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PubMed1987

The effect of chaotropic anions on the activation and the activity of spinach chloroplast fructose-1,6-bisphosphatase.

The effect of chaotropic anions was studied on processes that constitute the chloroplast fructose-1,6-bisphosphatase reaction, i.e. enzyme activation and catalysis. The specific activity of chloroplast fructose-1,6-bisphosphatase was enhanced by preincubation with dithiothreitol, fructose 1,6-bisphosphate, Ca2+, and a chaotropic anion. When chaotropes were ranked in the order of increasing concentrations required for maximal activation they followed a lyotropic (Hofmeister) series: SCN- less than Cl3C-COO- less than ClO4- less than I- less than Br- less than Cl- less than SO4(2-). On the contrary, salts inhibited the catalytic step. The stimulation of chloroplast fructose-1,6-bisphosphatase by chaotropic anions arose from a decrease of the activation kinetic constants of both fructose 1,6-bisphosphate and Ca2+; on the other hand, in catalysis neutral salts caused a decrease of kcat because the S0.5 for both fructose 1,6-bisphosphate and Mg2+ remained unaltered. The molecular weight of chloroplast fructose-1,6-bisphosphatase did not change after the activation by incubation with dithiothreitol, fructose 1,6-bisphosphate, Ca2+, and a chaotrope; consequently, the action of these modulators altered the conformation of the enzyme. Modification in the relative position of aromatic residues of chloroplast fructose-1,6-bisphosphatase was detected by UV differential spectroscopy. In addition, the concerted action of modulators made the enzyme more sensitive to (a) trypsin attack and (b) S-carboxymethylation by iodoacetamide. These results provide a new insight on the mechanism of light-mediated regulation of chloroplast fructose-1,6-bisphosphatase; concurrently to the action of a sugar bisphosphate, a bivalent cation, and a reductant, modifications of hydrophobic interactions in the structure of chloroplast fructose-1,6-bisphosphatase play a crucial role in the enhancement of the specific activity.

Anions↗
PubMed1985

Des-1-25-fructose-1,6-bisphosphatase, a nonallosteric derivative produced by trypsin treatment of the native protein.

Limited tryptic digestion of pig kidney fructose-1,6-bisphosphatase in the presence of magnesium ions results in the formation of an active enzyme derivative which is no longer inhibited by the allosteric effector AMP. The presence of AMP during incubation of fructose-1,6-bisphosphatase with trypsin protects against the loss of AMP inhibition. By contrast, the presence of the nonhydrolyzable substrate analog fructose 2,6-bisphosphate accelerates the rate of formation of that form of fructose-1,6-bisphosphatase which is insensitive to AMP inhibition. Sodium dodecyl sulfate-polyacrylamide electrophoresis of samples taken during trypsin treatment shows that the loss of AMP inhibition parallels the conversion of the native 36,500 molecular weight fructose-1,6-bisphosphatase subunit into a 34,000 molecular weight species. Automated Edman degradation of trypsin-treated fructose-1,6-bisphosphatase following gel filtration shows a single sequence beginning at Gly-26 in the original enzyme, but no changes in the COOH-terminal region of fructose-1,6-bisphosphatase. Thus, the proteolytic product has been characterized as "des-1-25-fructose-1,6-bisphosphatase." A comparison of the kinetic properties of control enzyme and des-1-25-fructose-1,6-bisphosphatase reveals some differences in properties (pH optimum, Ka for Mg2+, K+ activation, inhibition by fructose 2,6-bisphosphate) between the two enzymes, but none is so striking as the complete loss of AMP sensitivity shown by des-1-25-fructose-1,6-bisphosphatase. The loss of AMP inhibition is due to the loss of AMP-binding capacity, but it is not known at this stage whether residues of the AMP site are present in the 25-amino acid NH2-terminal region or the removal of this region leads to a conformational change that abolishes the function of an AMP site located elsewhere in the molecule.

Adenosine Monophosphate↗
PubMed1983

Studies on the hysteretic properties of chloroplast fructose-1,6-bisphosphatase.

Chloroplast fructose-1,6-bisphosphatase hysteresis in response to modifiers was uncovered by carrying out the enzyme assays in two consecutive steps. The activity of chloroplast fructose-1,6-bisphosphatase, assayed at low concentrations of both fructose-1,6-bisphosphatase and Mg2+, was enhanced by preincubating the enzyme with dithiothreitol, thioredoxin f, fructose 1,6-bisphosphate, and Ca2+. In the time-dependent activation process, fructose 1,6-bisphosphate and Ca2+ could be replaced by other sugar biphosphates and Mn2+, respectively. Once activated, chloroplast fructose-1,6-bisphosphatase hydrolyzed fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate in the presence of Mg2+, Mn2+, or Fe2+. The A0.5 for fructose 1,6-bisphosphate (activator) was lowered by reduced thioredoxin f and remained unchanged when Mg2+ was varied during the assay of activity. On the contrary, the S0.5 for fructose 1,6-bisphosphate (substrate) was unaffected by reduced thioredoxin f and depended on the concentration of Mg2+. Ca2+ played a dual role on the activity of chloroplast fructose-1,6-bisphosphatase; it was a component of the concerted activation and an inhibitor in the catalytic step. Provided dithiothreitol was present, the activating effectors were not required to maintain the enzyme in the active form. Considered together these results strongly suggest that the regulation of fructose-1,6-bisphosphatase in chloroplast occurs at two different levels, the activation of the enzyme and the catalysis.

Calcium↗
PubMed1996

Fructose-1,6-bisphosphate as a metabolic substrate in hog ileum smooth muscle during hypoxia.

Exogenously applied fructose-1,6-bisphosphate has been reported to be effective in preventing some damage to the small intestine during ischemia. To determine whether exogenously applied fructose-1,6-bisphosphate protects ileum smooth muscle from damage from hypoxia and from reoxygenation, we examined the effect of fructose-1,6-bisphosphate on the ability of hog ileum smooth muscle to maintain isometric force during hypoxia and to generate isometric force after reoxygenation in the presence of 5 mM glucose. After 180 min of hypoxia, tissues incubated with 20 mM fructose-1,6-bisphosphate maintained significantly greater levels of isometric force than tissues incubated in the absence of exogenous substrate (23% of pre-hypoxia force compared to 16%). During the first contraction following reoxygenation there was a significantly greater force generation in tissues incubated with 20 mM fructose-1,6-bisphosphate during the hypoxia period compared to tissues with no exogenous substrate included during the hypoxia period (29% of pre-hypoxia force compared to 19%). However, glucose always was a better metabolic substrate compared to fructose-1,6-bisphosphate under all experimental conditions. The presence of fructose-1,6-bisphosphate during hypoxia likely improved tissue function by fructose-1,6-bisphosphate entering the cells and acting as a glycolytic intermediate, since during a 120 min period of hypoxia, unmounted ileum smooth muscle metabolized 1,6-13C-fructose-1,6-bisphosphate to 3-13C-lactate. This conversion of 1,6-13C-fructose-1,6-bisphosphate to 3-13C-lactate was inhibited by the addition of 1 mM iodoacetic acid, a glycolytic inhibitor. We conclude that exogenously provided fructose-1,6-bisphosphate does provide modest protection of ileum smooth muscle from hypoxic damage by functioning as a glycolytic intermediate and improving the cellular energy state.

Animals↗
PubMed2003

Effects of fructose-1,6-bisphosphate on morphological and functional neuronal integrity in rat hippocampal slices during energy deprivation.

D-fructose-1,6-bisphosphate, a high energy glycolytic intermediate, attenuates ischemic damage in a variety of tissues, including brain. To determine whether D-fructose-1,6-bisphosphate serves as an alternate energy substrate in the CNS, rat hippocampal slices were treated with D-fructose-1,6-bisphosphate during glucose deprivation. Unlike pyruvate, an endproduct of glycolysis, 10 mM D-fructose-1,6-bisphosphate did not preserve synaptic transmission or morphological integrity of CA1 pyramidal neurons during glucose deprivation. Moreover, during glucose deprivation, 10-mM D-fructose-1,6-bisphosphate failed to maintain adenosine triphosphate levels in slices. D-fructose-1,6-bisphosphate, however, attenuated acute neuronal degeneration produced by 200 microM iodoacetate, an inhibitor of glycolysis downstream of D-fructose-1,6-bisphosphate. Because (5S, 10R)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine, an antagonist of N-methyl-D-aspartate receptors, exhibited similar protection against iodoacetate damage, we examined whether (5S, 10R)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine and D-fructose-1,6-bisphosphate share a common neuroprotective mechanism. Indeed, D-fructose-1,6-bisphosphate diminished N-methyl-D-aspartate receptor-mediated synaptic responses and partially attenuated neuronal degeneration induced by 100-microM N-methyl-D-aspartate. Taken together, these results indicate that D-fructose-1,6-bisphosphate is unlikely to serve as an energy substrate in the hippocampus, and that neuroprotective effects of D-fructose-1,6-bisphosphate are mediated by mechanisms other than anaerobic energy supply.

Adenosine Triphosphate↗
PubMed1992

Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. The role of surface loop basic residues in substrate binding to the fructose-2,6-bisphosphatase domain.

Lys-356 has been implicated as a critical residue for binding the C-6 phospho group of fructose 2,6-bisphosphate to the fructose-2,6-bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (Li, L., Lin, K., Correia, J., and Pilkis, S. J. (1992) J. Biol. Chem. 267, 16669-16675). To ascertain whether the three other basic residues (Arg-352, Arg-358, and Arg-360), which are located in a surface loop (residues 331-362) which contains Lys-356, are important in substrate binding, these arginyl residues were mutated to Ala, and each arginyl mutant was expressed in Escherichia coli and purified to homogeneity. The far UV circular dichroism spectra of the mutants were identical to that of the wild-type enzyme. The kinetic parameters of 6-phosphofructo-2-kinase of the mutants revealed only small changes. However, the Km for fructose 2,6-bisphosphate, Ki for fructose 6-phosphate, and Ka for inorganic phosphate of fructose-2,6-bisphosphatase for Arg352Ala were, respectively, 2,800-, 4,500-, and 1,500-fold higher than those for the wild-type enzyme, whereas there was no change in the maximal velocity or the Ki for inorganic phosphate. The Km for fructose 2,6-bisphosphate and Ki for inorganic phosphate of Arg360Ala were 10- and 12-fold higher, respectively, than those of the wild-type enzyme, whereas the maximal velocity and Ki for fructose 6-phosphate were unchanged. In addition, substrate inhibition was not observed with Arg352Ala and greatly reduced with Arg360Ala. The properties of the Arg358Ala mutant were identical to those of the wild-type enzyme. The results demonstrate that in addition to Lys-356, Arg-352 is another critical residue in fructose-2,6-bisphosphatase for binding the C-6 phospho group of fructose 2,6-bisphosphate and that Arg-360 binds the C-2 phospho group of fructose 2,6-bisphosphate in the phosphoenzyme.fructose 2,6-bisphosphate complex. The results also provide support for Arg-352, Lys-356, and Arg-360 constituting a specificity pocket for fructose-2,6-bisphosphatase.

Amino Acid Sequence↗
PubMed1994

In vivo and in vitro 31P magnetic resonance spectroscopic studies of the hepatic response of healthy rats and rats with acute hepatic damage to fructose loading.

The hepatic response to a fructose challenge for control rats, and rats subjected to an acute sublethal dose of carbon tetrachloride (CCl4) or bromobenzene (BB), was compared using dynamic in vivo 31P MRS. Fructose loading conditions were used in which control rats showed only a modest increase in hepatic phosphomonoester (PME), and a small decrease in ATP, Pi, and intracellular pH after fructose administration. Both CCl4 and BB-treated rats showed a much greater fructose-induced accumulation of PME than did controls. Trolox C, a free radical scavenger, prevented most of this PME increase. BB-treated rats, given sufficient time to recover from the hepatotoxic insult, responded to the fructose load similarly to controls. Liver aldolase activities of control, toxicant-treated rats, and toxicant plus Trolox C-treated rats correlated inversely with PME accumulation after fructose loading (correlation coefficient: -0.834, P < 0.05). Perchloric acid extracts of rat livers studied by in vitro 31P MRS confirmed that the PME accumulation after fructose loading is mainly due to an increase in fructose 1-phosphate. These studies are consistent with the aldolase-catalyzed cleavage of fructose 1-phosphate being rate-limiting in hepatic fructose metabolism, and that the CCl4 and BB treatment modify and inactivate the aldolase enzyme.

Adenosine Triphosphate↗
PubMed1980

Metabolic effects of large fructose loads in different parts of the rat nephron.

Rats were given large parenteral loads of fructose and the different segments of single nephrons then analyzed for fructose metabolites, fructose metabolizing enzymes, and nucleotide high energy phosphates. Fructokinase and fructose-1-P aldolase activities, and all the major metabolite and nucleotide effects, were confined to the proximal tubule. The proximal straight segment had the highest fructokinase and suffered the greatest changes. In this segment, fructose-1-P rose to 60 mmol/kg (dry weight basis) and glycerol-3-P and glucose-6-P reached 8 and 12 mmol/kg, respectively. ATP fell 80% and GTP (judging from the changes in GTP plus GDP) fell by the same percentage, but UTP was less affected. Total adenylate decreased 50%. In the proximal convoluted tubule, where fructokinase was lower and fructose-1-P aldolase higher than in the straight segment, fructose-1-P rose ony one-fourth as much and glucose-6-P was almost unchanged. In contrast, glycerol-3-P rose more, reaching 16 mmol/kg. Other substances measured along the nephron were glycerol-3-P dehydrogenase, fructose-1,6-bisphosphate aldolase, fructose, glucose, fructose bisphosphate, triose phosphate, and 6-P-gluconate. Control ATP levels were found to be highest in the distal tubule.

Adenine Nucleotides↗
PubMedDate not supplied

Fructose-mediated non-enzymatic glycation: sweet coupling or bad modification.

The Maillard reaction is a process in which reducing sugars react spontaneously with amino groups in proteins to advanced glycation end products (AGEs). Although an elevated level of glucose had been thought to play a primary role in the Maillard reaction, on a molecular basis, glucose is among the least reactive sugars within biological systems. The formation of AGEs is now also known to result from the action of various metabolites other than glucose, which are primarily located intracellularly and participate in the non-enzymatic glycation reaction at a much faster rate, such as fructose, trioses and dicarbonyl compounds. In this review, we considered the glycation reaction with particular attention to the potential role of fructose and fructose metabolites. The two sources for fructose are an exogenous supply from the diet and the endogenous formation from glucose through the aldose reductase pathway. Despite its approximately eightfold higher reactivity, the contribution of extracellular glycation by fructose is considerably less than that by glucose, because of the low plasma concentration of fructose (5 mmol/L glucose vs 35 micro mol/L fructose). Intracellularly, fructose is elevated in a number of tissues of diabetic patients in which the polyol pathway is active. In the cells of these tissues, the concentrations of fructose and glucose are of the same magnitude. Although direct evidence is not yet available, it is likely that the high reactivity of fructose and its metabolites may substantially contribute to the formation of intracellular AGEs and may contribute to alterations of cellular proteins, dysfunction of cells and, subsequently, to vascular complications.

Animals↗
PubMed1997

Cytoprotection by fructose and other ketohexoses during bile salt-induced apoptosis of hepatocytes.

Toxic bile salts cause hepatocyte necrosis at high concentrations and apoptosis at lower concentrations. Although fructose prevents bile salt-induced necrosis, the effect of fructose on bile salt-induced apoptosis is unclear. Our aim was to determine if fructose also protects against bile salt-induced apoptosis. Fructose inhibited glycochenodeoxycholate (GCDC)-induced apoptosis in a concentration-dependent manner with a maximum inhibition of 72% +/- 10% at 10 mmol/L. First, we determined if fructose inhibited apoptosis by decreasing adenosine triphosphate (ATP) and intracellular pH (pHi). Although fructose decreased ATP to <25% of basal values, oligomycin (an ATP synthase inhibitor) did not inhibit apoptosis despite decreasing ATP to similar values. Fructose (10 mmol/L) decreased intracellular pH (pHi) by 0.2 U. However, extracellular acidification (pH 6.8), which decreased hepatocyte pHi 0.35 U and is known to inhibit necrosis, actually potentiated apoptosis 1.6-fold. Fructose cytoprotection also could not be explained by induction of bcl-2 transcription or metal chelation. Because we could not attribute fructose cytoprotection to metabolic effects, alterations in the expression of bcl-2, or metal chelation, we next determined if the poorly metabolized ketohexoses, tagatose and sorbose, also inhibited apoptosis; unexpectedly, both ketohexoses inhibited apoptosis. Because bile salt-induced apoptosis and necrosis are inhibited by fructose, these data suggest that similar processes initiate bile salt-induced hepatocyte necrosis and apoptosis. In contrast, acidosis, which inhibits necrosis, potentiates apoptosis. Thus, ketohexose-sensitive pathways appear to initiate both bile salt-induced cell apoptosis and necrosis, whereas dissimilar, pH-sensitive, effector mechanisms execute these two different cell death processes.

Adenosine Triphosphate↗
PubMed2004

High glucose inhibits fructose uptake in renal proximal tubule cells: involvement of cAMP, PLC/PKC, p44/42 MAPK, and cPLA2.

The precise signal that regulates fructose transport in renal proximal tubule cells (PTCs) under high glucose conditions is not yet known although fructose has been recommended as a substitute for glucose in the diets of diabetic people. Thus, we investigated that effect of high glucose on fructose uptake and its signaling pathways in primary cultured rabbit renal PTCs. Glucose inhibited the fructose uptake in a time- and dose-dependent manner. A maximal inhibitory effect of glucose on fructose uptake was observed at 25 mM glucose after 48 h, while 25 mM mannitol and l-glucose did not affect fructose uptake. Indeed, 25 mM glucose for 48 h decreased GLUT5 protein level. Thus, the treatment of 25 mM glucose for 48 h was used for this study. Glucose-induced (25 mM) inhibition of fructose uptake was blocked by pertussis toxin (PTX), SQ-22536 (an adenylate cyclase inhibitor), and myristoylated amide 14-22 (a protein kinase A inhibitor). Indeed, 25 mM glucose increased the intracellular cAMP content. Furthermore, 25 mM glucose-induced inhibition of fructose uptake was prevented by neomycin or U-73122 (phospholipase C inhibitors) and staurosporine or bisindolylmaleimide I (protein kinase C inhibitors). In fact, 25 mM glucose increased the total PKC activity and translocation of PKC from the cytosolic to membrane fraction. In addition, PD 98059 (a p44/42 mitogen-activated protein kinase (MAPK) inhibitor) but not SB 203580 (a p38 MAPK inhibitor) and mepacrine or AACOCF3 (phospholipase A2 inhibitors) blocked 25 mM glucose-induced inhibition of fructose uptake. Results of Western blotting using the p44/42 MAPK and GLUT5 antibodies were consistent with the results of uptake experiments. In conclusion, high glucose inhibits the fructose uptake through cAMP, PLC/PKC, p44/42 MAPK, and cytosolic phospholipase A2 (cPLA2) pathways in the PTCs.

Animals↗
PubMed1977

Different degradation pathways for glucose and fructose in Rhodopseudomonas capsulata.

In Rhodopseudomonas capsulata the enzymes of the Entner-Doudoroff pathway and the Embden-Meyerhof pathway have been examined. Fructose-grown cells contained inducible activities of phosphoenolpyruvate-fructosephospho-transferase and 1-phosphofructokinase and only low levels of fructokinase and 6-phosphofructokinase. Although fructose-grown cells contained, in addition, all the enzymes of the Entner-Doudoroff pathway together with fructose-1,6-diphosphatase and phosphoglucose isomerase, the Entner-Doudoroff pathway was not operative in fructose catabolism and served only the degradation of glucose. The functional separation of glucose and fructose catabolism via the Entner-Doudoroff and a modified Embden-Meyerhof pathway, respectively, was confirmed by different approaches: 1. Radiorespirometric experiments with glucose and fructose labelled in positions 1, 2, 3, 3+4 and 6 have been carried out. The pattern of 14CO2-evolution from position-labelled glucose was characteristic for the Entner-Doudoroff pathway, that from position-labelled fructose for the Embden-Meyerhof pathway. 2. In the presence of arsenite up to 50% of glucose- and fructose-carbon was excreted as pyruvate. Using 1-14C-glucose, 86% of the pyruvate was labelled in the carboxyl group, whereas using 1-14C-fructose only 19% of the pyruvate was labelled in the carboxyl group. 3. A glucose-6-phosphate dehydrogenase-deficient mutant was isolated which lacked a functional Entner-Doudoroff pathway but which was unaltered in its ability to grow on fructose.

Fructose↗
PubMedDate not supplied

Pancreatic islet metabolism and redox state during stimulation of insulin secretion with glucose and fructose.

The mechanism of potentiation of insulin secretion by fructose was investigated. Twenty mM fructose + 3 mM glucose stimulated insulin secretion in a biphasic manner similar to what is found during stimulation with 20 mM glucose, whereas 20 mM fructose alone did not affect secretion. Fructose utilization was measured as formation of tritiated water from 5-3H-fructose. At 27.8 mM fructose the utilization rate was 258 pmol/2 h/10 islets, which is less than the utilization rate of 2.8 mM glucose. 20 mM glucose increased the islet NADH/NAD+ and NADPH/NADP+ redox ratios as well as islet concentration of ATP and PEP. 20 mM fructose + 3 mM glucose did not affect the concentration of ATP and PEP or the NADH/NAD+ redox ratio. The NADPH/NADP+ ratio was significantly decreased (60%) after 2.5 min incubation with .20 mM fructose + 3 mM glucose. It is concluded that fructose potentiation of insulin secretion is not primarily dependent on fructose metabolism and that any conceivable effect on plasma membrane ion fluxes as caused by a reduction of plasma membrane disulfides, may be caused by mechanisms other than a mere increase in the pyridine nucleotide substrates for the transhydrogenation process.

Adenosine Triphosphate↗
PubMed1993

Dietary fructose vs glucose lowers copper solubility in the digesta in the small intestine of rats.

The hypothesis was tested that dietary fructose vs glucose lowers copper solubility in the digesta in the small intestine of rats, which in turn causes a decreased copper absorption. Male rats were fed adequate-copper (5 mg Cu/kg) diets containing either fructose or glucose (709.4 g monosaccharide/kg) for a period of 5 wk. Fructose vs glucose significantly lowered copper concentrations in plasma and the liver, but did not alter hepatic copper mass. Fructose feeding resulted in a significantly lesser intestinal solubility of copper as based on either a smaller soluble fraction of copper in the liquid phase of small intestinal contents or a lower copper concentration in the liquid phase. The latter fructose effect can be explained by the observed fructose-induced increase in volume of liquid phase of intestinal digesta. After administration of a restricted amount of diet extrinsically labeled with 64Cu, rats fed fructose also had significantly lower soluble 64Cu fraction in the digesta of the small intestine. Although this study shows that fructose lowered intestinal copper solubility, only a slight reduction of apparent copper absorption was observed. It is suggested that the fructose-induced lowering of copper status in part counteracted the fructose effect on copper absorption at the level of the intestinal lumen.

Animals↗
PubMed2006

Effect of anthocyanin-rich extract from black rice (Oryza sativa L. indica) on hyperlipidemia and insulin resistance in fructose-fed rats.

This study was designed to evaluate the effect of an anthocyanin-rich extract from black rice on hyperlipidemia and insulin resistance in fructose-fed rats. Rats fed fructose diet for 4 weeks exhibited significantly higher plasma insulin levels and lower insulin sensitivity than the control rats fed AIN-93G diet. Dietary supplementation with the anthocyanin-rich extract (5 g/kg of high-fructose diet) prevented the development of fructose-induced insulin resistance. After fructose-induced insulin resistance had been established, 4-week treatment with the anthocyanin-rich extract (5 g/kg of high-fructose diet) or pioglitazone (270 mg/kg of high-fructose diet) ameliorated the glucose intolerance and hyperlipidemia, but the extract failed to reverse the fructose-induced hyperinsulinemia as pioglitazone did. In addition, rats supplemented by the extract exhibited lower oxidative stress than the fructose-fed controls, as indicated by the lower concentrations of plasma thiobarbituric acid reactive substances and blood oxidized glutathione. Overall, these results suggest that the anthocyanin-rich extract from black rice improves certain metabolic abnormalities associated with diets high in fructose.

Animals↗
PubMed1976

Direct effects of fructose metabolism on fatty acid oxidation in a recombined rat liver mitochondria-hish speed supernatant system.

The effects of fructose on the oxidation of [1-(14)C]palmitate in a rat liver mitochondria-high speed supernatant system have been investigated. This model system permitted study of the direct effects of fructose and the metabolism of fructose on fatty acid oxidation in the near absence of fatty acid esterification. Fructose inhibited the utilization of albumin-bound [1-(14)C] palmitate in the mitochondria-supernatant system, but did not affect fatty acid utilization by isolated liver mitochondria. Although fructose decreased the ATP content in the mitochondrial-supernatant system, the level of ATP throughout the incubation period was sufficient for maximal fatty acid activation. Fructose decreased the conversion of [1-(14)C]palmitate to 14CO2 and depressed the formation of total labeled oxidation products (14CO2 + 14C-labeled ketone bodies) in this system. The results suggest that fructose metabolism inhibited fatty acid oxidation in the mitochondria-supernatant system by competitive substrate oxidation and thereby decreased utilization of the added [1-(14)C]palmitate. The ihibition of L-[L-(14)C]palmitoylcarnitine oxidation, fructose was in all respects similar to its inhibition of palmitate oxidation, indicating that the site of fructose interaction was within the beta-oxidation sequence. These observations support the concept (Ontko, J.A. [1972] J. Biol. Chem. 247, 1788-1800) that the reciprocal changes in esterification and oxidation of palmitate caused by fructose in liver cells are primarily mediated via inhibitory effects on long-chain fatty acid oxidation.

Acetoacetates↗
PubMed1990

Role of fructose-sorbitol malabsorption in the irritable bowel syndrome.

Because even after low doses of fructose and sorbitol, fructose-sorbitol malabsorption has been found in a high number of patients with the irritable bowel syndrome, an etiological role of fructose-sorbitol malabsorption in the irritable bowel syndrome has been suggested. However, these studies have been uncontrolled. Therefore, a controlled study of fructose-sorbitol malabsorption in the irritable bowel syndrome compared with healthy controls was performed. Seventy-three patients, 23 men and 50 women with a mean age 43.1 +/- 1.7 years (range, 18-66 years) with the irritable bowel syndrome were compared with 87 age- and sex-matched control subjects. Fructose-sorbitol malabsorption was determined by a breath-hydrogen test (Lactoscreen, Hoek Loos, Schiedam, The Netherlands) following an oral load of 25 g fructose and 5 g sorbitol after a 10-hour fast. Fructose-sorbitol malabsorption, as shown by an H2 peak of 20 ppm over basal values, was found in 22 (30.1%) of the patients and 35 (40.2%) of the control subjects. With a lower peak level of 10 ppm over basal values, these percentages were 45.2% and 57.5%, respectively. Also, the highest H2 peak values (15.2 +/- 2.3 ppm vs. 21.5 +/- 2.6 ppm), time to reach peak levels (110.7 +/- 5.4 min vs. 107.1 +/- 5.9 min), and area under the H2 curve (1310 +/- 219 ppm.min vs. 1812 +/- 255 ppm.min) did not discriminate between patients and controls. During the test, symptoms developed in 31 of 70 patients and in 3 of 85 control subjects (P less than 0.0001). Symptomatic patients did not differ from asymptomatic patients regarding the presence or absence of fructose-sorbitol malabsorption, H2 peak values, and area under the curve. No differences could be identified between male and female patients or controls. In conclusion, fructose-sorbitol malabsorption is frequently seen in patients with irritable bowel syndrome, but this is not different from observations in healthy volunteers. Therefore, fructose-sorbitol malabsorption does not seem to play an important role in the etiology of irritable bowel syndrome.

Adult↗
PubMed1993

GLUT2 is the transporter for fructose across the rat intestinal basolateral membrane.

BACKGROUND: The exact roles of disaccharidases and GLUT5 in the brush border membrane and GLUT2 in the basolateral membrane in the absorption of fructose across the intestine have not been fully determined. This paper describes characterization of fructose transport across the jejunal basolateral membrane using isolated membrane vesicles. METHODS: Transport of fructose was measured using rapid filtration of vesicles. Luminal perfusion in vivo with glucose and fructose before vesicle preparation was used to assess modulation of GLUT2 activity. Western blotting measured the abundance of GLUT2 in the membrane. RESULTS: The maximal rate of transport for fructose was 1100 pmol/mg protein/s and the Michaelis constant was 16 mmol/L. Fructose and glucose could completely inhibit the transport of each other. Perfusion of the intestinal lumen with fructose or glucose saline for 4 hours produced a fourfold increase in maximal fructose transport. CONCLUSIONS: These data indicate that the one transport protein, GLUT2, is responsible for moving both fructose and glucose out of the enterocyte across the basolateral membrane under basal conditions. The activity of this, or a closely related carrier, is rapidly upregulated by the presence of hexoses in the intestinal lumen, explaining the potentiation of fructose absorption by luminal glucose and obviating any need to involve apical disaccharidases.

Amino Acid Sequence↗