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PubMed1992

Interconversion of D-fructose 1,6-bisphosphate and triose phosphates in human erythrocytes.

Aldolase and triose phosphate isomerase both display strict specificity towards the enantiomers of [1-3H]glycerone 3-phosphate. The enantiomer generated from D-[1-3H]glyceraldehyde 3-phosphate produces 3HOH in the aldolase reaction, whilst the other enantiomer generated from D-[3-3H]fructose 1,6-bisphosphate is solely detritiated in the reaction catalyzed by triose phosphate isomerase. Advantage was taken of such a specificity to assess, in human erythrocytes exposed to either D-[3-3H]glucose or D-[3,4-3H]glucose, the extent of D-glyceraldehyde 3-phosphate sequential conversion to glycerone 3-phosphate and D-fructose 1,6-bisphosphate, relative to net glycolytic flux. At 37 degrees C and in the presence of 5.6 mM D-glucose, only 55% of the metabolites of D-[4-3H]glucose underwent detritiation in the reactions catalyzed by triose phosphate isomerase and aldolase. Such a percentage was further decreased at low temperature (8 degrees C) or lower concentrations of D-glucose (0.2 and 1.0 mM). However, when the erythrocytes were exposed to menadione, the increase in 3HOH production from either D-[3-3H]glucose or D-[3,4-3H]glucose indicated that the majority of the 3H atoms initially located on the C4 of D-glucose were recovered as 3HOH upon circulation through the pentose phosphate pathway. These findings suggest that, under physiological conditions, a large fraction of D-glyceraldehyde 3-phosphate generated from exogenous D-glucose may undergo enzyme-to-enzyme channelling in the glycolytic pathway.

Blood Glucose↗
PubMed1992

Phosphorylation of pyruvate kinase type K is restricted to the dimeric form.

In the absence of glycolytic intermediate, fructose-1,6-bisphosphate, pyruvate kinase type K exists in the dimeric form and is readily phosphorylated, whereas in the same sample and the same conditions pyruvate kinase type M is present as a tetramer and is not phosphorylated. Addition of fructose-1,6-bisphosphate results in the association of dimeric K2 molecules to a tetrameric K4 enzyme as determined by gel filtration and cellulose acetate electrophoresis, with concomitant loss of the capacity of the K isozyme to become phosphorylated. Phosphorylated K2 dimers can also tetramerize, but with a low recovery of the radiolabel, suggesting a fructose-1,6-bisphosphate induced dephosphorylation or selective degradation. The dimeric K isozyme is enzymatically active; inactive K-type monomers can be detected by immunoblot analysis in the absence of fructose-1,6-bisphosphate, but no phosphorylated pyruvate kinase is present in this fraction. The formation of K4 tetramers can not be accomplished by the substrate phosphoenolpyruvate. Fructose-1,6-bisphosphate is an allosteric activator of pyruvate kinase type K and induces hyperbolic saturation curves for phosphoenolpyruvate. In contrast, in the absence of effectors, pyruvate kinase type M exhibits Michaelis-Menten kinetics, but sigmoidal curves can be induced by the amino acid phenylalanine. However, even in the presence of phenylalanine, the M-type maintained its tetrameric configuration and did not serve as a substrate in the phosphorylation reaction. These findings argue for the importance of subunit interaction in the regulation of phosphorylation of pyruvate kinase.

Astrocytoma↗
PubMed1996

Electrophoretic pattern of cytosolic pyruvate kinase fractions A and B (type L and M2) from normal rat liver and Morris hepatoma 7777.

Cytosolic pyruvate kinase fractions A and B obtained by salting out procedure from normal rat liver and Morris hepatoma 7777, purified by affinity chromatography on Blue Sepharose CL-6B, have shown similar electrophoretic patterns in polyacrylamide gel at pH 8.3 to previously studied pyruvate kinase extracts from chromatin of cell nuclei. Three variants (alpha 1, beta 1, gamma 1) from normal rat liver pyruvate kinase fraction A (type L) had the greatest electrophoretic mobility, showed sigmoidal kinetics in relation to 2-phosphoenolpyruvate (PEP), and sensitivity to ATP and fructose 1,6-diphosphate (FDP). The fraction A dominated over normal liver fraction B (type M2), which in electrophoresis showed a slower gamma 2 variant, similar to the fraction A of hepatoma. All variants from fractions B of normal liver and A of hepatoma had linear kinetics and were sensitive to ATP but not to FDP. The greatest differences showed pyruvate kinase fraction B from Morris hepatoma. Its all variants alpha 2, beta 2, gamma 3 were cathodic and had linear kinetics in relation to PEP. They all were insensitive to normal signal molecules (ATP and FDP). The gamma 3 alkaline variant acquired sensitivity to inhibition by L-cysteine. Showing several-fold higher activity, much greater affinity to the main substrate, and a lack of sensitivity to feed-back inhibition by ATP, it was responsible for a high rate of aerobic glycolysis and diminution of the Pasteur effect in metabolic studies. It was probably encoded during oncogene activation and plays a special role in different metabolic strategies of tumour cells.

Adenosine Triphosphate↗
PubMed1986

Regulation of fructose 2,6-bisphosphate metabolism in human fibroblasts.

In the present work, the mechanism involved in the regulation of fructose 2,6-bisphosphate (fructose-2,6-P2) metabolism in human fibroblasts has been studied. Various agents like serum, insulin and adrenaline known to affect glycolysis have been investigated for their ability to influence fructose 2,6-P2 metabolism in confluent human fibroblasts. Serum appears to be the most potent activator of fructose-2,6-P2 levels and capable of inducing a marked increase in 6-phosphofructo-2-kinase (ATP: D-fructose-6-phosphate-2-phosphotransferase), EC 2.7.1. 105). To a lesser extent insulin has the same effects. The increase in enzyme activity elicited by serum and insulin does not require de novo protein synthesis since the process is insensitive to cycloheximide. Incubation of fibroblasts in the presence of adrenaline is responsible for a significant rise in fructose-2,6-P2 levels without affecting 6-phosphofructo-2-kinase. Similar experiments performed on glucose-starved or cytochalasin B-treated cells show that the effects elicited by all the agents are strictly dependent on glucose availability.

Cells, Cultured↗
PubMed1989

pH sensitivity of the thrombin-induced rise in fructose 2,6-bisphosphate content of human platelets.

The stimulation of human platelets with thrombin results in a rapid and sustained increase in the fructose 2,6-bisphosphate content which may play an important role in the potentiation of glycolytic flux induced by the agonist. The investigation of the effect of pH on thrombin-induced rise in platelet fructose 2,6-bisphosphate content is reported here. The results indicate that the early intracellular alkalinization which follows platelet stimulation may contribute to mediate the positive effect of thrombin on the regulatory metabolite.

Blood Platelets↗
PubMed1991

Glucose: a more powerful modulator of fructose 2,6-bisphosphate levels than insulin in human hepatocytes.

This study provides the first experimental evidence of the short-term control of fructose 2,6-bisphosphate (Fru(2,6)P2) levels in adult human hepatocytes. (1) In hepatocytes whose metabolic status resembles the fed state (glycogen-rich), exposure to glucagon (10(-8) M) caused a drastic decrease in the levels of this effector and a significant fall in lactate production rate. Adrenaline, isoprenaline (a beta-adrenergic agonist) and lactate exerted a similar action decreasing Fru(2,6)P2 concentration. (2) In glucagon pre-treated, glycogen- and Fru(2,6)P2-depleted cells (a situation that mimics the fasted state), Fru(2,6)P2 re-synthesis was strictly dependent on glucose availability. (3) Insulin did not seem to exert a direct action on the control of Fru(2,6)P2 in human hepatocytes. The hormone--which failed to enhance Fru(2,6)P2 in glucose-starved cells--did not further increase Fru(2,6)P2 content nor its time-course evolution as compared to hepatocytes incubated with glucose alone. (4) Lactate caused a significant delay in the glucose-induced increase in Fru(2,6)P2 content that could not be prevented by insulin. (5) Data indicate that in human hepatocytes glucose is a more powerful modulator of Fru(2,6)P2 than insulin, and that variations in blood lactate concentration may also play a role in the control of hepatic Fru(2,6)P2 levels during the fasted-to-fed transition in humans.

Cells, Cultured↗
PubMed1991

Glucocorticoids inhibit fructose 2,6-bisphosphate synthesis in rat thymocytes. Opposite effect of cycloheximide.

The content of fructose 2,6-bisphosphate (Fru(2,6)P2) and lactate production in triamcinolone acetonide-treated rats thymocytes was studied. The effect in vitro of corticosterone and dexamethasone on normal thymocytes was also examined. Glucocorticoids produced a marked decrease in Fru(2,5)P2 content and lactate production. The largest effect was observed with triamcinolone acetonide (7.5 mg per kg body weight), which after 20 h of treatment produced over 90% of inhibition. This change was accompanied by the decrease of both phosphofructokinase-1 and -2 activities and ATP levels, without modifications of hexoses phosphate content. The inhibitory actions of glucocorticoids were abolished by cycloheximide, an inhibitor of protein synthesis. Furthermore this drug, by itself, increased Fru(2,6)P2 content by more than 50% compared with the controls.

Animals↗
PubMed1991

Thimerosal induces calcium mobilization, fructose 2,6-bisphosphate synthesis and cytoplasmic alkalinization in rat thymus lymphocytes.

The effect of thimerosal on intracellular calcium ([Ca2+]i), pH (pHi) and fructose 2,6-bisphosphate (Fru 2,6-P2) in thymus lymphocytes was investigated. The effect of thimerosal on cell growth was also examined. Thimerosal produced a dose-dependent increase in [Ca2+]i, pHi and in the level of fructose 2,6-bisphosphate. Thimerosal was, however, unable to produce cell proliferation and inhibited [3H]thymidine incorporation when cells were challenged with PHA and costimulator. In the absence of external calcium, thimerosal produced only a slight increase in [Ca2+]i. In Na(+)-containing buffer, thimerosal induced an initial acidification (0.05 +/- 0.01 pH units), followed by an alkalinization of 0.08 pH units/min, whereas in Na(+)-free media, pHi decreased 0.2 +/- 0.02 units and this acidification was maintained for more than 40 min. When external calcium was removed the initial acidification was unchanged and no further increase in pHi was observed. Polymyxin B, an inhibitor of protein kinase C, did not modify the initial thimerosal-induced acidification although pH returned to basal levels after 10 min. It was concluded that alkalinization induced by thimerosal is probably due to activation of the Na+/H+ exchanger and that changes in internal Ca2+, pH and metabolic rate are not sufficient to induce cellular proliferation. The mechanism by which thimerosal inhibits thymocyte proliferation remains to be clarified.

Animals↗
PubMed1992

Stimulation of glycolysis by corticotropin and phorbol ester in cultured neurons.

Incubation of cultured neurons from chick embryo forebrain with corticotropin (ACTH) or the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) stimulates the production of lactate. The stimulation is seen after 2 h of treatment and is maximal after 12 h. Both ACTH (1-24) and TPA increase the concentration of fructose 2,6-bisphosphate (Fru-2,6-P2), a metabolic activator of 6-phosphofructo-1-kinase (PFK-1). This effect is concentration-dependent and is maximal after 4 h of treatment. PFK-1 activity is increased in a dose-dependent manner by ACTH (1-24) or TPA. This increase is not visible during the first 6 h and reaches its maximum after 18 h of treatment. The stimulation of PFK-1 activity is not due the increase of Fru-2,6-P2 by ACTH (1-24) or TPA, since saturating concentrations of Fru-2,6-P2 are present in the PFK-1 assay medium. Thus, it appears that ACTH (1-24) and TPA regulate glycolysis through two modes with different time responses: increase in Fru-2,6-P2 is the main mechanism operating during the first 6 h following the treatments and increase in the amount, or stable increase in activity of PFK-1, takes place during the later phase. It is suggested that the action of corticotropin on glycolysis is part of the mechanism of the neurotrophic activity of this hormone.

Adenosine Triphosphate↗
PubMed1992

Activation of in situ glycolytic flux by bisphosphorylated compounds: studies in porous rat adipocytes.

By carefully permeabilizing eukaryotic cells such that intracellular enzymes are largely retained, an opportunity is created to explore the regulation of in situ flux. This is particularly important since the latter may not be accurately represented by kinetic measurements of isolated, solubilized enzymes from disrupted cells. In this study the action of fructose 2,6-diphosphate (F2,6DP) and other bisphosphorylated sugars which purportedly activate phosphofructokinase-1 (PFK-1; EC 2.7.1.11) were studied. Using porous adipocytcs and initiating flux with radiolabeled glucose 6-phosphate, the regulation of lactate production under both 0.1 and 1.0 mM ATP conditions by F2,6DP, glucose 1,6-diphosphate (G1,6DP), ribulose 1,5-diphosphate (R1,5DP), 2,3 diphosphoglycerate (2,3DPG), and mannose 6-phosphate (M6P) was examined. Studied at 1, 5, and 25 microM concentrations, F2,6DP and 2,3DPG significantly (and to the same extent) augmented glycolysis compared to control (at 0.1 mM ATP, the respective glycolytic rates--as % above control--at these three above-mentioned concentrations for F2,6DP were 60, 84, and 77%, whereas for 2,3DPG they were 84, 105, and 179%; at 1 mM ATP, the F2,6DP effect was 88, 99, and 121%, and for 2,3DPG it was 52, 89, and 96%). Stimulation by these compounds was less obvious at higher glycolytic flux rates (saturating amounts of G6P). Amongst this group, and only at 1.0 mM ATP, the sole other positive effector was 25 microM R1,5DP. The measured fat cell content of G1,6DP was 24 +/- 4 microM (n = 3); at this concentration no significant effect on glycolysis was observed. Examining the effects of 2,3DPG (25 microM) on proximal glycolysis (to triose phosphates) revealed there was a modest, but significant, 41% increase over basal; in contrast, under the exact same conditions, F2,6DP caused a 123% increase. Separate experiments also examined the effect of F2,6DP, 2,3DPG, and G1,6DP on glycolysis at 5 and 25 microM in the presence of a physiologic cytosolic ATP/ADP ratio and free cation concentrations. Under these conditions, F2,6DP and 2,3DPG remained pre-eminent in their stimulatory prowess, inducing 27-71% increases over control, while G1,6DP remained ineffectual. These studies support a locus of action of 2,3DPG on overall glycolysis which is distal to the triose phosphates. M6P was ineffective at all concentrations. In conclusion, F2,6DP is the pre-eminent in situ regulator of in situ adipocyte glycolysis, especially at higher ATP levels, although other sugars containing two phosphoryl groups may under certain conditions cause activation.

2,3-Diphosphoglycerate↗
PubMed1994

Cyclosporin A antagonizes phenylephrine, oxytocin and angiotensin effects on glucose metabolism in rat thymus lymphocytes.

Effects of phenylephrine, oxytocin and angiotensin on fructose 2,6-bisphosphate (Fru 2,6-P2) content and glycolytic parameters were studied in incubated thymus lymphocytes. These hormones modified Fru 2,6-P2 content dependent upon the energetic status of the cells. In non-preincubated thymus lymphocytes (with relatively high levels of glycogen and ATP), phenylephrine, oxytocin and angiotensin depressed Fru 2,6-P2 content in a dose-dependent manner. The opposite was found when the cells were preincubated for 2 h without substrates (low levels of ATP and glycogen). Changes in lactate release were less evident, but significant. Phenylephrine did not modify the maximal activities of phosphofructokinase (PFK)-1 or PFK-2. However, both submaximal PFK-1 and PFK-2 activities were inhibited by phenylephrine, and the response to exogenous Fru 2,6-P2 on PFK-1 was also altered. The activities of Fru 1,6-P2 and pyruvate kinase were not modified by phenylephrine or A23187 treatment. Simultaneous presence of Cyclosporin A (CsA), an immunosuppressive drug, antagonizes the alpha-adrenergic effect on Fru 2,6-P2 content. CsA alone did not alter basal levels of ATP, hexose phosphate or Fru 2,6-P2, and its opposing effect to alpha-agonist was dose-dependent. CsA cannot change the positive action of PMA or the negative action of A23187 on Fru 2,6-P2 content. The present data suggest that CsA acts prior to calcium liberation and protein kinase C activation. Different possible molecular models are discussed.

Adenosine Triphosphate↗
PubMed1994

Bradykinin stimulates fructose 2,6-bisphosphate metabolism in human fibroblasts.

Bradykinin (BK), a peptide released during inflammatory response, has been investigated for its ability to regulate glucose metabolism in human fibroblasts. The peptide is able to significantly increase glycolytic flux in these cells. The strict relationship between the glycolytic rate and the levels of fructose 2,6-bisphosphate (Fru-2,6-P2) strongly suggests that the metabolite plays a key role in the regulation of glucose metabolism by bradykinin. The mechanism by which bradykinin increases Fru-2,6-P2 content involves the activation of 6-phosphofructo-2-kinase (PFK-2), the enzyme responsible for the synthesis of the metabolite. The study of the multiple signalling systems triggered by bradykinin demonstrates the involvement of the rise in intracellular Ca2+ concentration and of protein kinase C mediated pathway in the mechanism by which bradykinin increases Fru-2,6-P2 content and PFK-2 activity.

Bradykinin↗
PubMed1996

5-Iodotubercidin and proglycosyn: a comparison of two glycogenic compounds in hepatocytes from fasted rats.

5-Iodotubercidin (Itu) and proglycosyn (Pro) have similar glycogenic properties. To compare their mechanisms of action, we tested them in hepatocytes from fasted rats. We show that both compounds are similar in that they stimulated glycogen synthesis, increased the concentration of synthase a, decreased that of phosphorylase a and lowered the concentration of F-2,6-P2 in the presence of glucose, lactate-pyruvate and amino acids. However, when amino acids were absent, Pro was the better stimulator of glycogenesis than Itu and in combination they elevated glycogen and synthase a concentrations synergistically. Further they differ in that (1) Itu enhanced the levels of cyclic AMP whereas Pro did not; (2) Pro depressed glucose production from gluconeogenic substrates, whereas Itu stimulated this process; (3) the inhibition of F-2,6-P2 formation and glycolysis by Pro became much weaker than that by Itu when glucose concentrations were raised from 10 to 20 mM. Inhibition of glycolysis but not that of glycogen synthesis was partly due to a phosphorylated metabolite of Itu. The present study indicates that despite their similar glycogenic effects, Itu and Pro do not share a common mechanism of action. Further, the inhibition of glycolysis and F-2,6-P2 formation by Itu cannot be explained if it acts solely as a general inhibitor of protein kinases.

Animals↗
PubMed1991

Possible mechanism of proteolysis for the extrapancreatic action of tolbutamide.

In order to assess the mode of the extrapancreatic action of the sulfonylureas, we evaluated the contribution of a proteolytic mechanism for sulfonylurea action by analyzing the effects of a protease inhibitor on insulin- or tolbutamide-stimulated liver fructose-2,6-bisphosphate (F-2,6-P2) formation using isolated rat hepatocytes. The F-2,6-P2 level in hepatocytes was significantly increased by the addition of insulin or tolbutamide. The stimulatory effect of insulin on the F-2,6-P2 formation was most significant when its level was reduced by the addition of 2 microM of forskolin. Insulin action on F-2,6-P2 formation was inhibited by the addition of a protease inhibitor, p-tosyl-L-arginine methyl ester hydrochloride (TAME). Tolbutamide (2 mM) significantly increased hepatocyte F-2,6-P2 level (P less than 0.01 vs the control level). In the presence of TAME, the stimulatory effect of tolbutamide was also suppressed. The present data suggest that a proteolytic mechanism is important in both insulin and tolbutamide action on the F-2,6-P2 formation, and it may be hypothesized that, like insulin, the chemical mediator of tolbutamide action is formed proteolytically.

Animals↗
PubMed1994

Regional distribution of glycogen, glucose and phosphorylated sugars in rat brain after intoxicating doses of ethanol.

Ethanol and anaesthetics increase glycogen levels in the brain. However, no data have been reported about the effect of ethanol on glycogen and glucose metabolism in specific brain regions. We have studied the concentrations of glycogen, glucose, glucose 6-P, glucose 1,6-P2 and fructose 2,6-P2 and the activities of glycogen synthase, glycogen phosphorylase and glycogen phosphorylase kinase in seven brain regions of starved rats following treatment with a single dose or several doses of ethanol. Our results show that: (1) the effect of ethanol on glucose metabolism depends on whether it is given in one single dose or in a series of doses; (2) glycogen concentration increases after a single dose of ethanol but not after long exposure; (3) glucose, glucose 6-P in some areas, and the bisphosphorylated sugar, fructose 2,6-P2 significantly increase after prolonged exposure to ethanol; and (4) the enzymatic activities of glycogen metabolism are not modified after a long exposure to ethanol. In summary, these data show that ethanol may modify the use of glycogen, glucose and derivatives in brain. Moreover, the changes produced depend on the pattern of ethanol intake and the brain area considered.

Alcoholic Intoxication↗
PubMed1995

Effects of pharmacological interventions on emetine cardiotoxicity in isolated perfused rat hearts.

The cardiotoxicity of emetine continues to be a significant clinical problem. The purpose of this study was to investigate the effect of several mechanistic interventions, including ICRF-187, an iron-chelating agent which protects against doxorubicin toxicity, atropine, and fructose-1,6-bisphosphate (FBP) on the toxicity of emetine in our isolated, perfused rat heart model. The model includes functional, electrocardiographic, and biochemical determinations in the same preparation. Atropine and ICRF-187 had no effect on the time needed for emetine to induce ventricular asystole, while FBP significantly increased this time. Administration of 47 microM atropine, 300 microM FBP, or 1 mM FBP decreased the release of lactate dehydrogenase (LDH) into the coronary effluent, while ICRF-187 had no effect. These pharmacological interventions variably changed the amplitude of the biphasic response of the coronary flow to emetine. Finally, FBP was very effective in slowing the rate of QRS-waveform degeneration in the perfused hearts. Emetine caused PR- and QRS-prolongation which was not altered by FBP.

Animals↗
PubMed1996

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

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

Acetaminophen↗
PubMedDate not supplied

Human erythrocyte phosphoglucomutase: comparison of the kinetic properties of PGM1 and PGM2 isoenzymes.

Kinetic properties of PGM1 and PGM2 phosphoglucomutase "primary" isoenzymes from human erythrocytes were studied. The two enzyme forms share a "ping-pong" kinetic mechanism and show similar Km for substrate (glucose 1-P) and cofactor (glucose 1,6-P2). Micromolar concentrations of fructose 1,6-P2 and glycerate 2,3-P2 inhibit both PGM1 and PGM2 isoenzymes to a similar extent. The sole PGM2 form is affected by ribose monophosphates (ribose 1-P and ribose 5-P) that act as mutase inhibitors vs. glucose 1,6-P2 and as apparent activators vs. glucose 1-P. The interaction between PGM2 isoenzyme and ribose monophosphates is discussed in the light of the ability of this form to also display phosphoribomutase activity.

2,3-Diphosphoglycerate↗