Association of Enzymes with Rat Liver Glycogen Isolated by Rate-zonal Centrifugation

Acid hydrolysis and molecular density of phytoglycogen and liver glycogen helps understand the bonding in glycogen α (composite) particles

Phytoglycogen (from certain mutant plants) and animal glycogen are highly branched glucose polymers with similarities in structural features and molecular size range. Both appear to form composite α particles from smaller β particles. The molecular size distribution of liver glycogen is bimodal, with distinct α and β components, while that of phytoglycogen is monomodal. This study aims to enhance our understanding of the nature of the link between liver-glycogen β particles resulting in the formation of large α particles. It examines the time evolution of the size distribution of these molecules during acid hydrolysis, and the size dependence of the molecular density of both glucans. The monomodal distribution of phytoglycogen decreases uniformly in time with hydrolysis, while with glycogen, the large particles degrade significantly more quickly. The size dependence of the molecular density shows qualitatively different shapes for these two types of molecules. The data, combined with a quantitative model for the evolution of the distribution during degradation, suggest that the bonding between β into α particles is different between phytoglycogen and liver glycogen, with the formation of a glycosidic linkage for phytoglycogen and a covalent or strong non-covalent linkage, most probably involving a protein, for glycogen as most likely. This finding is of importance for diabetes, where α-particle structure is impaired.

Changes in glycogen structure over feeding cycle sheds new light on blood-glucose control

Liver glycogen, a highly branched polymer of glucose, is important for maintaining blood-glucose homeostasis. It was recently shown that db/db mice, a model for Type 2 diabetes, are unable to form the large composite glycogen α particles present in normal, healthy mice. In this study, the structure of healthy mouse-liver glycogen over the diurnal cycle was characterized using size exclusion chromatography and transmission electron microscopy. Glycogen was found to be formed as smaller β particles, and then only assembled into large α particles, with a broad size distribution, significantly after the time when glycogen content had reached a maximum. This pathway, missing in diabetic animals, is likely to give optimal blood-glucose control during the daily feeding cycle. Lack of this control may contribute to, or result from, diabetes. This discovery suggests novel approaches to diabetes management.

The Molecular Size Distribution of Glycogen and its Relevance to Diabetes

Glycogen is a highly branched polymer of glucose, functioning as a blood-glucose buffer. It comprises relatively small β-particles, which may be joined as larger aggregate α-particles. The size distributions from size-exclusion chromatography (SEC, also known as GPC) of liver glycogen from non-diabetic and diabetic mice show that diabetic mice have impaired α-particle formation, shedding new light on diabetes. SEC data also suggest the type of bonding holding β-particles together in α-particles. SEC characterisation of liver glycogen at various time points in a day/night cycle indicates that liver glycogen is initially synthesised as β-particles, and then joined by an unknown process to form α-particles. These α-particles are more resistant to degradation, presumably because of their lower surface area-to-volume ratio. These findings have important implications for new drug targets for diabetes management.

Measurement of glycogen synthase activity in crude extracts by CE

Glycogen synthase catalyzes the incorporation of UDP-glucose into glycogen. The activity of the enzyme is usually measured either by a spectrophotometric method or by a radioassay. The first one is not suitable because of the difficulties regarding the use of coupled enzymes in crude extracts, while the second is a time-consuming method involving glycogen isolation and manipulation of radioactivity. We have used a CZE technique as a novel approach to measure glycogen synthase activity. The separations were performed at 22 kV (36 microA) in uncoated capillaries (53 cmx50 microm). Sample injection time was 30 s and nucleotides were monitored at 254 nm. Best resolution was achieved in 20 mM tetraborate buffer, pH 9.2. Curves of absorbance as a function of UDP and UDP-glucose concentration were linear. Enzyme activity in oocyte extracts was linear with respect to time (up to15 min) and enzyme concentration. The K(m app.) for UDP-glucose was 0.87 mM, a value identical to the one reported using the radioassay. CZE enables easy quantitation of compounds, high sensitivity, and automation of the process. Small sample sizes are required, interferences by auxiliary enzymes and manipulation of radioactivity are avoided, and analysis time is significantly diminished.

Restoration of the glycogen-forming function of hepatocytes in rats with liver cirrhosis is facilitated by a high-carbohydrate diet

Using cytofluorimetric and biochemical methods, the content of glycogen and its labile and stable fractions, as well as activities of glucose-6-phosphatase (EC 3.1.3.9), glycogen phosphorylase (EC 2.4.1.1) and glycogen synthase (EC 2.4.1.11) were determined in the rat liver for 6 months after chronic poisoning of the animals with CCl4 and then at 1, 3, and 6 months after the end of the poisoning. One group of rats was given a standard diet, the other, a high-carbohydrate diet. The 6-month long chronic intoxication with CCl4 was shown to produce development of typical liver cirrhosis characterized by a 2·8-fold increase in the total glycogen content in hepatocytes as compared with normal cells, by a fall in the glycogen labile fraction (from 85 to 53 % of the total glycogen) as well as by decreases in the activities of glycogen phosphorylase and glucose-6-phosphatase by 25 and 82% respectively. The structural rehabilitation occurred faster and more completely at the cellular level than at the tissue level. Functional variables of the cirrhotic liver tissue also recovered, after cessation of poisoning, faster and more completely than the liver structure at the tissue level: glycogen levels in hepatocytes fell dramatically, the labile: stable glycogen fraction ratio recovered completely, and the activity of glycogen phosphorylase rose to the level characteristic of the normal liver. Use of the high-carbohydrate diet promoted a somewhat faster and more complete recovery of hepatic structure and function.

Glycogen-forming function of hepatocytes in cirrhotically altered rat liver after treatment with chorionic gonadotropin

Using cytofluorimetric and biochemical studies on serial supravital liver punctate biopsies, effects of chorionic gonadotropin (CG) on recovery of hepatocyte glycogen-forming function in the cirrhotically altered rat liver were analyzed. The biopsies were taken first from rats with experimental cirrhosis produced by their 6-month-long poisoning with the hepatotoxic poison CCl4, then from the same animals in 1, 3, and 6 month after cessation of their poisoning, either on treatment with CG or with no treatment. In smears of isolated hepatocytes, the contents of the total glycogen (TG) and of its labile and stable fractions (LF and SF, respectively) were measured. In liver homogenates, activities of glucose-6-phosphatase (G6Pase), glycogen phosphorylase, and glycogen synthetase were determined. It was found that the threefold increased TG content in hepatocytes of cirrhotic liver returned to the normal level in 3 months without treatment, while as soon as in 1 month in the case of the treatment with CG. The CG treatment for 3 months resulted in normalization of the glycogen fraction composition that had been changed in cirrhotic liver, whereas without treatment, the glycogen LF/SF ratio remained changed even after 6 months after cessation of the poisoning with CCl4. Activity of G6Pase was fourfold reduced in cirrhosis; in 3 months after the end of poisoning, under effect of CG, the activity increased to the normal level, but somewhat decreased subsequently. In the animals that were not treated with CG, the decrease in the G6Pase activity after the cessation of the CCl4 poisoning was even more marked than in the CG-treated rats. Activities of two other enzymes of glycogen metabolism did not differ statistically significantly from the norm throughout the entire experiment. The data obtained indicate that the use of CG for rehabilitation of the glycogen-forming function of the cirrhotically altered liver is more efficient than other ways of treatment studied previously, such as partial hepatectomy or a high-carbohydrate diet.

Translocation and aggregation of hepatic glycogen synthase during the fasted-to-refed transition in rats

Changes in the activation state and intracellular distribution of liver glycogen synthase have been studied during the fasted-to-refed transition in rats. Glycogen synthase activity and activation state were measured in supernatants and pellets obtained after centrifugation of liver homogenates at 9200 g. Upon refeeding, the glycogen synthase activity ratio increased, in a time-dependent manner, in both fractions. The total activity of the enzyme decreased in supernatants and was quantitatively recovered in the pellets. Therefore, refeeding induced both the activation of glycogen synthase and its translocation from the soluble to the pelletable fraction. Immunocytochemical evidence indicates that refeeding induced the formation of clusters of glycogen synthase, which were recovered in the 9200 g sediments. However, the enzyme clusters did not locate with the glycogen particles in the pelletable fraction. The glycogen synthase activation state responded almost as an of-off switch to changes in the intracellular glucose 6-phosphate concentration in the range 0.2-0.3 mM. The amount of enzyme present in the pellets correlated linearly with the intracellular glucose 6-phosphate levels. These results indicate that glucose 6-phosphate is the key signal for both the activation and changes in intracellular localization of hepatic glycogen synthase in vivo.

Fractionation of particulate glycogen and bound enzymes using high-performance liquid chromatography

During a study aimed at the specificity of binding of phosphorylase and glycogen synthase to glycogen particles we investigated the separation of these particles with the technique of high-performance liquid chromatography using several media. The technique allowed relatively rapid fractionations with little, if any, loss of enzyme activity. The basis of some separations was molecular weight of the particle. Of the molecular exclusion media used, Superose 6 had an exclusion limit large enough to accommodate most glycogens but excluded mouse liver and bovine liver glycogen particles. Toyopearl HW-75 F was a medium that could accommodate liver glycogen and gave us reasonably well-defined separation of the particles that carried phosphorylase and glycogen synthase. Separations on ion-exchange columns, that were obviously based on the overall charge of proteins associated with the glycogen particles, were not very effective in terms of allowing the separation of different types of particle. Of all the media used the hydrophobic interaction column was best in terms of separating what appeared to be distinct populations of particles that had definite preferences in terms of binding phosphorylase and glycogen synthase. As expected, these separations were based on the hydrophobic interaction of the proteins associated with the glycogen particles, since particles that had been deproteinized by a cold water extraction procedure did not bind to the column.

Patterns of glycogen turnover in liver characterized by computer modeling

We used a computer model of liver glycogen turnover to reexamine the data of Devos and Hers, who reported the time course of accumulation in and loss from glycogen of label originating in [1-14C]galactose injected at different times after the start of refeeding of 40-h fasted mice or rats. In the present study computer representation of individual glycogen molecules was utilized to account for growth and degradation of glycogen according to specific hypothetical patterns. Using this model we could predict the accumulation and localization within glycogen of labeled glucose residues and compare the predictions with the previously published data. We considered three specific hypotheses of glycogen accumulation during refeeding: 1) simultaneous, 2) sequential, and 3) accelerating growth. Hypothetical patterns of glycogen degradation were 1) ordered and 2) random degradation. The pattern of glycogen synthesis consistent with experimental data was a steadily increasing number of growing glycogen molecules, whereas during degradation glycogen molecules are exposed to degrading enzymes randomly, rather than in a specific reverse order of synthesis. These patterns predict the existence of a specific mechanism for the steadily increasing "seeding" of new glycogen molecules during synthesis.

Compartmentation of glycogen metabolism in the liver

The incorporation of radioactivity into liver glycogen has been shown not only to be a metabolically inhomogeneous process but also to depend critically on the nature of the precursor. D-Galactose is incorporated into glycogen by a mechanism which is separate from that associated with the incorporation of D-glucose. D-Galactose is favoured for incorporation into high-molecular-weight glycogen and consequently is affected more by treatment of the animal with the antibiotic tunicamycin, since high-molecular-weight glycogen is preferentially found in the lysosomal compartment.

Glycogen of high molecular weight from mammalian muscle

Glycogen of high molecular weight has been isolated from mammalian muscle, in contrast to the material of low molecular weight commonly described. The large polysaccharide is similar to liver glycogen in the structure of its individual beta-particles and also, partially, in the mode of assembly into the gross alpha-particles. The large particles may be disrupted by 2-mercaptoethanol, but not to the same extent as their liver counterparts.

Effects of prolonged space flight on rat skeletal muscle

The effect of a 20-day space flight on water, Na+, K+, Mg2+, Ca2+ and glycogen contents as well as on activities of glycogen metabolism enzymes--glycogen synthetase and glycogen phosphorylase--of rat skeletal muscles was studied. This data is regarded as an integral test characterizing the state of contractile tissue of the animals at the final stage of flight aboard biosatellites. The measurements indicate that there were no significant changes of cations and glycogen contents nor of the enzymic activities in fast-twitch muscles during the 20-day spaceflight. At the same time dehydration in these muscles was observed, which disappeared on the 25th postflight day. In slow-twitch antigravitational skeletal muscle (m. soleus) there was a decrease of K+ and increase of Na+ in the tissue contents. The changes disappeared at the end of the on-earth readaptation period. From the pattern of these observations, we can conclude that the 20-day space flight leads to some reversible biochemical changes of the rat skeletal muscles. A conclusion can be drawn about necessity of creating, aboard the spaceship, an artificial load on antigravitational skeletal muscles.

Glycogen synthase in the rat tapeworm, Hymenolepis diminuta--II. Control of enzyme activity by glucose and glycogen

1. The proportion of activity in the physiologically active I form of glycogen synthase in Hymenolepis diminuta (Cestoda) decreased in the worm when the rat host was fasted and was greatly increased in the cestode 1 hr after a 24 hr fasted rat was refed. 2. The increase in glycogen synthase I activity was due to glucose present in the host gut after feeding, not to other physiological changes in the rat intestine due to meal consumption. 3. Incubation of intact H. diminuta in vitro with glucose also resulted in the conversion of glycogen synthase D to I. 4. Glucose does not appear to affect the glycogen synthase complex directly, because neither the total synthase converted to I nor the rate of conversion was affected by glucose in a partially purified homogenate. 5. High concentrations of glycogen inhibited the synthase D to I conversion and high mol. wt glycogen was a more effective inhibitor than low mol. wt glycogen.

Purkinje fibre glycogen. A morphologic and biochemical study of glycogen particles isolated from the cow conducting system

Glycogen from the cow conducting system was extracted by crude and mild methods. For comparison similar extractions were also performed on cow ordinary ventricular tissue. Glycogen particles from the conducting system, isolated by a mild method, were characterized by a low molecular weight (3-5 X 10(6)) and small dimensions (average diameter about 30 nm). 3.5-7% protein was firmly bound to the glycogen. The glycogen, based on spectrophotometric analysis, appeared to be in a native state. Glycogen as a polysaccharide-protein complex can thus be obtained from the cow conducting system and is proposed to be useful for analysis of the structure and function of glycogen in the conducting system.

Macromolecular structure and morphology of native glycogen particles isolated from Candida albicans

A polysaccharide-rich particulate fraction was isolated from cytoplasmic extracts of Candida albicans by a procedure using differential centrifugation. The polysaccharide particles obtained after purification with deoxycholate treatment were essentially free of nitrogen and were identified chemically as polyglucosan, in which the glucosidic links were of alpha type. Both the response to amylolytic enzymes and the spectral characteristics of the iodine complexes of the polysaccharide particles were similar to those of rabbit liver glycogen. They also precipitated with concanavalin A, the glycogen value being assessed at 1.04. These data strongly indicated that the polysaccharide particles have the macromolecular structure characteristic of glycogen. The sedimentation analysis revealed that they were polydisperse, with a weight average sedimentation coefficient of 340S. In negatively stained specimens, the glycogen particles were seen to form rosette-like structures consisting of a complex unit 40 to 150 nm in diameter. Such complex particles were composed of smaller globules that were fairly uniform in size with an average diameter of 32 nm.

Chemical and biochemical changes in subcellular fractions of guinea pig liver during infection with Coxiella burneti

Livers of uninfected guinea pigs and of guinea pigs infected with Coxiella burneti were fractionated into smooth endoplasmic reticulum, rough endoplasmic reticulum (RER), pellet, and cell sap fractions. The ribonucleic acid (RNA) and protein of each fraction were determined, and the phosphorylase, glucose-6-phosphatase, and glucosyl transferase (glycogen synthetase) activities of each fraction were measured. Decreased RNA, protein, and enzyme activities were found in the RER and pellet fractions of infected livers, with the greatest differences in the RER. The evidence indicates a solubilization of the phosphorylase and synthetase, with the enzymes moving from the RER and glycogen-containing pellet fraction to the cell sap. The data suggest the RER as a target during Q fever.