Effect of nordihydroguaiaretic acid on glucose absorption, metabolism and (Na(+)+K+)-ATPase activity in rat jejunum

A regulatory role of endogenously synthesized eicosanoids on the absorption, transmural transport and metabolism of glucose in perfused, isolated loops of jejunum in vitro was investigated using the lipoxygenase/cyclooxygenase inhibitor, nordihydroguaiaretic acid (NDGA). NDGA diminished glucose absorption over the range 100-500 microM: maximal inhibition at 500 microM NDGA was 52 +/- 9 and 64 +/- 9% (mean +/- SE, P < 0.001) for jejuna from fed rats and rats maintained on glucose water for 48 hr, respectively. In each instance, transmural transport was effectively abolished. The vectorial disposition of lactate release was also changed such that the ratio of luminal to serosal production was increased from 0.19 +/- 0.02 to 1.72 +/- 0.12 (P < 0.001) in fed rats, indicating inhibition of the Na+ pump. NDGA inhibited (Na(+)+K+)-ATPase activity in whole mucosal homogenates with a concentration dependence similar to that observed for glucose absorption. However, NDGA also inhibited Mg(2+)-ATPase activity in whole homogenates and purified rabbit skeletal muscle phosphofructokinase under the same conditions. The results are discussed in terms of the dissipation of the transmembrane Na+ gradient via direct inhibition of the (Na(+)+K+)-ATPase by NDGA. Inhibition of the ATPase precludes the use of NDGA as a suitable drug with which to investigate the role of endogenously synthesized eicosanoids in the regulation of intestinal function.

Substrate- and effector-induced conformational changes in phosphofructokinase from white muscle of rainbow trout (Oncorhynchus mykiss): a fluorescence study

Results of activity and spectral studies using fluorescence show that AMP and fructose 2,6-bisphosphate (F2,6P2) activate muscle phosphofructokinase (PFK) from rainbow trout (Oncorhynchus mykiss) through specific and similar conformational changes. Inorganic compounds, such as ammonium and phosphate ions, also increase enzyme activity allosterically; however, the structural alterations in the enzyme caused by these effectors are quite different from those caused by AMP and F2,6P2. No effects of the inorganic compounds on the environment of tryptophan residues of the enzyme were observed. Mg-ATP, a substrate of the enzyme, acts as an allosteric inhibitor at high concentrations. Although Mg-ATP and citrate inhibit the enzyme activity in a synergistic way, the conformational effects of these negative effectors are different. Mg-ATP caused a drastic decrease in fluorescence intensity of the enzyme, whereas citrate did not.

Inhibition of glycolysis by amino acids in ascites tumor cells. Specificity and mechanism

The effect of glutamine and asparagine on glucose metabolism has been studied in ascites tumor cells. Either of these amino acids decreased the glycolytic flux about 80%. Half-maximal effects were obtained with 0.14 mM glutamine and 0.087 mM asparagine. Among the 20 L-amino acids, only glutamate produced a similar effect. Glutamine and asparagine caused a 70% increase of hexose monophosphates and a large decrease of fructose-1,6-P2 and triose phosphates, evidencing a strong inhibition of the phosphofructokinase (EC 2.7.11) reaction. Analysis of the levels of various phosphofructokinase effectors revealed that fructose-2,6-P2 and AMP decreased 4-fold, phosphoenolpyruvate, citrate, and ATP increased 4-, 3-, and 1.8-fold, respectively, and that there was no change in ADP, Pi, and intracellular pH. Assay of phosphofructokinase at concentrations of substrates and effectors determined to be in the cells showed that the low activity of this enzyme could be accounted for by the change in the concentration of effectors, the major mechanism being the change in adenine nucleotides. The decrease in fructose-2,6-P2 contributed very little to the inhibition of phosphofructokinase activity. The effects of amino acids were prevented by amino-oxyacetate, suggesting that transamination was an obligatory step for these changes.

The mechanism of ATP inhibition of wild type and mutant phosphofructo-1-kinase from Escherichia coli

Escherichia coli 6-phosphofructo-1-kinase was inhibited by high concentrations of ATP at alkaline pH. The mechanism of the inhibition was studied with two mutants generated by site-directed mutagenesis; I126A, with a Km for fructose-6-P that was more than two orders of magnitude higher than that of wild type but with minimal changes in kcat and Km for ATP, and R72H, with little change in substrate half-saturation concentrations but with a kcat that was 300-fold lower that of wild type enzyme. ATP and fructose-6-P interacted in a mutually antagonistic manner; that is ATP decreased the apparent affinity for fructose-6-P and vice versa. The half-saturation concentrations for both substrates, most strikingly fructose-6-P, increased with increasing pH while the kcat increased. Studies with I126A suggested that ATP inhibition was not dependent on a dissociable group with a pK in the alkaline range and that the inhibition was not caused by abortive binding of substrate to the wrong substrate site. Inhibition was not the result of differential affinity of ATP for the R and T states of the enzyme. The low kcat mutant, R72H, did not display ATP inhibition. These data indicate that ATP inhibition results from substrate antagonism coupled with a steady state random mechanism wherein the high rate of catalysis does not permit equilibration of substrates.

[In vitro decrease of the cytolytic effect of E. histolytica by inhibition of its phosphofructokinase]

The C14 radioactive label of PPi analogues was incorporated to E. histolytica after 24 hours of incubation at 37 degrees C; more than 90% of trophozoites remained viable. The PPi dependent phosphofructokinase was isolated in order to determine its kinetic parameters. With PPi, the Km was 18.06 +/- 0.91 micromol/mL-1. Using three different PPi analogues (tetrasodium salts) of (I) 1,1 hydroxy-methyl diphosphonate; (II) 1,1 hydroxy ethylene diphosphonate; (III) 1,1 hydroxy-nonano diphosphonate, KiI was 35.19 +/- 1.74; KiII was 42.65 +/- 0.65, and KiIII 2as 62.81 +/- 0.27 micromol/mL-1. The graphic expression of these results shows that the enzyme was competitively inhibited by the three analogues. When trophozoites were incubated with each one of the three inhibitors, a correlation was observed between the concentration and the cytolytic inhibition with an r = 0.98. Nevertheless, the slope obtained was different for each one of them. The smallest concentration of inhibitor to achieve a 50% lysis inhibition of trophozoites was that of inhibitor III. In addition, it was demonstrated that the incubation of the trophozoites with this inhibitor increased the time needed to destroy CHO cells. We conclude that enzymatic inhibition of the PPi dependent phosphofructokinase caused by the PPi analogues was responsible for the modification of the lytic capacity of trophozoites, possibly by altering the metabolic pathway of carbohydrates.

Kinetics of phosphofructokinase from granulocytes isolated from patients with insulin resistance

Phosphofructokinases from granulocytes isolated from insulin-resistant patients, mainly those from type II diabetics where the degree of insulin resistance was more pronounced, exhibit some changes in their kinetic behavior when assayed under allosteric conditions, characterized by an increased affinity for fructose-6-phosphate, being more resistant to ATP inhibition while it became more sensitive to citrate inhibitory effect. Those changes are suggestive of a isozymic modification to a more L-type enriched enzyme with a loss of the F-type component, probably present in the normal granulocyte.

Influence of long-term hypoxia on the energy metabolism of the haemoglobin-containing bivalve Scapharca inaequivalvis: critical O2 levels for metabolic depression

The oxygen consumption rate of Scapharca inaequivalvis measured under normoxic conditions over 48 h showed a significant daily cycle with lowest values occurring shortly after the dark period; all hypoxia exposure experiments were carried out during the declining part of the cycle. Animals were exposed to a constant level of hypoxia for a 12-h period in a series of 14 experiments, each at a different oxygen tension. The oxygen consumption was measured continuously, and the extent of accumulation of end-products (succinate and propionate), and the inhibitory effect of adenosine triphosphate on phosphofructokinase were determined at the end of exposures. All three parameters (oxygen consumption, end-product accumulation, phosphofructokinase inhibition) showed a remarkable correlation with major changes occurring between 2.5 and 1.5 ppm (7 and 4 kPa) O2. The oxygen consumption rates showed a drop to 6% of the normoxic rate, but a consistent low consumption remained below 2 ppm (5.5 kPa) which partly recovered over the 12-h exposure period by about three-fold. Succinate and propionate accumulated progressively between 2.5 and 1.5 ppm (7 and 4 kPa); at [O2] less than 1.5 ppm (4 kPa) the concentration did not increase further, indicating that anaerobic metabolism had reached a maximum. Over the same range, phosphofructokinase showed an increased sensitivity for adenosine triphosphate, the lower inhibitor concentration at 50% Vmax value pointing to depression of glycolytic rate. Despite the activation of anaerobic metabolism and the evident depression of aerobic metabolism, simple calculation demonstrates that Scapharca inaequivalvis relies mainly on aerobic metabolism even during severe hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

The pleiotypic response to amino acid deprivation is the result of interactions between components of the glycolysis and protein synthesis pathways

Several diverse metabolic events become compromised when mammalian cells are made deficient in essential amino acids or when charging of their tRNA is blocked by amino acid analogs. This rapid general demise of cell function can be due to inhibition of phosphofructokinase (PFK) by uncharged tRNA. It has now been demonstrated that when tRNA is added to PFK in an assay dependent upon the reassociation of inactive, dissociated enzyme subunits, nanomolar concentrations cause complete inhibition. The model for control suggests that charged tRNA becomes associated with EF-1, which is specific for aminoacyl-tRNAs and is present in sufficiently high concentrations in cells to sequester that charged forms from an inhibitory role. Support for this model include: (1) the rapid onset of inhibition of glycolysis and glucose uptake upon amino acid deficiency; (2) the unique role of the product of PFK activity, fructose-1,6-diphosphate, in reactions of peptide chain initiation, particularly its role as a co-factor for purified eIF-2B, the GDP/GTP exchange factor; (3) the correlations of this interaction with the cellular and molecular lesions of insulin insufficiency; (4) the recognition that the anomalous role of high concentrations of cAMP as a stimulant of peptide chain initiation in energy depleted or gel-filtered cell lysates correlates with its stimulatory action on PFK as an analog for the positive effector, adenosine-5'-monophosphate; and (5) the role of fructose-1,6-diphosphate in the formation of glyceraldehyde-3-phosphate, a substrate for synthesis of ribose-5-phosphate via the non-oxidative portion of the pentose phosphate pathway, which, as a precursor of phosphoribosylpyrophosphate, is essential for nucleic acid synthesis.

A conformational transition involved in antagonistic substrate binding to the allosteric phosphofructokinase from Escherichia coli

The binding of fructose 6-phosphate, ATP or its nonhydrolyzable analogue adenylyl 5'-(beta,gamma-methylenediphosphonate), ADP, and phosphoenolpyruvate to Escherichia coli phosphofructokinase has been studied by changes in the protein fluorescence and/or equilibrium dialysis. The results lead to the following conclusions: (1) tetrameric phosphofructokinase can bind four ATP but only two fructose-6-phosphate, and this binding occurs without cooperativity; (2) only two conformational states, T and R, with respectively a high and a low fluorescence, seem accessible to phosphofructokinase, which exists as a mixture of one-third R and two-third T states in the absence of ligand; (3) the substrate fructose 6-phosphate and the allosteric activator ADP bind preferentially to the low-fluorescence R state, while the other substrate, ATP [or its nonhydrolyzable analogue adenylyl 5'-(beta,gamma-methylenediphosphonate)], and the allosteric inhibitor phosphoenolpyruvate bind to the high-fluorescence T state; (4) the binding of a given ligand is cooperative, with a Hill coefficient of 2, only when this binding is accompanied by a complete shift from one state to the other; for instance, the binding of the ATP analogue adenylyl 5'-(beta,gamma-methylenediphosphonate) to the T state is cooperative only in the presence of fructose 6-phosphate which favors the R state. This behavior is qualitatively consistent with a concerted transition, but quite different from that described earlier for phosphofructokinase from steady-state activity measurements (Blangy et al., 1968). This discrepancy suggests that the allosteric properties of phosphofructokinase are due in part to ligand binding and in part to the kinetics of the enzymatic reaction.

Temperature- and concentration-dependence of compatibility of the organic osmolyte beta-dimethylsulfoniopropionate

The effects of the organic osmolyte beta-dimethylsulfoniopropionate (DMSP) on the structural stability of three model proteins were examined to determine whether DMSP, like the structurally similar solute dimethyl sulfoxide (DMSO), is compatible with native protein structure at low, but not elevated, temperatures. DMSP stabilized phosphofructokinase under conditions of cold-induced denaturation. Thus, DMSP, like DMSO, may be an effective protein cryoprotectant. However, DMSP was not an effective stabilizer of protein structure under conditions of heat denaturation. Whereas low (0.2 M) concentrations of DMSP stabilized lactate dehydrogenase against inactivation at 50 degrees C, higher DMSP concentrations were ineffective. DMSP favored the denaturation of glutamate dehydrogenase at all DMSP concentrations tested. DMSP may be a compatible osmotic solute only under conditions of moderate temperature and low, yet physiological, concentrations. The mechanistic basis of DMSP's temperature- and concentration-dependent effects and the possible roles played by adaptation temperature and severity of osmotic stress in the evolutionary selection of organic osmolytes are discussed.

Inhibition of key enzymes of carbohydrate metabolism in regenerating mouse liver by ascorbic acid

The effect of ascorbic acid on the key enzymes of carbohydrate metabolism e.g. hexokinases, phosphofructokinase, pyruvate kinase, lactate dehydrogenase, glucose-6-phosphate dehydrogenase and malic enzyme was determined in regenerating mouse liver. All the enzymes showed a significant increase in the activity during regeneration. Ascorbic acid reduced the activities of the enzymes in regenerating liver. A decrease in liver weight in ascorbic acid treated animals may be correlated with its effect on these enzymes as glycolytic pathway is the main source of energy required by the dividing cells.

Allosteric control of 6-phosphofructo-1-kinase from rat lung

1. The regulation of 6-phosphofructo-1-kinase (PFK) in the rat lung of normally fed (control), 72 hr-starved and streptozotocin-induced diabetic rats was investigated. 2. No significant changes in the total enzyme activities and the activity ratios [activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 (v0.5/V)] of rat lung were observed between the control and 72 hr-starved or streptozotocin-induced diabetic rats. 3. Rat lung PFK was highly stimulated by fructose 2,6-bisphosphate (Fru-2,6-P2) as the affinity of the enzyme for fructose 6-phosphate was highly increased by this metabolite and the enzyme inhibition by ATP was released. 4. Although rat liver and mucosal PFK were found to be highly sensitive to stimulation by Fru-2,6-P2, lung PFK was significantly more sensitive to the stimulation by this metabolite than the other tissues. 5. The enzyme was highly inhibited by citrate and was only slightly inhibited by phosphocreatine. 6. ADP, AMP and c-AMP were shown to be activators of lung PFK with c-AMP being the most effective activator. 7. As a rate limiting enzyme of glycolysis, rat lung PFK is highly controlled by its allosteric effectors, especially Fru-2,6-P2, possibly for surfactant lipid synthesis which usually requires a high rate of glycolysis.

Modification of the ATP inhibitory site of the Ascaris suum phosphofructokinase results in the stabilization of an inactive T state

Treatment of the Ascaris suum phosphofructokinase (PFK) with 2',3'-dialdehyde ATP (oATP) results in an enzyme form that is inactive. The conformational integrity of the active site, however, is preserved, suggesting that oATP modification locks the PFK into an inactive T state that cannot be activated. A rapid, irreversible first-order inactivation of the PFK is observed in the presence of oATP. The rate of inactivation is saturable and gives a KoATP of 1.07 +/- 0.27 mM. Complete protection against inactivation is afforded by high concentrations of ATP, and the dependence of the inactivation rate on the concentration of ATP gives a Ki of 326 +/- 26 microM for ATP which is 22-fold higher than the Km for ATP at the catalytic site but close to the binding constant for ATP to the inhibitory site. Fructose 6-phosphate, fructose 2,6-bisphosphate, and AMP provide only partial protection against modification. The pH dependence of the inactivation rate gives a pKa of 8.4 +/- 0.1. Approximately 2 mol of [3H]oATP is incorporated into a subunit of PFK concomitant with 90% loss of activity, and ATP prevents the derivatization of 1 mol/subunit. The oATP-modified enzyme is not activated by AMP or fructose 2,6-bisphosphate. oATP has no effect on the activity of a desensitized form of PFK in which the ATP inhibitory site is modified with diethyl pyrocarbonate but with the active site intact [Rao, G.S.J., Wariso, B.A., Cook, P.F., Hofer, H.W., & Harris, B.G. (1987) J. Biol. Chem. 262, 14068-14073].(ABSTRACT TRUNCATED AT 250 WORDS)

Homogeneous distribution of phosphofructokinase in the rat liver acinus: a quantitative histochemical study

A quantitative histochemical method was developed for the demonstration in rat liver of the activity of phosphofructokinase, one of the enzymes assumed to be rate-limiting for glycolysis. The procedure was based on the reduction of a tetrazolium salt as final electron acceptor and a multistep reaction using the exogenous or endogenous auxiliary enzymes aldolase, triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase. The highest activity was found in unfixed cryostat sections of rat liver when the incubation medium contained 17% (wt/vol) polyvinyl alcohol, 100 mmol/L Tris-maleate buffer (pH 8.4), 20 mmol/L fructose-6-phosphate, 2 mmol/L ATP, 2 mmol/L MgCl2, 5.9 mmol/L NAD+, 0.47 mmol/L 1-methoxyphenazine methosulfate, 5 mmol/L sodium azide and 5 mmol/L Nitro BT. The addition of auxiliary enzymes was not necessary to demonstrate maximum activity in rat liver. The specificity of the reaction was proven by the absence of any specific (test minus control) reaction when the incubation was performed in the presence of 25 mmol/L phosphoenolpyruvate, a competitive inhibitor of phosphofructokinase. Cytophotometric analysis revealed that linear relationships exist between the amount of specific reaction product formed and incubation time and the section thickness. The Km values for fructose-6-phosphate and the Vmax values were not significantly different in periportal and pericentral areas of livers from either normally fed or 24-hr-fasted rats. The homogeneous distribution of phosphofructokinase activity in the liver acinus is in line with biochemical findings using hepatocytes isolated from the two different areas showing that these cells contained similar amounts of enzyme activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate antagonism in the kinetic mechanism of E. coli phosphofructokinase-1

In the presence of its allosteric activator GDP, the major phosphofructokinase-1 from Escherichia coli K12 follows Michaelis-Menten kinetics. The kinetic behavior observed at steady-state using different concentrations of the substrates ATP and fructose-6-phosphate and the pattern of inhibition by the substrate analogs adenylyl-(beta, gamma-methylene)-diphosphonate and D-arabinose-5-phosphate are consistent with a random sequential mechanism in rapid equilibrium, rather than with an ordered binding as was suggested earlier. However, ATP and fructose-6-phosphate do not bind independently to the same active site, since the apparent affinity for one substrate is decreased about 20-fold when the other substrate is already bound. The antagonism between ATP and fructose-6-phosphate shows that a negative interaction occurs during the reaction with E. coli phosphofructokinase-1 which must be considered in addition to its allosteric properties.

Auranofin inactivates phosphofructokinase in human neutrophils, leading to depletion of intracellular ATP and inhibition of superoxide generation and locomotion

The effects of the oral gold compound auranofin (AF), at concentrations well within the therapeutic range (0.04-1.5 microM), on human neutrophil functions and energy metabolism were investigated in vitro. At the concentrations tested, this agent had minimal effects on neutrophil degranulation and phagocytosis. However, AF caused dose-related inhibition of neutrophil chemotaxis and stimulus-activated generation of superoxide, which was evident at concentrations as low as 0.04 microM. Inhibition of superoxide generation by activated neutrophils increased with the time of preincubation of the cells with AF at 37 degrees. At low concentrations of AF (less than 0.75 microM), early events (within 5 min) involved in the transduction, assembly, and activity of the neutrophil superoxide-generating enzyme NADPH oxidase appeared to be normal, but the cells were unable to sustain the level of oxygen consumption, superoxide production, and NADPH oxidase activity of the corresponding drug-free control cells. On a mechanistic level, coincubation of neutrophils with AF was associated with decreased glycolytic activity and depletion of intracellular ATP, apparently due to drug-mediated, dose-related inactivation of the glycolytic enzyme phosphofructokinase (PFK). Using purified PFK, the triethylphosphine gold (TEPG) moiety of AF, but not AF per se, caused dose-related inactivation of enzyme activity. These data indicate that the potent inhibition of neutrophil migration and reactive oxidant generation observed during treatment of neutrophils with low, therapeutically attainable concentrations of AF is related to TEPG-mediated inactivation of PFK and consequent interference with cellular energy metabolism and functions.

Modulation by citrate of glycolytic oscillations in skeletal muscle extracts

Oscillatory behavior of glycolysis in cell-free extracts of skeletal muscle involves repeated bursts of phosphofructokinase activity and associated oscillations in the [ATP]/[ADP] ratio. Addition of citrate, a potent physiological inhibitor of phosphofructokinase, decreased the frequency of the oscillations and delayed the first burst of phosphofructokinase activity in a dose-dependent manner. Citrate decreased the trigger point [ATP]/[ADP] ratio at which bursts of phosphofructokinase activity were initiated but had a much smaller effect on the average [ATP]/[ADP] ratio and did not decrease the peak values of the ratio. When oscillations were prevented by addition of fructose-2,6-P2, the decrease in the [ATP]/[ADP] ratio caused by citrate in the steady state system was similar to the decrease in the trigger point [ATP]/[ADP] ratio in the oscillatory system. The decrease in the average [ATP]/[ADP] ratio was greater in the steady state system than in the oscillating system. These results demonstrate advantages of oscillatory behavior of glycolysis in the regulation of carbohydrate utilization and the maintenance of a high [ATP]/[ADP] ratio.

Kinetic studies of human polymorphonuclear leukocyte phosphofructokinase

Phosphofructokinase from human polymorphonuclear leukocytes has low cooperativity and high affinity for its substrate, F-6-P. It is resistant to ATP inhibition at pH 8; however, at pH 7.1 it becomes sensitive to the effect of this compound. It is activated by F-1, 6-P2; it is not very sensitive to citrate inhibition and F-2, 6-P2 has no effect on its activity. With these kinetic characteristics we assume that perhaps the predominant L-type subunit is accompanied by an F-type component.

Evidence for a role of phosphofructokinase and tRNA in the polyribosome disaggregation of amino acid deficiency

The activity of rabbit muscle phosphofructokinase was inhibited by transfer ribonucleic acid. This inhibition was reduced by inclusion of an amino-acyl-tRNA charging system. The results are discussed in terms of the loss of ATP in amino acid deprived cells and in the critical role of fructose 1,6-diphosphate in peptide chain initiation.

Fructose 2,6-bisphosphate and AMP increase the affinity of the Ascaris suum phosphofructokinase for fructose 6-phosphate in a process separate from the relief of ATP inhibition

Kinetic data have been collected suggesting that heterotropic activation by fructose 2,6-bisphosphate and AMP is a result not only of the relief of allosteric inhibition by ATP but is also the result of an increase in the affinity of phosphofructokinase for fructose 6-phosphate. Modification of the Ascaris suum phosphofructokinase at the ATP inhibitory site produces a form of the enzyme that no longer has hysteretic time courses or homotropic positive (fructose 6-phosphate) cooperativity or substrate inhibition (ATP) (Rao, G.S. J., Wariso, B.A., Cook, P.F., Hofer, H.W., and Harris, B.G. (1987a) J. Biol. Chem. 262, 14068-14073). This form of phosphofructokinase is Michaelis-Menten in its kinetic behavior but is still activated by fructose 2,6-bisphosphate and AMP and by phosphorylation using the catalytic subunit of cyclic AMP-dependent protein kinase (cAPK). Fructose 2,6-bisphosphate activates by decreasing KF-6-P by about 15-fold and has an activation constant of 92 nM, while AMP decreases KF-6-P about 6-fold and has an activation constant of 93 microM. Double activation experiments suggest that fructose 2,6-bisphosphate and AMP are synergistic in their activation. The desensitized form of the enzyme is phosphorylated by cAPK and has an increased affinity for fructose 6-phosphate in the absence of MgATP. The increased affinity results in a change in the order of addition of reactants from that with MgATP adding first for the nonphosphorylated enzyme to addition of fructose 6-phosphate first for the phosphorylated enzyme. The phosphorylated form of the enzyme is also still activated by fructose 2,6-bisphosphate and AMP.

Protein-induced inactivation and phosphorylation of rabbit muscle phosphofructokinase

Several previously untested proteins promote the reversible inactivation of rabbit skeletal muscle phosphofructokinase. Grouped in decreasing order of effectiveness, they include the following: skeletal muscle troponin C greater than troponin, the two smooth muscle myosin light chains, alpha-actinin, and S-100 much greater than parvalbumin and soybean trypsin inhibitor. The efficiency of troponin C in this process may even exceed that previously reported for calmodulin. Sequences near calcium binding site III are apparently involved in the troponin C-phosphofructokinase interaction. Troponin C and calmodulin exert calcium-dependent effects on the physical and chemical properties of muscle phosphofructokinase. When calcium is present, comigration with either protein allows the enzyme to enter the stacking gel during urea-polyacrylamide gel electrophoresis. Both enhance the phosphorylation of phosphofructokinase catalyzed by the cAMP-dependent protein kinase, with phosphate incorporations approaching 2 mol of P/mol of protomer. Reaction occurs at Ser774 and at Ser376--a novel site whose phosphorylation is highly sensitive to troponin C and less so to calmodulin. Maximum phosphorylation has slight effect on the catalytic activity of the enzyme under standard assay conditions. The troponin C induced or calmodulin-induced phosphorylation of phosphofructokinase requires calcium and is strongly inhibited by either fructose 2,6-bisphosphate or fructose 1,6-bisphosphate. Inactivation occurs in the presence or absence of calcium, with generally higher concentrations of effectors required for protection in the latter case. Liver and yeast phosphofructokinases shows little activity loss in the presence of either calmodulin or troponin C. We have developed and tested a general mathematical model for the protein-induced inactivation of phosphofructokinase which may find application to other systems.

6-Phosphofructo-1-kinase of rat placenta

6-Phosphofructo-1-kinase (PFK) of rat placenta was purified to homogeneity with a recovery of 56% of the enzyme activity in the original extract. The purified enzyme is a tetramer and the Mr value of the subunit is 85,000 +/- 1500 as shown by gel filtration and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Considering the properties of the native rat placental PFK isoenzyme, it is clear that this tissue is a complex mixture of homotetramer and heterotetramer. Purified placenta PFK displayed little cooperativity at pH 7.0 with respect to fructose 6-phosphate and was markedly inhibited with high concentrations of ATP. The affinity of the enzyme for fructose 6-phosphate was increased by fructose 2,6-biphosphate. The purified enzyme was highly inhibited by citrate, whereas it was only slightly inhibited by phosphoenol pyruvate. ADP, AMP and fructose 2,6-bisphosphate showed little stimulation towards placental PFK. The present study suggests that the placental PFK is a relatively active enzymic form and it is also probably characterized with a high rate of glycolysis possibly because this tissue requires a high energy production for the development and maintenance of the fetus as the placenta tends to be a semipermeable membrane through which substances are exchanged between mother and fetus.

Vanadate promotes photooxidative cleavage and inactivation of muscle phosphofructokinase

During irradiation in the presence of decavanadate, the subunits of phosphofructokinase underwent progressive degradation to a fragment of about 78,000 daltons. This cleavage pattern was altered when the photoirradiation was performed in the presence of monomeric vanadate with formation of several smaller peptides. The specificity of the decavanadate induced cleavage was proved by the resistance of other enzymes to the treatment and by the effects of phosphofructokinase ligands. During irradiation, the activity of the enzyme declined. Differences between the rate of inactivation and of cleavage of enzyme subunits suggest the occurrence of multiple processes.

Mechanism of aluminum-induced inhibition of hepatic glycolysis: inactivation of phosphofructokinase

Aluminum, an abundant element in the earth's crust, has been implicated in various pathological disorders and low concentrations of this element have recently been shown to inhibit brain glycolysis. However, despite the fact that aluminum accumulates in high concentrations in the liver, potential effects of this metal on hepatic intermediary metabolism have not been explored. In perfused livers from untreated rats, maximal rates of production of lactate plus pyruvate (glycolysis) were 93 +/- 15 mumols/g/hr. Glycolysis was severely inhibited in livers from aluminum-treated rats (0.5 mg/kg, 6 hr before experiment) with maximal rates of only 23 +/- 4 mumols/g/hr. In contrast, glucose production (glycogenolysis) and hepatic oxygen uptake were not altered significantly by prior treatment with aluminum. In livers from fasted rats, pretreatment with aluminum did not influence gluconeogenesis or production of lactate and pyruvate from fructose (5 mM). This finding indicates that pyruvate kinase is not inhibited by aluminum and implicates phosphofructokinase, hexokinase and/or glucokinase as sites for the inhibitory effect of aluminum on glycolysis. In liver homogenates from untreated rats, increasing concentrations of aluminum did not show any appreciable effect on hexokinase or glucokinase activity but did cause progressive decreases in phosphofructokinase activity. Therefore, aluminum-induced inhibition of liver phosphofructokinase, an important control site in the glycolytic pathway, is most likely responsible for aluminum-induced inhibition of hepatic glycolysis.