Showing papers on "Glycolysis published in 1982"

Energy Metabolism of Human Neutrophils during Phagocytosis

Abstract: Detailed quantitative studies were performed on the generation and utilization of energy by resting and phagocytosing human neutrophils. The ATP content was 1.9 fmol/cell, was constant during rest, and was not influenced by the presence or absence of glucose in the medium. The intracellular content of phosphocreatine was less than 0.2 fmol/cell. In the presence of glucose, ATP was generated almost exclusively from lactate produced from glucose taken up from the surrounding medium. The amount of lactate produced could account for 85% of the glucose taken up by the cells, and the intracellular glycosyl store, glycogen, was not drawn upon. The rate of ATP generation as calculated from the rate of lactate production was 1.3 fmol/cell/min. During phagocytosis, there was no measurable increase in glucose consumption or lactate production, and the ATP content fell rapidly to 0.8 fmol/cell. This disappearance of ATP was apparently irreversible since no corresponding increase in ADP or AMP was observed. It therefore appears that this phagocytosis-induced fall in ATP concentration represents all the extra energy utilized in human neutrophils in the presence of glucose. In the absence of glucose, the rate of ATP generation in the resting cell was considerably smaller, 0.75 fmol/cell per min, as calculated from the rate of glycolysis, which is sustained exclusively by glycogenolysis. Under this condition, however, phagocytosis induces significant enhancement of glycogenolysis and the rate of lactate production is increased by 60%, raising the rate of ATP generation to 1.2 fmol/cell per min. Nonetheless, the ATP content drops significantly from 1.9 to 1.0 fmol/cell. Neutrophils from patients with chronic granulomatous disease have the same rate of glycolysis and the same ATP content as normal cells, thus confirming that the defective respiration of these cells does not affect their energy metabolism.

Metabolism of round spermatids from rats: lactate as the preferred substrate

Abstract: Round spermatids were prepared from rat testes and incubated with various substrates (glucose, fructose, pyruvate, lactate and acetate) to measure utilization of substrates and production of ATP in the presence of saturating levels of each substrate. By both criteria lactate is the preferred substrate by a factor of 3 or 4. Production of more than half of the ATP with lactate is substrate is prevented by addition of an inhibitor of alpha-ketoacid dehydrogenase (5-methoxyindole-2-carboxylic acid) Pyruvate and lactate are interconverted and pyruvate inhibits production of ATP from lactate. Synthesis of ATP with lactate and with pyruvate is inhibited by rotenone, rutamycin or 2,4-dinitrophenol. Utilization of glucose is limited by aldolase activity. These findings suggest that exogenous lactate is oxidized by lactate dehydrogenase followed by pyruvate dehydrogenase and Krebs; cycle enzymes under conditions which do not allow pyruvate to inhibit lactate dehydrogenase. ATP is synthesized through electron transport. Post-mitochondrial supernate from spermatids showed that high concentration of pyruvate (greater than 1 mM) inhibit lactate dehydrogenase with pyruvate as substrate and that with lactate as substrate, pyruvate behaves as a competitive inhibitor of lactate dehydrogenase. Evidently lactate is the preferred substrate for round spermatids and energy production is most efficient when this substance is present in high concentrations and pyruvate is present in low concentrations. Reasons are given for suggesting that Sertoli cells may provide the relatively large amounts of lactate required by round spermatids.

Maximum activities of some enzymes of glycolysis, the tricarboxylic acid cycle and ketone-body and glutamine utilization pathways in lymphocytes of the rat.

Abstract: 1. The maximum activity of hexokinase in lymphocytes is similar to that of 6-phosphofructokinase, but considerably greater than that of phosphorylase, suggesting that glucose rather than glycogen is the major carbohydrate fuel for these cells. Starvation increased slightly the activities of some of the glycolytic enzymes. A local immunological challenge in vivo (a graft-versus-host reaction) increased the activities of hexokinase, 6-phosphofructokinase, pyruvate kinase and lactate dehydrogenase, confirming the importance of the glycolytic pathway in cell division. 2. The activities of the ketone-body-utilizing enzymes were lower than those of hexokinase or 6-phosphofructokinase, unlike in muscle and brain, and were not affected by starvation. It is suggested that the ketone bodies will not provide a quantitatively important alternative fuel to glucose in lymphocytes. 3. Of the enzymes of the tricarboxylic acid cycle whose activities were measured, that of oxoglutarate dehydrogenase was the lowest, yet its activity (about 4.0mumol/min per g dry wt. at 37 degrees C) was considerably greater than the flux through the cycle (0.5mumol/min per g calculated from oxygen consumption by incubated lymphocytes). The activity was decreased by starvation, but that of citrate synthase was increased by the local immunological challenge in vivo. It is suggested that the rate of the cycle would increase towards the capacity indicated by oxoglutarate dehydrogenase in proliferating lymphocytes. 4. Enzymes possibly involved in the pathway of glutamine oxidation were measured in lymphocytes, which suggests that an aminotransferase reaction(s) (probably aspartate aminotransferase) is important in the conversion of glutamate into oxoglutarate rather than glutamate dehydrogenase, and that the maximum activity of glutaminase is markedly in excess of the rate of glutamine utilization by incubated lymphocytes. The activity of glutaminase is increased by both starvation and the local immunological challenge in vivo. This last finding suggests that metabolism of glutamine via glutaminase is important in proliferating lymphocytes.

Correlative histochemistry of some enzymes of carbohydrate metabolism in preneoplastic and neoplastic lesions in the rat liver

Abstract: The livers from a total of 51 Sprague-Dawley rats treated with different doses of N-nitrosomorpholine (80-120 mg/l in the drinking water) for up to 14 weeks together with the livers of 28 control animals were histochemically investigated at the cessation of carcinogenic insult and at varying periods thereafter for their glycogen content, basophilia and activities of various enzymes of carbohydrate metabolism: glycogen synthetase, glycogen phosphorylase, glucose-6-phosphatase, glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase The enzymatic patterns of normal tissue, preneoplastic and neoplastic lesions were characterized and compared with reference to the morphologically defined stages of tumor development in the liver The early appearing glycogen storing areas, localized in the peripheral and intermediate lobular regions, did not show significant changes in the histochemically demonstrable activities of the enzymes tested After cessation of the carcinogen treatment the more pronounced glycogen storage foci which developed within the aforementioned regions of the liver acinus usually showed a reduction in the activities of phosphorylase and glucose-6-phosphatase while the activity of glucose-6-phosphate dehydrogenase, a key enzyme for the pentose phosphate pathway, was increased The mixed cell foci, neoplastic nodules and tumors which emerged at later stages were characterized by a progressive shift away from glycogen metabolism towards glycolysis and the pentose phosphate pathway, as indicated by an increase in glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase activities These changes in enzyme pattern are supportive of a developmental sequence leading from glycogen storage foci through mixed cell foci and neoplastic nodules to hepatocellular carcinomas

A special fructose bisphosphate functions as a cytoplasmic regulatory metabolite in green leaves.

Abstract: Fructose 2,6-bisphosphate (Fru-2,6-P2), a regulatory metabolite discovered in animal cells and recently reported to occur in etiolated seedlings, was found to be present in the cytoplasmic fraction of leaves of spinach and peas (typical C3 plants, in which a three-carbon carboxylic acid is a major early photosynthetic product). At concentrations approximating those calculated to occur physiologically, Fru-2,6-P2 modulated two enzymes of the leaf cytoplasm: (i) Fructose-1,6-bisphosphatase (EC 3.1.3.11), a key enzyme of sugar synthesis, was competitively inhibited by Fru-2,6-P2, and (ii) pyrophosphate-linked phosphofructokinase (inorganic pyrophosphate-D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.90), a cytoplasmic enzyme that now seems important in glycolysis of C3 plants, was activated by Fru-2,6-P2. There was no indication of a role for Fru-2,6-P2 in photosynthesis of either chloroplasts or oxygenic prokaryotes. The results suggest that Fru-2,6-P2 functions in the regulation of glycolysis and gluconeogenesis (carbohydrate synthesis) in the cytoplasm of leaves of C3 plants.

Involvement of oxygen-sensitive pyruvate formate-lyase in mixed-acid fermentation by Streptococcus mutans under strictly anaerobic conditions.

Abstract: Streptococcus mutans JC2 produced formate, acetate, ethanol, and lactate when suspensions were incubated with an excess of galactose or mannitol under strictly anaerobic conditions. The galactose- or mannitol-grown cell suspensions produced more formate, acetate, and ethanol than the glucose-grown cells even when incubated with glucose. The levels of lactate dehydrogenase and fructose 1,6-bisphosphate were not significantly different in these cells, but the level of pyruvate formate-lyase was higher in the galactose- or mannitol-grown cells, and that of triose phosphate was lower in the galactose-grown cells. This suggests that the regulation of pyruvate formate-lyase may play a major role in the change of the fermentation patterns. The cells of S. mutans grown on glucose produced a significant amount of volatile products even in the presence of excess glucose under strictly anaerobic conditions. However, when the anaerobically grown cells were exposed to air, only lactate was produced from glucose. When cells were anaerobically grown on mannitol and then exposed to air for 2 min, only trace amounts of fermentation products were formed from mannitol under anaerobic conditions. It was found that the pyruvate formate-lyase in the cells was inactivated by exposure of the cells to air.

Intracellular pH mediates action of insulin on glycolysis in frog skeletal muscle

Abstract: In a glucose-free bicarbonate Ringer (5% CO2 in N2), insulin increased intracellular pH (pHi), as determined by [14C]dimethadione, by 0.12 +/- 0.02 and stimulated glycolysis, as monitored by anaerobic lactate production, by 42.9 +/- 3.5% in paired frog sartorius muscles. The effect of insulin on glycolysis was shown to vary approximately linearly with log [Na+]0, being converted in 0.12 mM Na+ Ringer to a 51.5 +/- 8.4% inhibition of glycolysis. As the Na+ free-energy gradient was varied by decreasing [Na+]0 from 104 to 6.8 mM, the changes in glycolytic flux produced by insulin consistently paralleled the changes in pHi produced by the hormone. The relationship between the change in pHi and percent change in glycolytic flux was the same regardless of whether the effects were produced by insulin or by changing CO2. When glycolysis was either stimulated or inhibited, intracellular levels of fructose 6-phosphate varied inversely with glycolytic flux. This indicates that the effect on glycolysis of either insulin or changes in CO2 is due to a change in the activity of phosphofructokinase. The results support the model that the acute effect of insulin on glycolysis is mediated by a change in pHi, consequent to activation by insulin of Na:H exchange at the plasma membrane.

Fructose-2,6-P2, chemistry and biological function.

Abstract: A new activator of phosphofructokinase, which is bound to the enzyme and released during its purification, has been discovered. Its structure has been determined as beta-D Fructose-2,6-P2 by chemical synthesis, analysis of various degradation products and NMR. D-Fructose-2,6-P2 is the most potent activator of phosphofructokinase and relieves inhibition of the enzyme by ATP and citrate. It lowers the Km for fructose-6-P from 6 mM to 0.1 mM. Fructose-6-P,2-kinase catalyzes the synthesis of fructose-2,6-P2 from fructose-6-P and ATP, and the enzyme has been partially purified. The degradation of fructose-2,6-P2 is catalyzed by fructose-2,6-bisphosphatase. Thus a metabolic cycle could occur between fructose-6-P and fructose-2,6-P2, which are catalyzed by these two opposing enzymes. The activities of these enzymes can be controlled by phosphorylation. Fructose-6-P,2-kinase is inactivated by phosphorylation catalyzed by either cAMP dependent protein kinase or phosphorylase kinase. The inactive, phospho-fructose-6,P,2-kinase is activated by dephosphorylation catalyzed by phosphorylase phosphatase. On the other hand, fructose-2,6-bisphosphatase is activated by phosphorylation catalyzed by cAMP dependent protein kinase. Investigation into the hormonal regulation of phosphofructokinase reveals that glucagon stimulates phosphorylation of phosphofructokinase which results in decreased affinity for fructose-2,6-P2 appears to be due to the decreased synthesis by inactivation of fructose-2,6-P2,2-kinase and increased degradation as a result of activation of fructose-2,6-bisphosphatase. Such a reciprocal change in these two enzymes has been demonstrated in the hepatocytes treated by glucagon and epinephrine. The implications of these observations in respect to possible coordinated controls of glycolysis and glycogen metabolism are discussed.

Intermediary carbohydrate metabolism in protoscoleces of Echinococcus granulosus (horse and sheep strains) and E. multilocularis.

Abstract: With few exceptions, the specific activities of the glycolytic enzymes and the steady-state content of glycolytic and associated intermediates in protoscoleces of the horse (E.g.H) and sheep (E.g.S) strains of Echinococcus granulosus and the closely related E. multilocularis (E.m.) are very similar. Phosphorylase, hexokinase, phosphofructokinase and pyruvate kinase catalyse non-equilibrium reactions and the patterns of activity for pyruvate kinase, phosphoenolpyruvate carboxykinase and malic enzyme are similar in the three organisms. The levels of tricarboxylic acid cycle intermediates in E.g.H., E.g.S. and E.m. are of the same order as those reported in tissues with an active cycle. Each has a complete sequence of cycle enzymes but there are substantial differences between the three parasites with regard to the activity of individual enzymes, The activities of NAD and NADP-linked isocitrate dehydrogenases are significantly lower in E.g.H. than in E.g.S. and particularly in E.m. which suggests that the tricarboxylic acid cycle may play a more important role in carbohydrate metabolism and energy production in the latter parasites. Nevertheless, the three organisms utilize fermentative pathways for alternative energy production, fix carbon dioxide via phosphoenolpyruvate carboxykinase and have a partial reversed tricarboxylic acid cycle. It is speculated that in vivo more carbon will be channelled towards oxaloacetate than pyruvate at the phosphoenolpyruvate branch point. The steady state content of ATP and the ATP/AMP ratios are low in the three organisms, suggesting a low rate of ATP utilization in each.

Role of fructose 2,6-bisphosphate in the stimulation of glycolysis by anoxia in isolated hepatocytes.

Abstract: 1. Incubation of hepatocytes from fed or starved rats with increasing glucose concentrations caused a stimulation of lactate production, which was further increased under anaerobic conditions. 2. When glycolysis was stimulated by anoxia, [fructose 2,6-bis-phosphate] was decreased, indicating that this ester could not be responsible for the onset of anaerobic glycolysis. In addition, the effect of glucose in increasing [fructose 2,6-bisphosphate] under aerobic conditions was greatly impaired in anoxic hepatocytes. [Fructose 2,6-bisphosphate] was also diminished in ischaemic liver, skeletal muscle and heart. 3. The following changes in metabolite concentration were observed in anaerobic hepatocytes: AMP, ADP, lactate and L-glycerol 3-phosphate were increased; ATP, citrate and pyruvate were decreased: phosphoenolpyruvate and hexose 6-phosphates were little affected. Concentrations of adenine nucleotides were, however, little changed by anoxia when hepatocytes from fed rats were incubated with 50 mM-glucose. 4. The activity of ATP:fructose 6-phosphate 2-phosphotransferase was not affected by anoxia but decreased by cyclic AMP. 5. The role of fructose 2,6-bisphosphate in the regulation of glycolysis is discussed.

Muscle phosphoglycerate rnutase deficiency

Abstract: A 52-year-old man complained since adolescence of cramps and pigmenturia after 15 to 30 minutes of intense exercise. There was no family history of neuromuscular diseases, and strength was normal. The rise of venous lactate after forearm ischemic exercise was abnormally low. Histochemical and ultrastructural studies of a muscle biopsy showed mild increase of glycogen, which was confirmed by biochemical analysis. Studies of anaerobic glycolysis in vitro showed decrease lactate formation with glycogen and with all hexosephosphate glycolytic intermediates, suggesting a defect below the phosphofructokinase reaction. Muscle phosphoglycerate mutase (PGAM) activity was 5.7% of the lowest control, while all other enzymes of glycolysis had normal activities. Electrophoretic, heat lability, and mercury inhibition studies showed that the small residual activity of PGAM in the patient's muscle was represented by the brain (BB) isoenzyme, suggesting a genetic defect of the M subunit that predominates in normal muscle. The prevalence of the BB isoenzyme in other tissues, including muscle culture, may explain why symptoms were confined to muscle.

The hydrogen ion in normal metabolism: a review.

Abstract: The production of hydrogen ions (H+) by metabolic processes is described, with particular emphasis on glycolysis and ketogenesis. Total metabolic production of H+ is approximately 150 g day-1 but utilization closely balances production, so that intracellular and extracellular H+ production is maintained within narrow limits. H+ is generated at several sites in glycolysis but no net H+ production occurs unless the ATP formed is hydrolysed. The other main source of metabolic H+ production is ketogenesis. Here H+ accumulation depends on both the relative dominance of ketone body production over utilization and the loss of base in urine. The H+ is produced during the synthesis of 3-hydroxy-3-methylglutaryl-CoA and not because of dissociation of acetoacetic acid. Lipolysis and re-esterification of fats are additional major producers of H+, while net H+ production also occurs with pathological accumulation and incomplete combustion of other organic acids. Many metabolic systems are sensitive to the changes in pH. These effects have been examined in vivo using an ammonium chloride acidaemia model in the rat. Severe insulin resistance and impaired glucose metabolism in liver and muscle were found. One mechanism involved inhibition, by H+, of the binding of insulin to its receptors. Further mechanisms include inhibition of key glycolytic enzymes including phosphofructokinase. It is concluded that too little attention is paid to metabolic production of hydrogen ions and to their effects, in turn, on metabolism.

Ultrastructure and metabolism of skeletal muscle fibres in the tench: effects of long-term acclimation to hypoxia.

Abstract: Tench (Tinca tinca) were acclimated to either aerated (P O 2 17.6 KPa) or hypoxic (P O 2 1.5 KPa) water for 6 weeks. Acclimation to hypoxia resulted in a decrease in mitochondrial volume fraction in both slow (22.9 to 15.0 %) and fast glycolytic (4.5 to 1.8 %) myotomal muscles fibres (P<0.01). Intermyofibrillar mitochondrial populations (4.4 to 1.2% slow; 0.6 to 0.04% fast fibres) were affected to a greater extent than those in the subsarcolemmal zone (18.5 to 13.8% slow; 3.9 to 1.8% fast fibres). After acclimation to hypoxia, cytochrome-oxidase activities decreased by 31 and 33 % in slow and fast fibres, respectively, but were maintained in the liver. Fibre size remained unchanged and actively differentiating fibres were observed in muscles from both groups of fish. Hypoxia resulted in a significant increase in myofibrillar volume fraction in both slow (43.1 to 56.1 %) and fast glycolytic fibres (73.1 to 82.7%) (P<0.05). Glycogen concentrations (mg/100g tissue) for liver (6616) slow muscle (1892) and fast muscle (334) were similar for fish acclimated to aerated or hypoxic water. Acclimation to hypoxia increased carnitine palmitoyl transferase activity (μmoles substrate utilised g·dry wt-1 min-1) in slow (0.42 to 1.1), fast glycolytic muscle (<0.01 to 0.15) and liver (1.1 to 3.7) indicating an enhanced capacity for fatty acid oxidation. Phosphofructokinase activities of fast glycolytic fibres were similar in fish acclimated to either aerated or hypoxic water, consistent with an unaltered capacity for anaerobic glycogenolysis. Hexokinase activities (μmoles substate utilised, g·dry wt-1 min-1) decreased in fast fibres (1.2 to 0.4) but were maintained in the slow muslce (2.1 to 2.5) and liver (4.5 to 4.8) of hypoxic fish. The activities of phosphofructokinase in slow muscle and phosphofructokinase, pyruvate kinase and lactate dehydrogenase in liver were two times higher in fish acclimated to hypoxia. An enhanced capacity for glycolysis in these tissues may reflect a reduced threshold for anaerobic metabolism during activity and/or an adaptation for acute exposure to anoxia in fish acclimated to hypoxia.

Regulation of glycolysis and fatty acid synthesis from glucose in sheep adipose tissue

Abstract: 1. The following were measured in adipose-tissue pieces, obtained from 7–9 month-old sheep, before or after the tissue pieces had been maintained in tissue culture for 24 h: the rates of synthesis from glucose of fatty acids, acylglycerol glycerol, pyruvate and lactate; the rate of glucose oxidation to CO2; the rate of glucose oxidation via the pentose phosphate pathway; the activities of hexokinase, glucose 6-phosphate dehydrogenase, phosphofructokinase, pyruvate kinase, pyruvate dehydrogenase and ATP citrate lyase; the intra- and extra-cellular water content; the concentration of various metabolites and ATP, ADP and AMP. 2. The proportion of glucose carbon converted into the various products in sheep adipose tissue differs markedly from that observed in rat adipose tissue. 3. There was a general increase in the rate of glucose utilization by the adipose-tissue pieces after maintenance in tissue culture; largest changes were seen in the rates of glycolysis and fatty acid synthesis from glucose. These increases are paralleled by an increase in pyruvate kinase activity. There was no change in the activities of the other enzymes as measured, although the net flux through all the enzymes increased. 4. Incubation of fresh adipose-tissue pieces for 2–6h led to an increase in the affinity of pyruvate kinase for phosphoenolpyruvate. 5. The rate of pyruvate production by glycolysis was greater than the activity of pyruvate dehydrogenase of the tissue. 6. The results suggest that both pyruvate kinase and pyruvate dehydrogenase have important roles in restricting the utilization of glucose carbon for fatty acid synthesis in sheep adipose tissue.

Anaerobic energy metabolism in isolated adductor muscle of the sea musselMytilus edulis L.

Abstract: Energy metabolism during anaerobiosis was investigated in the isolated posterior adductor muscle of the sea mussel. Metabolism appeared to be similar to that observed in the intact organism. Glycogen and aspartate are simultaneously utilized and levels of alanine, succinate, strombine and octopine increase. The sum of the adenylates remains constant, whereas phosphoarginine is dephosphorylated. The influence of iodoacetate, aminooxyacetate and hadacidin, inhibitors of glycolysis, transamination and purine nucleotide cycle, respectively, on the utilization of substrates and the interconversion of metabolites has been studied. The results suggest that the purine nucleotide cycle is not involved in the inverse correlation of changes in levels of aspartate and alanine, but that this exclusively depends on transamination reactions. Pyruvate (required for alanine formation) arises about equally from glycolysis and aspartate decarboxylation. When the utilization of aspartate is blocked by aminooxyacetate, glycolytically formed pyruvate is metabolized by reductive condensation with glycine and arginine to yield strombine and octopine. Under this condition phosphoarginine is dephosphorylated at a faster rate in order to maintain the energy status of the cell.

Aspects of biochemical effects by hyperthermia

Abstract: Hyperthermia caused an immediate decrease of DNA, RNA, and protein synthesis. The latter process was most sensitive. Initiation of DNA synthesis at the transition from G1-to S-phase and at the start of new replication units were inhibited. These effects were responsible for the growth delay of cells and were potentiated with irradiation. The immediate inhibition of protein synthesis was due mainly to a disaggregation of the synthesizing mechanisms. It led to a decrease of enzyme activities with a short biological half-life, e.g., ornithine decarboxylase. Lysosomal hydrolytic activities might be enhanced after hyperthermia and contribute to tissue damage. During hyperthermia, glycogen breakdown and glucose turnover through glycolysis and the citrate cycle were apparently increased, but after hyperthermia, respiration and glycolysis were reduced. No lactate accumulation occurred, but other acidic metabolites were enhanced and could induce a metabolic acidosis hours later. A glucose load potentiated the effects on respiration and glycolysis. Immediately after hyperthermia, a lactate accumulation was observed under these conditions. A formula is given by which the ratios of reduced to oxidized substrates might indicate the redox state in different cellular compartments, with oxygen pressure, and during other metabolic conditions. Such changes of the intracellular milieu are important for themore » thermosensitivity of cells.« less

Effect of Caffeine on Glucose‐Induced Inactivation of Gluconeogenetic Enzymes in Saccharomyces cerevisiae

Abstract: The mechanism of catabolite inactivation of three gluconeogenetic enzymes, fructose-1,6-bisphosphatase, cytoplasmic malate dehydrogenase and phosphoenolpyruvate carboxykinase, has been studied in the yeast Saccharomyces cerevisiae. The glucose-induced inactivation of the three enzymes is remarkably retarded by pre-incubation of the cells with different caffeine concentrations; however, a full conservation of activity has never been obtained, even at the highest drug concentration. Caffeine modifies the metabolic effects produced in the yeast cell by exposure to glucose. It reduces the consumption rate of glucose; changes the glycolytic intermediate pattern, giving rise to a crossover point at the level of the phosphofructokinase/fructose-bisphosphatase cycle; and increases the ATP level and the energy charge. Moreover, it substantially reduces the peak of intracellular cAMP content that immediately follows glucose entry; the magnitude of this effect is dependent on the drug concentration. The effect on the change of intracellular cAMP level appears, among all metabolic effects determined by caffeine, the only plausible one to explain the interference with catabolite inactivation of enzymes. Actually a strong negative correlation between residual activity of each of the three investigated enzymes and intracellular cAMP level has been demonstrated. The existence of a common mechanism of action of cAMP, as the mediating factor for catabolite inactivation of all three enzymes, is proposed.