In vitro demonstration of alternate stationary states in an open enzyme system containing phosphofructokinase

The kinetic properties of an open reconstituted enzyme system are investigated with the aim of demonstrating experimentally hysteretic transitions between alternate stationary states. The approach is based on a stirred flow-through reactor containing phosphofructokinase and pyruvate kinase entrapped in polyacrylamide gel. Through the reactor is pumped a solution containing fructose 6-phosphate, ATP, and phosphoenol pyruvate as well as adenylate kinase and glucose-6-phosphate isomerase. The latter two enzymes are in excess in respect to phosphofructokinase and pyruvate kinase. According to theoretical predictions the existence of multiple stationary states could be shown experimentally within precisely definable parameters. Switches between alternate stationary states have been caused by perturbations of flow rates and of reactant concentrations.     

Sustained oscillations in a reconstituted enzyme system containing phosphofructokinase and fructose 1,6-bisphosphatase

In a reconstituted open and homogeneous enzyme system containing phosphofructokinase, fructose 1,6-bisphosphatase, pyruvate kinase, adenylate kinase, and glucose-6-phosphate isomerase sustained oscillations could experimentally be generated. The approach is based on a stirred flow-through reaction chamber. The periodic motions of the reactants are mainly caused by the antagonistic allosteric effects of the adenine nucleotides on the activities of the phosphofructokinase and fructose 1,6-bisphosphatase.     

Non-linear dynamic phenomena in open reconstituted enzyme systems.

The dynamic pattern of an open reconstituted in-vitro enzyme system containing phosphofructokinase, pyruvate kinase and adenylate kinase has been investigated. The approach is experimentally based on a stirred flow-through reactor. Stationary concentrations of phosphofructokinase and pyruvate kinase are maintained by entrapment in polyacrylamide gel particles. The results fit to a theoretical model based on the initial kinetic responses of the enzymes involved. An S-shaped steady state input characteristic for fructose 6-phosphate has been experimentally obtained.     

Self-stabilization of the energy charge in a reconstituted enzyme system containing phosphofructokinase

The self-stabilization of the energy charge and of ATP was investigated in an open reconstituted enzyme system containing phosphofructokinase, pyruvate kinase, adenylate kinase, and glucose-6-phosphate isomerase. The experiments were performed in a stirred flow-through reactor containing gel-entrapped enzymes. The dynamics of the system were analyzed theoretically by a model based on the kinetic properties of the individual enzymes. The energy charge was identified as one of the essential variables of the system. According to the theoretical prediction, homoeostasis of the energy charge was observed experimentally when either the maximal activity of phosphofructokinase, the energy charge of the influx solution of the flow rate through the reaction chamber was varied. It is shown that the efficiency of stabilization of the energy charge is related to the occurrence of alternative stationary states.     

Interaction of immobilized phosphofructokinase with soluble muscle proteins

Selected glycolytic enzymes (including phosphoglucose isomerase, aldolase, glyceraldehyde phosphate dehydrogenase, enolase, pyruvate kinase and lactate dehydrogenase), as well as glycogen phosphorylase, creatine kinase, and adenylate kinase, bound to phosphofructokinase immobilized on an agarose gel. The affinity of phosphofructokinase to these various proteins differed, with phosphorylase exhibiting the strongest binding. Binding was reversed either by: (1) elution with high-ionic-strength buffer (0.4 M KCl); (2) the addition of a 5-10 mM concentration of ATP; or (3) high concentrations of fructose 6-phosphate (5 mM).     

Decreased phosphofructokinase activity in skeletal muscle of diabetic rats

The activities of phosphofructokinase, aldolase and pyruvate kinase were diminished in extracts from skeletal muscle of streptozotocin diabetic rats, whereas the activities of glucose phosphate isomerase and phosphoglucomutase were not changed. Treatment of diabetic rats with insulin restored the activity of phosphofructokinase to normal. A kinetic study of the partially purified enzyme from normal and diabetic rats showed identical Michaelis constants for ATP and equal sensitivity to inhibition by excess of this substrate. Extracts of quick frozen muscle from diabetic rats had higher levels of citrate (an inhibitor of phosphofructokinase) and lower levels of D-fructose-1,6-bisphosphate and D-glucose-1,6-bisphosphate (activators of this enzyme). The levels of D-fructose-6-phosphate, D-glucose-6-phosphate, ATP, ADP and AMP were the same for the two groups. Our data suggest that the in vivo decrease of phosphofructokinase activity in skeletal muscle of diabetic rats is due to a decrease in the level of the enzymatically active protein as well as to an unfavorable change in the level of several of its allosteric modulators.     

Use of Nicotinamide Adenine Dinucleotide (NAD)-Dependent Glucose-6-Phosphate Dehydrogenase in Enzyme Staining Procedures

Substitution of nicotinamide adenine dinucleotide dependent glucose-6-phosphate dehydrogenase for the nicotinamide adenine dinucleotide phosphate dependent enzyme has produced identical results in a number of enzyme-linked electrophoretic staining procedures. This substitution significantly reduces the cost of staining for adenylate kinase, creatine kinase, glucosephosphate isomerase, mannosephosphate isomerase, phosphoglucomutase, and pyruvate kinase activity by utilizing NAD rather than the more expensive NADP.     

Use of nicotinamide adenine dinucleotide (NAD)-dependent glucose-6-phosphate dehydrogenase in enzyme staining procedures

Substitution of nicotinamide adenine dinucleotide dependent glucose-6-phosphate dehydrogenase for the nicotinamide adenine dinucleotide phosphate dependent enzyme has produced identical results in a number of enzyme-linked electrophoretic staining procedures. This substitution significantly reduces the cost of staining for adenylate kinase, creatine kinase, glucosephosphate isomerase, mannosephosphate isomerase, phosphoglucomutase, and pyruvate kinase activity by utilizing NAD rather than the more expensive NADP.     

Regulation by glucagon of hepatic pyruvate kinase, 6-phosphofructo 1-kinase, and fructose-1,6-bisphosphatase

Glucagon stimulates gluconeogenesis in part by decreasing the rate of phosphoenolpyruvate disposal by pyruvate kinase. Glucagon, via cyclic AMP (cAMP) and the cAMP-dependent protein kinase, enhances phosphorylation of pyruvate kinase, phosphofructokinase, and fructose-1,6-bisphosphatase. Phosphorylation of pyruvate kinase results in enzyme inhibition and decreased recycling of phosphoenolpyruvate to pyruvate and enhanced glucose synthesis. Although phosphorylation of 6-phosphofructo 1-kinase and fructose-1,6-bisphosphatase is catalyzed in vitro by the cAMP-dependent protein kinase, the role of phosphorylation in regulating the activity of and flux through these enzymes in intact cells is uncertain. Glucagon regulation of these two enzyme activities is brought about primarily by changes in the level of a novel sugar diphosphate, fructose 2,6-bisphosphate. This compound is an activator of phosphofructokinase and an inhibitor of fructose-1,6-bisphosphatase; it also potentiates the effect of AMP on both enzymes. Glucagon addition to isolated liver systems results in a greater than 90% decrease in the level of this compound. This effect explains in large part the effect of glucagon to enhance flux through fructose-1,6-bisphosphatase and to suppress flux through phosphofructokinase. The discovery of fructose 2,6-bisphosphate has greatly furthered our understanding of regulation at the fructose 6-phosphate/fructose 1,6-bisphosphate substrate cycle.     

Induction of glycolytic enzyme synthesis in proliferating fibroblasts. Study of phosphofructokinase, glucose phosphate isomerase and pyruvate kinase.

Specific activity of phosphofructokinase is 7-8-fold higher in exponentially growing human fibroblasts than in quiescent cells, but the difference is considerably less pronounced for two other glycolytic enzymes, glucose phosphate isomerase and pyruvate kinase. The ratio of the F-type to L-type phosphofructokinase subunits is essentially the same in growing and resting cells, 4:1. F-type-phosphofructokinase-related antigen concentration is decreased in resting cells as compared with proliferating fibroblasts, but relatively less than the enzyme activity; the ratio of the enzyme activity to the antigen concentration (immunological specific activity) is therefore lower in resting than in growing fibroblasts. Synthesis of phosphofructokinase, as a percentage of the total protein synthesis, is about 30-fold greater during the proliferative phase than in quiescent cells, but this difference is only 3-4-fold for glucose phosphate isomerase and pyruvate kinase. Modulation of the synthesis of phosphofructokinase therefore seems to be responsible for the changes of its specific activity in function of cell proliferation. The appearance of some inactive cross-reacting material in quiescent cells is probably due to post-translational alteration of the pre-synthesized molecules. Compared with other glycolytic enzymes, such as glucose phosphate isomerase and pyruvate kinase, phosphofructokinase seems to be the (or one of the) preferential target of glycolytic induction in proliferating cells.     

Control of glycolysis in cerebral cortex slices

1. Intracellular concentrations of intermediates and cofactors of glycolysis were measured in guinea-pig cerebral cortex slices incubated under varying conditions. 2. Comparison of mass-action ratios with apparent equilibrium constants for the reactions of glycolysis showed that hexokinase, phosphofructokinase and pyruvate kinase catalyse reactions generally far from equilibrium, whereas phosphoglucose isomerase, aldolase, phosphoglycerate kinase, phosphoglycerate mutase, enolase, adenlyate kinase and creatine phosphokinase are generally close to equilibrium. The possibility that glyceraldehyde 3-phosphate dehydrogenase may catalyse a ;non-equilibrium' reaction is discussed. 3. Correlation of changes in concentrations of substrates for enzymes catalysing ;non-equilibrium' reactions with changes in rates of glycolysis caused by alteration of the conditions of incubation showed that hexokinase, phosphofructokinase, pyruvate kinase and possibly glyceraldehyde 3-phosphate dehydrogenase are subject to metabolic control in cerebral cortex slices. 4. It is suggested that the glycolysis is controlled by two regulatory systems, the hexokinase-phosphofructokinase system and the glyceraldehyde 3-phosphate dehydrogenase-pyruvate kinase system. These are discussed. 5. It is concluded that the rate of glycolysis in guinea-pig cerebral cortex slices is limited either by the rate of glucose entry into the slices or by the hexokinase-phosphofructokinase system. 6. It is concluded that addition of 0.1mm-ouabain to guinea-pig cerebral cortex slices causes inhibition of either glyceraldehyde 3-phosphate dehydrogenase or phosphoglycerate kinase or both, in a manner independent of the known action of ouabain on the sodium- and potassium-activated adenosine triphosphatase.     

The regulation of triglyceride synthesis and fatty acid synthesis in rat epididymal adipose tissue. Effects of insulin, adrenaline and some metabolites in vitro

1. Adipose tissues from rats fed a balanced diet were incubated in the presence of glucose (20mm) with the following additions: insulin, anti-insulin serum, insulin+acetate, insulin+pyruvate, insulin+lactate, insulin+phenazine methosulphate, insulin+oleate+albumin, insulin+adrenaline+albumin, insulin+6-N-2'-O-dibutyryl 3':5'-cyclic AMP+albumin. 2. Measurements were made of the whole tissue concentrations of adenine nucleotides, hexose phosphates, triose phosphates, glycerol 1-phosphate, 3 phosphoglycerate, 6-phosphogluconate, long-chain fatty acyl-CoA, acid-soluble CoA, citrate, isocitrate, malate and 2-oxoglutarate, and of the release into the incubation medium of lactate, pyruvate and glycerol after 1h of incubation. 3. Fluxes of [(14)C]glucose carbon through the major pathways of glucose metabolism were calculated from the yields of (14)C in various products after 2h of incubation. Fluxes of [(14)C]acetate, [(14)C]pyruvate or [(14)C]lactate carbon in the presence of glucose were also determined. 4. Measurements were also made of the whole-tissue concentrations of metabolites in tissues taken directly from Nembutal-anaesthetized rats. 5. Whole tissue mass-action ratios for phosphofructokinase, phosphoglucose isomerase and the combined (aldolasextriose phosphate isomerase) reaction were similar in vivo and in vitro. The reactants of phosphofructokinase appeared to be far from mass-action equilibrium. In vitro, the reactants of hexokinase also appeared to be far from mass-action equilibrium. 6. Correlation of observed changes in glycolytic flux with changes in fructose 6-phosphate concentration suggested that phosphofructokinase may show regulatory behaviour. The enzyme appeared to be activated in the presence of oleate or adrenaline and to be inhibited in the presence of lactate or pyruvate. 7. Evidence is presented that the reactants of lactate dehydrogenase and glycerol 1-phosphate dehydrogenase may be near to mass-action equilibrium in the cytoplasm. 8. No satisfactory correlations could be drawn between the whole-tissue concentrations of long-chain fatty acyl-CoA, citrate and glycerol 1-phosphate and the observed rates of triglyceride and fatty acid synthesis. Under the conditions employed, the concentration of glycerol 1-phosphate appeared to depend mainly on the cytoplasmic [NAD(+)]/[NADH] ratios. 9. Calculated hexose monophosphate pathway flux rates roughly correlated with fatty acid synthesis rates and with whole tissue [6-phosphogluconate]/[glucose 6-phosphate] ratios. The relative rates of production of NADPH for fatty acid synthesis by the hexose monophosphate pathway and by the ;malic enzyme' are discussed. It is suggested that all NADH produced in the cytoplasm may be used in that compartment for reductive synthesis of fatty acids, lactate or glycerol 1-phosphate.     

Deficiencies of glycolytic enzymes as a possible cause of hemolytic anemia

The critical minimum values of Na,K-ATPase and glycolytic enzyme activities at which the erythrocyte viability is lost were calculated using the mathematical model of the erythrocyte, which included all reactions of glycolysis, adenylate metabolism, ionic balance, and osmotic regulation of erythrocyte volume. The criterion for cell death was an increase in its volume to the level at which it is sequestrated from the circulation or is lysed. In hemolytic anemia associated with hexokinase or pyruvate kinase deficiency, activities of these enzymes measured in patient erythrocytes appeared to be close to the calculated critical values. By contrast, in hemolytic anemia associated with phosphofructokinase, glucosephosphate isomerase, triosephosphate isomerase, or phosphoglycerate kinase deficiency, activities of these enzymes measured in patient erythrocytes were significantly greater than the calculated critical values. In this case, if the deficient enzyme were stable, i.e. its activity in the cell were low, but constant in time, the deficiency observed would not account for the erythrocyte destruction observed and the development of hemolytic anemia. It was shown, however, that in phosphofructokinase, glucosephosphate isomerase, triosephosphate isomerase, or phosphoglycerate kinase deficiency, hemolytic anemia can arise because of the instability of these enzymes in time.     

Gene dosage effect in human triploid fibroblasts

Summary The activity of 13 cytoplasmic enzymes has been determined in fibroblast extracts from 9 triploid and 13 control lines. The results show a high activity for 2 X-linked enzymes, glucose 6-phosphate dehydrogenase and phosphoglycerate kinase. These data, together with cytogenetic observations, support the contention that 2 X chromosomes were active in the triploid lines.Abbreviations G6PD Glucose 6-phosphate dehydrogenase - 6PGD 6-phosphogluconate dehydrogenase - HK hexokinase - PGM phosphoglucomutase - PHI phosphohexoisomerase - PFK phosphofructokinase - ALD aldolase - TPI triosephosphate isomerase - PGK phosphoglycerate kinase - ENOL enolase - AK adenylate kinase - LDH lactic dehydrogenase - HBDH hydroxybutyrate dehydrogenase INSERM U. 129.INSERM U. 73.     

Enzymes of glycolysis and the pentose phosphate pathway during development of the rabbit stomach worm Obeliscoides cuniculi

The activities of glycolytic and other enzymes of carbohydrate metabolism were measured in free-living and parasitic stages of the rabbit stomach worm Obeliscoides cuniculi. Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, phosphoglucomutase, hexokinase, glucosephosphate isomerase, phosphofructokinase, aldolase, triosephosphate isomerase, α-glycerophosphatase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, phosphoglycerate mutase, enolase, pyruvate kinase, phosphoenol pyruvate carboxykinase, lactate dehydrogenase, alcohol dehydrogenase, and glucose-6-phosphatase activities were present in worms recovered 14, 20 and 190 days postinfection.The presence of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase indicates the possible function of a pentose phosphate pathway and a capacity for gluconeogenesis, respectively, in these worms.The ratio of pyruvate kinase (PK) to phosphoenol pyruvate carboxykinase (PEPCK) less than I in parasitic stages suggests that their most active pathway is that fixing CO₂ into phosphoenol pyruvate to produce oxaloacetate.Low levels of glucose-6-phosphate dehydrogenase, triosephosphate isomerase, PEPCK and PK were recorded in infective third-stage larvae stored at 5°C for 5 and 12 mos. The ratio of PK to PEPCK greater than 1 indicates that infective larvae preferentially utilize a different terminal pathway than the parasitic stages.     

Advances in hereditary red cell enzyme anomalies

Conclusions From the above report we can observe that recent advances in hereditary disorders of red cell enzymes concern molecular mechanisms of the defect and relationship between molecular anomalies and pathologic consequences. Moreover, congenital nonspherocytic hemolytic anemias of undertermined cause are becoming fewer.It seems to us that the perspectives opened in this field could develop in two ways. Firstly, the current possibility of obtaining homogeneous mutant enzymes (for pyruvate kinase, glucose phosphate isomerase, glucose-6-phosphate dehydrogenase, phosphoglycerate kinase) should enable the study of the structure-function relationship, as was done for hemoglobin. Secondly, the recent progress in genetic analysis and genetic engineering could provide a direct approach of the nature of the genetic defect at the DNA level. This should be fundamental to understand the nature of some enzyme deficiencies without any detectable abnormal product of a mutant gene (e.g., glucose phosphate isomerase deficiencies with silent gene and M-type phosphofructokinase deficiency).     

A model study of the fructose diphosphatase-phosphofructokinase substrate cycle

A fructose diphosphatase–phosphofructokinase substrate cycle has been reconstructed in vitro to provide a system that recycles fructose 6-phosphate and hydrolyses ATP to ADP and P_i. The concerted actions of glucose phosphate isomerase, phosphofructokinase, aldolase and triose phosphate isomerase catalysed the loss of ³H from [5-³H,U-¹⁴C]glucose 6-phosphate. This was used as the basis of a method for the estimation of the fructose diphosphatase–phosphofructokinase substrate cycle. For the reconstructed cycle, the rate of decrease of the ³H/¹⁴C ratio in [5-³H,U-¹⁴C]hexose 6-phosphate was proportional to the rate of fructose 6-phosphate substrate cycling. A detailed theoretical treatment of this relationship is developed, which enables the rate of substrate cycling to be determined in vivo.     

Irreversible transitions in the 6-phosphofructokinase/fructose 1,6-bisphosphatase cycle.

The dynamics of the fructose 6-phosphate fructose-1,6-bisphosphate cycle operating in an open and homogeneous system reconstituted from purified enzymes was extensively studied. In addition to 6-phosphofructokinase and fructose-1,6-bisphosphatase, pyruvate kinase, adenylate kinae and glucose-6-phosphate isomerase were involved. In that multi-enzyme system, the main source of non-linearity is the reciprocal effect of AMP on the activities of 6-phosphofructokinase and fructose-1,6-bisphosphatase. Depending upon the experimental parameter values, stable attractors, various types of multiple states and sustained oscillations were shown to occur. In the present report we show that irreversible transitions are also likely to occur for realistic operating conditions. Two parameters of the system, that is the adenylate energy charge of the influx and the fructose-1,6-bisphosphatase maximal activity, are potential candidates to provoke such irreversible transitions from one steady state to the other: (a) when varying the maximal activity of fructose-1,6-bisphosphatase, the system can jump irreversibly from a low to a high stable steady state, and (b) when the adenylate energy charge of the influx is the changing parameter, irreversible transitions occur from a high stable steady state to a stable oscillatory state (limit cycle motion). This behavior can be predicted by constructing the loci of limit points and Hopf bifurcation points.     

The effect of dietary and hormonal conditions on the activities of glycolytic enzymes in rat epididymal adipose tissue

1. Measurements were made of the activities of nine glycolytic enzymes in epididymal adipose tissues obtained from rats that had undergone one of the following treatments: starvation; starvation followed by re-feeding with bread or high-fat diet; feeding with fat without preliminary starvation; alloxan-diabetes; alloxan-diabetes followed by insulin therapy. 2. In general, the activities of the glycolytic enzymes of adipose tissue, unlike those of liver, were not greatly affected by the above treatments. 3. The ;key' glycolytic enzymes, phosphofructokinase and pyruvate kinase, were generally no more adaptive in response to physiological factors than other glycolytic enzymes such as glucose phosphate isomerase, fructose diphosphate aldolase, triose phosphate isomerase, glycerol 3-phosphate dehydrogenase, phosphoglycerate kinase and lactate dehydrogenase. 4. Adiposetissue pyruvate kinase did not respond to feeding with fat in a manner similar to the liver enzyme. 5. Glyceraldehyde phosphate dehydrogenase had a behaviour pattern unlike the other eight glycolytic enzymes studied in that its activity was depressed by feeding with fat and was not restored to normal by re-feeding with a high-fat diet after starvation. These results are discussed in relation to the requirements of adipose tissue for glycerol phosphate in the esterification of fatty acids. 6. A statistical analysis of the results permitted the writing of linear equations describing the relationships between the activities of eight of the enzymes studied. 7. Evidence is presented for the existence of two constant-proportion groups amongst the enzymes studied, namely (i) glucose phosphate isomerase, phosphoglycerate kinase and lactate dehydrogenase, and (ii) triose phosphate isomerase, fructose diphosphate aldolase and pyruvate kinase. 8. Mechanisms for maintaining the observed relationships between the activities of the enzymes in the tissue are discussed.     

Phosphofrucktokinase and glucose catabolism of Mucor and Penicillium species.

The primary catabolic pathways in the fungi Penicillium notatum and P. duponti, and Mucor rouxii and M. miehei were examined by measuring the relative rate of 14CO2 production from different carbon atoms of specifically labelled glucose. It was found that these organisms dissimilate glucose predominantly via the Embden--Meyerhof pathway in conjunction with the tricarboxylic acid cycle and to a lesser extent by the pentose phosphate pathway. Phosphofructokinase (EC 2.7.1.11) activity could not be detected initially in Penicillium species because of the interference from mannitol-1-phosphate dehydrogenase (EC 1.1.1.17) and NADH oxidase (EC 1.6.99.3). A combination of differential centrifuging and a heat treatment of Penicillium cell-free extracts in the presence of fructose-6-phosphate removed the interfering enzymes. The kinetic characteristics of phosphofructokinase from P. notatum and M. rouxii are described. The enzyme presents highly cooperative kinetics for fructose-6-phosphate. The kinetics for ATP show no cooperativity and inhibition by excess ATP is observed. The addition of AMP activated the P. notatum enzyme, relieving ATP inhibition; slight inhibition by AMP was observed with the M. rouxii enzyme. In contrast M. rouxii pyruvate kinase (EC 2.7.1.40) is activated 50-fold by fructose-1,6-diphosphate whereas pyruvate kinase from P. notatum and P. duponti were unaffected by fructose-1,6-diphosphate.