Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “fructose”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources
PubMed1982

Studies of the mechanism of fructose-induced hypertriglyceridemia in the rat.

Carbohydrate-induced hypertriglyceridemia is easily produced in the rat, and fructose has been shown to be particularly potent in this regard. In this study we have compared the effects of feeding rats diets high (66% of total calories) in fructose or glucose on various aspects of carbohydrate and lipid metabolism. The results confirmed previous observations that fructose (456 +/- 276 mg/dl) was more potent (p less than 0.001) in raising plasma TG concentration than was glucose (242 +/- 13 mg/dl), and indicated that the difference in magnitude of hypertriglyceridemia produced by the two carbohydrates was closely related to the ability of the test diets to increase VLDL-TG secretion (r = 0.85, p less than 0.001). Both glucose and fructose feeding led to comparable degrees of hyperinsulinemia, and plasma TG concentrations increased before hyperinsulinemia evolved in fructose-fed rats. Therefore, it was concluded that fructose can act directly on the liver to increase VLDL-TG secretion, and that fructose-induced hypertriglyceridemia can occur in the absence of hyperinsulinemia. On the other hand, the rise in plasma TG concentration produced by fructose was reduced dramatically in exercise-trained rats, and this was associated with a decrease in plasma insulin concentration. Based upon these observations, we suggest that fructose feeding produces hypertriglyceridemia by directly stimulating hepatic VLDL-TG secretion, as well as by producing insulin resistance and hyperinsulinemia, and that it is the combined effect of these two separate actions which accounts for the magnitude of fructose-induced hypertriglyceridemia.

Animals↗
PubMed1986

Nitrogen conservation in starvation revisited: protein sparing with intravenous fructose.

The provision of small amounts of glucose during fasting is known to spare body protein and to attenuate markedly the metabolic response to starvation. These actions, which are not shared by fat, are generally thought to depend on the ability of exogenous glucose to stimulate insulin secretion. To determine whether fructose, a very weak insulin secretagogue, will also conserve nitrogen and alter the response to fasting, we infused small amounts of fructose, 100 g/d (375 kcal), into 7 obese subjects during a 10-day fast: 4 received fructose days 7 to 10, and 3 received fructose days 1 to 7. Fructose virtually abolished (all P less than 0.05-0.01) the fasting induced: (a) fall in glucose and insulin and rise in glucagon, (b) fall in triiodothyronine, (c) ketosis and acidosis, (d) increased ammonia excretion, (e) hyperuricemia (and hypouricosuria), and (f) fall in plasma alanine and rise in branched chain amino acids. Fructose also significantly reduced urinary sodium loss. Moreover, fructose exerted a prominent protein-sparing action, even though plasma insulin concentrations never exceeded postabsorptive levels. Excretion of total nitrogen was reduced by 40% to 50% during periods of fructose infusion, reflecting significant suppression of both urea and ammonia generation (all P less than 0.05-0.01). Most plasma glucogenic amino acids rose significantly during fructose administration. We conclude that low-dose fructose infusion essentially abolishes the entire hormone-substrate response to fasting, and spares body protein without raising insulin above postabsorptive levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-Base Equilibrium↗
PubMed1991

Modulation of the interaction between aldolase and glycerol-phosphate dehydrogenase by fructose phosphates.

Kinetics of fructose-1,6-disphosphate aldolase (EC 4.1.2.13) catalyzed conversion of fructose phosphates was analyzed by coupling the aldolase reactions to the metabolically sequential enzyme, glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), which interacts with aldolase. At low enzyme concentration poly(ethylene glycol) was added to promote complex formation of aldolase and glycerol-phosphate dehydrogenase resulting in a 3-fold increase in KM of fructose-1,6-bisphosphate and no change in Vmax. Kinetic parameters for fructose-1-phosphate conversion changed inversely upon complex formation: Vmax increased while KM remained unchanged. Gel penetration and ion-exchange chromatographic experiments showed positive modulation of the interaction of aldolase and dehydrogenase by fructose-1,6-bisphosphate. The dissociation constant of the heterologous enzyme complex decreased 10-fold in the presence of this substrate. Fructose-1-phosphate or dihydroxyacetone phosphate had no effect on the dissociation constant of the aldolase-dehydrogenase complex. In addition, titration of fluorescein-labelled glycerol-phosphate dehydrogenase with aldolase indicated that both fructose-1,6-bisphosphate and fructose-2,6-biphosphate enhanced the affinity of aldolase to glycerol-phosphate dehydrogenase. The results of the kinetic and binding experiments suggest that binding of the C-6 phosphate group of fructose-1,6-bisphosphate to aldolase complexed with dehydrogenase is sterically impeded while saturation of the C-6 phosphate group site increases the affinity of aldolase for dehydrogenase. The possible molecular mechanism of the fructose-1,6-bisphosphate modulated interaction is discussed.

Animals↗
PubMed1992

The metabolism of fructose in the bivascularly perfused rat liver.

The metabolism of fructose was investigated in the bivascularly and hemoglobin-free perfused rat liver. Anterograde and retrograde perfusions were performed. In anterograde perfusion, fructose was infused at identical rates (19 mumols min-1 g-1) via the portal vein (all liver cells) or the hepatic artery (predominantly perivenous cells); in retrograde perfusion fructose was infused via the hepatic vein (all liver cells) or the hepatic artery (only periportal cells). The cellular water spaces accessible via the hepatic artery were measured by means of the multiple-indicator dilution technique. The following results were obtained. (i) Fructose was metabolized to glucose, lactate and pyruvate even when this substrate was infused via the hepatic artery in retrograde perfusion; oxygen consumption was also increased. (ii) When referred to the water spaces accessible to fructose via the hepatic artery in each perfusion mode, the rate of glycolysis was 0.99 +/- 0.14 mumols min-1 ml-1 in the retrograde mode; and, 2.05 +/- 0.19 mumols min-1 ml-1 in the anterograde mode (P = 0.002). (iii) The extra oxygen uptake due to fructose infusion via the hepatic artery was 1.09 +/- 0.16 mumols min-1 ml-1 in the retrograde mode; and, 0.51 +/- 0.08 mumols min-1 ml-1 in the anterograde mode (P = 0.005). (iv) Glucose production from fructose via the hepatic artery was 2.18 +/- 0.18 mumols min-1 ml-1 in the retrograde mode; and, 1.83 +/- 0.16 mumols min-1 ml-1 in the anterograde mode (P = 0.18). (v) Glucose production and extra oxygen uptake due to fructose infusion did not correlate by a single factor in all perfusion modes. It was concluded that: (a) rates of glycolysis are lower in the periportal area, confirming previous views; (b) extra oxygen uptake due to fructose infusion is higher in the periportal area; (c) a predominance of glucose production in the periportal area could not be demonstrated; and (d) extra oxygen uptake due to fructose infusion is not a precise indicator for glucose synthesis.

Animals↗
PubMed2006

Fructose malabsorption and symptoms of irritable bowel syndrome: guidelines for effective dietary management.

Dietary fructose induces abdominal symptoms in patients with fructose malabsorption, but there are no published guidelines on its dietary management. The objective was to retrospectively evaluate a potentially successful diet therapy in patients with irritable bowel syndrome and fructose malabsorption. Tables detailing the content of fructose and fructans in foods were constructed. A dietary strategy comprising avoidance of foods containing substantial free fructose and short-chain fructans, limitation of the total dietary fructose load, encouragement of foods in which glucose was balanced with fructose, and co-ingestion of free glucose to balance excess free fructose was devised. Sixty-two consecutively referred patients with irritable bowel syndrome and fructose malabsorption on breath hydrogen testing underwent dietary instruction. Dietary adherence and effect on abdominal symptoms were evaluated via telephone interview 2 to 40 months (median 14 months) later. Response to the diet was defined as improvement of all symptoms by at least 5 points on a -10- to 10-point scale. Forty-eight patients (77%) adhered to the diet always or frequently. Forty-six (74%) of all patients responded positively in all abdominal symptoms. Positive response overall was significantly better in those adherent than nonadherent (85% vs 36%; P<0.01), as was improvement in individual symptoms (P<0.01 for all symptoms). This comprehensive fructose malabsorption dietary therapy achieves a high level of sustained adherence and good symptomatic response.

Adolescent↗
PubMed1998

The actions of a novel lipoprotein lipase activator, NO-1886, in hypertriglyceridemic fructose-fed rats.

High circulating fasting and prandial triglyceride levels are associated with both insulin resistance and the development of cardiovascular disease. The aim of this investigation was to study the effects of NO-1886, a novel lipoprotein lipase (LPL) activator, on triglyceride levels, fat oxidation, and glucose tolerance in fructose-fed rats, a hypertriglyceridemic model of insulin resistance. Adult male Wistar rats were fed for 4 weeks with a high-starch diet or a high-fructose diet without and with NO-1886 (50 mg x kg[-1] x d[-1] orally). Fructose feeding increased plasma triglyceride levels, an effect that was ameliorated by NO-1886 treatment (week 1/week 4: starch-fed, 2.4 +/- 0.1/2.8 +/- 0.2 mmol/L; fructose-fed, 3.6 +/- 0.5/5.5 +/- 0.5; fructose + NO-1886, 2.7 +/- 0.2/3.6 +/- 0.3). The mean 24-hour respiratory quotient (RQ) was significantly lower in the fructose + NO-1886 group compared with fructose-fed rats, indicating increased oxidation of fat. Fructose feeding elevated liver triglyceride levels by 74% (P < .01), an effect not altered by NO-1886. Red and white quadriceps hindlimb muscle triglyceride levels were not different between groups. Glucose tolerance (intravenous test in long-term cannulated rats) was mildly deteriorated and fasting insulin and glucose levels were elevated in fructose-fed rats, effects which were ameliorated by NO-1886. In conclusion, in the fructose-fed rat model of hypertriglyceridemia and insulin resistance, addition of a LPL activator reduced circulating triglyceride levels without causing increased muscle triglyceride accumulation or deterioration in glucose tolerance. LPL activators may prove to be a fruitful avenue to explore in the search for new therapeutic agents in the treatment of dyslipidemias and insulin resistance.

Animals↗
PubMed2002

Dietary vitamin E and C supplementation prevents fructose induced hypertension in rats.

In fructose-induced hypertension in Wistar-Kyoto (WKY) rats, excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca2+ channels and increasing cytosolic free calcium and blood pressure. The thiol compound N-acetyl cysteine prevents fructose-induced hypertension by binding excess endogenous aldehydes and normalizing membrane Ca2+ channels and cytosolic free calcium. The aim of the present study was to investigate whether dietary supplementation of vitamin E and vitamin C which are known to increase tissue glutathione, a storage form of cysteine, prevents this hypertension and its associated biochemical and histopathological changes. Starting at 7 weeks of age, animals were divided into four groups of six animals each and treated as follows: control group, normal diet and normal drinking water; fructose group, normal diet and 4% fructose in drinking water; fructose + vitamin E group, diet supplemented with vitamin E (34 mg/ kg feed) and 4% fructose in drinking water; fructose + vitamin C group, diet supplemented with vitamin C (1,000 mg/kg feed) and 4% fructose in drinking water. At 14 weeks, systolic blood pressure, platelet [Ca2+]i and kidney and aortic aldehyde conjugates were significantly higher in the fructose group. These animals also displayed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary vitamin E and C supplementation in fructose-treated WKY rats prevented the increase in systolic blood pressure by normalizing cytosolic [Ca2+]i and kidney and aortic aldehyde conjugates and preventing adverse renal vascular changes.

Animals↗
PubMed2000

Hepatic intralobular mapping of fructose metabolism in the rat liver.

Detailed mapping of glucose and lactate metabolism along the radius of the hepatic lobule was performed in situ in rat livers perfused with 1.5 mM lactate before and during the addition of 5 mM fructose. The majority of fructose uptake occurred in the periportal region; 45% of fructose taken up in the periportal half of the lobular volume being converted into glucose. Periportal lactate uptake was markedly decreased by addition of fructose. Basal perivenous lactate output, which was derived from glucose synthesized periportally, was increased in the presence of fructose. During fructose infusion there was a small decrease in cell pH periportally, but acidification of up to 0.5 pH units perivenously. The evidence suggests that in situ the apparent direct conversion of fructose into lactate represents, to a substantial extent, the result of periportal conversion of fructose into glucose and the subsequent uptake and glycolysis to lactate in the perivenous zone of some of that glucose. (31)P NMR spectroscopy showed that the cellular concentration of phosphomonoesters changes very little periportally during fructose infusion, but there was an approximate twofold increase perivenously, presumably due to the accumulation of fructose 1-phosphate. It may be inferred that fructokinase activity is expressed throughout the hepatic lobule.

Animals↗
PubMed1971

Temperature and the regulation of enzyme activity in poikilotherms. Regulatory properties of fructose diphosphatase from muscle of the Alaskan king-crab.

1. The properties of fructose diphosphatase from skeletal muscle of the Alaskan king-crab (Paralithodes camtschatica) were examined over the physiological temperature range of the animal. 2. King-crab muscle fructose diphosphatase is first activated by Na(+) and NH(4) (+) and is then partially inhibited by these cations at concentrations higher than 10mm at 0 degrees , 8 degrees and 15 degrees C. Enzyme activity is stimulated by K(+) at 0 degrees C, but is curtailed at 8 degrees C and 15 degrees C, an effect that could render rate independent of temperature. 3. Affinity for substrate increases with decreasing temperature; below the temperature of acclimatization, K(m) for fructose 1,6-diphosphate increases, resulting in a complex U-shaped temperature-K(m) curve. 4. King-crab muscle fructose diphosphatase is inhibited by low concentrations of AMP. As with enzymes of other poikilotherms, inhibition by AMP is sensitive to temperature; the enzyme is least sensitive to inhibition by AMP near the temperature of acclimatization. 5. The affinity of fructose diphosphatase for fructose 1,6-diphosphate is enhanced by phosphoenolpyruvate, and this activation is temperature-sensitive; 0.5mm-phosphoenolpyruvate causes a sevenfold decrease in K(m) for fructose 1,6-diphosphate at 15 degrees C but a 25-fold decrease at 0 degrees C. 6. Phosphoenolpyruvate appears to decrease the affinity of king-crab muscle fructose diphosphatase for AMP at low temperature, whereas at the higher temperature it appears to enhance inhibition by AMP. Phosphoenolpyruvate was not observed to cause a reversal of inhibition of fructose diphosphatase activity by AMP. The identification of phosphoenolpyruvate as an activator of a rate-limiting step in gluconeogenesis permits the suggestion of a coupling of the controlling mechanisms of several steps in the glycolytic and gluconeogenic chains. 7. These findings suggest mechanisms for the maintenance and regulation of control of fructose diphosphatase activity in king-crab skeletal muscle at low temperature and under conditions that favour concomitant activity of phosphofructokinase.

Adenine Nucleotides↗
PubMed1980

Effects of fructose on the energy metabolism and acid-base status of the perfused starved-rat liver. A 31phosphorus nuclear magnetic resonance study.

Fructose metabolism has been studied with 31P n.m.r. in perfused livers from rats starved for 48h. The time course of changes in liver ATP, Pi and sugar phosphate (fructose l-phosphate) concentrations, and intracellular pH were followed in each perfusion after infusion of fructose to give an initial concentration of either 5mM or 10mM. Rapid falls in the concentrations of ATP and Pi and intracellular pH occurred after infusion of fructose, reaching a minimum after 4-5 min, which was lower in the 10mM group than in the 5mM group. These changes were accompanied by a rapid rise in fructose 1-phosphate, reaching a plateau also after 4-5 min. At both concentrations of fructose, after the early falls, some recovery of ATP, Pi and intracellular pH occurred; this was complete for Pi and intracellular pH in the 5mM-fructose experiments (within 12-30 min). Complete restoration of ATP to the pre-fructose value was not achieved in either the 5mM of 10mM groups. Measurements of the uptake of lactate by the liver indicated that the fall in intracellular pH was caused primarily by production of protons accompanying the formation of lactate from fructose with possibly a transient contribution generated during the rise in fructose 1-phosphate.

Animals↗
PubMed2003

Fructose modulates GLUT5 mRNA stability in differentiated Caco-2 cells: role of cAMP-signalling pathway and PABP (polyadenylated-binding protein)-interacting protein (Paip) 2.

In intestinal cells, levels of the fructose transporter GLUT5 are increased by glucose and to a greater extent by fructose. We investigated the mechanism by which fructose increases GLUT5 expression. In Caco-2 cells, fructose and glucose increased activity of the -2500/+41 GLUT5 promoter to the same extent. cAMP also activated the GLUT5 promoter. However, if a protein kinase A inhibitor was used to block cAMP signalling, extensive GLUT5 mRNA degradation was observed, with no change in basal transcription levels demonstrating the involvement of cAMP in GLUT5 mRNA stability. Indeed, the half-life of GLUT5 mRNA was correlated ( R2=0.9913) with cellular cAMP levels. Fructose increased cAMP concentration more than glucose, accounting for the stronger effect of fructose when compared with that of glucose on GLUT5 production. We identified several complexes between GLUT5 3'-UTR RNA (where UTR stands for untranslated region) and cytosolic proteins that might participate in mRNA processing. Strong binding of a 140 kDa complex I was observed in sugar-deprived cells, with levels of binding lower in the presence of fructose and glucose by factors of 12 and 6 respectively. This may account for differences in the effects of fructose and glucose. In contrast, the amounts of two complexes of 96 and 48 kDa increased equally after stimulation with either glucose or fructose. Finally, PABP (polyadenylated-binding protein)-interacting protein 2, a destabilizing partner of PABP, was identified as a component of GLUT5 3'-UTR RNA-protein complexes. We conclude that the post-transcriptional regulation of GLUT5 by fructose involves increases in mRNA stability mediated by the cAMP pathway and Paip2 (PABP-interacting protein 2) binding.

3' Untranslated Regions↗
PubMed1982

The effect of fructose on pyruvate kinase activity in isolated hepatocytes. Inhibition by allantoin and alanine.

1. Incubation of isolated hepatocytes with fructose at concentrations above 3 mM resulted in an apparent inhibition of pyruvate kinase assayed in crude extracts at sub-optimal phosphoenolpyruvate concentrations. 2. Fructose at concentrations below 3 mM caused an activation of the enzyme. 3. Increases in the hepatocyte contents of the positive effectors fructose 1.6-bisphosphate and fructose 1-phosphate were found at all concentrations of fructose up to 10mM. 4. Removal of the extrahepatocyte medium from the hepatocytes by washing resulted in an activation of the enzyme at all concentrations of fructose examined. 5. Inhibitors of the enzyme were shown to accumulate in the hepatocytes despite the depletion of ATP (a known negative effector) caused by higher concentrations of fructose. Indeed the inhibition of pyruvate kinase appeared to be correlated to the depletion of ATP. 6. Alanine (a known inhibitor) was shown to accumulate in hepatocytes as a consequence of incubation with fructose. 7. Allantoin and uric acid were shown to be inhibitors of a partially purified pyruvate kinase preparation assayed both in the presence and in the absence of fructose 1.6-bisphosphate. 8. Allantoin, but not uric acid, accumulated in the extrahepatocyte medium as a result of incubation of the cells with 10 mM-fructose.

Adenosine Triphosphate↗
PubMed1988

Enhancement of glycogen concentrations in primary cultures of rat hepatocytes exposed to glucose and fructose.

Glycogen synthesis in isolated hepatocytes can occur from glucose both by a direct mechanism and by an indirect process in which glucose is first metabolized to C3 intermediates before use for glycogenesis via gluconeogenesis. We studied the incorporation into glycogen of glucose and the gluconeogenic substrate, fructose, in primary cultures of hepatocytes from fasted rats. In the presence of insulin, both glucose and fructose promoted net deposition of glycogen; however, fructose carbon was incorporated into glycogen to a greater extent than that from glucose. When glucose and fructose were administered simultaneously, the glycogenic utilization of glucose was stimulated 2-3-fold, and that of fructose was increased by about 50%. At constant hexose concentrations, the total incorporation of carbon, and the total accumulation of glycogen mass, from glucose and fructose when present together exceeded that from either substrate alone. Fructose did not change the relative proportion of glucose carbon incorporated into glycogen via the indirect (gluconeogenic) mechanism. The synergism of glucose and fructose in glycogen synthesis in isolated rat hepatocytes in primary culture appears to result from a decrease in the rate of degradation of newly deposited glycogen, owing to (i) decreased amount of phosphorylase a mediated by glucose and (ii) noncovalent inhibition of residual phosphorylase activity by some intermediate arising from the metabolism of fructose, presumably fructose 1-phosphate.

Animals↗
PubMed2003

Insulin receptors and renal sodium handling in hypertensive fructose-fed rats.

BACKGROUND: Insulin resistance and hypertension are present in Sprague-Dawley rats fed a fructose-enriched diet. In these rats, insulin might elevate blood pressure via an antinatriuretic action. METHODS: To investigate the sodium-insulin interaction in fructose-fed rats, we compared insulin sensitivity, insulin receptor binding, and insulin receptor mRNA levels in the kidney and skeletal muscle of rats that were fed standard rat chow or a fructose-enriched diet (66%) with either low (0.07%), normal (0.3%), or high (7.5%) NaCl concentrations for 3 weeks. RESULTS: Systolic blood pressure increased in the fructose-fed rats receiving the normal and high-salt diet, but not the low-salt diet. When the rats were fed the low-salt diet, the rate of glucose infusion required to maintain euglycemia during a hyperinsulinemic clamp and insulin receptor number and mRNA levels in skeletal muscle were lower in fructose-fed than control rats. High-salt diet decreased significantly the rate of glucose disposal during the clamp and muscular insulin receptor number and mRNA levels in control, but not fructose-fed rats. During the low-salt diet, renal insulin receptor number and mRNA levels were comparable in fructose-fed and control rats and hyperinsulinemia had comparable acute antinatriuretic effects in the two groups; when the rats were maintained on the high-salt diet, the expected decrease in renal insulin receptor number and mRNA levels occurred in control but not fructose-fed rats and, consistent with this finding, the antinatriuretic response to hyperinsulinemia was blunted only in controls. An inverse relationship between dietary NaCl content and renal insulin receptor mRNA levels was observed in control but not fructose-fed rats. CONCLUSION: Fructose-fed rats appear to have lost the feedback mechanism that limits insulin-induced sodium retention through a down-regulation of the renal insulin receptor when the dietary NaCl content is increased. This abnormality might possibly contribute to the elevation of blood pressure in these rats.

Animals↗
PubMed1981

Effect of fructose feeding on the activity of enzymes of glycolysis, gluconeogenesis, and the pentose phosphate shunt in the liver and jejunal mucosa of rats.

The influence of fructose feeding for 1 to 12 days on the activity of enzymes of glycolysis and gluconeogenesis was studied in the jejunal mucosa and the liver of rats. In the jejunal mucosa fructose feeding leads to an increase in the activity of 6-phosphofructokinase (p less than 0.05) and fructose-1.6-bisphosphate aldolase (p less than 0.05), while the activity of hexokinase and glucose-6-phosphate dehydrogenase remains unchanged. Fructose feeding increases the activity of fructose-bisphosphatase in the jejunal mucosa, however, the absolute values of this enzyme remain low (less than 10%) when compared to those in the liver. In the liver fructose feeding is followed by a marked increase of the activity of fructose-bisphosphatase and glucose-6-phosphate dehydrogenase. In contrast, the activity of glucose-6-phosphatase decreases significantly under a fructose enriched diet. The enzyme activity rose to a maximum within 3 days; in the following time of observation no major changes occurred. The results are in accordance with the assumption that fructose feeding leads in the jejunal mucosa mainly to adaptive alterations of the activity of those enzymes which are involved in the breaking-down of fructose, whereas in the liver the activity of those enzymes is increased, which take part in the new synthesis of glucose-6-phosphate or which direct glucose-6-phosphate into the pentose-phosphate.

Animals↗
PubMed1996

Function of the duplicated IIB domain and oligomeric structure of the fructose permease of Escherichia coli.

The fructose permease of Escherichia coli, the fructose-specific Enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system (PTS), contains a duplicated IIB domain. The protein therefore consists of three distinct domains, B', B, and C (N-terminal to C-terminal), joined by flexible linkers and is thus designated FruB'BC. The N-terminal B' domain was removed using molecular genetic techniques, and the truncated Enzyme II (FruBC) was characterized relative to the wild-type enzyme both in vivo and in vitro. In vivo, FruBC exhibited depressed fermentation characteristics at low fructose concentrations. [14C]Fructose uptake measurements revealed reduced rates only when the permease was rate-limiting for transport. In vitro, FruBC exhibited a 10-fold lower affinity for its phosphoryl donating protein, the IIA-FPr diphosphoryl transfer protein (DTP), than was observed with the wild-type enzyme, and the maximal velocity of fructose phosphorylation was 7-fold depressed. Because the fructose-1-phosphate:[14C]fructose transphosphorylation reaction appeared normal, we conclude that the loss of the B' domain primarily affected phosphoryl transfer between the IIA and IIB domains of the permease. A mutant FruBC derivative with cysteine 112 replaced by serine (C112S FruBC) was inactive as a phosphoryl carrier and a sugar transport protein. Expression of the plasmid-encoded mutant protein inhibited the in vivo activity of the chromosomally encoded wild-type fructose permease, but it did not observably affect the activities of the mannitol or glucitol PTS permeases or of non-PTS sugar permeases. Further, the presence of the detergent extracted mutant protein inhibited the activity of the detergent solubilized wild-type or FruBC enzyme. In contrast, the wild-type FruB BC permease was apparently epistatic over the truncated FruBC permease in vivo. The experiments reported 1) show that the B' domain of the fructose permease functions to facilitate phosphoryl transfer between DTP and the permease, 2) establish the essentiality of cysteine 112 in the B domain of the permease, 3) provide evidence that a functional fructose permease consists of an oligomer in which both IIB domains must be active for the enzyme to catalyze normal rates of phosphoryl transfer and transport, 4) suggest that a single B' domain in the oligomeric Enzyme II is sufficient to allow high efficiency phosphoryl transfer between the IIA domain of DTP and the IIB domain of the permease, and 5) show that the B' domain is not important for oligomerization.

Bacterial Proteins↗
PubMed1994

Dietary fructose v. glucose in rats raises urinary excretion, true absorption and ileal solubility of magnesium but decreases magnesium retention.

Apparent Mg absorption, that is Mg intake minus faecal excretion, was found to be greater in rats fed on diets containing fructose instead of glucose. This effect of fructose was accompanied by enhanced urinary excretion and diminished retention of Mg. True Mg absorption was then determined with the use of oral and intraperitoneal administration of tracer doses of 28 Mg. True Mg absorption was significantly greater in rats fed on fructose. There was no significant effect of fructose v. glucose on faecal excretion of endogenous Mg. It was hypothesized that fructose enhances the solubility of Mg in the ileal lumen and thereby facilitates its absorption. The distribution of Mg between the solid and liquid phases of the ileum was determined in rats fed on either glucose or fructose. Fructose reduced the amount of Mg in the solid phase but raised both the amount and the concentration of Mg in the liquid phase. We conclude that the dietary-fructose-induced stimulation of Mg absorption in rats is caused by a raised solubility of ileal Mg, but the mechanism by which fructose exerts this effect and why it was not associated with a decrease in faecal excretion of endogenous Mg remain unknown. Dietary fructose v. glucose did not systematically affect the apparent absorption of Ca and P.

Animals↗
PubMed1977

Comparison of glucose and fructose tolerance before and after starvation.

Glucose and fructose were studied in eight healthy volunteers who fasted twice for 4 days. Before and after the fasts each subject received a 4-hr glucose or fructose infusion providing 0.5 g/kg/hr. Glucose infusion during starvation resulted in a mean maximal plasma glucose rise of 401 +/- 21 mg/100 ml (+/- SEM) as compared to 119 +/- 10 mg/100 ml before starvation. Insulin/glucose ratios were lower than normal in fasted subjects. Fructose infusion during fasting produced a maximal plasma glucose rise of 91 +/- 9 mg/100 ml as opposed to 5+/-1 mg/100 ml before starvation. During fructose infusion in the fasted state, plasma fructose levels were higher than control and the rise in blood lactate and pyruvate was delayed, but finally lactate concentrations were above control values. The antiketotic effects of intravenous glucose and fructose were similar during fasting but fructose was significantly less potent in reducing free fatty acid levels. After starvation, urinary carbohydrate losses during glucose infusion were 5 times higher than those observed during fructose infusion. Thus, fructose utillization was less impaired during fasting than was glucose utilization, although fasting induced abnormalities in both glucose and fructose metabolism.

Adult↗