Mice with elevated muscle glycogen stores do not have improved exercise performance

Skeletal muscle glycogen is considered to be an important source of energy for contraction and increasing the level of the glucose polymer is generally thought to improve exercise performance in humans. A genetically modified mouse model (GSL30), which overaccumulates glycogen due to overexpression of a hyperactive form of glycogen synthase, was used to examine whether increasing the level of the polysaccharide enhances the ability of mice to run on a treadmill. The skeletal muscle of the GSL30 mice had large deposits of glycogen. There were no significant increases in the work performed by GSL30 mice as compared to their respective wild type littermates when exercised to exhaustion. The amount of muscle glycogen utilized by GSL30 mice, however, was greater, while the amount of liver glycogen consumed during exhaustive exercise was less than wild type animals. This result suggests that increased muscle glycogen stores do not necessarily improve exercise performance in mice.     

Increased glycogen storage in yeast results in less branched glycogen

Glycogen is a branched polymer of glucose, synthesized as a reserve of both energy and carbon. The branched nature of glycogen is important for its function and polyglucosan bodies, particles that contain a glycogen-like polymer with reduced branching, are a feature of several disease states. The degree of glycogen branching is thought to be governed by the balance between glycogen synthesis and branching activities. However, there have been reports that the intrinsic properties of individual branching enzymes govern the degree of branching. To address the relationship between synthesis and branching more fully, we made use of the yeast Saccharomyces cerevisiae. The glycogen content of yeast cells was manipulated by using different growth conditions or by the introduction of specific mutations. Whenever glycogen storage was elevated, the polysaccharide formed was found to be less branched but normal branching could be restored by overexpression of branching enzyme.     

Glycogen metabolism in tissues from a mouse model of Lafora disease

Laforin, encoded by the EPM2A gene, by sequence is a member of the dual specificity protein phosphatase family. Mutations in the EPM2A gene account for around half of the cases of Lafora disease, an autosomal recessive neurodegenerative disorder, characterized by progressive myoclonus epilepsy. The hallmark of the disease is the presence of Lafora bodies, which contain polyglucosan, a poorly branched form of glycogen, in neurons, muscle and other tissues. Glycogen metabolizing enzymes were analyzed in a transgenic mouse over-expressing a dominant negative form of laforin that accumulates Lafora bodies in several tissues. Skeletal muscle glycogen was increased 2-fold as was the total glycogen synthase protein. However, the -/+glucose-6-P activity of glycogen synthase was decreased from 0.29 to 0.16. Branching enzyme activity was increased by 30%. Glycogen phosphorylase activity was unchanged. In whole brain, no differences in glycogen synthase or branching enzyme activities were found. Although there were significant differences in enzyme activities in muscle, the results do not support the hypothesis that Lafora body formation is caused by a major change in the balance between glycogen elongation and branching activities.     

Regulation of glycogen concentration and glycogen synthase activity in skeletal muscle of insulin-resistant rats

The aim of this study was to investigate the effect of insulin resistance on glycogen concentration and glycogen synthase activity in the red and white gastrocnemius muscles and to determine whether the inverse relationship existing between glycogen concentration and enzyme activity is maintained in insulin resistant state. These questions were addressed using 3 models that induce various degrees of insulin resistance: sucrose feeding, dexamethasone administration, and a combination of both treatments (dex+sucrose). Sucrose feeding raised triglyceride levels without affecting plasma glucose or insulin concentrations whereas dexamethasone and dex+sucrose provoked severe hyperinsulinemia, hyperglycemia and hypertriglyceridemia. Sucrose feeding did not alter muscle glycogen concentration but provoked a small reduction in the glycogen synthase activity ratio (-/+ glucose-6-phosphate) in red but not in white gastrocnemius. Dexamethasone administration augmented glycogen concentration and reduced glycogen synthase activity ratio in both muscle fiber types. In contrast, dex+sucrose animals showed decreased muscle glycogen concentration compared to dexamethasone group, leading to levels similar to those of control animals. This was associated with lower glycogen synthase activity compared to control animals leading to levels comparable to those of dexamethasone-treated animals. Thus, in dex+sucrose animals, the inverse relationship observed between glycogen levels and glycogen synthase activity was not maintained, suggesting that factors other than the glycogen concentration modulate the enzyme's activity. In conclusion, while insulin resistance was associated with a reduced glycogen synthase activity ratio, we found no correlation between muscle glycogen concentration and insulin resistance. Furthermore, our results suggest that sucrose treatment may modulate dexamethasone action in skeletal muscle.     

Interaction between glycogenin and glycogen synthase

Glycogen synthase plays a key role in regulating glycogen metabolism. In a search for regulators of glycogen synthase, a yeast two-hybrid study was performed. Two glycogen synthase-interacting proteins were identified in human skeletal muscle, glycogenin-1, and nebulin. The interaction with glycogenin was found to be mediated by the region of glycogenin which contains the 33 COOH-terminal amino acid residues. The regions in glycogen synthase containing both NH2- and COOH-terminal phosphorylation sites are not involved in the interaction. The core segment of glycogen synthase from Glu21 to Gly503 does not bind COOH-terminal fragment of glycogenin. However, this region of glycogen synthase binds full-length glycogenin indicating that glycogenin contains at least one additional interacting site for glycogen synthase besides the COOH-terminus. We demonstrate that the COOH-terminal fragment of glycogenin can be used as an effective high affinity reagent for the purification of glycogen synthase from skeletal muscle and liver.     

Effect of glycogen synthase overexpression on insulin-stimulated muscle glucose uptake and storage

Insulin-stimulated muscle glucose uptake is inversely associated with the muscle glycogen concentration. To investigate whether this association is a cause and effect relationship, we compared insulin-stimulated muscle glucose uptake in noncontracted and postcontracted muscle of GSL3-transgenic and wild-type mice. GSL3-transgenic mice overexpress a constitutively active form of glycogen synthase, which results in an abundant storage of muscle glycogen. Muscle contraction was elicited by in situ electrical stimulation of the sciatic nerve. Right gastrocnemii from GSL3-transgenic and wild-type mice were subjected to 30 min of electrical stimulation followed by hindlimb perfusion of both hindlimbs. Thirty minutes of contraction significantly reduced muscle glycogen concentration in wild-type (49%) and transgenic (27%) mice, although transgenic mice retained 168.8 +/- 20.5 micromol/g glycogen compared with 17.7 +/- 2.6 micromol/g glycogen for wild-type mice. Muscle of transgenic and wild-type mice demonstrated similar pre- (3.6 +/- 0.3 and 3.9 +/- 0.6 micromol.g(-1).h(-1) for transgenic and wild-type, respectively) and postcontraction (7.9 +/- 0.4 and 7.0 +/- 0.4 micromol.g(-1).h(-1) for transgenic and wild-type, respectively) insulin-stimulated glucose uptakes. However, the [14C]glucose incorporated into glycogen was greater in noncontracted (151%) and postcontracted (157%) transgenic muscle vs. muscle of corresponding wild-type mice. These results indicate that glycogen synthase activity is not rate limiting for insulin-stimulated glucose uptake in skeletal muscle and that the inverse relationship between muscle glycogen and insulin-stimulated glucose uptake is an association, not a cause and effect relationship.     

Stimulation of autophosphorylation of rabbit skeletal muscle phosphorylase kinase by glycogen synthase on glycogen particles

Glycogen synthase stimulated the autophosphorylation and autoactivation of phosphorylase kinase from rabbit skeletal muscle. This stimulation was additive to that by glycogen and the reaction was dependent on Ca2+. The effect by glycogen synthase was maximum within the activity ratio (the activity of enzyme without glucose-6-P divided by the activity with 10 mM glucose-6-P) of 0.3 and over 0.3 it was rather inhibitory. The results suggest that autophosphorylation of phosphorylase kinase in the presence of glycogen synthase on glycogen particles may be an important regulatory mechanism of glycogen metabolism in skeletal muscle.     

GNN is a self-glucosylating protein involved in the initiation step of glycogen biosynthesis in Neurospora crassa

The initiation of glycogen synthesis requires the protein glycogenin, which incorporates glucose residues through a self-glucosylation reaction, and then acts as substrate for chain elongation by glycogen synthase and branching enzyme. Numerous sequences of glycogenin-like proteins are available in the databases but the enzymes from mammalian skeletal muscle and from Saccharomyces cerevisiae are the best characterized. We report the isolation of a cDNA from the fungus Neurospora crassa, which encodes a protein, GNN, which has properties characteristic of glycogenin. The protein is one of the largest glycogenins but shares several conserved domains common to other family members. Recombinant GNN produced in Escherichia coli was able to incorporate glucose in a self-glucosylation reaction, to trans-glucosylate exogenous substrates, and to act as substrate for chain elongation by glycogen synthase. Recombinant protein was sensitive to C-terminal proteolysis, leading to stable species of around 31kDa, which maintained all functional properties. The role of GNN as an initiator of glycogen metabolism was confirmed by its ability to complement the glycogen deficiency of a S. cerevisiae strain (glg1 glg2) lacking glycogenin and unable to accumulate glycogen. Disruption of the gnn gene of N. crassa by repeat induced point mutation (RIP) resulted in a strain that was unable to synthesize glycogen, even though the glycogen synthase activity was unchanged. Northern blot analysis showed that the gnn gene was induced during vegetative growth and was repressed upon carbon starvation.     

Cloning and characterization of the glycogen branching enzyme gene existing in tandem with the glycogen debranching enzyme from Pectobacterium chrysanthemi PY35

The glycogen branching enzyme gene (glgB) from Pectobacterium chrysanthemi PY35 was cloned, sequenced, and expressed in Escherichia coli. The glgB gene consisted of an open reading frame of 2196bp encoding a protein of 731 amino acids (calculated molecular weight of 83,859Da). The glgB gene is upstream of glgX and the ORF starts the ATG initiation codon and ends with the TGA stop codon at 2bp upstream of glgX. The enzyme was 43-69% sequence identical with other glycogen branching enzymes. The enzyme is the most similar to GlgB of E. coli and contained the four regions conserved among the alpha-amylase family. The glycogen branching enzyme (GlgB) was purified and the molecular weight of the enzyme was estimated to be 84kDa by SDS-PAGE. The glycogen branching enzyme was optimally active at pH 7 and 30 degrees C.     

Skeletal muscle glycogen content, structure, and metabolism are normal in rats with hepatic glycogen phosphorylase kinase deficiency

Skeletal muscle glycogen content and structure, and the activities of several enzymes of glycogen metabolism are reported for the hepatic glycogen phosphorylase b kinase deficient (gsd/gsd) rat. The skeletal muscle glycogen content of the fed gsd/gsd rat is 0.50 +/- 0.11% tissue wet weight, and after 40 hours of starvation this value is lowered 40% to 0.30 +/- 0.05% tissue wet weight. In contrast the gsd/gsd rat liver has an elevated glycogen content which remains high after starvation. The skeletal muscle phosphorylase b kinase, glycogen phosphorylase, glycogen synthase and acid alpha-glucosidase activities are 17.2 +/- 2.9 units/g tissue, 119.9 +/- 6.4 units/g tissue, 12.2 +/- 0.4 units/g tissue and 1.4 +/- 0.4 milliunits/g tissue, respectively, with approx. 20% of phosphorylase and approx. 24% of synthase in the active form (at rest). These enzyme activities resemble those of Wistar skeletal muscle, and again this contrasts with the situation in the liver where there are marked differences between the Wistar and the gsd/gsd rat. Fine structural analysis of the purified glycogen showed resemblance to other glycogens in branching pattern. Analysis of the molecular weight distribution of the purified glycogen indicated polydispersity with approx. 66% of the glycogen having a molecular weight of less than 250 X 10(6) daltons and approx. 25% greater than 500 X 10(6) daltons. This molecular weight distribution resembles those of purified Wistar liver and skeletal muscle glycogens and differs from that of the gsd/gsd liver glycogen which has an increased proportion of the low molecular weight material.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rat liver glycogen metabolism in the perinatal period

The correlation between blood glucose levels, the concentration of glycogen, the activities of glycogen sythase and phosphorylase and their respective kinases and phosphatases was examined in liver of rat fetuses between day 18 of gestation and one day after birth. Between day 18 and 21 there is a rapid increase in the concentration of glycogen and in the activity of synthase a and a much slower increase in the activity of phosphorylase a. The activity of the respective kinases increased rapidly during this period and reached maximun on day 21. The activity of synthase phosphatase and phosphorylase phosphatase increased after day 18, to reach a maximum on day 19 and 20, respectively, but decreased again towards day 21. The possibility that the changes in glycogen concentration and enzyme activities were related to an effect of glucose of AMP on the respective phosphatases was considered. It was found that the Km of phosphatase for glucose in the prenatal period was 5–7 mM, as in the adult. Since the level of blood glucose during this period was constant (2.8 mM), an effect of glucose on phosphatase activity seems unlikely. AMP concentration increased between day 18 and 21 from 6–15 nmol/g. In view of the low level of phosphorylase a activity during this period, the increase in AMP concentration is not considered to be important in the regulation of glycogen breakdown at this time.Immediately after birth blood glucose levels dropped to 5 mg/dl. This was accompanied by a rapid decrease in glycogen concentration and in the activity of glycogen synthase and a rise in phosphorylase activity. Blood glucose levels returned to the initial level within 1 h after birth, whereas the changes in glycogen concentration and enzyme activities continued for at least 3 h after birth. On day 22 all parameters examined had reached the level found in adult rat liver.It is suggested that the rapid changes observed immediately after birth are due to an effect of hypoglycemia mediated by hormones and cannot be ascribed to direct effects of metabolites on the enzyme systems involved.     

Genetic Depletion of the Malin E3 Ubiquitin Ligase in Mice Leads to Lafora Bodies and the Accumulation of Insoluble Laforin

Approximately 90% of cases of Lafora disease, a fatal teenage-onset progressive myoclonus epilepsy, are caused by mutations in either the EPM2A or the EPM2B genes that encode, respectively, a glycogen phosphatase called laforin and an E3 ubiquitin ligase called malin. Lafora disease is characterized by the formation of Lafora bodies, insoluble deposits containing poorly branched glycogen or polyglucosan, in many tissues including skeletal muscle, liver, and brain. Disruption of the Epm2b gene in mice resulted in viable animals that, by 3 months of age, accumulated Lafora bodies in the brain and to a lesser extent in heart and skeletal muscle. Analysis of muscle and brain of the Epm2b^−/− mice by Western blotting indicated no effect on the levels of glycogen synthase, PTG (type 1 phosphatase-targeting subunit), or debranching enzyme, making it unlikely that these proteins are targeted for destruction by malin, as has been proposed. Total laforin protein was increased in the brain of Epm2b^−/− mice and, most notably, was redistributed from the soluble, low speed supernatant to the insoluble low speed pellet, which now contained 90% of the total laforin. This result correlated with elevated insolubility of glycogen and glycogen synthase. Because up-regulation of laforin cannot explain Lafora body formation, we conclude that malin functions to maintain laforin associated with soluble glycogen and that its absence causes sequestration of laforin to an insoluble polysaccharide fraction where it is functionally inert.     

Phosphorylation and Inactivation of Brain Glycogen Synthase by a Multifunctional Calmodulin-Dependent Protein Kinase

Glycogen synthase was partially purified from canine brain to about 70% purity. The purified enzyme showed differences from the properties of the skeletal muscle enzyme with respect to molecular weights of the holoenzyme and subunit and phosphopeptide mapping. The multifunctional calmodulin-dependent protein kinase from the brain phosphorylated brain glycogen synthase with concomitant inactivation of the enzyme. Although about 1.3 mol of phosphate/mol subunit was maximally incorporated into glycogen synthase, 0.4 mol of phosphate/mol subunit was sufficient for the maximal inactivation of the enzyme. The results indicate that brain glycogen synthase is regulated in a calmodulin-dependent manner similarly to the skeletal muscle enzyme, but that the brain enzyme is different from the skeletal muscle enzyme.     

Mode of action of glycogen branching enzyme from Neurospora crassa

Neurospora crassa branching enzyme [EC 2.4.1.18] acted on potato amylopectin or amylose to convert them to highly branched glycogen-type molecules which consisted of unit chains of six glucose units. The enzyme also acted on the amylopectin beta-limit dextrin, indicating that the enzyme acted on internal glucose chains as well as outer chains. By the combined action of N. crassa glycogen synthase [EC 2.4.1.11] and the branching enzyme, a glycogen-type molecule was formed from UDP-glucose. In the presence of primer glycogen, the glucose transfer reaction was accelerated by the addition of branching enzyme. On the other hand, the glucose transfer reaction by glycogen synthase did not occur without primers. When the branching enzyme was added, the glucose transfer occurred after a short time lag. This recovery of the glucose transfer reaction did not occur upon addition of the inactivated branching enzyme. The structure of the product formed by the combined action of the two enzymes was different from that of the intact N. crassa glycogen with respect to the distribution patterns of the unit chains.     

Control of glycogen deposition

Traditionally, glycogen synthase (GS) has been considered to catalyze the key step of glycogen synthesis and to exercise most of the control over this metabolic pathway. However, recent advances have shown that other factors must be considered. Moreover, the control of glycogen deposition does not follow identical mechanisms in muscle and liver. Glucose must be phosphorylated to promote activation of GS. Glucose-6-phosphate (Glc-6-P) binds to GS, causing the allosteric activation of the enzyme probably through a conformational rearrangement that simultaneously converts it into a better substrate for protein phosphatases, which can then lead to the covalent activation of GS. The potency of Glc-6-P for activation of liver GS is determined by its source, since Glc-6-P arising from the catalytic action of glucokinase (GK) is much more effective in mediating the activation of the enzyme than the same metabolite produced by hexokinase I (HK I). As a result, hepatic glycogen deposition from glucose is subject to a system of control in which the 'controller', GS, is in turn controlled by GK. In contrast, in skeletal muscle, the control of glycogen synthesis is shared between glucose transport and GS. The characteristics of the two pairs of isoenzymes, liver GS/GK and muscle GS/HK I, and the relationships that they establish are tailored to suit specific metabolic roles of the tissues in which they are expressed. The key enzymes in glycogen metabolism change their intracellular localization in response to glucose. The changes in the intracellular distribution of liver GS and GK triggered by glucose correlate with stimulation of glycogen synthesis. The translocation of GS, which constitutes an additional mechanism of control, causes the orderly deposition of hepatic glycogen and probably represents a functional advantage in the metabolism of the polysaccharide.     

Combined action of Escherichia coli glycogen synthase and branching enzyme in the so-called "unprimed" polyglucoside synthesis.

Mutants of Escherichia coli which are unable to synthesize glycogen were used to study the so-called “unprimed” synthesis of glycogen. The glycogen synthase has been partially purified from these mutants. During the purification, attempts were made to separate the activity which requires the addition of an exogenous primer (primed activity) from the activity which does not require a primer but is highly dependent on the presence of some salts such as citrate and EDTA (unprimed activity). No separation between these two activities could be achieved but the results obtained by chromatography on DEAE-Sephadex indicate that there is a single form of glycogen synthase which is responsible for both unprimed and primed activity. The evidence that a single protein was necessary to catalyze these two reactions was given by the findings that mutants defective in glycogen synthase activity were unable to catalyze glucosyl transfer without added primer. At low concentration, the glycogen synthase purified from a branching enzyme negative mutant catalyzed the unprimed reaction at a slow rate even in presence of salts. A protein activator of this reaction was found in mutants lacking glycogen synthase but not in mutants lacking branching enzyme. The hypothesis that this activator is the branching enzyme itself was supported by the observation that it co-purified with the branching enzyme from a E. coli strain defective in glycogen synthase activity. EDTA or Triton X-100 increased the stimulation of the unprimed synthesis by the branching enzyme. The apparent affinity of the glycogen synthase for glycogen was increased twofold in the presence of EDTA but the branching enzyme further increased the effect of EDTA. The combined action of the glycogen synthase and the branching enzyme on the endogenous glucan associated with the synthase may account for the unprimed activity observed in vitro.     

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.     

Differences between glycogen synthases from rat and rabbit skeletal muscle as indicated by phosphopeptide maps

Glycogen synthase I was purified from rat skeletal muscle. On sodium dodecyl sulfate polyacrylamide gel electrophoresis, the enzyme migrated as a major band with a subunit Mr of 85,000. The specific activity (24 units/mg protein), activity ratio (the activity in the absence of glucose-6-P divided by the activity in the presence of glucose-6-P X 100) (92 +/- 2) and phosphate content (0.6 mol/mol subunit) were similar to the enzyme from rabbit skeletal muscle. Phosphorylation and inactivation of rat muscle glycogen synthase by casein kinase I, casein kinase II (glycogen synthase kinase 5), glycogen synthase kinase 3 (kinase FA), glycogen synthase kinase 4, phosphorylase b kinase, and the catalytic subunit of cAMP-dependent protein kinase were similar to those reported for rabbit muscle synthase. The greatest decrease in rat muscle glycogen synthase activity was seen after phosphorylation of the synthase by casein kinase I. Phosphopeptide maps of glycogen synthase were obtained by digesting the different 32P-labeled forms of glycogen synthase by CNBr, trypsin, or chymotrypsin. The CNBr peptides were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and the tryptic and chymotryptic peptides were separated by reversed-phase HPLC. Although the rat and rabbit forms of synthase gave similar peptide maps, there were significant differences between the phosphopeptides derived from the N-terminal region of rabbit glycogen synthase and the corresponding peptides presumably derived from the N-terminal region of rat glycogen synthase. For CNBr peptides, the apparent Mr was 12,500 for rat and 12,000 for the rabbit. The tryptic peptides obtained from the two species had different retention times. A single chymotryptic peptide was produced from rat skeletal muscle glycogen synthase after phosphorylation by phosphorylase kinase whereas two peptides were obtained with the rabbit enzyme. These results indicate that the N-terminus of rabbit glycogen synthase, which contains four phosphorylatable residues (Kuret et al. (1985) Eur. J. Biochem. 151, 39-48), is different from the N-terminus of rat glycogen synthase.     

Mechanism of the reaction catalyzed by glycogen synthetase I in rabbit skeletal muscle

1.5-Gluconolactone was shown to exert a strong inhibiting effect on the activity of rabbit skeletal muscle glycogen synthase I. The Ki values determined according to Dixon (0.13 mM) and Chuang and Bell (0.14 mM) coincide with the Km value for UDPG. Within the pH range of 5.4-7.0, N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide (less than or equal to 3 mM) specifically inhibits the carboxyl group, which was supported by the reactivation of the enzyme under mild alkaline conditions. The reversible competitive inhibitor of glycogen synthase and the UDP reaction product as well as 1.5-gluconolactone afford an effective protective effect. It is supposed that the reaction catalyzed by rabbit skeletal muscle glycogen synthase I results in the formation of an intermediate carbonium ion. An essential role in the enzyme activity belongs to the carboxylic group of the active center.