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PubMed1991

Non reductive activation of spinach chloroplastic fructose-1,6-bisphosphatase: evidence for structural modification of the enzyme.

Preincubation of chloroplastic fructose-1,6-bisphosphatase (FBPase) in the presence of Ca2+/fructose-1,6-bisphosphate (FBS) gives rise to an active enzyme. This non-reductive activation at pH 8 occurs in the same range of time (min) as the well known reductive activation by thioredoxins and this process is reversible. A conformational change of the enzyme occurs upon the activation by Ca2+/FBP. Indeed, the circular dichroism and the fluorescence spectra of the inactive and active enzymes are different. The titration of the sulfhydryl groups of both enzymes indicates that one -SH group per monomer is unmasked upon activation, and the isoelectrofocusing pattern shows that the pI of inactive FBPase is shifted from 4.26 to 4.56 upon this non-reductive process.

Calcium↗
PubMed1992

Effect of experimental hypothyroidism on the control of 6-phosphofructo-1-kinase activity in rat jejunal mucosa.

Changes in the activity of 6-phosphofructo-1-kinase (PFK, EC 2.7.1.11) from the epithelial cells of rat small intestine during experimental hypothyroidism were studied. Hypothyroidism resulted in significant decreases in the plasma concentrations of total tri-iodothyronine, free tri-iodothyronine, total thyroxine, free thyroxine and insulin. These changes were associated with a significant increase in the plasma concentration of thyrotropin. The total activity and activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 (v0.5/V)) of jejunal PFK of hypothyroid rats were significantly diminished as compared to control rats. PFK of hypothyroid rats was more sensitive to inhibition by ATP. The mucosal enzyme of both control and hypothyroid state was sensitive to stimulation by AMP and fructose 2,6-bisphosphate. It is concluded that during hypothyroidism the rate of glycolytic pathway in the small intestine is reduced as a result of a fall in glucose uptake, and the subsequent kinetic changes of PFK are primarily to maintain the concentrations of fructose 6-phosphate (and glucose 6-phosphate) during the reduced glycolytic flux. These changes in PFK activity may be caused by changes in plasma insulin concentrations, glucose utilization and fructose 2,6-bisphosphate concentrations.

Adenosine Monophosphate↗
PubMed1989

Effects of fructose 1,6-diphosphate on splanchnic artery occlusion shock in the rat.

Splanchnic artery occlusion (SAO) shock, produced by clamping splanchnic arteries for 45 min followed by the release of occlusion, was induced in male rats, treated 15 min before surgery, with fructose 1,6-diphosphate (FDP) or with equivalent doses of fructose or inorganic phosphate. Survival rate, peritoneal macrophage phagocytosis and plasma levels of myocardial depressant factor (MDF) were measured. Shocked animals pretreated with vehicle exhibited 24.6 +/- 0.9% phagocytic activity, 110 +/- 3.9 units/ml MDF plasma levels and 0% survival. Sham animals showed the following values: survival 100%; phagocytosis, 49.5 +/- 1.3%; MDF, 22 +/- 2.9 units/ml. Pretreatment with FDP (25 mg/kg/i.v.) significantly improved survival rate (50%) and macrophage phagocytosis (37.9 +/- 0.4%) and reduced plasma MDF levels (77 +/- 3 units/ml). Equivalent doses of fructose and inorganic phosphate did not improve survival, as well as lower doses of FDP. These results suggest a beneficial effect of FDP in SAO shock.

Animals↗
PubMed1991

The effects of fructose-1,6-diphosphate on myocardial damage in acute coronary artery occlusion.

Acute myocardial infarction can result from thrombosis of a coronary artery. The purpose of this study was to evaluate the ability of fructose-1,6-diphosphate (FDP; Esafosfina) to reduce myocardial necrosis during acute thrombosis of a coronary artery. A canine model of acute myocardial infarction was used to produce intraluminal thrombosis by placement of a coil of wire in a coronary artery. After developing a coronary thrombosis of the left anterior descending artery, dogs were injected intravenously with 90 mg/kg, 175 mg/kg, or 350 mg/kg of FDP or normal saline (controls). Hemodynamic, biochemical and electrocardiographic parameters were evaluated before, and 30 min and 4 h after occlusion. Four hours after acute coronary occlusion, the animals were sacrificed, and the weights of ischemic and necrotic myocardial tissue were quantified using a histologic-staining method. There were no significant differences between control and treated animals in biochemical or hemodynamic parameters. All animal groups treated with FDP demonstrated significant reductions in the amount of necrotic and ischemic tissue compared to controls (P less than 0.05). However, only the 175 mg/kg group had a significant reduction compared to controls in necrotic tissue weight as a percentage of ischemic myocardium (24 +/- 15% vs. 72 +/- 22%, respectively, P less than 0.01). These data suggest that FDP may have a role in limiting the amount of myocardial damage after an acute coronary artery occlusion.

Animals↗
PubMed1982

Flux regulation in glycogen-induced oscillatory glycolysis in cell-free extracts of Saccharomyces carlsbergensis.

To localise the controlling point of the glycolytic system, the temporal changes in concentrations of glycolytic intermediates have been analysed after addition of glycogen to a substrate-depleted yeast extract. Three sequential metabolic states are clearly observable: a transition state at which there is continuous accumulation of the intermediates before the glyceraldehydephosphate dehydrogenase (GAPDH, EC 1.2.1.12) step; a stationary state with all glycolytic intermediates having concentrations oscillating at nearly stationary mean values; and a depletion state at which the intermediates before the GAPDH step are being depleted due to the exhaustion of glycogen. In all these states, the mean ethanol production rate and the concentration of ATP and the intermediates beyond the GAPDH-step are maintained fairly constant, while the glycogen consumption rate and intermediate concentrations of the upper part of the glycolytic system changes considerably: the glycogen consumption rate varies 4-fold and fructose-bis-phosphate concentration more than 10-fold. Doubling of the initial glycogen concentration and the addition of a great excess of fructose-bis-phosphate do not affect the ethanol production rate and the mean glycerate-3-phosphate (3-PGA) and pyruvate levels. By contrast, ethanol production was accelerated by an increase of the net ATP consumption rate resulting from either the addition of apyrase or by substitution of trehalose for glycogen. Neither the mean absolute ATP level nor the adenylate energy charge were measurably affected, however, all this data can be interpreted in terms of a very strong stoichiometric regulation and stabilization of the lower part of the glycolytic system.

Adenosine Triphosphate↗
PubMed1990

The impairment of respiration by glycolysis in the Lewis lung carcinoma.

Isolated vegetative tumour cells from mice bearing the Lewis lung carcinoma showed low rates of basal respiration with both low oxygen uptake rates and cytochrome-c oxidase activity. The cells were affected by a marked Crabtree effect and a high rate of lactate production in the presence of 10 mM glucose. The glycolytic capacity of the tumour was also assessed through the measurement of the maximum activities for hexokinase, phosphofructokinase, pyruvate kinase and lactate dehydrogenase. These activities were similar to the ones found in other fast-growing, undifferentiated tumours. The concentration of fructose-2,6-bisphosphate in the tumour was 2,3 nmoles/g fresh tissue wt., a value which is of the same order of magnitude as that found in other types of highly glycolytic cells. It is concluded that the Lewis lung carcinoma follows the same pattern as other undifferentiated tumours with a high capacity for both glucose and amino acid utilization.

Animals↗
PubMed1978

Some effects of glucose concentration and anoxia on glycolysis and metabolite concentrations in the perfused liver of fetal guinea pig.

Effects of glucose concentration and anoxia upon the metabolite concentrations and rates of glycolysis and respiration have been investigated in the perfused liver of the fetal guinea pig. In most cases the metabolite concentrations in the perfused liver were similar to those observed in vivo. Between 50 days and term there was a fall in the respiratory rate and in the concentration of ATP and fructose 1,6-diphosphate and an increase in the concentration of glutamate, glycogen and glucose. Reducing the medium glucose concentration from 10 mM to 1 mM or 0.1 mM depressed lactate production and the concentration of most of the phosphorylated intermediates (except 6-phosphogluconate) in the liver of the 50-day fetus. This indicates a fall in glycolytic rate which is not in accord with the known kinetic properties of hexokinase in the fetal liver. Anoxia increased lactate production by, and the concentrations of, the hexose phosphates ADP and AMP in the 50-day to term fetal liver, while the concentration of ribulose 5-phosphate, ATP and some triose phosphates fell. These results are consistent with an activation of glycolysis, particularly at phosphofructokinase and of a reduction in pentose phosphate pathway activity, particularly at 6-phosphogluconate dehydrogenase. The calculated cytosolic NAD+/NADH ratio for the perfused liver was similar to that measured in vivo and evidence is presented to suggest that the dihydroxyacetone phosphate/glycerol 3-phosphate ratio gives a better indication of cytosolic redox than the lactate/pyruvate ratio. The present observations indicate that phosphofructokinase hexokinase and possibly pyruvate kinase control the glycolytic rate and that glyceraldehyde-3-phosphate dehydrogenase is at equilibrium in the perfused liver of the fetal guinea pig.

Adenine Nucleotides↗
PubMed1979

Metabolism of exogenous N-acetylglucosamine in extracts of rat kidney, liver and hepatoma.

1. The metabolism of exogenous N-acetylglucosamine (GlcNAc) in rat kidney extracts was greatly stimulated by fructose 1,6-diphosphate (Fru-1,6-P2) and to a lesser extent by phosphoenolpyruvate. They served as a generator of ATP. Under these conditions, the majority of metabolized GlcNAc was recovered in the form of glycolytic intermediates. 2. The metabolism of exogenous GlcNAc in rat liver extracts was stimulated by phosphoenolpyruvate but not by Fru-1,6P2. With phosphoenolpyruvate present, most of the metabolized GlcNAc was recovered as sialic acid. 3. The metabolism of exogenous GlcNAc in rat hepatoma (AH-130) extracts was stimulated by Fru-1,6-P2 and to a lesser extent by phosphoenolpyruvate. Even with phosphoenolpyruvate present, the synthesis of sialic acid was extremely small. In these respects, hepatoma extracts resemble kidney extracts rather than those of liver.

Acetylglucosamine↗
PubMed1984

Alterations in 32P-labelled intermediates during flux activation of human platelet glycolysis.

Using a newly developed isotopic tracer technique for the measurement of 32P-labelled intermediates in glycolysis and nucleotide metabolism in platelets, we studied the variations in 32P-labelled intermediates during activation of the glycolytic flux by cyanide and platelet-activating agents. The major variations occurred in [32P]Fru-1,6-P2, dihydroxy acetone phosphate, ATP and Pi. There was a quantitative covariance between the increase in lactate production and the rise in [32P]Fru-1,6-P2 induced by different platelet-activating agents. In contrast, cyanide induced weaker activation of the flux and greater accumulation of [32P]Fru-1,6-P2. Variations in 32P-labelled intermediates were apparent 5 s after flux activation, but the major changes in [32P]Fru-1,6-P2 occurred much later and fell in periods in which a constant lactate formation was maintained. The cyanide-induced changes in 32P-labelled intermediates depended on the extracellular level of glucose, showing a predominant ATP----Pi conversion in glucose-depleted medium that shifted to an ATP----Fru-1,6-P2 conversion at excess glucose. At about 50 microM glucose, flux activation occurred without major changes in [32P]Fru-1,6-P2, dihydroxy acetone phosphate and Pi, with only a small fall in [32P]ATP. The data provide evidence for a role of the aldolase reaction in flux control and demonstrate rapid changes in Fru-1,6-P2 and ATP during flux activation with an additional role for Fru-1,6-P2 as an energy buffer during post-activation periods.

Adenosine Diphosphate↗
PubMed1985

Inhibition of phosphoglucomutase by fructose 2,6-bisphosphate.

Fructose 2,6-bisphosphate inhibits phosphoglucomutase. The inhibition is mixed with respect to glucose 1,6-bisphosphate and non-competitive with respect to glucose 1-phosphate. In contrast with fructose 1,6-bisphosphate and glycerate 1,3-bisphosphate, which also possess inhibitory effect, fructose 2,6-bisphosphate does not phosphorylate phosphoglucomutase. Fructose 2,6-bisphosphate preparations contain contaminants which can explain artefactual results previously reported.

Animals↗
PubMed1987

Phosphofructokinase in rat lung during perinatal development: characterization of subunit composition and regulation by fructose 2,6-bisphosphate and glucose 1,6-bisphosphate.

The subunit composition of phosphofructokinase (ATP: D-fructose-6-phosphate-1-phosphotransferase, EC 2.7.1.11) was studied in rat lung during perinatal development. No change in subunit composition during this period was observed. The three subunits of phosphofructokinase (L, M and C) were present in a ratio of approx. 65:25:10, respectively. In addition the levels of two effectors of phosphofructokinase were determined in rat lung during perinatal development: glucose 1,6-bisphosphate and fructose 2,6-bisphosphate. Until day 20 of gestation (term is 22 days) the glucose 1,6-bisphosphate level remains relatively constant (approx. 0.55 mumol/g protein), decreases before birth and increases sharply up to 1.04 mumol/g protein 2 days after birth. The amount of fructose 2,6-bisphosphate in rat lung shows a different developmental profile. A small peak is shown at day 17 of gestation whereas a larger peak up to 36.4 nmol/g protein is shown at days 20 and 21 of gestation. The time of maximal fructose 2,6-bisphosphate content corresponds with the time of glycogen breakdown and acceleration of surfactant synthesis in prenatal rat lung. Both glucose 1,6-bisphosphate and fructose 2,6-bisphosphate stimulate lung phosphofructokinase. Half maximal stimulations occur in the range of 24.1-70.9 microM glucose 1,6-bisphosphate and 0.17-0.34 microM fructose 2,6-bisphosphate.

Age Factors↗
PubMed1987

Regulation of fructose 2,6-bisphosphate levels in Neurospora crassa.

Both wild type and cr-1 mutant (adenylate cyclase and cyclic AMP-deficient) strains of Neurospora crassa contain fructose 2,6-bisphosphate at levels of 27 nmol/g dry tissue weight. This level decreases by about 50% in both strains upon depriving the cells of carbon or nitrogen sources for 3 h. An increase in cyclic AMP levels produced by addition of lysine to nitrogen-starved cells produced no increase in fructose 2,6-bisphosphate levels. Both strains respond to short-term addition of salicylate, acetate, or 2,4-dinitrophenol with an increase in fructose 2,6-bisphosphate. Thus, the above-described regulation of fructose 2,6-bisphosphate levels is cyclic AMP-independent. A suspension of the wild type produces a transient increase of fructose 2,6-bisphosphate in response to administration of glucose, whereas the mutant strain does not respond unless it is fed exogenous cyclic AMP. Substitution of acetate for sucrose as a sole carbon source for growth leads to a differential decrease in fructose 2,6-bisphosphate levels between the two strains: the wild type strain has 63% and the cr-1 mutant strain has 37% of the levels of fructose 2,6-bisphosphate on acetate as compared to sucrose-grown controls. This may be the basis for an advantage of cr-1 over wild type in growth on acetate. Thus, although most regulation of fructose 2,6-bisphosphate is cyclic AMP-independent, the levels can be regulated by a combination of carbon source and cyclic AMP levels.

Acetates↗
PubMed1990

Changes in the concentration of fructose 2,6-bisphosphate in Aspergillus niger during stimulation of acidogenesis by elevated sucrose concentration.

The presence of fructose 2,6-bisphosphate (Fru-2,6-P2) and phosphofructokinase 2 (PFK 2) were established in the citric-acid-producing filamentous fungus Aspergillus niger. Fru-2,6-P2 levels were around 3.0 (+/- 0.8) nmol per g dry weight during growth on sucrose, and half of this in mycelia grown on citrate as a carbon source. PFK 2 was detected with a specific activity of 150 mU/mg protein and a Km for fructose 6-phosphate of 40 microM. Induction of citric acid accumulation (acidogenesis) in A. niger by cultivation on high concentrations of sucrose, or replacement on 14% (w/v) sucrose correlated with an increase in the intracellular concentration of Fru-2,6-P2. A similar correlation was obtained when A. niger was cultivated on different carbon sources, which induced different rates of acidogenesis. The increase in Fru-2,6-P2 during transfer to 14% (w/v) sucrose was not correlated with the behaviour of mycelial concentrations of cyclic AMP, a potential regulator of Fru-2,6-P2 formation in other organisms, nor with that of Fru-6-P and ATP, the precursors of its formation. The extracellular addition of cyclic AMP and theophylline, an inhibitor of cellular cyclic AMP breakdown, increased both Fru-2,6-P2 concentration and acidogenesis in mycelia cultivated in 1% (w/v) sucrose medium. It is concluded that Fru-2,6-P2 controls citric acid accumulation by enabling increased rates of glucolysis, a prerequisite to acidogenesis.

Adenosine Triphosphate↗
PubMed1991

Regulation of 6-phosphofructo-1-kinase from the epithelial cells of rat small intestine during pregnancy and lactation.

The regulation of 6-phosphofructo-1-kinase (PFK) in the epithelial cells of rat small intestine was studied during pregnancy and lactation. The total activities and activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 (nu 0.5/V] of the partially purified mucosal PFK were found to increase initially in early pregnant rats (11-12 days of gestation) and to fall back to normal in late pregnant rats (19-20 days of gestation). These changes in enzyme activity during pregnancy were associated with similar changes in the circulating levels of progesterone. The maximal activity and activity ratio (nu 0.5/V) were increased in male and female rats injected with progesterone. An increase in the total activity and activity ratio of mucosal PFK was also obtained in lactating rats. However, the enzyme was not strongly activated by inorganic phosphate, fructose 2,6-bisphosphate or glucose 1,6-bisphosphate either in early pregnant or lactating rats. These results indicate that mucosal PFK is already present as an active form during early pregnancy and lactation. Therefore, it is suggested that female sex hormones increase the circulating levels of insulin during early pregnancy which, in turn, positively affect the activity of mucosal PFK which could be also stimulated by the increased levels of fructose 2,6-bisphosphate. The increased activity of PFK in the peak lactating rats could be possible because of an increased demand for lactate production from glucose together with the stimulation of PFK by the increased concentrations of fructose 2,6-bisphosphate which therefore increases the rate of glycolysis.

Animals↗