The effects of exogenous glucose, uracil, and inorganic phosphate on differentiation in Dictyostelium discoideum.

The cellular slime mold was exposed to exogenous glucose, uracil, and inorganic phosphate for either 900 or 90 min to determine their effects on the cellular levels of glucose 6-phosphate (glucose-6-P), UDP-glucose, glycogen, trehalose, and cellulose. Glucose, and phosphate to a lesser extent, increase the levels of glucose-6-P and trehalose, whereas glycogen levels are increased only by glucose. Uracil inhibits glucose-6-P and trehalose accumulation, and this inhibition is reversed by glucose or phosphate. Uracil, especially in the presence of glucose, stimulates the accumulation of UDP-glucose and cellulose. In an attempt to understand the dynamics of the biochemical mechanisms underlying these experimental observations, fluxes of the same metabolites were imposed on a kinetic model of this system. The effects of glucose, uracil, and phosphate either singly or in various combinations on the accumulation of glycogen and trehalose can be predicted quantitatively by applying the appropriate external flux(es) of these additives to the model; the predicted effects on glucose-6-P levels are qualitatively consistent with the observations, but are greater in magnitude, suggesting compartmentation of glucose-6-P. Matching the observed and simulated results requires a lower level of additive in the simulated system than in the actual experiment, which is consistent with earlier studies on the cellular permeability of these metabolites.It is concluded that the complex of flux changes induced in the model by the perturbing metabolites may also occur in vivo, and that endogenous glucose availability is a critical variable controlling the rate and cessation of differentiation as well as the relative amounts of the saccharide end products of differentiation.     

Transient repression of beta-galactosidase synthesis by glucose-6-phosphate in a mutant of Escherichia coli lacking enzyme II specific for glucose in the phosphoenolpyruvate-sugar phosphotransferase system.

The effects of glucose and glucose-6-phosphate in initiating the repression of beta-galactosidase synthesis were studied using a mutant of Escherichia coli K12 which lacks glucose-specific enzyme II of the phosphoenolpyruvate-sugar phosphotransferase system. It was found that glucose-6-phosphate causes transient repression of beta-galactosidase synthesis but glucose does not cause transient repression in this mutant. Evidence was obtained that both the presence of an active transport system for glucose-6-phosphate in the cells and glucose-6-phosphate in the medium are necessary for the initiation of transient repression. No metabolism of glucose-6-phosphate is required. Upon depletion of glucose-6-phosphate in the medium the transient repression was reversed. After the reversal the rate of enzyme synthesis was high in the cells which had been exposed to a high concentration of glucose-6-phosphate. It was concluded that the translocation of glucose-6-phosphate across the membranes is the primary event which affects both the initiation of and the recovery from the transient repression. During the transient repression the cellular content of cyclic adenosine 3',5'-monophosphate decreased significantly.     

Antagonistic regulation of the glucose/glucose 6-phosphate cycle by insulin and glucagon in cultured hepatocytes.

Flux through the glucose/glucose 6-phosphate cycle in cultured hepatocytes was measured with radiochemical techniques. Utilization of [2-3H]glucose was taken as a measure of glucokinase flux. Liberation of [14C]glucose from [U-14C]glycogen and from [U-14C]lactate, as well as the difference between the utilization of [2-3H]glucose and of [U-14C]glucose, were taken as measures of glucose-6-phosphatase flux. At constant 5 mM-glucose and 2 mM-lactate concentrations insulin increased glucokinase flux by 35%; it decreased glucose-6-phosphatase flux from glycogen by 50%, from lactate by 15% and reverse flux from external glucose by 65%, i.e. overall by 40%. Glucagon had essentially no effect on glucokinase flux; it enhanced glucose-6-phosphatase flux from glycogen by 700%, from lactate by 45% and reverse flux from external glucose by 20%, i.e. overall by 110%. At constant glucose concentrations cellular glucose 6-phosphate concentrations were essentially not altered by insulin, but were increased by glucagon by 230%. In conclusion, under basic conditions without added hormones the glucose/glucose 6-phosphate cycle showed only a minor net glucose uptake, of 0.03 mumol/min per g of hepatocytes; this flux was increased by insulin to a net glucose uptake of 0.21 mumol/min per g and reversed by glucagon to a net glucose release of 0.22 mumol/min per g. Since the glucose 6-phosphate concentrations after hormone treatment did not correlate with the glucose-6-phosphatase flux, it is suggested that the hormones influenced the enzyme activity directly.     

The concentrations of glucose 1,6-bisphosphate and other regulatory metabolites, and the activities of enzymes of the glycogen metabolism in the perfused rabbit psoas muscle

The following parameters were determined in the rabbit psoas muscle after perfusion in the presence of either insulin, propranolol, or isoproterenol: Concentrations of cyclic AMP, glucose 1,6-bisphosphate, fructose 2,6-bisphosphate, glucose-1-phosphate, glucose 6-phosphate, and fructose-1,6-bisphosphate. Maximum and "regulatory" activities of the enzymes glycogen phosphorylase, glycogen synthase, phosphofructokinase, and histone-phosphorylating protein kinase.     

Glucose conversion by multiple pathways in brain extract: theoretical and experimental analysis

Experimental and model studies were performed to characterize the flux of glucose metabolism and the sharing of glucose-6-phosphate (Glu6P) by the upper parts of glycolytic and pentosephosphate pathways in the brain extract. A mathematical model based upon the kinetic equations of the individual enzymes was evaluated to fit the experimental data. Glucose is converted to glucose-6-phosphate by hexokinase that controls almost exclusively the glucose metabolism. Experiments showed that this crossroad-metabolite was shared between glycolysis and pentosephosphate pathway in the brain extract in a ratio of 1.5:1. This ratio was favorable to the pentosephosphate pathway by the addition of high excess of exogenous glucose-6-phosphate dehydrogenase, standardly used for the activity assay of hexokinase, but still a significant part (17+/-3%) of the common intermediate was converted into the direction of glycolysis. Stimulation of glucose-6-phosphate formation via moderate (30-50%) increase of hexokinase activity by adding exogenous hexokinase or tubulin resulted in the slight increase of the relative flux into direction of glycolysis. The model correctly described all of these observations. However, when the activity of hexokinase was doubled with exogenous enzyme, significantly less glucose-6-phosphate was converted into direction of glycolysis than predicted. This discrepancy shows that the system did not behave in this case as an ideal one, which could be due to the formation of distinct pools for the intermediate.     

The effect of glucose on the control of carbohydrate metabolism in ripening bananas

The effect of exogenous glucose on the major fluxes of carbohydrate metabolism in cores of climacteric fruit of banana (Musa cavendishii Lamb ex Paxton) was determined with the intention of using the effects in the application of top-down metabolic control analysis. Hands of bananas, untreated with ethylene, were allowed to ripen in the dark at 21 °C. Cores were removed from climacteric fruit and incubated in 100 or 200 mM glucose for 4 or 6 h. The rates of starch breakdown, sucrose and fructose accumulation and CO₂ production were measured. The steady-state contents of hexose monophosphates, adenylates and pyruvate were determined. In addition, the detailed distribution of label was determined after supply of the following: [U-¹⁴C]-, [1-¹⁴C]-, [3,4¹⁴C]and [6-¹⁴C]glucose, and [U-¹⁴C]glycerol. The data were used to estimate the major fluxes of carbohydrate metabolism. Supply of exogenous glucose led to increases in the size of the hexose-monophosphate pools. There was a small stimulation of the rate of sugar synthesis and a major increase in the rate of starch synthesis. Starch breakdown was inhibited. Respiration responded to the demand for ATP by sugar synthesis. The effect of glucose on fluxes and metabolite pools is discussed in relation to our understanding of the control and regulation of carbohydrate metabolism in ripening fruit.Abbreviations Glc6P glucose-6-phosphate - Glc1P glucose-1-phosphate - Fru6P fructose-6-phosphate - AEC adenylate energy charge We thank Geest Foods Group, Great Dunmow, Essex, UK for giving us the bananas. SAH thanks the managers of the Broodbank Fund for a fellowship.     

Synergism of glucose and fructose in net glycogen synthesis in perfused rat livers

Synergism of glucose and fructose in net glycogen synthesis was studied in perfused livers from 24-h fasted rats. With either glucose or fructose alone, net glycogen deposition did not occur (p greater than 0.10 for each), whereas the addition of both together resulted in significant glycogen accumulation (net glycogen accumulation was 0.21 +/- 0.03 mumol of glucose/g of liver/min at 2 mM fructose and 30 mM glucose, p less than 0.001). To better understand this synergism, intermediary substrate levels were compared at steady state with various glucose levels in the absence and in the presence of 2 mM fructose. Independent of fructose, hepatic glucose and glucose 6-phosphate increased proportionally when glucose level in the medium was raised (r = 0.86, p less than 0.001). Unlike glucose 6-phosphate, UDP-glucose did not consistently increase with glucose (p greater than 0.10); in fact, there was a small decrease at a very high glucose level (30 mM), a result consistent with the well-established activation of glycogen synthase by glucose. With elevated glucose, the level of glucose 6-phosphate was strongly correlated with glycogen content (r = 0.71, p less than 0.01, slope = 32). Adding fructose increased the "efficiency" of glucose 6-phosphate to glycogen conversion: the effect of a given increment in glucose 6-phosphate upon glycogen accumulation was increased 2.6-fold (r = 0.73, p less than 0.01, slope = 86). A kinetic modeling approach was used to investigate the mechanisms by which fructose synergized glycogen accumulation when glucose was elevated. Based on steady-state hepatic substrate levels, net hepatic glucose output, and net glycogen synthesis rate, the model estimated the rate constants of major enzymes and individual fluxes in the glycogen metabolic pathway. Modeling analysis is consistent with the following scenario: glycogen synthase is activated by glucose, whereas glucose-6-phosphatase was inhibited. In addition, the model supports the hypothesis that fructose synergizes net glycogen accumulation due to suppression of phosphorylase. Overall, our analysis suggests that glucose enhances the metabolic flux to glycogen by inducing a build up of glucose 6-phosphate via combined effects of mass action and glucose-6-phosphatase inhibition and activating glycogen synthase and that fructose enhances glycogen accumulation by retaining glycogen via phosphorylase inhibition.     

Evidence for the cerebral uptake in vivo from two pools of glucose and the role of glucose-6-phosphatase in removing excess substrate from brain

We propose the following scheme for cerebral uptake and overall metabolism of glucose in vivo: that brain selects from two pools of glucose anomers in arterial blood, that it takes up excess glucose, that glucose enters the brain tissue as glucose-6-phosphate through the actions of mutarotase and hexokinase, that some glucose-6-phosphate becomes metabolized to CO₂ and some becomes incorporated into brain carbon pools, and that excess glucose-6-phosphate leaves brain through glucose-6-phosphatase and mutarotase activities. This results from our observations in arterio-venous studies for the determination of cerebral metabolism in humans in vivo that the cerebral uptake of [¹⁴C]glucose often appeared to differ from that of unlabeled glucose. With rapidly falling arterial radioactivity, unlabeled glucose uptake was more than [¹⁴C]glucose. With rising arterial radioactivity, [¹⁴C]glucose extraction extraction exceeded unlabeled glucose. Studies with [¹⁴C]glucose-6-phosphate suggested that glucose-6-phosphatase in brain removes excess substrate by dephosphorylation. However, when arterial [¹⁴C]glucose increased slowly, [¹⁴C]glucose uptake varied considerably and the data resembled human cerebral metabolism of glucose anomers. An experiment employing [¹³C]glucose and NMR provided further support for our proposed scheme.     

Glycogen is the primary source of glucose during the lag phase of E. coli proliferation

In the studies of Escherichia coli (E. coli), metabolomics analyses have mainly been performed using steady state culture. However, to analyze the dynamic changes in cellular metabolism, we performed a profiling of concentration of metabolites by using batch culture. As a first step, we focused on glucose uptake and the behavior of the first metabolite, G6P (glucose-6-phosphate). A computational formula was derived to express the glucose uptake rate by a single cell from two kinds of experimental data, extracellular glucose concentration and cell growth, being simulated by Cell Illustrator. In addition, average concentration of G6P has been measured by CE-MS. The existence of another carbon source was suggested from the computational result. After careful comparison between cell growth, G6P concentration, and the computationally obtained curve of glucose uptake rate, we predicted the consumption of glycogen in lag phase and its accumulation as an energy source in an E. coli cell for the next proliferation. We confirmed our prediction experimentally. This behavior indicates the importance of glycogen participation in the lag phase for the growth of E. coli. This article is part of a Special Issue entitled: Computational Methods for Protein Interaction and Structural Prediction.     

Characterization of 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) as an inhibitor of brain glycogen shunt activity

The pharmacological properties of 1,4-dideoxy-1,4-imino- d -arabinitol (DAB), a potent inhibitor of glycogen phosphorylase and synthase activity in liver preparations, were characterized in different brain tissue preparations as a prerequisite for using it as a tool to investigate brain glycogen metabolism. Its inhibitory effect on glycogen phosphorylase was studied in homogenates of brain tissue and astrocytes and IC₅₀-values close to 400 nM were found. However, the concentration of DAB needed for inhibition of glycogen shunt activity, i.e. glucose metabolism via glycogen, in intact astrocytes was almost three orders of magnitude higher. Additionally, such complete inhibition required a pre-incubation period, a finding possibly reflecting a limited permeability of the astrocytic membrane. DAB did not affect the accumulation of 2-deoxyglucose-6-phosphate indicating that the transport of DAB is not mediated by the glucose transporter. DAB had no effect on enzymes involving glucose-6-phosphate, i.e. glucose-6-phosphate dehydrogenase, phosphoglucoisomerase and hexokinase. Furthermore, DAB was evaluated in a functional preparation of the isolated mouse optic nerve, in which its presence severely reduced the ability to sustain evoked compound action potentials in the absence of glucose, a condition in which glycogen serves as an important energy substrate. Based on the experimental findings, DAB can be used to evaluate glycogen shunt activity and its functional importance in intact brain tissue and cells at a concentration of 300–1000 μM and a pre-incubation period of 1 h.     

Metabolic impact of overexpression of liver glycogen synthase with serine-to-alanine substitutions in rat primary hepatocytes

To investigate the effect of elevation of liver glycogen synthase (GYS2) activity on glucose and glycogen metabolism, we performed adenoviral overexpression of the mutant GYS2 with six serine-to-alanine substitutions in rat primary hepatocytes. Cell-free assays demonstrated that the serine-to-alanine substitutions caused constitutive activity and electrophoretic mobility shift. In rat primary hepatocytes, overexpression of the mutant GYS2 significantly reduced glucose production by 40% and dramatically induced glycogen synthesis via the indirect pathway rather than the direct pathway. Thus, we conclude that elevation of glycogen synthase activity has an inhibitory effect on glucose production in hepatocytes by shunting gluconeogenic precursors into glycogen. In addition, although intracellular compartmentation of glucose-6-phosphate (G6P) remains unclear in hepatocytes, our results imply that there are at least two G6P pools via gluconeogenesis and due to glucose phosphorylation, and that G6P via gluconeogenesis is preferentially used for glycogen synthesis in hepatocytes.     

Relationship of Glycolytic Intermediates, Glycolytic Enzymes, and Ammonia to Glycogen Metabolism During Sporulation in the Yeast Saccharomyces cerevisiae

To identify the factors which control glycogen synthesis in Saccharomyces cerevisiae, we have studied the regulation of glycogen metabolism during sporulation, since in vivo glycogen has been reported to undergo significant changes in concentration during this process. We examined the concentration of a number of key glycolytic intermediates and enzymes in strains that sporulate at different rates and those that are deficient in sporulation. There were no significant changes found in the adenylate energy charge or cyclic AMP levels throughout sporulation. Although significant alterations occurred in the levels of glucose-6-phosphate, fructose-6-phosphate, fructose-1,6-bisphosphate, phosphoenolpyruvate, and ATP during sporulation, only the fourfold increase in fructose-1,6-bisphosphate appeared to correlate with glycogen synthesis in all of the strains examined. Only limited changes occurred in the level of a number of glycolytic and gluconeogenic enzymes which were examined during this process. Intracellular glucose content underwent a dramatic 30- to 40-fold increase in sporulating cells. Comparison of strains with different rates of sporulation demonstrated that this increase in glucose content coincides with the time of glycogen degradation in each strain. Both the increase in glucose content and the degradation of accumulated glycogen were not observed in nonsporulating alpha/alpha strains, or in cells incubated in NH(4) (+) supplemented sporulation medium. Although glucose appears to be the direct product of glycogen degradation, a 10-fold increase in a nonspecific alkaline phosphatase occurs at this time, which may be degrading phosphorylated sugars to glucose. All of the strains examined released extracellular glucose while suspended in acetate sporulation medium. It is concluded that most of the changes in the glycolytic pathway that occur during sporulation, with the exception of glycogen degradation and the concomitant increase in intracellular glucose pools, are a response to the transfer to sporulation medium and are independent of sporulation-specific processes. Inhibition of sporulation with ammonium ions resulted in a different pattern of change in all of the glycolytic intermediates examined, including a twofold increase in cyclic AMP levels. Ammonia did not interfere with glycogen synthesis, but prevented sporulation-specific glycogen degradation. The levels of the glycolytic enzymes examined were not affected by ammonia.     

Insulin action in intact mouse diaphragm I.

Summary The incubation of intact mouse diaphragms with insulin caused a dose and time dependent increase in the independent activity of glycogen synthase in tissue extracts. 2-deoxyglucose (2–10 mm) alone markedly stimulated the conversion of glycogen synthase to the independent activity under conditions in which tissue ATP concentrations were not affected. The incubation of diaphragms with both insulin and 2-deoxyglucose resulted in a greater than additive effect. Insulin stimulated the uptake of 2-deoxyglucose into mouse diaphragms, accumulating as 2-deoxyglucose-6-phosphate. The accumulation of 2-deoxyglucose-6-phosphate correlated well with the increase in the independent activity of glycogen synthase and with the activation of glycogen synthase phosphatase in tissue extracts. The uptake of 3-0-methyl glucose was also markedly stimulated by insulin, without affecting the activity of glycogen synthase. Both glucose-6-phosphate and 2-deoxyglucose-6-phosphate stimulated the activation of endogenous glycogen synthase phosphatase activity in muscle homogenates. We conclude that insulin, in addition to its effects in the absence of exogenous sugars, increases the independent activity of glycogen synthase through increased sugar transport resulting in increased concentrations of sugar-phosphates which promote the activity of glycogen synthase phosphatase.Abbreviations GS Glycogen synthase - GS-I Glycogen synthase activity independent of G6P - GS-D Glycogen synthase activity dependent on G6P - G6P Glucose-6-phosphate - ATP Adenosine triphosphate - EDTA Ethylene diamine tetracetic acid - Mops Morpholinopropane sulfonic acid - 2DG 2-Deoxy glucose - 3-0-MG 3-0-Methyl glucose - tricine N-tris(Hydroxymethyl)methyl glycine Enzymes: Glycogen Synthase — UDPGlucose — Glycogen Glucosyl — Transferase (EC 2.4.1.11) J. Larner is an established investigator of the American Diabetes Association.     

Phosphoglucomutase1 is necessary for sustained cell growth under repetitive glucose depletion

Phosphoglucomutase (PGM)1 catalyzes the reversible conversion reaction between glucose-1-phosphate (G-1-P) and glucose-6-phosphate (G-6-P). Although both G-1-P and G-6-P are important intermediates for glucose and glycogen metabolism, the biological roles and regulatory mechanisms of PGM1 are largely unknown. In this study we found that T553 is obligatory for PGM1 stability and the last C-terminal residue, T562, is critical for its activity. Interestingly, depletion of PGM1 was associated with declined cellular glycogen content and decreased rates of glycogenolysis and glycogenesis. Furthermore, PGM1 depletion suppressed cell proliferation under long-term repetitive glucose depletion. Our results suggest that PGM1 is required for sustained cell growth during nutritional changes, probably through regulating the balance of G-1-P and G-6-P in order to satisfy the cellular demands during nutritional stress.     

Metabolism of glucose by unicellular blue-green algae

Summary A facultative photo- and chemoheterotroph, the unicellular bluegreen alga Aphanocapsa 6714, dissimilates glucose with formation of CO₂ as the only major product. A substantial fraction of the glucose consumed is assimilated and stored as polyglucose (probably glycogen). The oxidation of glucose proceeds through the pentose phosphate pathway. The first enzyme of this pathway, glucose-6-phosphate dehydrogenase, is partly inducible. In addition, the rate of glucose oxidation is controlled, at the level of glucose-6-phosphate dehydrogenase function, by the intracellular level of an intermediate of the Calvin cycle, ribulose-1,5-diphosphate, which is a specific allosteric inhibitor of this enzyme. As a consequence, the rate of glucose oxidation is greatly reduced by illumination, an effect reversed by the presence of DCMU, an inhibitor of photosystem II.Two obligate photoautotrophs, Synechococcus 6301 and Aphanocapsa 6308, produce CO₂ from glucose at extremely low rates, although their levels of pentose pathway enzymes and of hexokinase are similar to those in Aphanocapsa 6714. Failure to grow with glucose appears to reflect the absence of an effective glucose permease. A general hypothesis concerning the primary pathways of carbon metabolism in blue-green algae is presented.Abbreviations A (U)DPG ADP-glucose or UDP-glucose - G-1-P glucose-1-phosphate - G-6-P glucose-6-phosphate - G_(int.) intracellular glucose - F-6-P fructose-6-phosphate - 6-PG 6-phosphogluconate - Ru-5-P ribulose-5-phosphate - RUDP ribulose-1,5-diphosphate - PGA 3-phosphoglycerate - GAP glyceraldehyde-3-phosphate     

Do non-equilibrium reactions in the glucose-to-triose phosphate pathway exist in the liver?

The circadian changes in the contents of intermediates of the initial reactions of the glycolytic pathway in pigeon liver were studied. the concentrations of glucose, glucose-6-phosphate, fructose-6-phosphate, fructose-1,6-diphosphate and triose phosphates were found to change synchronously, being maximal at the dark time and minimal during the light daytime. The glycogen content in the liver decreased steadily between 12.00 and 09.00. The diurnal variations in the concentrations of metabolite pairs (glucose and glucose-6-phosphate, glucose-6-phosphate and fructose-6-phosphate, fructose-6-phosphate and fructose-1.6-diphosphate, fructose-1.6-diphosphate and triose phosphates) appeared to correlate significantly. The results obtained suggest that in the liver at least there are no limiting i. e. physiologically non-equilibrium reactions in the carbohydrate metabolic pathway from glucose to triose phosphates.     

Glycogen synthetase and the control of glycogen synthesis in the cellular slime mould Dictyostelium discoideum during the growth (myxamoebal) phase

1. Myxamoebae of the cellular slime mould Dictyostelium discoideum Ax-2 that are grown in axenic medium containing 86mm-glucose have seven times the glycogen content of the same myxamoebae grown in the same medium but lacking added carbohydrate. 2. During the transition from the exponential to the stationary phase of growth in axenic medium containing glucose myxamoebae preferentially synthesize glycogen and can have as much as three times the glycogen content during the stationary phase as they have during the exponential phase of growth. 3. The rate of glycogen degradation by myxamoebae is, under all conditions of growth, small compared with the rate of glycogen accumulation and the changes in glycogen content thus reflect altered rates of glycogen synthesis. 4. There is no correlation between the rate of glycogen synthesis by myxamoebae and the glycogen synthetase content of the myxamoebae. 5. The activity of glycogen synthetase of D. discoideum is inhibited by a physiological concentration of ATP and this inhibition is overcome by glucose 6-phosphate. Both effects are especially marked at physiological concentrations of UDP-glucose. 6. The rate of glycogen accumulation by myxamoebae growing exponentially in axenic media can be satisfactorily accounted for in terms of the known intracellular concentrations of glucose 6-phosphate, UDP-glucose and glycogen synthetase. The rate-limiting factors controlling glycogen synthesis by the myxamoebae are apparently the substrate (UDP-glucose) and effector (glucose 6-phosphate and ATP) concentrations rather than the amount of the enzyme.     

Molecular pathology of glucose-6-phosphatase

It was known in the 1950s that hepatic microsomal glucose-6-phosphatase plays an important role in the regulation of blood glucose levels. All attempts since then to purify a single polypeptide with glucose-6-phosphatase activity have failed. Until recently, virtually nothing was known about the molecular basis of glucose-6-phosphatase or its regulation. Recent studies of the type 1 glycogen storage diseases, which are human genetic deficiencies that result in impaired glucose-6-phosphatase activity, have greatly increased our understanding of glucose-6-phosphatase. Glucose-6-phosphatase has been shown to comprise at least five different polypeptides, the catalytic subunit of glucose-6-phosphatase with its active site situated in the lumen of the endoplasmic reticulum; a regulatory Ca2+ binding protein; and three transport proteins, T1, T2, and T3, which respectively allow glucose-6-phosphate, phosphate, and glucose to cross the endoplasmic reticulum membrane. Purified glucose-6-phosphatase proteins, immunospecific antibodies, and improved assay techniques have led to the diagnosis of a variety of new type 1 glycogen storage diseases. Recent studies of the type 1 glycogen storage diseases have led to a much greater understanding of the role and regulation of each of the glucose-6-phosphatase proteins.     

Calcium control of glycogen synthase activities in mouse diaphragms, rat adipocytes and rat hepatocytes

The following article provides evidence that cellular calcium controls the activity of glycogen synthase in all three major glycogen storage tissues; muscle, fat, and liver. Depletion of cellular calcium resulted in a moderate increase of glycogen synthase %I activities in intact mouse diaphragms, in isolated rat adipocytes, and in rat hepatocytes. The increase in %I activity of glycogen synthase was more pronounced when the uridine di-phosphoglucose concentration in the glycogen synthase assay was lowered from 4.4 mM to 0.2 mM. Calcium depletion resulted in an approximately two-fold decrease in the Ka values for glucose-6-phosphate in all three tissues. The activities of glycogen synthase also correlated well with the content of cell-associated calcium in rat hepatocytes. The glucose-6-phosphate independent activities of glycogen synthase in extracts of calcium-replete and calcium-depleted tissue approached the same value following the exposure to crude phosphoprotein phosphatase. The activities of glycogen phosphorylase decreased in calcium-depleted tissues and cells. Insulin stimulated the activity of glycogen synthase in muscle and fat in the absence of added sugar and in the absence of extracellular calcium. It is concluded that glycogen synthase is under the control of calcium in the three main glycogen storage tissues. The actions of calcium are probably mediated through the actions of calcium-sensitive protein kinase(s).