Functional and structural characterization of the catalytic domain of the starch synthase III from Arabidopsis thaliana

Glycogen and starch are the major energy storage compounds in most living organisms. The metabolic pathways leading to their synthesis involve the action of several enzymes, among which glycogen synthase (GS) or starch synthase (SS) catalyze the elongation of the alpha-1,4-glucan backbone. At least five SS isoforms were described in Arabidopsis thaliana; it has been reported that the isoform III (SSIII) has a regulatory function on the synthesis of transient plant starch. The catalytic C-terminal domain of A. thaliana SSIII (SSIII-CD) was cloned and expressed. SSIII-CD fully complements the production of glycogen by an Agrobacterium tumefaciens glycogen synthase null mutant, suggesting that this truncated isoform restores in vivo the novo synthesis of bacterial glycogen. In vitro studies revealed that recombinant SSIII-CD uses with more efficiency rabbit muscle glycogen than amylopectin as primer and display a high apparent affinity for ADP-Glc. Fold class assignment methods followed by homology modeling predict a high global similarity to A. tumefaciens GS showing a fully conservation of the ADP-binding residues. On the other hand, this comparison revealed important divergences of the polysaccharide binding domain between AtGS and SSIII-CD.     

Crystal structure of an archaeal glycogen synthase: insights into oligomerization and substrate binding of eukaryotic glycogen synthases

Glycogen and starch synthases are retaining glycosyltransferases that catalyze the transfer of glucosyl residues to the non-reducing end of a growing alpha-1,4-glucan chain, a central process of the carbon/energy metabolism present in almost all living organisms. The crystal structure of the glycogen synthase from Pyrococcus abyssi, the smallest known member of this family of enzymes, revealed that its subunits possess a fold common to other glycosyltransferases, a pair of beta/alpha/beta Rossmann fold-type domains with the catalytic site at their interface. Nevertheless, the archaeal enzyme presents an unprecedented homotrimeric molecular arrangement both in solution, as determined by analytical ultracentrifugation, and in the crystal. The C-domains are not involved in intersubunit interactions of the trimeric molecule, thus allowing for movements, likely required for catalysis, across the narrow hinge that connects the N- and C-domains. The radial disposition of the subunits confers on the molecule a distinct triangular shape, clearly visible with negative staining electron microscopy, in which the upper and lower faces present a sharp asymmetry. Comparison of bacterial and eukaryotic glycogen synthases, which use, respectively, ADP or UDP glucose as donor substrates, with the archaeal enzyme, which can utilize both molecules, allowed us to propose the residues that determine glucosyl donor specificity.     

Phosphorylation and inactivation of liver glycogen synthase by liver protein kinases

A rapid method for purifying glycogen synthase a from rat liver was developed and the enzyme was tested as a substrate for nine different protein kinases, six of which were isolated from rat liver. The enzyme was phosphorylated on a 17-kDa CNBr fragment to approximately 1 phosphate/87-kDa subunit by phosphorylase b kinase from muscle or liver with a decrease in the activity ratio (-Glc-6-P/+Glc-6-P) from 0.95 to 0.6. Calmodulin-dependent glycogen synthase kinase from rabbit liver produced a similar phosphorylation pattern, but a smaller activity change. The catalytic subunit of beef heart cAMP-dependent protein kinase incorporated greater than 1 phosphate/subunit initially into a 17-kDa CNBr peptide and then into a 27-30-kDa CNBr peptide, with an activity ratio decrease to 0.5. Glycogen synthase kinases 3, 4, and 5 and casein kinase 1 were purified from rat liver. Glycogen synthase kinase 3 rapidly phosphorylated liver glycogen synthase to 1.5 phosphate/subunit with incorporation of phosphate into 3 CNBr peptides and a decrease in the activity ratio to 0.3. Glycogen synthase kinase 4 produced a pattern of phosphorylation and inactivation of liver synthase which was very similar to that caused by phosphorylase b kinase. Glycogen synthase kinase 5 incorporated 1 phosphate/subunit into a 24-kDa CNBr peptide, but did not alter the activity of the synthase. Casein kinase 1 phosphorylated and inactivated liver synthase with incorporation of phosphate into a 24-kDa CNBr peptide. This kinase and glycogen synthase kinase 4 were more active against muscle glycogen synthase. Calcium-phospholipid-dependent protein kinase from brain phosphorylated liver and muscle glycogen synthase on 17- and 27-kDa CNBr peptides, respectively. However, there was no change in the activity ratio of either enzyme. The following conclusions are drawn. 1) Liver glycogen synthase a is subject to multiple site phosphorylation. 2) Phosphorylation of some sites does not per se control activity of the enzyme under the assay conditions used. 3) Liver contains most, if not all, of the protein kinases active on glycogen synthase previously identified in skeletal muscle.     

Adrenaline-mediated glycogen phosphorylase activation is enhanced in rat soleus muscle with increased glycogen content

The effect of glycogen content on the activation of glycogen phosphorylase during adrenaline stimulation was investigated in soleus muscles from Wistar rats. Furthermore, adrenergic activation of glycogen phosphorylase in the slow-twitch oxidative soleus muscle was compared to the fast-twitch glycolytic epitrochlearis muscle. The glycogen content was 96.4 +/- 4.4 mmol (kg dw)(-1) in soleus muscles. Three hours of incubation with 10 mU/ml of insulin (and 5.5 mM glucose) increased the glycogen content to 182.2+/-5.9 mmol (kg dw)(-1) which is similar to that of epitrochlearis muscles (175.7+/-6.9 mmol (kg dw)(-1)). Total phosphorylase activity in soleus was independent of glycogen content. Adrenaline (10(-6) M) transformed about 20% and 35% (P < 0.01) of glycogen phosphorylase to the a form in soleus with normal and high glycogen content, respectively. In epitrochlearis, adrenaline stimulation transformed about 80% of glycogen phosphorylase to the a form. Glycogen synthase activation was reduced to low level in soleus muscles with both normal and high glycogen content. In conclusion, adrenaline-mediated glycogen phosphorylase activation is enhanced in rat soleus muscles with increased glycogen content. Glycogen phosphorylase activation during adrenaline stimulation was much higher in epitrochlearis than in soleus muscles with a similar content of glycogen.     

The protein phosphatases involved in cellular regulation. Evidence that dephosphorylation of glycogen phosphorylase and glycogen synthase in the glycogen and microsomal fractions of rat liver are catalysed by the same enzyme: protein phosphatase-1

Glycogen synthase (labelled in sites-3) and glycogen phosphorylase from rabbit skeletal muscle were used as substrates to investigate the nature of the protein phosphatases that act on these proteins in the glycogen and microsomal fractions of rat liver. Under the assay conditions employed, glycogen synthase phosphatase and phosphorylase phosphatase activities in both subcellular fractions could be inhibited 80-90% by inhibitor-1 or inhibitor-2, and the concentrations required for half-maximal inhibition were similar. Glycogen synthase phosphatase and phosphorylase phosphatase activities coeluted from Sephadex G-100 as broad peaks, stretching from the void volume to an apparent molecular mass of about 50 kDa. Incubation with trypsin decreased the apparent molecular mass of both activities to about 35 kDa, and decreased their I50 for inhibitors-1 and -2 in an identical manner. After tryptic digestion, the I50 values for inhibitors-1 and -2 were very similar to those of the catalytic subunit of protein phosphatase-1 from rabbit skeletal muscle. The glycogen and microsomal fractions of rat liver dephosphorylated the beta-subunit of phosphorylase kinase much faster than the alpha-subunit and dephosphorylation of the beta-subunit was prevented by the same concentrations of inhibitor-1 and inhibitor-2 that were required to inhibit the dephosphorylation of phosphorylase. The same experiments performed with the glycogen plus microsomal fraction from rabbit skeletal muscle revealed that the properties of glycogen synthase phosphatase and phosphorylase phosphatase were very similar to the corresponding activities in the hepatic glycogen fraction, except that the two activities coeluted as sharp peaks near the void volume of Sephadex G-100 (before tryptic digestion). Tryptic digestion of the hepatic glycogen and microsomal fractions increased phosphorylase phosphatase about threefold, but decreased glycogen synthase phosphatase activity. Similar results were obtained with the glycogen plus microsomal fraction from rabbit skeletal muscle or the glycogen-bound form of protein phosphatase-1 purified to homogeneity from the same tissue. Therefore the divergent effects of trypsin on glycogen synthase phosphatase and phosphorylase phosphatase activities are an intrinsic property of protein phosphatase-1. It is concluded that the major protein phosphatase in both the glycogen and microsomal fractions of rat liver is a form of protein phosphatase-1, and that this enzyme accounts for virtually all the glycogen synthase phosphatase and phosphorylase phosphatase activity associated with these subcellular fractions.     

Seasonal variations in frog liver glycogen metabolism (author's transl)

Glycogen metabolism in frog (Rana ridibunda) liver is subject to seasonal variations. Hepatic glycogen and glycogen synthase levels are highest in the fall and winter months and lowest in the summer months, whereas glycogen phosphorylase activity is highest in spring and summer and lowest in fall and winter months. Blood glucose levels show a clear increase during the months of March, June-July and November over the mean level for the rest of year (19.0 +/- 5.5 mg glucose/100 ml serum). Results indicate that the animal accumulated glycogen in the fall to be consumed during the winter. Glycogen levels are in direct proportion to glycogen synthase activity levels (I-form and total activity) and in inverse proportion to glycogen phosphorylase (phosphorylated form) activity levels, which would suggest that these enzymes exercise a direct control over glycogen levels.     

A new enzymatic activity participating in the initiation of glycogen biosynthesis in rat brain

A rat brain extract, able to synthesize from UDP-Glc an alpha-1,4-glucan covalently bound to a protein in the absence of added primer is described. The compound formed is precipitable by dilute trichloroacetic acid (TCA). In the presence of glycogen, added as primer, this molecule is enlarged and is not precipitable by TCA. Unprimed and primed activities differ in several aspects, such as the behavior in the presence of some effectors, and the optimum pH. Umprimed and primed activities presented two pHs optima, both sharing only one. The proteoglucans synthesized under the different pHs gave different patterns after analysis under denaturing PAGE and the oligosaccharides synthesized on the protein backbone differ in the glucosyl length. It is concluded that also in rat brain, the initiation process of glycogen biosynthesis is mediated through the formation of a glycoprotein. Our present results showed that the step of the putative "Glycogen Initiator" proposed by use before, requires two enzymes UDPGlc-transglucosylating activities, Glycogen Initiator 1 and Glycogen Initiator 2, before Glycogen Synthase in the alpha-1,4-glucosidic linkages formation.     

Isolation and characterization of cyclic AMP-independent glycogen synthase kinase from rat skeletal muscle

Glycogen synthase kinase was isolated from rat skeletal muscle. This kinase, which is cyclic nucleotide-independent and calcium-independent, was separated from phosphorylase kinase, cyclic AMP-dependent protein kinase and phosvitin kinase by phosphocellulose chromatography. Gel filtration on Sephadex G-100 resolved the glycogen synthase kinase into two fractions with apparent molecular weights of 68 000 (peak I) and 52 000 (peak II). This step also separated glycogen synthase kinase from the catalytic subunit of the cyclic AMP-dependent protein kinase, which had an apparent molecular weight of 39 000. Peak II glycogen synthase kinase activity was not affected by the addition of calcium, EGTA or a number of cyclic nucleotides. In addition to ATP, dATP would serve as the phosphate donor. Other trinucleotides tested were either poor or ineffective substrates. Activity was about 5-fold greater with Mg2+ than with Mn2+. Glycogen stimulated activity about 25%. Modifications of the methods of Soderling et al. ((1970) J. Biol. Chem. 245, 6317--6328) and Nimmo et al. ((1976) Eur. J. Biochem. 68, 21--30) were developed for purification of glycogen synthease (UDPglucose:glycogen 4-alpha D-glucosyltransferase, EC 2.4.1.11) to specific activity of 35 units/mg of protein. Using this preparation of glycogen synthase as substrate, the phosphorylation and inactivation catalyzed by glycogen synthase kinase was compared to that catalyzed by cyclic AMP-dependent protein kinase or phosphorylase kinase. Each of the kinases had different specificities for phosphorylation sites on glycogen synthase.     

Development of glycogen and phospholipid metabolism in fetal and newborn rat lung.

Glucose, a major metabolic substrate for the mammalian fetus, probably makes significant contributions to surface active phospholipid synthesis in adult lung. We examined the developmental patterns of glycogen content, glycogen synthase activity, glycogen phosphorylase activity and glucose oxidation in fetal and newborn rat lung. These patterns were correlated with the development of phosphatidylcholine synthesis, content and the activities of enzymes involved in phosphatidylcholine synthesis. Fetal lung glycogen concentration increased until day 20 of gestation (term is 22 days) after which it declined to low levels. Activity of both glycogen synthase I and total glycogen synthase (I + D) in fetal lung increased late in gestation. Increased lung glycogen concentration preceded changes in enzyme activity. Glycogen phosphorylase a and total glycogen phosphorylase (a + b) activity in fetal lung increased during the period of prenatal glycogen depletion. The activity of the pentose phosphate pathway, as measured by the ratio of CO2 derived from oxidation of C1 and C6 of glucose, declined after birth. Fetal lung total phospholipid, phosphatidycholine and disaturated phosphatidylcholine content increased by 60, 90 and 180%, respectively, between day 19 of gestation and the first postnatal day. Incorporation of choline into phosphatidylcholine and disaturated phosphatidylcholine increased 10-fold during this time. No changes in phosphatidylcholine enzyme activities were noted during gestation, but both choline phosphate cytidylyltransferase and phosphatidate phosphatase activity increased after birth. The possible contributions of carbohydrate derived from fetal lung glycogen to phospholipid synthesis are discussed.     

Inactivation of phosphorylase is a major component of the mechanism by which insulin stimulates hepatic glycogen synthesis.

Multiple signalling pathways are involved in the mechanism by which insulin stimulates hepatic glycogen synthesis. In this study we used selective inhibitors of glycogen synthase kinase-3 (GSK-3) and an allosteric inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a, to determine the relative contributions of inactivation of GSK-3 and dephosphorylation of phosphorylase a as alternative pathways in the stimulation of glycogen synthesis by insulin in hepatocytes. GSK-3 inhibitors (SB-216763 and Li+) caused a greater activation of glycogen synthase than insulin (90% vs. 40%) but a smaller stimulation of glycogen synthesis (30% vs. 150%). The contribution of GSK-3 inactivation to insulin stimulation of glycogen synthesis was estimated to be less than 20%. Dephosphorylation of phosphorylase a with CP-91149 caused activation of glycogen synthase and translocation of the protein from a soluble to a particulate fraction and mimicked the stimulation of glycogen synthesis by insulin. The stimulation of glycogen synthesis by phosphorylase inactivation cannot be explained by either inhibition of glycogen degradation or activation of glycogen synthase alone and suggests an additional role for translocation of synthase. Titrations with the phosphorylase inactivator showed that stimulation of glycogen synthesis by insulin can be largely accounted for by inactivation of phosphorylase over a wide range of activities of phosphorylase a. We conclude that a signalling pathway involving dephosphorylation of phosphorylase a leading to both activation and translocation of glycogen synthase is a critical component of the mechanism by which insulin stimulates hepatic glycogen synthesis. Selective inactivation of phosphorylase can mimic insulin stimulation of hepatic glycogen synthesis.     

Effect of Diabetes on Glycogen Metabolism in Rat Retina

Glucose is the main fuel for energy metabolism in retina. The regulatory mechanisms that maintain glucose homeostasis in retina could include hormonal action. Retinopathy is one of the chemical manifestations of long-standing diabetes mellitus. In order to better understand the effect of hyperglycemia in retina, we studied glycogen content as well as glycogen synthase and phosphorylase activities in both normal and streptozotocin-induced diabetic rat retina and compared them with other tissues. Glycogen levels in normal rat retina are low (46 +/- 4.0 nmol glucosyl residues/mg protein). However, high specific activity of glycogen synthase was found in retina, indicating a substantial capacity for glycogen synthesis. In diabetic rats, glycogen synthase activity increased between 50% and 100% in retina, brain cortex and liver of diabetic rats, but only retina exhibited an increase in glycogen content. Although, total and phosphorylated glycogen synthase levels were similar in normal and diabetic retina, activation of glycogen synthase by glucose-6-P was remarkable increased. Glycogen phosphorylase activity decreased 50% in the liver of diabetic animals; it was not modified in the other tissues examined. We conclude that the increase in glycogen levels in diabetic retina was due to alterations in glycogen synthase regulation.     

The activity of glycogen synthase phosphatase limits hepatic glycogen deposition in the adrenalectomized starved rat.

Hepatocytes from adrenalectomized 48 h-starved rats responded to increasing glucose concentrations with a progressively more complete inactivation of phosphorylase. Yet no activation of glycogen synthase occurred, even in a K+-rich medium. Protein phosphatase activities in crude liver preparations were assayed with purified substrates. Adrenalectomy plus starvation decreased synthase phosphatase activity by about 90%, but hardly affected phosphorylase phosphatase activity. Synthase b present in liver extracts from adrenalectomized starved rats was rapidly and completely converted into the a form on addition of liver extract from a normal fed rat. Glycogen synthesis can be slowly re-induced by administration of either glucose or cortisol to the deficient rats. In these conditions there was a close correspondence between the initial recovery of synthase phosphatase activity and the amount of synthase a present in the liver. The latter parameter was strictly correlated with the measured rate of glycogen synthesis in vivo. The decreased activity of synthase phosphatase emerges thus as the single factor that limits hepatic glycogen deposition in the adrenalectomized starved rat.     

NMR measurements of in vivo myocardial glycogen metabolism

Using 13C and 1H NMR we measured the rate of glycogen synthesis (0.23 +/- 0.10 mumol/min gram wet weight tissue (gww) in rat heart in vivo during an intravenous infusion of D-[1-13C]glucose and insulin. Glycogen was observed within 10 min of starting and increased linearly throughout a 50-min infusion. This compared closely with the average activity of glycogen synthase I (0.22 +/- 0.03 mumol/min gww) measured at physiologic concentrations of UDP-glucose (92 microM) and glucose-6-phosphate (110 microM). When unlabeled glycogen replaced D-[1-13C]glucose in the infusate after 50 min the D-[1-13C]glycogen signal remained stable for another 60 min, indicating that no turnover of the newly synthesized glycogen had occurred. Despite this phosphorylase a activity in heart extracts from rats given a 1 h glucose and insulin infusion (3.8 +/- 2.4 mumol/min gww) greatly exceeded the total synthase activity and if active in vivo should promote glycogenolysis. We conclude that during glucose and insulin infusion in the rat: (a) the absolute rate of myocardial glycogen synthesis can be measured in vivo by NMR; (b) glycogen synthase I can account for the observed rates of heart glycogen synthesis; (c) there is no futile cycling of glucose in and out of heart glycogen; and (d) the activity of phosphorylase a measured in tissue extracts is not reflected in vivo. These studies raise the question whether significant regulation of phosphorylase a activity in vivo is mediated by factors in addition to its phosphorylation state.     

Control of glycogen synthesis is shared between glucose transport and glycogen synthase in skeletal muscle fibers

The effects of transgenic overexpression of glycogen synthase in different types of fast-twitch muscle fibers were investigated in individual fibers from the anterior tibialis muscle. Glycogen synthase was severalfold higher in all transgenic fibers, although the extent of overexpression was twofold greater in type IIB fibers. Effects of the transgene on increasing glycogen and phosphorylase and on decreasing UDP-glucose were also more pronounced in type IIB fibers. However, in any grouping of fibers having equivalent malate dehydrogenase activity (an index of oxidative potential), glycogen was higher in the transgenic fibers. Thus increasing synthase is sufficient to enhance glycogen accumulation in all types of fast-twitch fibers. Effects on glucose transport and glycogen synthesis were investigated in experiments in which diaphragm, extensor digitorum longus (EDL), and soleus muscles were incubated in vitro. Transport was not increased by the transgene in any of the muscles. The transgene increased basal [(14)C]glucose into glycogen by 2.5-fold in the EDL, which is composed primarily of IIB fibers. The transgene also enhanced insulin-stimulated glycogen synthesis in the diaphragm and soleus muscles, which are composed of oxidative fiber types. We conclude that increasing glycogen synthase activity increases the rate of glycogen synthesis in both oxidative and glycolytic fibers, implying that the control of glycogen accumulation by insulin in skeletal muscle is distributed between the glucose transport and glycogen synthase steps.     

Activity of glycogen metabolizing enzymes in glucose deprived HT 29 adenocarcinoma cell-line

When deprived of glucose, the cultured HT 29 adenocarcinoma cells are able to mobilize their glycogen within 4 hours. Glycogen phosphorylase is strongly activated during the first hour of glucose starvation. Then, while the a/a + b ratio for phosphorylase is declining, glycogen synthase is partially converted into the a form; this conversion does occur although glycogen phosphorylase is far from being totally inactivated. After 4 hours, activity of both a and total forms of glycogen synthase decrease. Cell UDP-glucose and glucose-6-P levels are declining during the 24 hours period of glucose starvation. Cell ATP content decreases by only 50 percent over the same period of time.     

Activity of enzymes regulating glycogen metabolism in perfused muscle-cuticle sections of Ascaris suum (Nematoda)

A new perfusion system has been developed in which muscle-cuticle sections of Ascaris suum were perfused, enabling study of enzymes in vitro. Using this technique the activity of the regulatory enzymes glycogen synthase and glycogen phosphorylase was determined, and the level of glycogen in the muscle was assessed. During starvation, 98% of glycogen synthase was in the inactive D-form, and 80% of the glycogen phosphorylase activity was in the active a-form. When the ascarid muscle section was perfused with 27 mM glucose, 13.1% of the glycogen synthase was in the active I-form, whereas phosphorylase a-levels dropped to 46% and glycogen was synthesized at a linear rate of 12 mg/g/hr or 1.23 mumoles/min/g muscle-cuticle. ATP levels (3.71 +/- 0.32 mM) remained unchanged over a 4-hr perfusion period with an adenylate energy charge of 0.82. Fructose supported glycogen synthesis, though not as well as glucose. Galactose, mannose, and trehalose did not support glycogen synthesis. The new perfusion system should be useful in future, similar studies on Ascaris.     

On the activities of glycogen phosphorylase and glycogen synthase in the liver of the rat

A procedure was developed for determination of glycogen synthase and phosphorylase activities in liver after various in vivo physiological treatments. Liver samples were obtained from anaesthetised rats by freeze-clamping in situ. Other procedures were shown to stimulate the activity of phosphorylase and depress the activity of glycogen in the liver. The direction of glycogen metabolism appears to be regulated by the relative proportions of the two enzymes, as shown by a strong positive correlation between total activities and active forms of phosphorylase and synthase. The enzyme activities responded as expected to stimuli such as insulin and glucose, which depressed phosphorylase and increased synthase activity, and glucagon, which increased phosphorylase and decreased synthase activity. In fasted animals approximately 50% of each enzyme was in the active form, which suggests the existence of a potential futile cycle for glycogen metabolism. The role for such a cycle in the regulation of glycogen synthesis and degradation is discussed.     

C-Glucosylated malonitrile as a key intermediate towards carbohydrate-based glycogen phosphorylase inhibitors

Glycogen utilization involves glycogen phosphorylase, an enzyme which appears to be a potential target for the regulation of glycaemia, as the liver isoform is a major player for hepatic glucose output. A single C-glucosylated malonitrile allowed for the synthesis of three glucose-based derivatives namely bis-oxadiazoles, bis-amides and a C-glucosylated tetrahydropyrimidin-2-one. When evaluated as glycogen phosphorylase inhibitors, two of the synthesized compounds displayed inhibition in the sub-millimolar range. In silico studies revealed that only one out of the bis-amides obtained and the C-glucosylated tetrahydropyrimidin-2-one may bind at the catalytic site.     

Comparative purification and characterization of invertebrate muscle glycogen synthase from the porcine parasite Ascaris suum

Glycogen synthase has been purified from the obliquely striated muscle of the swine parasite Ascaris suum. The muscle contains a concentration of glycogen synthase and glycogen which is 20-fold and 15-fold, respectively, greater than rabbit skeletal muscle. The enzyme could not be solubilized with salivary amylase, but partial solubilization was achieved by activation of endogenous phosphorylase. The enzyme was purified to 85-90% homogeneity (specific activity = 4.3 units/mg) by DEAE-cellulose, Sepharose 4B, and glucosamine 6-phosphate chromatography. The purified glycogen synthase was substantially similar to rabbit skeletal muscle enzyme with respect to Mr (gel electrophoresis and gel filtration), pH dependence, aggregation properties, temperature dependence, and kinetic constants for substrates and activators. Glycogen synthase I was converted to glycogen synthase D by the cyclic AMP-dependent protein kinase. The cyclic AMP-dependent protein kinase catalyzed the incorporation of 1.3 mol of phosphate into each glycogen synthase I subunit and the concomitant interconversion to glycogen synthase D. Since glycogen is the sole fuel utilized by this organism during nonfeeding periods of the host, the characterization of this enzyme provides further insight into the regulatory mechanisms which determine glycogen turnover.     

On the activities of glycogen phosphorylase and glycogen synthase in the liver of the rat.

A procedure was developed for determination of glycogen synthase and phosphorylase activities in liver after various in vivo physiological treatments. Liver samples were obtained from anaesthetised rats by freeze-clamping in situ. Other procedures were shown to stimulate the activity of phosphorylase and depress the activity of glycogen in the liver. The direction of glycogen metabolism appears to be regulated by the relative proportions of the two enzymes, as shown by a strong positive correlation between total activities and active forms of phosphorylase and synthase. The enzyme activities responded as expected to stimuli such as insulin and glucose, which depressed phosphorylase and increased synthase activity, and glucagon, which increased phosphorylase and decreased synthase activity. In fasted animals approximately 50% of each enzyme was in the active form, which suggests the existence of a potential futile cycle for glycogen metabolism. The role for such a cycle in the regulation of glycogen synthesis and degradation is discussed.