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PubMed1978

In vivo glucose-, glucagon-, and cAMP-induced changes in liver glycogen synthase phosphatase activity.

In normal fed rats, glycogen synthase D phosphatase activity in a glycogen pellet preparation was only partially inhibited (approximately 50%) by high concentrations of EDTA. However, the proportion of phosphatase activity inhibited by EDTA was markedly and rapidly (15 s) increased following glucagon or cAMP administration. Epinephrine administration did not alter the proportion of activity inhibited by EDTA. Glucose administration rapidly (2 min) reduced the proportion of synthase phosphatase activity inhibitable by EDTA. That is, the effect of glucose was just the opposite of that produced by glucagon or cAMP. Insulin administration had no effect on phosphatase activity. Synthase phosphatase activity assayed in the absence of EDTA was similar in all groups except for a moderate increase after glucose administration. Addition of Mg2+ completely reversed EDTA inhibition. Phosphorylase phosphatase activity in each group was not modified by addition of EDTA, although the percentage of phosphorylase in the alpha form was higher in glucagon-treated and lower in the glucose-treated animals as expected. These data suggest the presence of rapidly interconvertible forms of either synthase phosphatase or its substrate synthase D, detectable as a change in EDTA inhibitability and subject to glucose and glucagon control.

Animals↗
PubMed1981

Liver glycogen metabolism in chicken. Activation prior to triiodothyronine-induced protein synthesis enhancement.

The depletion in glycogen levels, before enhancement in proteins synthesis, after a single Triiodothyronine (T3) injection has been previously shown in chickens and rats. The same injection of 4 microgram T3 into newborn chicken increases phosphorylase a activity in liver within 4 to 10 hours, showing a maximum at 6 hours. The lag time and the maximum relate well to glycogen level depletion in chicken. However, when phosphorylase is measured in the presence of AMP, the activity is not affected by the injection to T3. Liver cAMP levels are also increased 4 h after T3 injection, but the increase is lower than for the phosphorylase a. Glucose-6-Phosphatase an enzyme of the liver carbohydrate metabolism, which is known to be activated by a single T3 injection after the protein synthesis enhancement in rats, has a lag time of 14 hours, which occurs later than the depletion in glycogen levels.

Animals↗
PubMed1997

Isolation and nucleotide sequence of human liver glycogen debranching enzyme mRNA: identification of multiple tissue-specific isoforms.

Glycogen storage disease type III (GSD-III) is caused by a deficiency of glycogen debranching enzyme (AGL) activity. Patients are found to have deficient AGL activity in both muscle and liver, and also enzyme deficiency in the liver, but not in muscle. To determine the molecular basis of enzymatic variability in GSD-III and to elucidate the mechanism for control of tissue-specific expression of AGL, we previously cloned and sequenced the human muscle AGL cDNA. Here we report the isolation and nucleotide sequence of liver AGL cDNA and the tissue distribution of the isoform mRNAs. The predominant form of human liver AGL cDNA (isoform 1) contained 400 bp of 5' untranslated region, 4596 bp of coding region, and 2371 bp of 3' untranslated region. The liver AGL mRNA sequence was identical to the previously published muscle sequence (isoform 5) for most of the length, except for the 5' end, in which the liver sequence diverged completely from the muscle sequence. The divergence began with the transcription start point and extended 82 nucleotides downstream from the translation initiation codon. Six isoforms of AGL mRNA were identified and sequenced from liver and muscle. These isoforms differed only at the 5' end. Tissue distribution studies showed that liver, kidney and lymphoblastoid cells expressed predominantly isoform 1; whereas muscle and heart expressed not only isoform 1, but also muscle-specific isoform mRNAs (isoforms 2, 3 and 4). Defining tissue-specific AGL isoform mRNAs is an important step toward understanding the molecular basis of enzymatic variability in GSD-III.

Amino Acid Sequence↗
PubMed1985

In vivo phosphorylation of liver glycogen synthase. Isolation of the 32P-labeled enzyme and studies on the nature of the bound [32P]phosphates.

Our previous study (Tan, A. W. H., and Nuttall, F. Q. (1983) J. Biol. Chem. 258, 9624-9630) indicated that liver synthase D contained a large number of endogenous phosphates, 12 of which were stable and 6 labile to alkali treatment. We wished to investigate the nature of the phosphates on synthase which became isotopically labeled when inorganic [32P]phosphate was given either to intact rats or to isolated liver cells. An antibody against liver synthase D was used for the isolation of synthase. The antibody recognized both the phosphorylated and dephosphorylated form of the enzyme, native as well as partially cleaved species. A large enzyme form, with Mr of 90,000 as well as one with Mr of 73,000 was observed. A 61% decrease in [32P]phosphate was found in synthase when prelabeled liver cells were treated with glucose, whereas a 25% increase was seen in cells treated with glucagon. After [32P]synthase D was converted to synthase I by synthase phosphatase, 95% of the [32P]phosphate was lost. All of the bound [32P]phosphates were found to be labile to alkali. Thus, under the in vivo conditions used, the [32P]phosphates incorporated into synthase were characterized by their fast turnover rate, alkali lability and susceptibility to the action of synthase phosphatase, both in vivo and in vitro. These criteria serve to distinguish them from the slower turning-over, alkali-stable phosphates found previously in both synthases D and I.

Animals↗
PubMed1983

Endogenous phosphates on liver glycogen synthase D and synthase I. Studies on the number and location.

Synthase D and synthase I of high specific activity have been isolated from rabbit liver, with subunit Mr = 78,000 and 85,000, respectively. Minor bands of Mr = 78,000 and 71,000 were also observed, and these were shown to be proteolytic products containing synthase activity. Incubation with trypsin resulted in no activity changes, but there was a conversion of synthase I to the Mr = 78,000 species. Incubation with chymotrypsin produced no change in total synthase activity, but both enzymes were converted to the Mr = 71,000 species. Synthase I, after chymotrypsin treatment, became dependent on glucose-6-P for activity. Kinetic studies indicated that the chymotrypsin-treated synthase D and synthase I were not the same species. Synthase D contained 5.7 mol of alkali-labile phosphate/subunit and synthase I, 2.9 mol/subunit. With trypsin treatment there was little loss of phosphate from synthase. With chymotrypsin treatment, there was a loss of 3.1 and 1.5 mol of phosphate/subunit from synthase D and I, respectively. Some conversion to synthase I still occurred when chymotrypsin-treated synthase D was incubated with crude extract, suggesting that the phosphates which remained were still removable by synthase phosphatase. The presence of additional alkali-stable phosphates was indicated when the synthase samples were ashed. The total phosphates were 17.3 mol/subunit for synthase D and 13.4 mol/subunit for synthase I. Our results indicate that the liver synthase enzymes are highly phosphorylated and extremely susceptible to endogenous and exogenous proteolysis. The alkali-labile phosphates are distributed in chymotrypsin-sensitive as well as chymotrypsin-insensitive sites. Both sites are involved in the conversion of synthase D to synthase I.

Animals↗
PubMed1988

The effect of glucose on liver glycogen synthase phosphatase activity in the presence of ATP-Mg.

Glycogen particle synthase phosphatase activity is stimulated by glucose with an A0.5 of approximately 27 mM. The A0.5 is higher than the usual concentrations present in the liver. However, in vitro, certain methylxanthines such as caffeine or theophylline reduce the glucose A0.5 to approximately 10 mM, a concentration well within the normal range of liver glucose concentrations. Methylxanthines do not affect the maximum stimulation by glucose (2.3-fold greater than control rate). The phosphatase reaction also is inhibited by ATP-Mg (I0.5 = 0.1 mM). In the present studies, we have determined the interaction of these effectors. The presence of ATP-Mg at a concentration of 3 mM only slightly reduced the maximal stimulation by glucose. The A0.5 for glucose was unaffected (24 mM). The synergistic effect of caffeine with glucose also was not changed by the presence of ATP-Mg. The A0.5 for glucose was reduced to 11 mM, similar to that in the absence of ATP-Mg. In addition, maximum stimulation by glucose was unchanged. Similar results were obtained when theophylline replaced caffeine. We conclude that the ATP-Mg binding site on either the phosphatase or its substrate, synthase D, does not influence the glucose and methylxanthine binding sites. Effectively, ATP-Mg increased the range over which glucose stimulates the phosphatase activity. In the presence of ATP-Mg, the maximum stimulation by glucose is approximately 7-fold; whereas, in the absence of ATP-Mg it is approximately 2.3-fold. Thus, ATP-Mg may serve to increase the sensitivity of the synthase phosphatase reaction to glucose regulation under in vivo conditions.

Adenosine Triphosphate↗
PubMed1986

Sequence analysis of the cDNA encoding human liver glycogen phosphorylase reveals tissue-specific codon usage.

We have cloned the cDNA encoding glycogen phosphorylase (1,4-alpha-D-glucan:orthophosphate alpha-D-glucosyl-transferase, EC 2.4.1.1) from human liver. Blot-hybridization analysis using a large fragment of the cDNA to probe mRNA from rabbit brain, muscle, and liver tissues shows preferential hybridization to liver RNA. Determination of the entire nucleotide sequence of the liver message has allowed a comparison with the previously determined rabbit muscle phosphorylase sequence. Despite an amino acid identity of 80%, the two cDNAs exhibit a remarkable divergence in G+C content. In the muscle phosphorylase sequence, 86% of the nucleotides at the third codon position are either deoxyguanosine or deoxycytidine residues, while in the liver homolog the figure is only 60%, resulting in a strikingly different pattern of codon usage throughout most of the sequence. The liver phosphorylase cDNA appears to represent an evolutionary mosaic; the segment encoding the N-terminal 80 amino acids contains greater than 90% G+C at the third codon position. A survey of other published mammalian cDNA sequences reveals that the data for liver and muscle phosphorylases reflects a bias in codon usage patterns in liver and muscle coding sequences in general.

Animals↗
PubMed1988

Electrophoretic analysis of liver glycogen phosphorylase activation in the freeze-tolerant wood frog.

As an adaptation for overwinter survival, the wood frog, Rana sylvatica is able to tolerate the freezing of extracellular body fluids. Tolerance is made possible by the production of very high amounts of glucose in liver which is then sent to other organs where it acts as a cryoprotectant. Cryoprotectant synthesis is under the control of glycogen phosphorylase which in turn is activated in response to ice formation. To determine the mechanism of phosphorylase activation, a quantitative analysis of phosphorylase protein concentration and enzymatic activity in liver was carried out following separation of the phosphorylated a and nonphosphorylated b forms of the enzyme on native polyacrylamide gels. The results suggest that in gels, the b form is completely inactive, even in the presence of AMP and sodium sulfate, whereas the a form is active and stimulated 3-fold by these substances. Further, phosphorylase activation appears to arise solely from conversion of the b to a form of the enzyme without an increase in phosphorylase concentration or activation of a second isozyme. The quantitative analysis presented here should prove generally useful as a simple and rapid method for examining the physiological and genetic regulation of phosphorylase in animal cells.

Animals↗