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PubMed1986

Effect of tolbutamide on fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase in rat liver.

The effects of tolbutamide on the activities of fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase were examined using rat hepatocytes. Tolbutamide stimulated fructose-6-phosphate,2-kinase activity and inhibited fructose-2,6-bisphosphatase activity, resulting in an increase of fructose-2,6-bisphosphate level. Changes in the activities of the enzyme by tolbutamide were due to variation in the Km value, but not dependent on alteration of Vmax. Glucagon inhibition of fructose-2,6-bisphosphate formation resulting from an inactivation of fructose-6-phosphate,2-kinase and an activation of fructose-2,6-bisphosphatase was released by tolbutamide. Tolbutamide stimulation of fructose-2,6-bisphosphate formation through regulation of fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase may produce enhancement of glycolysis and inhibition of gluconeogenesis in the liver.

Animals↗

PubMed1982

A kinetic study of pyrophosphate: fructose-6-phosphate phosphotransferase from potato tubers. Application to a microassay of fructose 2,6-bisphosphate.

Pyrophosphate : fructose-6-phosphate phosphotransferase (PPi-PFK) has been purified 150-fold from potato tubers and the kinetic properties of the purified enzyme have been investigated both in the forward and the reverse direction. Saturation curves for fructose 6-phosphate and also for fructose 1,6-bisphosphate were sigmoidal whereas those for PPi and Pi were hyperbolic. In the presence of fructose 2,6-bisphosphate, the affinity for fructose 6-phosphate and for fructose 1,6-bisphosphate were greatly increased and the kinetics became Michaëlian. The effect of fructose 2,6-bisphosphate was increased by the presence of fructose 6-phosphate and decreased by the presence of Pi. Consequently, the Ka for fructose 2,6-bisphosphate was as low as 5 nM for the forward reaction and reached 150 nM for the reverse reaction. On the basis of these properties, a procedure allowing one to measure fructose 2,6-bisphosphate in amounts lower than a picomole, is described.

Fructosediphosphates↗

PubMed1995

Human fructose-1,6-bisphosphatase gene (FBP1): exon-intron organization, localization to chromosome bands 9q22.2-q22.3, and mutation screening in subjects with fructose-1,6-bisphosphatase deficiency.

Fructose-1,6-bisphosphatase (EC 3.1.3.11) is a key regulatory enzyme of gluconeogenesis that catalyzes the hydrolysis of fructose-1,6-bisphosphate to generate fructose-6-phosphate and inorganic phosphate. Deficiency of fructose-1,6-bisphosphatase is associated with fasting hypoglycemia and metabolic acidosis because of impaired gluconeogenesis. We have cloned and characterized the human liver fructose-1,6-bisphosphatase gene (FBP1). FBP1, localized to chromosome bands 9q22.2-q22.3 by fluorescence in situ hybridization, consists of seven exons that span > 31 kb, and the six introns are in the same position as in the rat gene. FBP1 was screened for mutations in two subjects with fructose-1,6-bisphosphatase deficiency. Four nucleotide substitutions were identified, two of which were silent mutations in the codons for Ala-216 (GCT-->GCC) and Gly-319 (GGG-->GGA). The other substitutions were in intron 3, a C-->T substitution 7 nucleotides downstream from the splice donor site, and in the promoter region, an A-->T substitution 188 nucleotides upstream from the start of transcription. These nucleotide substitutions were also found in normal unaffected subjects and thus are not the cause of fructose-1,6-bisphosphatase deficiency in the two subjects studied. The molecular basis of hepatic fructose-1,6-bisphosphatase deficiency in these subjects remains undetermined but could result from unidentified mutations in the promoter that decrease expression or from mutations in another gene that indirectly lead to decreased fructose-1,6-bisphosphatase activity.

Amino Acid Sequence↗

PubMed1993

[Repeated perioperative administration of fructose and sorbitol in a female patient with hereditary fructose intolerance [HFI)].

The present paper reports on an adult female patient whose hereditary fructose intolerance (HFI) was at first not diagnosed and who, within the space of 2 years after repeated elective surgery and the perioperative administration of fructose and sorbitol, developed "hepatic and renal failure of unclear origin." At a later stage we were able to establish the diagnosis of HFI by means of a fructose tolerance test in both she and her brother, for whom intolerance to fruit and desserts had been known since early childhood. In addition, literature references to fatalities following the parenteral application of fructose and sorbitol were analyzed. During the course of fructose infusion in both the patient and her brother with HFI, the following metabolic changes were noted: hypoglycemia, elevated rise in the blood fructose concentration, hyperlactacidemia, elevated rise in the blood fructose concentration, hyperlactacidemia, and hyperammonemia. These metabolic changes proved to be reversible after discontinuing the fructose infusion. Analysis of the literature on the fatalities following parenteral fructose administration established that fruit and dessert intolerance was known for all collated patients with HFI, and that, clearly, no regular metabolic tests had been conducted.

Acute Kidney Injury↗

PubMed1996

Intravenous load of fructose and fructose 1,6-diphosphate: effects on uricemia in patients with nonalcoholic liver disease.

OBJECTIVES: The i.v. load of fructose causes a significantly higher adenosine triphosphate (ATP) degradation and uric acid production in cirrhotic patients than in healthy controls. Resynthesis of ATP from adenosine diphosphate (ADP) may be facilitated by the phosphorylated compound fructose 1,6-diphosphate, which is used as energy support in parenteral nutrition. The aim of our research was to evaluate: 1) The 1-h uricemic effect of i.v. fructose (0.5 g/kg body weight) in 10 healthy controls and in 78 patients with differenct stages of non-alcoholic chronic liver damage associated or not with malnutrition or hepatocellular carcinoma; and 2) the effect of fructose 1,6-diphosphate (5 g/50 ml) administered i.v. after fructose infusion on the induced uricemia in a subgroup of 13 patients with well compensated cirrhosis. RESULTS AND CONCLUSIONS: The increase of uricemia above the basal level after fructose infusion was significantly higher (p < 0.01) in cirrhotics (3 mg/dl) than in controls (1.2 mg/dl) and in patients with chronic hepatitis (1.9 mg/dl) and was completely reversed by fructose 1,6-diphosphate in the patients tested. Neither Child-Pugh classes of cirrhosis nor malnutrition (present in about 50% of the patients) or hepatocarcinoma significantly affected the fructose-induced uricemia. Therefore, the fructose test efficiently differentiates cirrhotics from chronic hepatitis patients and healthy subjects, but it does not distinguish the various stages of the progression of cirrhosis or its complications.

Adult↗

PubMed1993

Modification of Cys-128 of pig kidney fructose 1,6-bisphosphatase with different thiol reagents: size dependent effect on the substrate and fructose-2,6-bisphosphate interaction.

Treatment of fructose 1,6-bisphosphatase with N-ethylmaleimide was shown to abolish the inhibition by fructose 2,6-bisphosphate, which also protected the enzyme against this chemical modification [Reyes, A., Burgos, M. E., Hubert, E., and Slebe, J. C. (1987), J. Biol. Chem. 262, 8451-8454]. On the basis of these results, it was suggested that a single reactive sulfhydryl group was essential for the inhibition. We have isolated a peptide bearing the N-ethylmaleimide target site and the modified residue has been identified as cysteine-128. We have further examined the reactivity of this group and demonstrated that when reagents with bulky groups are used to modify the protein at the reactive sulfhydryl [e.g., N-ethylmaleimide or 5,5'-dithiobis-(2-nitrobenzoate)], most of the fructose 2,6-bisphosphate inhibition potential is lost. However, there is only partial or no loss of inhibition when smaller groups (e.g., cyanate or cyanide) are introduced. Kinetic and ultraviolet difference spectroscopy-binding studies show that the treatment of fructose 1,6-bisphosphatase with N-ethylmaleimide causes a considerable reduction in the affinity of the enzyme for fructose 2,6-bisphosphate while affinity for fructose 1,6-bisphosphate does not change. We can conclude that modification of this reactive sulfhydryl affects the enzyme sensitivity to fructose 2,6-bisphosphate inhibition by sterically interfering with the binding of this sugar bisphosphate, although this residue does not seem to be essential for the inhibition to occur. The results also suggest that fructose 1,6-bisphosphate and fructose 2,6-bisphosphate may interact with the enzyme in a different way.

Amino Acids↗

PubMed2005

Lysine and glutamate production by Corynebacterium glutamicum on glucose, fructose and sucrose: roles of malic enzyme and fructose-1,6-bisphosphatase.

In the biotechnological production of L-lysine and L-glutamate by Corynebacterium glutamicum media based on glucose, fructose or sucrose are typically used. Glutamate production by C. glutamicum was very similar on glucose, fructose, glucose plus fructose and sucrose. In contrast, lysine production of genetically defined C. glutamicum strains was significantly higher on glucose than on the other carbon sources. To test whether malic enzyme or fructose-1,6-bisphosphatase might limit growth and lysine on fructose, glucose plus fructose or sucrose, strains overexpressing either malE which encodes the NADPH-dependent malic enzyme or the fructose-1,6-bisphosphatase gene fbp were generated. Overexpression of malE did not improve lysine production on any of the tested carbon sources. Upon overexpression of fbp lysine yields on glucose and/or fructose were unchanged, but the lysine yield on sucrose increased twofold. Thus, fructose-1,6-bisphosphatase was identified as a limiting factor for lysine production by C. glutamicum with sucrose as the carbon source.

Bacterial Proteins↗

PubMed1992

Fructose metabolism in the human erythrocyte. Phosphorylation to fructose 3-phosphate.

In human erythrocytes, the first step in the metabolism of fructose is generally thought to be phosphorylation to fructose 6-phosphate catalysed by hexokinase. In variance with this assumption, we show here that fructose in these cells is metabolized primarily to fructose 3-phosphate by a specific 3-phosphokinase. This process has an overall estimated Km of 30 mM with respect to extracellular fructose and an apparent Vmax. of 0.6 mumol/h per ml. At a fixed concentration of fructose in the medium, the accumulation of fructose 3-phosphate was linearly dependent on the duration of incubation up to 5 h and was not affected by glucose. Once accumulated, fructose 3-phosphate appears to be degraded and/or relatively slowly metabolized, decreasing by only approximately 30% after a 12 h incubation in a fructose-free medium.

Erythrocytes↗

PubMed1986

Plasma fructose, uric acid, and inorganic phosphorus responses of hyperinsulinemic men fed fructose.

We fed 12 men with abnormally high insulin responses to a sucrose load and 12 controls normal diets containing 0, 7.5, or 15% of the calories as pure fructose for 5 weeks each in a crossover design. Purified wheat starch replaced the fructose in the 0 and 7.5% diets. The two groups were matched for age, height, and weight. At the beginning of the study and at the end of each of the three 5-week periods, plasma responses to a sucrose load (2 gm/kg body weight) were measured. Initially there were no significant differences in the plasma fructose, uric acid, or inorganic phosphorus responses of the two groups. Plasma fructose responses to a sucrose load were significantly higher after the men consumed the 7.5 and 15% fructose diets than after the 0% diet. Uric acid responses tended to be greater in the hyperinsulinemic men than in controls and increased as the levels of fructose in the diet increased. The inorganic phosphorus levels after a sucrose load were higher in the hyperinsulinemic men than in controls when they consumed the 7.5 and 15% fructose diets. These results indicate that moderate levels of dietary fructose can affect plasma fructose, uric acid, and inorganic phosphorus levels, especially in hyperinsulinemic men.

Adult↗

PubMed2004

Plasma D-glucose, D-fructose and insulin responses after oral administration of D-glucose, D-fructose and sucrose to normal rats.

OBJECTIVE: To assess whether oral D-fructose modifies the plasma D-glucose and insulin responses to oral D-glucose administration in normal rats. DESIGN: Oral D-glucose (1.7, 3.5, 6.9 or 13.9 micromol/g body weight), D-fructose (6.9 micromol/g), both D-glucose and D-fructose (1.7 or 3.5 micromol/g of each hexose) or sucrose (3.7 micromol/g) were administered intragastrically to overnight fasted rats and the plasma concentration of D-glucose, D-fructose and insulin measured over the ensuing 120 minutes. Control experiments were conducted after oral administration of H(2)O or saline. RESULTS: The administration of D-fructose, given as the free hexose or as sucrose, instead of augmenting the plasma D-glucose concentration evoked by the concomitant administration of D-glucose, tended both to improve the insulin response of the pancreatic B-cell and to minimize hyperglycemia, when compared to the results of experiments including the administration of equimolar amounts of D-glucose alone. For instance, the area under the plasma D-glucose curve was comparable in the rats receiving both D-glucose and D-fructose (3.5 micromol/g of each hexose) and the rats receiving only D-glucose (3.5 micromol/g), averaging respectively 836 +/- 32 and 850 +/- 34 mM . min each. Likewise, the paired ratio between the areas under the plasma insulin and D-glucose curves, when corrected for the threshold concentration for the insulinotropic action of the hexose (2.05 +/- 0.10 mM), averaged 44.3 +/- 3.0 nmol/mol in the 16 rats receiving D-fructose alone, sucrose alone or both D-glucose and D-fructose, as compared to 37.7 +/- 2.9 nmol/mol in the 22 rats receiving increasing amounts of D-glucose alone. CONCLUSIONS: The intake of D-fructose, as the free hexose or as sucrose, favours D-glucose homeostasis. This is likely to be attributable to the reciprocal effects of the aldose and ketose upon their respective phosphorylation by glucokinase in both hepatocytes and insulin-producing pancreatic islet cells.

Administration, Oral↗

PubMed1988

Enzymes as biosensors. 2. Hysteretic response of chloroplastic fructose-1,6-bisphosphatase to fructose 2,6-bisphosphate.

Oxidized chloroplastic fructose-bisphosphatase is almost totally inactive at pH 7.5, that is under pH conditions that prevail in the chloroplast stroma. When preincubated for different time periods with fructose 2,6-bisphosphate and assayed in the absence of this ligand, it displays an activity which is directly related to the duration of the preincubation phase. This implies that fructose 2,6-bisphosphate induces enzyme conformers that appear in sequence and may be competent for catalytic activity. Upon desorption of fructose 2,6-bisphosphate the enzyme may retain its active conformation for a time period whose duration depends on magnesium concentration. It thus appears that reduction of the enzyme is not an obligatory prerequisite for its activity. Fructose 2,6-bisphosphate behaves as a competitive inhibitor of the reduced, active enzyme, with respect to the real substrate. When assayed with the oxidized enzyme, however, it behaves as an activator. Moreover the apparent steady-state rate that may be measured experimentally depends on both fructose 2,6-bisphosphate concentration and the direction of a concentration change. The reaction velocity experimentally measured is thus a meta-steady-state rate and depends on the initial conditions of the system. The fructose-bisphosphatase system thus displays, with respect to fructose 2,6-bisphosphate, a hysteresis loop and may then sense whether the concentration of that ligand is increased or decreased. A model has been proposed which allows one to explain these results. This model is based on the view that the substrate and fructose 2,6-bisphosphate compete for the same site of the enzyme and that this latter ligand stabilizes a conformation competent for enzyme activity. After the ligand has been chased away, the enzyme retains the active conformation for a while and slowly relapses to the initial inactive conformation. The time-scale of this slow relaxation overlaps that of the steady state of product appearance and this generates meta-steady-state kinetics, which is dependent on the initial state and therefore on the history of the system.

Binding, Competitive↗

PubMed2005

Fructose-induced increases in neonatal rat intestinal fructose transport involve the PI3-kinase/Akt signaling pathway.

Expression of rat glucose transporter-5 (GLUT5) is tightly regulated during development. Expression and activity are low throughout the suckling and weaning stages, but perfusion of the small intestinal lumen with fructose solutions during weaning precociously enhances GLUT5 activity and expression. Little is known, however, about the signal transduction pathways involved in the substrate-induced precocious GLUT5 development. We found that wortmannin and LY-294002, inhibitors of phosphatidylinositol 3-kinase (PI3-kinase) specifically inhibited the increase in fructose uptake rate and brush-border GLUT5 protein abundance but not GLUT5 mRNA abundance. Perfusion of EGF, an activator of PI3-kinase, also resulted in a marked wortmannin-inhibitable increase in fructose uptake. Perfusion of fructose for 4 h increased cytosolic immunostaining of phosphatidylinositol-3,4,5-triphosphate (PIP(3)), the primary product of PI3-kinase, mainly in the mid- to upper-villus regions in which the brush-border membrane also stained strongly with GLUT5. Perfusion of glucose for 4 h had little effect on fructose or glucose uptake and PIP(3) or GLUT5 staining. SH-5, an Akt inhibitor, prevented the increase in fructose uptake and GLUT5 protein induced by fructose solutions, and had no effect on glucose uptake. The PI3-kinase/Akt signaling pathway may be involved in the synthesis and/or recruitment to the brush border of GLUT5 transporters by luminal fructose in the small intestine of weaning rats. Increases in fructose transport during the critical weaning period when rats are shifting to a new diet may be modulated by several signaling pathways whose cross talk during development still needs to be elucidated.

Androstadienes↗

PubMed2002