Glucagon and N6,O2'-dibutyryl adenosine 3':5'-monophosphate inhibition of lipogenesis and phosphofructokinase activity of hepatocytes from meal-fed rats

Glucagon and N6,O2'-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cyclic AMP) inhibit net glucose utilization, lactate plus pyruvate accumulation and fatty acid synthesis by isolated hepatocytes prepared from meal-fed rats. A crossover in the metabolite profile of the glycolytic intermediates occurs between fructose-6-phosphate and fructose-1,6-bisphosphate, suggesting either inhibition of phosphofructokinase or activation of fructose diphosphatase, or both. Direct assay of the enzymes in cell-free extracts of the hepatocytes indicates that dibutyryl cyclic AMP inhibits phosphofructokinase but has no effect upon fructose diphosphatase. The assay for phosphofructokinase was modified by the use of ITP in place of ATP for the phosphate donor as the ATP-linked assay is complicated by an apparent time-dependent activation of the enzyme. These findings strongly suggest that cyclic AMP inhibition of phosphofructokinase explains in part cyclic AMP inhibition of aerobic glycolysis and lipogenesis by rat liver hepatocytes.

Increased ATP inhibition of liver phosphofructokinase from genetically diabetic mice

Phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) was partially purified from the livers of genetically diabetic mice (C57BL/KsJ-db) and their lean littermates (C57BL/KsJ). These genetically diabetic mice have been shown to be hyperglucagonemic and to exhibit symptoms resembling those of maturity-onset diabetes in humans. Two isoenzymes of phosphofructokinase were obtained after DEAE-Sephadex chromatography of extracts of livers from either normal or diabetic animals. One of these isozymes, peak II, from the genetically diabetic mice was shown to be more sensitive to ATP inhibition at physiological pH than the peak II isozyme from the normal animals. In addition, the peak II isozyme from the diabetic mice exhibited decreased affinity for fructose 6-phosphate. The altered kinetic properties of phosphofructokinase from diabetic animals are markedly similar to those recently reported for liver phosphofructokinase isolated from normal animals after glucagon treatment. Our results suggest that increased glucagon levels in diabetes may lead to altered regulation of phosphofructokinase in this disease.

Control in vivo of rat liver phosphofructokinase by glucagon and nutritional changes

Glucagon (250 microgram/kg body wt.) intravenously injected into normal fed rats produces within 5 min a marked inactivation of liver phosphofructokinase, only observed when the enzyme activity is measured at subsaturating concentrations of fructose 6-phosphate. Since half-maximal inactivation is observed at a dose of glucagon of 0.32 microgram/body wt., a dose within the range of the physiological concentrations of the hormone, the inactivation of phosphofructokinase can occur in vivo in response to physiological changes in the concentration of glucagon. In gluconeogenic conditions (starved rats or high-protein-diet-fed rats), there is a marked inactivation of liver phosphofructokinase at subsaturating concentrations of fructose 6-phosphate similar to that found in normal fed rats after glucagon treatment. In these gluconeogenic conditions a 50% decrease in the Vmax. of the enzyme is also observed. No significant changes in phosphofructokinase activity either at subsaturating concentrations of fructose 6-phosphate or in the Vmax. of the enzyme are observed when rats are fed on a high-carbohydrate diet. In the last dietary condition, glucagon treatment produces similar effects to that described in the normal fed rats. Similar results have been obtained in the above condtions for pyruvate kinase L activity when measured at subsaturating concentrations of phosphoenolpyruvate.

Inhibitory effects of Zn2+ on muscle glycolysis and their reversal by histidine

Of a number of divalent cations investigated, Zn2+ strongly inhibited lactate production from glucose 6-phosphate in rat muscle cytosol fraction. The I50 value for lactate production influenced by Zn2+ was 10 microM and was increased to 200 microM and 18 microM by the addition of 10mM histidine and 10mM carnosine, respectively. The inhibitory effect of 50 microM Zn2+ on lactate production was completely reversed by the addition of 1.2mM histidine and the apparent Km was found to be 0.34mM. The inhibitory site for Zn2+ was investigated by the estimation of glycolytic intermediates. It occurred at point between fructose 6-phosphate and fructose 1,6-bisphosphate. Purified rat muscle phosphofructokinase was inhibited by Zn2+. The I50 values for Zn2+ were calculated to be 12, 5.5 and 1.5 microM in the presence of 3, 0.3 and 0.06mM fructose 6-phosphate, respectively. The addition of histidine removed the inhibitory effect of Zn2+ on the glycolytic key enzyme. These studies indicate that in rat skeletal muscle the inhibitory effect of Zn2+ on lactate production might result from the inhibition of phosphofructokinase and that histidine could remove the effect.

[What is fluoroacetate diabetes?]

The opinion is incorrect that the monofluor carbon-induced hyperglucosaemia which in literature is cited under the term fluor acetate diabetes is caused by insulin deficiency due to lesion of the pancreatic beta-cells. The cause of the fluor acetate diabetes is a disturbance of the glucose degradation by inhibition of the enzyme phosphofructokinase. Insulin applications have no causal influence on the monofluor carbon intoxications and no symptomatic influence on the fluor acetate diabetes.

Inactivation of phosphofructokinase by glucagon in rat hepatocytes

Kinetic evidence of a time- and dose-dependent inactivation of phosphofructokinase by glucagon in isolated rat hepatocytes is reported. This inactivation, which persists after gel filtration of a cell-free extract on Sephadex G-25 and after 400-fold purification of the enzyme on agarose-ATP, is observed when the enzyme activity is measured at subsaturating concentrations of fructose 6-phosphate, while there is no change in Vmax. Phosphofructokinase inactivation by glucagon parallels the known inactivation of pyruvate kinase L and activation of glycogen phosphorylase alpha. Exogenous cyclic AMP mimics the effect of this hormone. Half-maximal effect for both phosphofructokinase and pyruvate kinase L is caused by a similar dose of glucagon (1 x 10(-10) M). The inactivation of phosphofructokinase by nonsaturating concentration of glucagon is reversed spontaneously within 40 min of incubation and this reversion is accelerated by insulin.

Binding of regulatory ligands to rabbit muscle phosphofructokinase. A model for nucleotide binding as a function of temperature and pH

The binding of nucleoside triphosphates to rabbit muscle phosphofructokinase has been determined in 0.05 M phosphate buffers by changes in intrinsic protein fluorescence and by direct binding measurements. These experiments have been performed over a wide range of pH, temperature, and effector concentration. Quenching of protein fluorescence is shown to measure binding of nucleotides to a site which is not the active site but rather a site responsible for inhibition of the kinetic activity. This site is relatively specific for either ATP or MgATP with free ATP binding about 10-fold more tightly than MgATP. A model to describe binding to this site as a function of pH and temperature is proposed. This model assumes that the apparent affinity for ATP is determined by protonation of two ionizable groups (per subunit) and that ATP binds exclusively to protonated enzyme forms. Several ligands which affect the apparent affinity for nucleotide binding at the inhibitory site act by shifting the apparent pK of the ionizable groups. NH4+ and citrate do not influence nucleotide binding to the inhibitory site. At pH 6.9 in 0.05 M phosphate, low concentrations of MgATP or MgGTP enhance the protein fluorescence due to binding at the active site. The fluorescence studies and direct binding studies show that there is one active site and one inhibitory site per subunit. As described elsewhere (Pettigrew, D. W., and Frieden, C. (1978) J. Biol. Chem. 253, 3623-3627), there is a third nucleotide binding site on each subunit which is specific for cAMP, AMP, and ADP.

[The method of Hill's coefficient calculation for the region of inhibition of an enzyme by excess of the substrate. Inhibition of phosphofructokinase by excess of ATP]

The new method for Hill's coefficient (nH) calculation in the region of substrate concentrations where the latter acts as an inhibitor has been developed. The method does not need preliminary determination of maximum value of enzyme reaction rate (V) for ascending branch of the plot of enzyme reaction rate versus substrate concentration and allows to avoid over-estimation of value of nH when the magnitude of optimal reaction rate is less than value of V. The literature data for inhibition of phosphofructokinase by excess of ATP are used for illustration of applicability of the suggested method of Hill's coefficient calculation.

Effects of prolonged starvation on cardiac energy metabolism in the rat

Experiments were carried out on two series of adult male rats (ad libitum-fed control and starved) for 7 days, at the end of which time components of the glycolytic, citric acid cycle, and associated metabolic pathways in the heart were examined. Levels of myocardial and arterial plasma metabolites in vivo were determined by fluoroenzymatic assays. Activities of enzymes in heart extracts and isolated mitochondria were measured in vitro spectrophotometrically. In starved rats, decreases were observed in heart tissue glucose, fructose-1,6-diphosphate, lactate, alanine, glutamate, and ADP; increases occurred in fructose-6-phosphate, beta-hydroxybutyrate, acetoacetate, and ATP. Slight to moderate elevations were noted in citric acid cycle metabolites. States of marked hypoglycemia, hyperketonemia, and hypocitricemia also developed. Evidence indicates that flux through the glycolytic pathway is diminished in prolonged starvation as a result of PFK inhibition. Elevated ATP and decreased AMP are suggested as possible factors in PFK inhibition; citrate is believed to have little effect. It is also postulated that amino acid utilization in the heart increases and that dependence on lipids as fuels of oxidation decreases. The latter occurs despite the high levels of circulating ketone bodies. There is little indication from a profile of citric acid cycle metabolites and analyses of mitochondrial enzyme activities that regulation of cycle activity is significantly altered.

Molecular biology of metabolic disease: defects in the regulation of enzymic activity

The Jacob-Monod model for the regulation of enzymic activity has been used in the analysis of some types of metabolic disease. Three lesions have been considered: (1) loss of allosteric inhibition of phosphofructokinase by citrate in the condition of lipomatosis; (2) failure of covalent modification of triglyceride lipase from inactive to active forms in the condition of triglyceride storage disease, and (3) failure of repression of HMG-CoA reductase by a mutant low-density liprotein in a new variant-of familial hypercholesterolaemia. Defects in enzymic regulation are contrasted with catalytic defects (the inborn errors); the major difference being an accumulation of a normal metabolic end-product of an unregulated pathway, rather than accumulation of an unusual intermediary metabolite as in the inborn errors.

Metabolic control of glycolysis in normal and tumor permeabilized cells

Our previous reports have presented evidence suggesting the existence in tumor cells of a second control site of glycolysis of pyruvate kinase as a competition for adenosine diphosphate between this enzyme and mitochondria, which is responsible for the Crabtree effect. Now, by using cells partially permeabilized to nucleotides and phosphorylated substrates, we provide evidence supporting the existence in hepatocytes of a partial control by adenosine triphosphate at phosphofructokinase, which is followed by the total control by adenosine triphosphate at pyruvate kinase. The partial or nonoperation of this second site in Ehrlich ascites tumor cells appears to be the cause for the characteristic aerobic glycolysis, Crabtree effect, and low Pasteur effect of these cells.

Effect of substrates and effectors on the reversible inactivation of pig spleen phosphofructokinase by adenosine triphosphate

1. To investigate the mechanism of the reversible inactivation of pig spleen phosphofructokinase by ATP, the effect of order of addition of reactants (substrates, effectors and enzyme solution) was studied by preincubating the enzyme before assay with various combinations of its substrates and effectors. 2. Preincubation of the enzyme with MgATP or ATP at pH7.0 before addition of fructose 6-phosphate caused a rapid and much greater inhibition of activity than that observed when the reaction (carried out at identical substrate concentrations) was initiated with enzyme. 3. The rapid inhibition caused by preincubation with ATP, together with the sigmoidal response to fructose 6-phosphate and activation by AMP, were all blocked by prior photo-oxidation of the enzyme with Methylene Blue, which selectively destroys the inhibitory binding site for ATP [Ahlfors & Mansour (1969) J. Biol. Chem.244, 1247-1251]. 4. Fructose 6-phosphate, but not Mg(2+), protected phosphofructokinase from inhibition during preincubation with ATP in a manner that was sigmoidally dependent on the fructose 6-phosphate concentration. 5. Mg(2+), by protecting the enzyme from the inhibitory effect of preincubation at low pH (7.0) and by preventing its activation during preincubation with fructose 6-phosphate, demonstrated both a weak activating effect in the absence of the other substrates and a stronger inhibitory effect in the presence of fructose 6-phosphate. 6. Positive effectors (K(+), NH(4) (+), AMP and aspartate) protected the enzyme from inhibition during preincubation with MgATP in proportion to their potency as activators, but citrate potentiated the ATP inhibition. P(i) significantly slowed the inactivation process without itself acting as a positive effector. 7. The non-linear dependence of the initial rate of the unmodified enzyme on protein concentration (associated with increased positive homotropic co-operativity to fructose 6-phosphate) was intensified by preincubation with ATP and abolished by photo-oxidation. 8. The results are interpreted in terms of an association-dissociation model which postulates that protonation, at low pH, of a photo-oxidation-sensitive inhibitory site for ATP allows more rapid dissociation of an active tetramer to an inactive dimeric species.