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At least 685 records · Page 38Linked to original sources
PubMed1984

The synergistic action of caffeine or adenosine on glucose stimulation of liver glycogen synthase phosphatase activity.

Recent studies indicate that glucose directly stimulates synthase phosphatase activity in vitro but only at high, non-physiological concentrations. Present results demonstrate that at a physiological concentration glucose can be stimulatory, provided that an appropriate second effector is present. Caffeine and adenosine are examples of such effectors which act synergistically with glucose to enhance synthase phosphatase activity. Caffeine but not adenosine enhances glucose stimulation of phosphorylase phosphatase activity. In the absence of glucose, caffeine but not adenosine stimulates both synthase and phosphorylase phosphatase reactions. Thus, glucose regulation of glycogen synthase activation in vivo could require a second effector. Neither the identity nor source of such an effector is known. The putative regulator could be a mediator for a hormone such as insulin. The present work suggests that the chemical nature of the effector might be that of a derivatized purine of which nucleosides are an example.

Adenine↗
PubMed1985

Glucagon-stimulated phosphorylation of rat liver glycogen synthase in isolated hepatocytes.

Addition of glucagon (20 nM) to the isolated hepatocytes from 24-h starved male rats results in an inactivation of glycogen synthase. The A0.5 for glucose-6-P is increased 2-fold over the control but the S0.5 for UDP-glucose is not significantly affected. The glucagon-stimulated inactivation of glycogen synthase is also accompanied by a 60-120% increase in the phosphorylation of the synthase. Glycogen synthase labeled with 32P by incubation of the hepatocytes with [32P] PO4(3-) was recovered by immunoprecipitation and the resulting immunoprecipitate was subjected to tryptic digestion. Analysis of the 32P-labeled peptides reveals that the sites corresponding to those phosphorylated by cAMP-dependent protein kinase and glycogen synthase (casein) kinase-1 (Itarte, E., and Huang, K.-P. (1979) J. Biol. Chem. 254, 4052-4057) are rapidly phosphorylated in response to glucagon. These results demonstrate that glucagon not only triggers the activation of cAMP-dependent protein kinase through an increase in the intracellular level of cAMP but also, by an unknown mechanism, activates a Ca2+- and cAMP-independent protein kinase.

Animals↗
PubMed1988

A minimal model of liver glycogen metabolism; feasibility for predicting flux rates.

A minimal model of glycogen metabolism can allow the estimation of the flux rates in the glycogen pathway from the time course of the intermediates in the pathway, measured during substrate administration and hormonal stimulation. The comprehensive model of El-Refai & Bergman (Am. J. Physiol. 231, 1608, 1976) consisting of six compartments and 26 non-estimable parameters has successfully accounted for the responses of hepatic glycogenic intermediates in response to a glucose load in hepatocytes (Katz et al., J. biol. Chem. 253, 4530, 1978), in perfused liver (Nordlie et al., J. biol. Chem. 255, 1834, 1980) and during refeeding in vivo (Van DeWerve & Jeanrenaud, Am. J. Physiol. 247, E271, 1984). The comprehensive model is here reduced to a minimal model, consisting of five compartments representing extracellular and intracellular glucose, glucose-phosphate, uridine diphosphate glucose (UDPG), glycogen, and five parameters estimated from the hepatic response to a given stimulus. Estimation of these parameters requires the measurement of the net hepatic glucose balance, the net gluconeogenic flux, and the time course of glycogenic intermediates responding to a hormone or substrate stimulus. The hepatic glycogenolytic response predicted by the comprehensive model in response to an increase in glucagon is closely fitted by the minimal model. When Gaussian distributed random error was added, 0-5% SD in the glucose and glycogen compartments and 0-10% SD in the glucose-phosphate and UDPG compartments, the hepatic response predicted by the minimal model was virtually free of the added error, and the model parameters were found to be within 30% of their true values. When the minimal model was used to interpret the experimental response to an increase in glucose concentration it predicted that: (1) glucokinase can phosphorylate glucose at rates similar to maximal rates of net glycogen synthesis; (2) futile cycling at the glycogen/glucose-1-phosphate level can limit glycogen synthesis; and (3) glucose-6-phosphatase inhibition by glucose has a significant role in net glycogen synthesis.

Animals↗
PubMed1998

Characterization of human glycogenin-2, a self-glucosylating initiator of liver glycogen metabolism.

Glycogenin-2 is a recently described self-glucosylating protein potentially involved in the initiation of glycogen biosynthesis (Mu, J., Skurat, A. V., and Roach, P. J. (1997) J. Biol. Chem. 272, 27589-27597). In human liver extracts, most of the glycogenin-2 was only detectable after treatment with alpha-amylase. Similarly, purifed high Mr glycogen was only detected after release by alpha-amylase treatment. Based on analysis by polymerase chain reaction, the predominant isoform in liver was glycogenin-2beta. Glycogenin-2 was found in Ewing's sarcoma RD-ES cells where, however, it was not associated with high Mr carbohydrate. Both human liver and human RD-ES cell extracts also contained glycogenin-1. Glycogenin-1 and glycogenin-2 interact with one another, based on in vitro interactions and co-immunoprecipitation from liver and cell extracts. Mutation of Tyr-196 in glycogenin-2 to a Phe residue abolished the ability of glycogenin-2 to self-glucosylate but not to interact with glycogenin-1. Stable overexpression of glycogenin-2alpha in Rat-1 fibroblast cells resulted in a 5-fold increase in the level of glycogen present in the low speed pellet but little change in the low speed supernatant. This result is important since it indicates that the level of glycogenin-2 can determine glycogen accumulation and hence has the potential to control glycogen synthesis.

Animals↗
PubMed1987

Effect of epinephrine infusions imitating plasma epinephrine levels in immobilization stress and of fasting on the activity of key enzymes of liver glycogen metabolism.

In adult male rats anaesthetized with pentobarbital, intravenous infusion of 0.25 microgram kg-1 min-1 of epinephrine increased plasma epinephrine levels to more than 10 pmol ml-1 within 5 min, the values remaining constant during the next 20 min. On the other hand, a maximal increase of liver phosphorylase a was found after 5 min and this response was attenuated at 10 and 20 minutes in the fed rats. Fasting for 24 h extinguished the greatest part of this response. During infusion, a decrease of liver synthase a activity was observed in the fasted and of total synthase activity in both the fed and fasted animals. In fed rats an i.v. bolus of 0.50 microgram.kg-1 or doubling of the infusion rate for 5 min, both immediately after 20 min of infusion, led to maximal phosphorylase a activity again and to a decrease of synthase a activity. The levels of cAMP in the liver were increased by the infusion in one series and unchanged in another.

Animals↗
PubMed1993

Comparative analysis of species-independent, isozyme-specific amino-acid substitutions in mammalian muscle, brain and liver glycogen phosphorylases.

Mammalian glycogen phosphorylases exist as three isozymes, muscle, brain and liver, that exhibit different responses to activation by phosphorylation and AMP, regardless of species. To identify species-independent, amino-acid substitutions that may be important determinants in differential isozyme control, we have sequenced cDNAs containing the entire protein coding regions of rat muscle and brain phosphorylases. Nucleotide sequence comparisons with rat liver, rabbit muscle, and human muscle, brain and liver phosphorylase genes, indicate that muscle and brain isozymes are more related to each other than to the liver isozyme. Unlike the human isozymes, there is little difference in GC content of codons in the rat isozymes. In relation to the rabbit muscle isozyme three-dimensional structure, amino-acid sequence comparisons indicate that very few nonconservative isozyme-specific substitutions occur in buried and dimer contact residues. There is strict conservation of active site, pyridoxal-phosphate-binding site and nucleoside inhibitor site residues, as well as CAP loop and helix-2 residues that comprise the phosphorylation activation and part of the AMP binding sites. In contrast, five liver isozyme-specific substitutions occur between residues 313-325 and another at residue 78 which may be important determinants in the poor activation of this isozyme by AMP. Substitutions in the brain isozyme at residues 21-23, 405 and 435 may play a role in its poor response to activation by phosphorylation.

Amino Acid Sequence↗
PubMed1987

Regulation of rat liver glycogen phosphorylase concentration by in vivo relative levels of glucagon and insulin.

The concentrations of glycogen phosphorylase protein were determined by rocket immunoelectrophoresis in liver extracts from rats that had artificially induced altered hormonal patterns. These levels were compared with measurements of total phosphorylase activity. Minipump-induced chronic hyperglucagonemia and streptozotocin-induced diabetes resulted in 47% and 67% decreases, respectively, in total phosphorylase activity along with corresponding 52% and 68% drop, respectively, in phosphorylase protein levels. Insulin replacement in diabetic rats returned both parameters to control values. Minipump-induced hyperinsulinemia or injection of glucagon antiserum, T3, or propylthiouracil had no effect. The results of this study indicate that conditions which lead to an elevation of the glucagon to insulin molar ratio to values higher than 1.0 cause a significant decrease in the liver phosphorylase protein level.

Animals↗
PubMed1990

Oral administration of vanadate to streptozotocin-diabetic rats restores the glucose-induced activation of liver glycogen synthase.

Isolated hepatocytes from streptozotocin-diabetic rats failed to respond to a glucose load with an activation of glycogen synthase. This lesion was associated with severely decreased activities of glycogen-synthase phosphatase and of glucokinase. All these defects were abolished after consumption for 13-18 days of drinking water containing Na3VO4 (0.7 mg/ml), and they were partially restored after 3.5 days, when the blood glucose concentration was already normalized. In all conditions the maximal extent of activation of glycogen synthase in cells closely parallelled the activity of glycogen-synthase phosphatase.

Animals↗