino acid and lipid metabolism regulation.

Allosteric Regulation↗
PubMed1983

Oscillations in the phosphofructokinase-fructose 1,6-bisphosphatase cycle. I. Purification and kinetic characterization of fructose 1,6-bisphosphatase from pig liver.

A rapid and effective purification procedure for pig liver fructose 1,6-bisphosphatase in neutral form is described. The procedure involves heat treatment and chromatography with CM-Sephadex with specific elution of the enzyme by fructose 1,6-bisphosphate and AMP. The enzyme was found suitable for integration into a reconstituted enzyme system in which the generation of oscillations is investigated. The kinetic properties of fructose 1,6-bisphosphatase are studied under conditions compatible to those applied for the investigation of the dynamic behaviour of the reconstituted system (pH 6.6, presence of inorganic phosphate). The enzyme is significantly inhibited by AMP and fructose 6-phosphate. The substrate fructose 1,6-bis phosphate has a high affinity to the enzyme and was found weakly inhibiting even at high concentrations. The kinetic results are interpreted in terms of a mathematical model which reflects the interaction of the various effectors with the enzyme.

Animals↗
PubMed1991

Dietary regulation of fructose metabolism in the intestine and in the liver of the rat. Duration of the effects of a high fructose diet after the return to the standard diet.

In this research on metabolic effects of a high fructose diet, we studied the duration of these effects by measuring the specific activity of 8 enzymes stimulated by such a diet, on days, 0, 3, 6, 9, 15, after the return to a normal diet. In the intestinal mucosa, ketohexokinase, aldolase, triokinase, fructose-diphosphatase, and glucose-6-phosphatase specific activities were still entirely or partially stimulated on the 15th day after return to the standard diet. The stimulation of glucose-6-phosphatase and pyruvate kinase specific activities stopped quickly. In the liver, with the exception of fructose-diphosphatase, the return to basic values was much quicker than in intestine. In 7 enzymes out of 8 it was realized in 9 days or less. When a high fructose diet gives way to a normal one, return to basic values comes so much the quicker as activation has needed longer to appear.

Animals↗
PubMed1989

Presence of a fructose-2,6-bisphosphate-insensitive pyrophosphate: fructose-6-phosphate phosphotransferase in the anaerobic protozoa Tritrichomonas foetus, Trichomonas vaginalis and Isotricha prostoma.

Extracts of the anaerobic protozoa Tritrichomonas foetus, Trichomonas vaginalis and Isotricha prostoma contained a high activity (0.5-1 mumol min-1 (mg protein)-1) of pyrophosphate:fructose-6-phosphate phosphotransferase (PPi-PFK), but no detectable ATP: fructose-6-phosphate phosphotransferase. PPi-PFK from I. prostoma was purified close to homogeneity by adsorption on phospho-Ultrogel and elution with fructose-1,6-bisphosphate, and subsequent anion-exchange chromatography. The enzyme had an Mr of 95,000 as determined by gel filtration and consisted of subunits of Mr 48,000. PPi-PFK from I. prostoma and from T. foetus displayed hyperbolic kinetics with respect to their substrates and were not affected by fructose-2,6-bisphosphate. In sharp contrast with what has been found in other eukaryotes, no evidence could be found for the presence of fructose-2,6-bisphosphate in the two trichomonads, in I. prostoma and in Entamoeba histolytica.

Animals↗
PubMed1985

Studies of the structure of fructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase.

Some physicochemical properties of a homogeneous preparation of a bifunctional enzyme, fructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase, were determined. The molecular weight of the enzyme is 101 000 as determined by high-speed sedimentation equilibrium. The molecular weight of dissociated enzyme is 55 000 in 6 M guanidinium chloride by sedimentation equilibrium and in sodium dodecyl sulfate by polyacrylamide gel electrophoresis. A value of 4.7 was observed for the isoelectric point. Tryptic peptide maps and high-performance liquid chromatography of the trypsin-digested enzyme revealed approximately 60 peptides. Amino acid analysis of the enzyme shows that it contains 27 lysine and 36 arginine residues per 55 000 daltons. No free N-terminal amino acid residue was detectable, suggesting that it is blocked. Hydrolysis of the enzyme by carboxypeptidases A and B releases tyrosine followed by histidine and arginine, indicating that the amino acid sequence at the carboxyl terminus is probably -Arg-His-Tyr. Tryptic digestion of [32P]phosphofructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase yields a 32P-labeled peptide detected by tryptic peptide mapping and high-performance liquid chromatography. Thermolysin digestion of CNBr-cleaved 32P-enzyme also yields a single 32P-peptide. These results indicate that fructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase is a dimer of 55 000 daltons and the subunits are very similar, if not identical.

Amino Acids↗
PubMed1986

Fructose 1,6-Bisphosphatase Form B from Synechococcus leopoliensis Hydrolyzes both Fructose and Sedoheptulose Bisphosphate.

The substrate specificity of purified fructose bisphosphatase form B from Synechococcus leopoliensis (EC 3.1.3.11; cf. K-P Gerbling, M Steup, E Latzko 1985 Eur J Biochem 147: 207-215) has been investigated. Of the phosphate esters tested only fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate were hydrolyzed by the enzyme. Both sugar bisphosphates were cleaved at the carbon 1-ester. Fructose- and sedoheptulose bisphosphate stabilized the activated (i.e. tetrameric) state of the enzyme and prevented a slow inactivation that is observed in the absence of sugar bisphosphates. With the activated enzyme, kinetic constants (half-saturating substrate concentrations, maximal reaction velocity, and the catalytical constant) were similar for both fructose- and sedoheptulose bisphosphate. The data suggest that fructose bisphosphatase form B from Synechococcus leopoliensis can catalyze both bisphosphatase reactions within the reductive pentose phosphate cycle.

Journal Article↗
PubMed1996

Complete Sucrose Metabolism Requires Fructose Phosphotransferase Activity in Corynebacterium glutamicum To Ensure Phosphorylation of Liberated Fructose.

Sucrose uptake by Corynebacterium glutamicum involves a phosphoenolpyruvate-dependent sucrose phosphotransferase (PTS), but in the absence of fructokinase, further metabolism of the liberated fructose requires efflux of the fructose and reassimilation via the fructose PTS. Mutant strains lacking detectable fructose-transporting PTS activity accumulated fructose extracellularly but consumed sucrose at rates comparable to those of the wild-type strain.

Journal Article↗