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PubMed1976

Oral fructose tolerance, gastric emptying and absorption: a compartmental model.

Gastric emptying and intestinal absorption regulate transfer of ingested fructose from stomach to plasma. Using conscious rhesus monkeys we have developed a compartmental model which describes this system. Fructose tolerance tests were performed in groups of monkeys by intragastrically administering 2 g/kg of 10.5% D-fructose solution; fructose concentration in arterial plasma, [fructose], and intragastric volume were measured at intervals afterward. One group was pretreated with atropine; stomachs imptied with a time constant, tau, of 67 min. Another group received fructose solution with trisodium citrate added; tau was 18 min. Another group received only fructose; tau was 30 min. Using these constants, a model was developed to describe the [fructose] data. In this model k1 related amount of fructose in intestine to absorption rate and Ae represented absorption efficiency. K1 = 0.03 and Ae = 89% provided a good fit for data from the atropinized group. k1 = 0.018 and Ae = 56% provided a good fit for data from the other groups. Differences were explained by considering effects of atropine on gastrointestinal secretion. The model adequately describes our [fructose] data and may be adapted for tests utilizing other substances.

Animals↗

PubMed1976

[Utilization of glucose and fructose in human liver and muscle].

Hepatic metabolic balances derived from arterio-hepatic-venous substrate differences and hepatic blood flow exhibited similar results during fructose infusion (10 g/5 min., 0.5 g/kg x h) as compared to those from the isolated perfused organ: a portion of 55% of splanchnic fructose uptake was accountable for splanchnic glucose production, 25% for splanchnic lactate and pyruvate production and 20% for glycogen - of fat - synthesis and endoxidation. In account of this hepatic metabolism of fructose, glucose homeostasis and insulin secretion almost maintained. However, the small insulin response did not reduce hepatic free fatty acid utilization to such an extent as to increase pyruvate oxidation. Accordingly, splanchnic production of lactate and pyruvate, descending from fructose was enhanced. Since hepatic-venous lactate: pyruvate substrate ratios did not change, the rise of the arterial redox couple could not be due to hepatic fructose metabolism. According to the findings from the isolated perfused organ, fructose seemed to be phosphorylated at a rate which was found to be 2 to 3-fold that of glucose as calculated from splanchnic utilization rates. This high rate of fructose uptake was reduced with increasing insulin levels. The rapid ATP breakdown was followed by an increment of splanchnic oxygen consumption. The larger energy demand seemed to be satisfied by an enhanced endoxidation of free fatty acids. Since diabetics failed to increase their insulin production, their hepatic fructose phosphorylation rate and consequently their hepatic oxygen demand were not limited. Preliminary results concerning metabolic balances from the human forearm during a steady state of fructose supply, underline the notion that fructose has some effects on intermediary metabolism of skeletal muscle. Accordingly, in juvenile diabetics an enhancement of glucose-, lactate- and pyruvate-uptake could be demonstrated.

Abdomen↗

PubMed2002

[Fructose and glucose in follicular fluid and serum of women undergoing stimulation in an in vitro fertilization program].

OBJECTIVE: To find out if there is some amount of fructose in the follicular fluid of IVF stimulated patients. If so then to compare fructose and glucose levels with IVF outcome, type of stimulation and infertility factors. DESIGN: Retrospective study. SETTING: Department of Gynaecology and Obstetrics, Charles University, the Czech Republic. METHODS: 202 women undergoing IVF treatment since 11/1999 till 02/2001 were evaluated. We measured a level of fructose and glucose in a sample of serum and follicular fluid (FF) obtained at ovum pick-up. Then the results were compared with the IVF outcome, the length of stimulation and infertility factors. The control group consists of 23 women with a pure andrologic factor of infertility. RESULTS: At the day of pick-up the serum concentration of glucose and fructose taken on an empty stomach is on average 5.43 +/- 1.35 mmol/l and 25.05 +/- 9.26 micrograms/ml compared to 5.26 +/- 1.00 mmol/l and 23.69 +/- micrograms/ml in control group. The concentration of both glucose and fructose in FF correlates with their serum level. The FF concentration of glucose is on average 3.75 +/- 1.32 mmol/l (70% of serum concentration) and fructose 11.34 +/- 5.29 micrograms/ml (48% of serum concentration) by comparison with 3.78 +/- 0.91 mmol/l (73%) and 12.37 +/- 7.06 micrograms/ml (53%) in control group. CONCLUSION: There is a possible influence of follicular fluid saccharides like glucose and fructose in process of fertilization. We derived this hypothesis from studies evaluating the role of saccharides in endometrial secretion, cervical mucous and seminal plasma. We assumed that patients with a successful IVF cycle have a lower fructose concentration in FF. The clinical pregnancy is really mildly lower but this difference is not statistically significant. We confirmed FF fructose is in close correlation with its serum concentration. But we did not confirm the FF glucose and FF fructose concentration is related to the IVF outcome.

Adult↗

PubMed1977

Diet-induced adaptation of intestinal fructose absorption in the rat.

The effect of dietary sucrose, fructose and glucose on the intestinal absorption of fructose and glucose was investigated in adult rats in vivo: Glucose absorption was not affected by the type of dietary carbohydrate, while the absorption of fructose was increased by the ingestion of the sucrose or fructose diet, as compared with the glucose diet. An almost maximal increase of fructose absorption was already observed when the quarter of the total dietary carbohydrates was replaced by fructose. Faecal fructose elimination declined during the feeding experiment. The enhanced intestinal absorption of the fructose load in rats fed the fructose diet was manifested by higher concentrations of fructose, but also of glucose and lactate in the hepatic portal blood.

Adaptation, Physiological↗

PubMed1992

Fructose protects rat hepatocytes from anoxic injury. Effect on intracellular ATP, Ca2+i, Mg2+i, Na+i, and pHi.

The effects of fructose on the intracellular ionic changes evoked by anoxia were studied in freshly isolated rat hepatocytes maintained in agarose gel threads and perfused with Krebs-Henseleit bicarbonate buffer (KHB). Cytosolic free calcium (Ca2+i) was measured with aequorin, intracellular sodium (Na+i) with sodium-binding benzofuran isophthalate, intracellular pH (pHi) with 2'-7'-bis(carboxyethyl)-5,6-carboxyfluorescein, lactic dehydrogenase (LDH) by the increase in NADH absorbance during lactate oxidation to pyruvate, and viability by trypan blue exclusion. ATP, Pi, phosphomonoesters, and the cell phosphorylation potential assessed by the reciprocal of the Pi/ATP ratio were measured by 31P NMR spectroscopy in real time. Intracellular free Mg2+ (Mg2+i) was calculated from the chemical shift of beta-ATP relative to alpha-ATP in the NMR spectra. Anoxia was induced by perfusing the cells with KHB saturated with 95% N2, 5% CO2. When the perfusate contained 5 mM glucose as substrate, anoxia caused a fall in ATP, a rise in Pi, and in the Pi/ATP ratio, a biphasic increase in Ca2+i that reached 1.45 +/- 0.42 microM and a 6-fold increase in LDH. When 15 mM fructose was used as substrate during the anoxic period, intracellular ATP decreased much faster than with glucose, Pi did not increase, and the concentration of phosphomonoesters increased 2.5-fold. During the first hour of anoxia, the Pi/ATP ratio was higher in the fructose than in the glucose group indicating that the hepatocyte phosphorylation potential and ATP decreased faster and to lower levels with fructose than with glucose. On the other hand, ATP and the phosphorylation potential of the fructose group increased during the second hour of anoxia, in contrast to their continuous decline in the glucose group. The major surge in Ca2+i was depressed 52% when glucose was replaced by fructose: Ca2+i reached only 0.7 +/- 0.2 microM instead of 1.45 +/- 0.42 microM (p less than 0.01). Anoxia also caused an increase in Na+i and an intracellular acidosis. The rise in Na+i was significantly greater with fructose than with glucose. Na+i rose from a control value of 15.9 +/- 2.4 to 32.2 +/- 0.4 mM with glucose and to 48.7 +/- 0.7 mM with fructose (p less than 0.001). The decrease in pHi from a control value of 7.43 +/- 0.03 was consistently greater and faster with fructose than with glucose: 6.59 +/- 0.03 and 7.04 +/- 0.01, respectively. At the same time, fructose completely suppressed LDH release and reduced the loss of viability produced by anoxia from 27.7 +/- 2.9 to 14 +/- 3.1% (p less than 0.05).

Adenosine Triphosphate↗

PubMed2005

Adverse effects of dietary fructose.

The consumption of fructose, primarily from high-fructose corn syrup (HFCS), has increased considerably in the United States during the past several decades. Intake of HFCS may now exceed that of the other major caloric sweetener, sucrose. Some nutritionists believe fructose is a safer form of sugar than sucrose, particularly for people with diabetes mellitus, because it does not adversely affect blood-glucose regulation, at least in the short-term. However, fructose has potentially harmful effects on other aspects of metabolism. In particular, fructose is a potent reducing sugar that promotes the formation of toxic advanced glycation end-products, which appear to play a role in the aging process; in the pathogenesis of the vascular, renal, and ocular complications of diabetes; and in the development of atherosclerosis. Fructose has also been implicated as the main cause of symptoms in some patients with chronic diarrhea or other functional bowel disturbances. In addition, excessive fructose consumption may be responsible in part for the increasing prevalence of obesity, diabetes mellitus, and non-alcoholic fatty liver disease. Although the long-term effects of fructose consumption have not been adequately studied in humans, the available evidence suggests it may be more harmful than is generally recognized. The extent to which a person might be adversely affected by dietary fructose depends both on the amount consumed and on individual tolerance. With a few exceptions, the relatively small amounts of fructose that occur naturally in fruits and vegetables are unlikely to have deleterious effects, and this review is not meant to discourage the consumption of these healthful foods.

Diabetes Mellitus↗

PubMed1987

Fructose effect to suppress hepatic glycogen degradation.

The effect of fructose on glycogen degradation was examined by measuring the flux of 14C from prelabeled glycogen in perfused rat livers. During 2-h refeeding of 24-h-fasted rats, newly synthesized hepatic glycogen was labeled by intraperitoneal injection of [U-14C] galactose (0.1 mg and 0.02 microCi/g of body weight). The livers of refed rats were then perfused in a nonrecirculating fashion for an initial 30 min with glucose alone (10 mM) for the following 60 min with glucose (10 mM) without (n = 5) or with fructose (1, 2, or 10 mM; n = 5 for each). When livers were exposed to fructose, release of label into the perfusate immediately declined and remained markedly suppressed through the end of perfusion (p less than 0.05). The suppression was dose-dependent; at steady state (50-70 min), label release was suppressed 45, 64, and 72% by 1, 2, and 10 mM fructose, respectively (p less than 0.0001). Suppression was not accompanied by significant changes in the activities of glycogen synthase or phosphorylase assessed in vitro. These results suggest the existence of allosteric inhibition of phosphorylase in the presence of fructose. Fructose 1-phosphate (Fru-1-P) accumulated in proportion to fructose (0.11 +/- 0.01 without fructose, 0.86 +/- 0.03, 1.81 +/- 0.18, and 8.23 +/- 0.60 mumol/g of liver with 1, 2, and 10 mM fructose, respectively; p less than 0.0001). Maximum inhibition of label release was 82%; the Fru-1-P concentration for half inhibition was 0.57 mumol/g of liver, well within the concentration of Fru-1-P attained during refeeding. We conclude that fructose enhances net glycogen accumulation in liver by suppressing glycogenolysis and that the suppression is presumably caused by allosteric inhibition of phosphorylase by Fru-1-P.

Animals↗

PubMed1986

Activation of ethanol metabolism in humans by fructose: importance of experimental design.

Fed human male subjects 18 to 30 years of age received 0.85 g/kg of ethanol p.o. as vodka followed by a second drink (0.3-0.4 g/kg) given 3 to 4 hr later. Blood ethanol levels reached approximately 100 mg% after both doses. Breath samples were taken every 20 to 30 min, and the rate of ethanol elimination was determined. Subjects returned 1 to 2 weeks later and the study was repeated with fructose (1.0 g/kg). When ethanol and fructose were given simultaneously, the peak blood ethanol values reached were reduced significantly. This resulted in calculated increases in the apparent volume of distribution of ethanol; however, the slope (beta) of the ethanol elimination curve was not increased. Thus, under these conditions, fructose appeared to have minimal effects on rates of ethanol elimination (ca. 10%). In another experimental design, subjects received 1 g/kg of fructose 1 hr after ethanol. Under these conditions, fructose did not affect the peak blood ethanol concentration or the apparent volume of distribution; however, the slopes of the ethanol elimination curves (beta) were increased significantly by fructose. Further, the time estimated to eliminate the ethanol completely was reduced by 90 min with fructose. Under these conditions, the stimulation of ethanol elimination by fructose was 20 to 30%. This increase in the rate of ethanol elimination was observed in every subject studied. Similar results were obtained with rats. To understand why blood ethanol values were lower when ethanol and fructose were given simultaneously, female Sprague-Dawley rats were given ethanol alone (1.5 g/kg) or ethanol and fructose (1 g/kg) simultaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1982

Regulation of rat liver fructose 2,6-bisphosphatase.

An enzyme activity that catalyzes the hydrolysis of phosphate from the C-2 position of fructose 2,6-bisphosphate has been detected in rat liver cytoplasm. The S0.5 for fructose 2,6-bisphosphate was about 15 microM and the enzyme was inhibited by fructose 6-phosphate (Ki 40 microM) and activated by Pi (KA 1 mM). Fructose 2,6-bisphosphatase activity was purified to homogeneity by specific elution from phosphocellulose with fructose by specific elution from phosphocellulose with fructose 6-phosphate and had an apparent molecular weight of about 100,000, 6-phosphofructo 2-kinase activity copurified with fructose 2,6-bisphosphatase activity at each step of the purification scheme. Incubation of the purified protein with [gamma-32P]ATP and the catalytic subunit of the cAMP-dependent protein kinase resulted in the incorporation of 1 mol of 32P/mol of enzyme subunit (Mr = 50,000). Concomitant with this phosphorylation was an activation of the fructose 2,6-bisphosphatase and an inhibition of the 6-phosphofructo 2-kinase activity. Glucagon addition to isolated hepatocytes also resulted in an inhibition of 6-phosphofructo 2-kinase and activation of fructose 2,6-bisphosphatase measured in cell extracts, suggesting that the hormone regulates the level of fructose 2,6-bisphosphate by affecting both synthesis and degradation of the compound. These findings suggest that this enzyme has both phosphohydrolase and phosphotransferase activities i.e. that it is bifunctional, and that both activities can be regulated by cAMP-dependent phosphorylation.

Animals↗

PubMed1990

Fructose 1-6 diphosphate prevents intestinal ischemic reperfusion injury and death in rats.

This study of ischemic and postischemic reperfusion intestinal injury in rats evaluates the potential therapeutic value of fructose 1-6 diphosphate on the basis of its ability to enhance anaerobic carbohydrate metabolism during ischemia and to prevent additional tissue injury after reestablishing blood flow by inhibiting the neutrophils to produce oxygen free radicals. In pursuit of this goal, 28 rats were randomized into 4 groups: pretreated with fructose 1-6 diphosphate (n = 7); pretreated with glucose (n = 7); post-reperfusion treated with fructose 1-6 diphosphate (n = 7); and post-reperfusion treated with saline (n = 7). Five additional rats were sham operated. Following 30 min occlusion of the superior mesenteric artery, all rats received their respective treatments for 5 days. Post-reperfusion arterial pressure was significantly lower in the control rats (p less than 0.001) as well as when compared with the fructose 1-6 diphosphate groups (p less than 0.001). Significant increase in white blood cell counts occurred in the controls (p less than 0.001), whereas in the fructose 1-6 diphosphate groups white blood cell counts were no different from preischemic values. All control rats that died in less than 5 days had transmural intestinal necrosis, whereas in 3 of the controls that survived 5 days, partial intestinal necrosis was noted. Only one fructose 1-6 diphosphate-treated rat had partial intestinal necrosis. The overall 5-day survival was 100% for sham-operated rats, 93% for fructose 1-6 diphosphate-treated rats, and 21% for controls (fructose 1-6 diphosphate vs. controls, p less than 0.001; fructose 1-6 diphosphate vs. sham, NS). The results are discussed and explained in terms of the postulated mechanism based on the pharmacological properties of fructose 1-6 diphosphate.

Animals↗

PubMed1989

6-Phosphofructo-2-kinase and fructose-2,6-bisphosphatase from Saccharomyces cerevisiae.

In permeabilized yeast cells 6-phosphofructo-2-kinase and fructose-2,6-bisphosphatase are studied during growth. It is shown that in yeast at least two fructose 2,6-bisphosphate degrading enzyme activities occur, differing in pH profile and in their substrate affinities. The activities of 6-phosphofructo-2-kinase and of fructose-2,6-bisphosphatases drop in the exponential and the transition phase while the activity of the alkaline phosphatases steadily increases. In the stationary phase the activities of 6-phosphofructo-2-kinase and of the low Km fructose-2,6-bisphosphatase increase again. Yeast 6-phosphofructo-2-kinase and fructose-2,6-bisphosphatase were purified and separated from each other. The purified 6-phosphofructo-2-kinase was found to exhibit a very high specific activity (1.3 U/mg). The enzyme is efficiently inhibited by ATP. The ATP inhibition is most pronounced at low concentrations of magnesium and fructose-6-phosphate. Phosphoenolpyruvate and sn-glycerol 3-phosphate are inhibitors of the enzyme. The high-affinity yeast fructose-2,6-bisphosphatase releases inorganic phosphate from the 2-position of fructose 2,6-bisphosphate. It displays hyperbolic kinetics towards fructose 2,6-bisphosphate (Km = 0.3 microM) and is strongly inhibited by fructose 6-phosphate. The inhibition is counteracted by sn-glycerol 3-phosphate. The enzyme is shown to be inactivated by cAMP-dependent phosphorylation and reactivated by the action of protein phosphatase 2A.

Cell Membrane Permeability↗

PubMed1976

Studies on heart phosphofructokinase. Use of fructose 6-sulfate as an alternative substrate to study the mechanism of action and active site specificity.

Fructose 6-sulfate was synthesized by direct sulfurylation of fructose and was isolated by two selective steps: (a) conversion of the 6-sulfuryl ester to fructose 1-phosphate-6-sulfate with phosphofructokinase; (b) conversion of fructose 1-phosphate-6-sulfate to fructose 6-sulfate by fructose-1,6-diphosphatase. Utilizing crystalline sheep heart phosphofructokinase, kinetic studies with the alternative substrate were carried out at pH 8.2 which is optimal for nonallosteric kinetics. The data are consistent with an ordered addition of the two substrates with the first, MgATP, being at thermodynamic equilibrium. The Vmax and Km obtained with fructose 6-sulfate were 0.03- and 100-fold, respectively, that obtained with the natural substrate. The study suggests that the divalent phosphoryl moiety is intimately involved in the active site conformation. Identification of the product of the reaction, fructose 1-phosphate-6-sulfate, was confirmed through studies with aldolase, fructose-1,6-diphosphatase, and by 31P NMR. The utilization of fructose 6-sulfate as a substrate by yeast glucose-6-phosphate isomerase could not be demonstrated.

Animals↗

PubMed1986

Kinetic effects of fructose-1,6-bisphosphate on yeast phosphofructokinase.

Yeast phosphofructokinase is known to be effectively activated by fructose-2,6-bisphosphate and AMP. In the absence of the two effectors, fructose-1,6-bisphosphate activates or inhibits the enzyme according to the concentrations of the substrates and of inorganic phosphate. At cellular concentrations of the substrates, however, the effects of fructose-1,6-bisphosphate are negligible. Whereas the activation of the enzyme by AMP is not affected by fructose-1,6-bisphosphate, the latter was found to diminish strongly the activity of the fructose-2,6-bisphosphate-activated enzyme. Inorganic phosphate amplifies the activating effect of fructose-2,6-bisphosphate and augments also the deactivation of the fructose-2,6-bisphosphate-activated enzyme. The deactivating action of fructose-1,6-bisphosphate with respect to fructose-2,6-bisphosphate dominates at low concentrations of fructose-6-phosphate and high levels of ATP and might be of regulatory significance.

Enzyme Activation↗

PubMed1987

Regulation of adult and fetal myocardial phosphofructokinase. Relief of cooperativity and competition between fructose 2,6-bisphosphate, ATP, and citrate.

To clarify the physiological role of fructose 2,6-bisphosphate in the perinatal switching of myocardial fuels from carbohydrate to fatty acids, the kinetic effects of fructose 2,6-bisphosphate on phosphofructokinase purified from fetal and adult rat hearts were compared. For both enzymes at physiological pH and ATP concentrations, 1 microM fructose 2,6-bisphosphate induced a greater than 10-fold reduction in S0.5 for fructose 6-phosphate and it completely eliminated subunit cooperativity. Fructose 2,6-bisphosphate may thereby reduce the influence of changes in fructose 6-phosphate concentration on phosphofructokinase activity. Based on double-reciprocal plots and ATP inhibition studies, adult heart phosphofructokinase activity is more sensitive to physiological changes in ATP and citrate concentrations than to changes in fructose 2,6-bisphosphate concentrations. Fetal heart phosphofructokinase is less sensitive to ATP concentration above 5 mM and equally sensitive to citrate inhibition. The fetal enzyme has up to a 15-fold lower affinity for fructose 2,6-bisphosphate, rendering it more sensitive to changes in fructose 2,6-bisphosphate concentration than adult heart phosphofructokinase. Together, these factors allow greater phosphofructokinase activity in fetal heart while retaining sensitive metabolic control. In both fetal and adult heart, fructose 2,6-bisphosphate is primarily permissive: it abolishes subunit cooperativity and in its presence phosphofructokinase activity is extraordinarily sensitive to both the energy balance of the cell as reflected in ATP concentration and the availability of other fuels as reflected in cytosolic citrate concentration.

Adenosine Triphosphate↗

PubMed1981

A novel enzyme catalyzes the synthesis of activation factor from ATP and D-fructose-6-P.

We have recently discovered an activator for phosphofructokinase termed "activation factor" (Furuya, E., and Uyeda, K. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 5861-5864). In this paper, we investigated the enzyme responsible for its synthesis. We have found an enzyme from rat liver which catalyzes the formation of activation factor from fructose-6-P and ATP-Mg and it has been identified as fructose-2,6-P2. Fructose-1,6-P2, fructose-1-P, or fructose does not serve as a substrate. This enzyme has been partially purified and shown to be different from phosphofructokinase. Several lines of evidence indicate that the in vitro synthetic product is identical with chemically synthesized fructose-2,6-P2: (a) it is active in our assay for activation factor which is based on counteraction of ATP inhibition of phosphofructokinase; (b) it is acid labile as is fructose-2,6-P2; and (c) it shows the same mobility as synthetic fructose-2,6-P2 upon paper chromatography and the acid hydrolysis product has been identified as fructose-6-P. Thus, this new enzyme catalyzes the synthesis of the activation factor from fructose-6-P and ATP-Mg.

Adenosine Triphosphate↗

PubMed2003

Fructose intolerance: an under-recognized problem.

OBJECTIVES: Although the role of lactose intolerance in the pathogenesis of abdominal symptoms is well known, the role of fructose intolerance is unclear. Our aims were 1) to examine the prevalence of fructose intolerance in patients with unexplained abdominal symptoms, and 2) to explore whether fructose concentration influences fructose breath test. METHODS: Over 2 yr, patients with unexplained symptoms answered questionnaires and underwent fructose breath tests. Patients received 50 g fructose in 150 ml water (33% solution). Breath samples were collected for hydrogen and methane. In a second study, breath test was performed after giving either 10%, 20%, or 33% fructose solution. Data were analyzed retrospectively. RESULTS: A total of 183 patients (50 male, 133 female) had breath tests, of whom 134 (73%) were positive. Among these, 119 (89%) had elevated H(2), and 15 (11%) had elevated CH(4) or both gases. Questionnaires showed that flatus (83%), pain (80%), bloating (78%), belching (70%), and altered bowel habit (65%) were the most common symptoms. Breath test reproduced symptoms in 101 patients (75%). In the second study, 14/36 (39%) tested positive with a 10% solution, 23/33 (70%) with a 20% solution, and 16/20 (80%) with a 33% solution (10% versus 20% or 33%, p < 0.01). CONCLUSIONS: Fructose intolerance may cause unexplained GI symptoms. The higher yield of positive tests in our initial study may be due to referral bias or testing conditions; lower test dose produced a lower yield. Nonetheless, recognition and treatment of fructose intolerance may help many patients.

Adult↗

PubMed1974

Transport and catabolism of D-fructose by Spirillum itersomii.

Spirillum itersonii ATCC 12639 utilized d-fructose but neither d-glucose nor d-gluconate as a sole source of carbon and energy. The substrate saturation kinetics for d-fructose and d-glucose uptake by whole cells indicated the presence of a carrier-mediated transport system for d-fructose but not for d-glucose. The d-fructose uptake activity was induced (10- to 12-fold increase) during growth on d-fructose-Casamino Acids (CA) or d-glucose-CA medium, but not CA alone. d-Fructose uptake activity was stimulated by Na(+) or Li(+), but was inhibited by KCN, NaN(3), 2,4-dinitrophenol, and p-chloromercuribenzoate. High specific activities of glucokinase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydratase, and 2-keto-3-deoxy-6-phosphogluconate aldolase were detected in extracts of cells cultured on d-fructose-CA medium. These enzymatic activities were undetectable in extracts of cells grown in CA or succinate-CA medium. No decrease in the maximally induced specific activities of these enzymes occurred after the addition of succinate to cells during exponential growth on d-fructose-CA. Fructose 1,6-diphosphate aldolase and glucose-6-phosphate isomerase specific activities were approximately the same irrespective of cultural conditions. These results indicated that d-glucose was not utilized by cells of S. itersonii because this bacterium was impermeable to this hexose.

Alcohol Oxidoreductases↗

PubMed2002

Glucose-to-fructose conversion at high temperatures with xylose (glucose) isomerases from Streptomyces murinus and two hyperthermophilic Thermotoga species.

The conversion of glucose to fructose at elevated temperatures, as catalyzed by soluble and immobilized xylose (glucose) isomerases from the hyperthermophiles Thermotoga maritima (TMGI) and Thermotoga neapolitana 5068 (TNGI) and from the mesophile Streptomyces murinus (SMGI), was examined. At pH 7.0 in the presence of Mg(2+), the temperature optima for the three soluble enzymes were 85 degrees C (SMGI), 95 degrees to 100 degrees C (TNGI), and >100 degrees C (TMGI). Under certain conditions, soluble forms of the three enzymes exhibited an unusual, multiphasic inactivation behavior in which the decay rate slowed considerably after an initial rapid decline. However, the inactivation of the enzymes covalently immobilized to glass beads, monophasic in most cases, was characterized by a first-order decay rate intermediate between those of the initial rapid and slower phases for the soluble enzymes. Enzyme productivities for the three immobilized GIs were determined experimentally in the presence of Mg(2+). The highest productivities measured were 750 and 760 kg fructose per kilogram SMGI at 60 degrees C and 70 degrees C, respectively. The highest productivity for both TMGI and TNGI in the presence of Mg(2+) occurred at 70 degrees C, pH 7.0, with approximately 230 and 200 kg fructose per kilogram enzyme for TNGI and TMGI, respectively. At 80 degrees C and in the presence of Mg(2+), productivities for the three enzymes ranged from 31 to 273. A simple mathematical model, which accounted for thermal effects on kinetics, glucose-fructose equilibrium, and enzyme inactivation, was used to examine the potential for high-fructose corn syrup (HFCS) production at 80 degrees C and above using TNGI and SMGI under optimal conditions, which included the presence of both Co(2+) and Mg(2+). In the presence of both cations, these enzymes showed the potential to catalyze glucose-to-fructose conversion at 80 degrees C with estimated lifetime productivities on the order of 2000 kg fructose per kilogram enzyme, a value competitive with enzymes currently used at 55 degrees to 65 degrees C, but with the additional advantage of higher fructose concentrations. At 90 degrees C, the estimated productivity for SMGI dropped to 200, whereas, for TNGI, lifetime productivities on the order of 1000 were estimated. Assuming that the most favorable biocatalytic and thermostability features of these enzymes can be captured in immobilized form and the chemical lability of substrates and products can be minimized, HFCS production at high temperatures could be used to achieve higher fructose concentrations as well as create alternative processing strategies.

Aldose-Ketose Isomerases↗