Pro- and macroglycogenolysis: relationship with exercise intensity and duration.

We examined the net catabolism of two pools of glycogen, proglycogen (PG) and macroglycogen (MG), in human skeletal muscle during exercise. Male subjects (n = 21) were assigned to one of three groups. Group 1 exercised 45 min at 70% maximal O(2) uptake (VO(2 max)) and had muscle biopsies at rest, 15 min, and 45 min. Group 2 exercised at 85% VO(2 max) to exhaustion (45.4 +/- 3.4 min) and had biopsies at rest, 10 min, and exhaustion. Group 3 performed three 3-min bouts of exercise at 100% VO(2 max) separated by 6 min of rest. Biopsies were taken at rest and after each bout. Group 1 had small MG and PG net glycogenolysis rates (ranging from 3.8 +/- 1.0 to 2.4 +/- 0.6 mmol glucosyl units. kg(-1). min(-1)) that did not change over time. In group 2, the MG glycogenolysis rate remained low and unchanged over time, whereas the PG rate was initially elevated (11.3 +/- 2.3 mmol glucosyl units. kg(-1). min(-1)) and declined (P < or = 0.05) with time. During the first 10 min, PG concentration ([PG]) declined (P < or = 0.05), whereas MG concentration ([MG]) did not. Similarly, in group 3, in both the first and the second bouts of exercise [PG] declined (P < or = 0.05) and [MG] did not, although by the end of the second exercise period the [MG] was lower (P < or = 0.05) than the rest level. The net catabolic rates for PG in the first two exercises were 22.6 +/- 6.8 and 21.8 +/- 8.2 mmol glucosyl units. kg(-1). min(-1), whereas the corresponding values for MG were 17.6 +/- 6.0 and 10.8 +/- 5.6. The MG pool appeared to be more resistant to mobilization, and, when activated, its catabolism was inhibited more rapidly than that of PG. This suggests that the metabolic regulation of the two pools must be different.     

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.     

Regulation of glycogen phosphorylase and PDH during exercise in human skeletal muscle during hypoxia

The present study examined the acute effects of hypoxia on the regulation of skeletal muscle metabolism at rest and during 15 min of submaximal exercise. Subjects exercised on two occasions for 15 min at 55% of their normoxic maximal oxygen uptake while breathing 11% O(2) (hypoxia) or room air (normoxia). Muscle biopsies were taken at rest and after 1 and 15 min of exercise. At rest, no effects on muscle metabolism were observed in response to hypoxia. In the 1st min of exercise, glycogenolysis was significantly greater in hypoxia compared with normoxia. This small difference in glycogenolysis was associated with a tendency toward a greater concentration of substrate, free P(i), in hypoxia compared with normoxia. Pyruvate dehydrogenase activity (PDH(a)) was lower in hypoxia at 1 min compared with normoxia, resulting in a reduced rate of pyruvate oxidation and a greater lactate accumulation. During the last 14 min of exercise, glycogenolysis was greater in hypoxia despite a lower mole fraction of phosphorylase a. The greater glycogenolytic rate was maintained posttransformationally through significantly higher free [AMP] and [P(i)]. At the end of exercise, PDH(a) was greater in hypoxia compared with normoxia, contributing to a greater rate of pyruvate oxidation. Because of the higher glycogenolytic rate in hypoxia, the rate of pyruvate production continued to exceed the rate of pyruvate oxidation, resulting in significant lactate accumulation in hypoxia compared with no further lactate accumulation in normoxia. Hence, the elevated lactate production associated with hypoxia at the same absolute workload could in part be explained by the effects of hypoxia on the activities of the rate-limiting enzymes, phosphorylase and PDH, which regulate the rates of pyruvate production and pyruvate oxidation, respectively.     

Adrenoceptor-mediated activation of liver glycogen phosphorylase: effects of thyroid state.

Fetal mouse tissues on gestational days 14, 16 and 19 were incubated with ¹⁴C-corticosterone (B) and ³H-11-dehydrocorticosterone (A), and the steroids were separated chromatograhically to determine the ratio reduction/dehydrogenation. In placenta the ratio remained high, >6, throughout this period, while it rose from 0.06 to 0.6 in brain and from 0.1 to 0.5 in gut. In liver, the ratio increased from 0.29 on day 14 to 3.54 on day 16; in lung it rose from 0.29 on day 16 to 6.7 on day 19. Injection of mothers 16 hr earlier with dexamethasone increased the ratio in lung and placenta on day 16 but not earlier. The unchanged ³H-corticosterone recovered from fetal tissues 15 min after injection into mothers was <3% on day 14 but increased manyfold in all tissues by day 19. It is concluded that corticosteroids are regulated in fetal mouse tissues by the interconversion of the hormone and its 11-dehydro metabolite, and that the pattern varies in different tissues and changes with gestational age.     

An engineered liver glycogen phosphorylase with AMP allosteric activation

Liver and muscle glycogen phosphorylases, which are products of distinct genes, are both activated by covalent phosphorylation, but in the unphosphorylated (b) state, only the muscle isozyme is efficiently activated by the allosteric activator AMP. The different responsiveness of the phosphorylase isozymes to allosteric ligands is important for the maintenance of tissue and whole body glucose homeostasis. In an attempt to understand the structural determinants of differential sensitivity of the muscle and liver isozymes to AMP, we have developed a bacterial expression system for the liver enzyme, allowing native and engineered proteins to be expressed and characterized. Engineering of the single amino acid substitutions Thr48Pro, Met197Thr and the double mutant Thr48Pro, Met197Thr in liver phosphorylase, and Pro48Thr in muscle phosphorylase, did not qualitatively change the response of the two isozymes to AMP. These sites had previously been implicated in the configuration of the AMP binding site. However, when nine amino acids among the first 48 in liver phosphorylase were replaced with the corresponding muscle phosphorylase residues (L1M2-48L49-846), the engineered liver enzyme was activated by AMP to a higher maximal activity than native liver phosphorylase. Interestingly, the homotropic cooperativity of AMP binding was unchanged in the engineered phosphorylase b protein, and heterotropic cooperativity between the glucose-1-phosphate and AMP sites was only slightly enhanced. The native liver, native muscle and L1M2-48L49-846 phosphorylases were converted to the a form by treatment with purified phosphorylase kinase; the maximal activity of the chimeric a enzyme was greater than the native liver a enzyme and approached that of muscle phosphorylase a. From these results we suggest that tissue-specific phosphorylase isozymes have evolved a complex mechanism in which the N-terminal 48 amino acids modulate intrinsic activity (Vmax), probably by affecting subunit interactions, and other, as yet undefined regions specify the allosteric interactions with ligands and substrates.     

Inhibition of the glycogen phosphorylase system during ochratoxicosis in chickens.

Graded doses of ochratoxin A incorporated into the diet (0, 0.5, 1.0, 2.0, 4.0, and 8.0 micrograms/g) of broiler chickens significantly (P < 0.05) inhibited activity of protein kinase, the initiator enzyme of the glycogen phosphorylase system, in the livers at all dose levels. Only the highest dose, 8.0 micrograms/g, significantly reduced the total activity of phosphorylase kinase, which is activated by protein kinase. The total activity of phosphorylase, which is activated by phosphorylase kinase, was unaltered by ochratoxin A at any level. Additon of ochratoxin A to liver extracts control birds inhibited protein kinase but not phosphorylase kinase. When added to extracts of livers from control birds, cyclic adenosine 3',5'-monophosphate stimulated protein kinase but not phosphorylase kinase. The cyclic adenosine 3',5'-monophosphate had no effect when added to extracts from birds fed ochratoxin A. These results suggest that ochratoxin A affects primarily the cyclic adenosine 3',5'-monophosphate-dependent protein kinase which initiates the enzymatic cascade leading to glycogenolysis. Furthermore, these results conform an earlier assignment on morphological criteria of the glycogenosis of ochratoxicosis as a type X glycogen storage disease.     

Particulate glycogen of mammalian liver: specificity in binding phosphorylase and glycogen synthase.

The glycogen particle - glycogen metabolizing enzyme complex was investigated to gain some understanding of its physiological significance. Fractionations of populations of particles from mouse liver were carried out utilising open column and high performance liquid chromatography, and based either on the molecular weight of the particles or the hydrophobic interactions of the glycogen-associated proteins. The activities of glycogen phosphorylase and glycogen synthase were measured in these fractions. Fractionations were of tissue in different stages of glycogen deposition or mobilization. In animals fed ad libitum, glycogen synthase was associated with the whole spectrum of molecular weights, while the glycogen phosphorylase distribution was skewed in favour of the lower molecular weight species. Under conditions of glycogen mobilization, the phosphorylase distribution changed to include all molecular weights. The hydrophobic interaction separations demonstrated that glycogen synthase binds to a specific subpopulation of particles that is a minor proportion of the total. In general, there was a direct relationship of the total amount of phosphorylase and synthase bound during periods of mobilization and deposition, respectively. Two notable exceptions were the large amounts of glucose-6-P dependent synthase present during the early period of glycogen mobilization and the high amounts of active phosphorylase appearing shortly after food withdrawal, in spite of interim glycogen deposition from presumably already ingested food.     

Muscle glycogen oxidation during prolonged exercise measured with oral [13C]glucose: comparison with changes in muscle glycogen content.

Plasma glucose and muscle glycogen oxidation during prolonged exercise [75-min at 48 and 76% maximal O(2) uptake (Vo(2 max))] were measured in eight well-trained male subjects [Vo(2 max) = 4.50 l/min (SD 0.63)] using a simplified tracer technique in which a small amount of glucose highly enriched in (13)C was ingested: plasma glucose oxidation was computed from (13)C/(12)C in plasma glucose (which was stable beginning at minute 30 and minute 15 during exercise at 48 and 76% Vo(2 max), respectively) and (13)CO(2) production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% Vo(2 max) [15.1% (SD 3.8) and 45.9% (SD 5.8)] and at 76% Vo(2 max) [15.4% (SD 3.6) and 59.8% (SD 9.2)] were well in line with data previously reported for similar work loads and exercise durations using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and postexercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than at 48% Vo(2 max). However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two work loads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.     

A hypersensitivity of glycogen phosphorylase activation in hearts of diabetic rats

This study was initiated to determine whether glycogen phosphorylase activation was defective in hearts of alloxan diabetic rats. When hearts were perfused by gravity flow for 1 to 10 min with various concentrations of epinephrine, activation of glycogen phosphorylase in the diabetic was significantly greater at every time and epinephrine concentration than that seen in the normal. Cyclic AMP accumulation and protein kinase activation by epinephrine in the diabetic were not appreciably different or were lower than the normal responses to the hormone. The effects of epinephrine on cAMP and protein kinase were blocked in both normal and diabetic hearts by propranolol. While the beta blocker prevented phosphorylase activation in the normal hearts, it did not block phosphorylase activation by epinephrine in the diabetic hearts. Likewise, the alpha agonist phenylephrine activated phosphorylase in the diabetic but not in the normal hearts. While glucagon produced the same phosphorylase hypersensitivity in diabetic hearts, the cAMP and protein kinase responses were not altered by diabetes. Phosphorylase phosphatase activity was found to be unaltered by either epinephrine or diabetes, whereas phosphorylase kinase activation by epinephrine in the diabetic was double the normal response. These data are consistent with a diabetes-related unmasking of an alpha effect on cardiac phosphorylase activation and an unexplained increase in the sensitivity of phosphorylase kinase activation by protein kinase.     

A new interpretation of sulfate activation of rabbit muscle glycogen phosphorylase

It is widely known that sulfate ion at high concentration serves like an allosteric activator of glycogen phosphorylase (GP). Based on the crystallographic studies on GP, it has been assumed that the sulfate ion is bound close to the phosphorylatable Ser¹⁴ site of nonactivated GP, causing a conformational change to catalytically-active GP. However, there are also reports that sulfate ion inhibits allosterically-activated GP by preventing the phosphate substrate from attaching to the catalytic site. In the present study, using a high concentration of sulfate ion, significant enhancement of GP activity was observed when macromolecular glycogen was used as substrate but not when smaller maltohexaose was used. In glycogen solution, nonreducing-end glucose residues are localized on the surface of glycogen and are not distributed homogenously in the solution. Using cyclodextrin-immobilized column chromatography, we found that sulfate at high concentration promoted GP–dextrin binding through the dextrin-binding site (DBS) located away from the catalytic site. This result is consistent with the properties of the DBSs found in glycogen-debranching enzyme and β-amylase. Therefore, we propose a new interpretation of the sulfate activation of GP, wherein sulfate ions at high concentration promote glycogen-binding to the DBS directly, and glycogen-binding to the catalytic site indirectly. Our findings were successfully applied to the affinity purification of porcine brain GP.     

Interaction between glycogen and glycogen phosphorylase of Tetrahymena

The glycogen phosphorylase of Tetrahymena pyriformis complexes with glycogen as judged by its elution pattern from columns of Sepharose 6B. Complex formation does not occur with starch, amylose, or amylopectin, and neither do these polyglucans serve as primers for the enzyme. To study the association between the phosphorylase and glycogen particles in situ, Tetrahymena were grown under differing physiological conditions, phosphorylase was isolated and chromatographed on a Sepharose 6B column. Phosphorylase activity isolated from cells grown in the absence of glucose was only partially associated with glycogen, while in cells exposed to glucose for 30 min or more all the phosphorylase activity was associated with glycogen. The effects of culture age and anaerobiosis on the relative amounts of free and glycogen-bound enzyme in the cells were also studied. It was concluded from the in vivo experiments that there was no simple relation between the fraction of enzyme bound to glycogen and between cell glycogen content.     

The liver angiotensin receptor involved in the activation of glycogen phosphorylase

Specific angiotensin binding to rat hepatocytes and purified liver plasma membranes was measured by using biologically active [(3)H]angiotensin (sp. radioactivity 14Ci/mmol). The kinetic parameters for angiotensin binding to hepatocytes are: K(+1) (association rate constant). 100mum(-1).min(-1); K(-1) (dissociation rate constant), 2min(-1); K(d) (dissociation constant). 30nm; maximal binding capacity, 0.42pmol/10(6) cells or 260000 sites/cell. Angiotensin binding to membranes is profoundly affected by GTP (0.1mm) and NaCl (100mm); these regulatory compounds greatly enhance both the rate of association and of dissociation and also the extent of dissociation. K(d) amounts to 10nm in the presence of GTP+NaCl and to 1.5nm in their absence; maximal binding capacity is 0.70pmol/mg of protein, both with or without GTP+NaCl. The relative affinities of 11 angiotensin structural analogues were deduced from competition experiments for [(3)H]angiotensin binding to hepatocytes and to membranes (in the latter case, GTP + NaCl were not included, in order to study the higher affinity state of the receptor). These are highly correlated with their biological activity (activation of glycogen phosphorylase in hepatocytes). Binding to membranes occurs in the same concentration range as the biological effect. On the other hand, the existence of numerous spare receptors is suggested by the observation that binding of the agonists to hepatocytes requires 25-fold higher concentrations than those needed for their biological activity. These data clearly suggest that the detected binding sites correspond to the physiological receptors involved in the glycogenolytic action of angiotensin on rat liver.     

Schistosoma haematobium: histochemistry of glycogen, glycogen phosphorylase a and glycogen branching enzyme in niridazole-treated females.

The body posterior to the ovary of Schistosoma haematobium females was investigated. Glycogen, glycogen phosphorylase a (EC 2.4.1.1) and glycogen branching enzyme (EC 2.4.1.18) activities were detected in the subtegumental muscle system, parenchyma and mature vitelline cells, whereas no activities were detected in the tegument and immature vitelline cells of the parasite. Administration of a single niridazole dose of 250 mg kg-1 to the pouched mouse (Saccostomus camestris) produced the following changes in S. haematobium females: a relatively rapid depletion of glycogen stores due to disruption of the absorptive surface of the parasite, and to an increase in the activity of glycogen phosphorylase a; a reduction in the phosphorylase a to phosphorylase b-conversion capacity of glycogen phosphorylase phosphatase (EC 3.1.3.17); a decrease in glycogen branching enzyme activity; and a relatively rapid degeneration of parasite cells possibly due to their loss of endogenous energy reserves.