Skeletal muscle glycogen phosphorylase a kinetics: effects of adenine nucleotides and caffeine.

This study aimed to determine physiologically relevant kinetic and allosteric effects of P(i), AMP, ADP, and caffeine on isolated skeletal muscle glycogen phosphorylase a (Phos a). In the absence of effectors, Phos a had Vmax = 221 +/- 2 U/mg and Km = 5.6 +/- 0.3 mM P(i) at 30 degrees C. AMP and ADP each increased Phos a Vmax and decreased Km in a dose-dependent manner. AMP was more effective than ADP (e.g., 1 microM AMP vs. ADP: Vmax = 354 +/- 2 vs. 209 +/- 8 U/mg, and Km = 2.3 +/- 0.1 vs. 4.1 +/- 0.3 mM). Both nucleotides were relatively more effective at lower P(i) levels. Experiments simulating a range of contraction (exercise) conditions in which P(i), AMP, and ADP were used at appropriate physiological concentrations demonstrated that each agent singly and in combination influences Phos a activity. Caffeine (50-100 microM) inhibited Phos a (Km approximately 8-14 mM, approximately 40-50% reduction in activity at 2-10 mM P(i)). The present in vitro data support a possible contribution of substrate (P(i)) and allosteric effects to Phos a regulation in many physiological states, independent of covalent modulation of the percentage of total Phos in the Phos a form and suggest that caffeine inhibition of Phos a activity may contribute to the glycogen-sparing effect of caffeine.

Thermodynamic characterization of 5'-AMP binding to bovine liver glycogen phosphorylase a.

The binding of adenosine 5'-monophosphate to liver glycogen phosphorylase a (EC 2.4.1.1) has been studied by size exclusion high performance liquid chromatography and isothermal titration microcalorimetry at pH 6.9 over a temperature range of 25 to 35 degrees C. The results are compared with those of the binding of the same nucleotide to the muscle isozyme and to liver phosphorylase b. Calorimetric measurements in various buffer systems with different ionization heats suggest that protons are released during the binding of the nucleotide. The dimer of liver glycogen phosphorylase a has been shown to have two equal and independent sites for 5'-AMP, which would correspond to the activator sites identified in the muscle isozyme. The binding constants as well as the changes in Gibbs energy, enthalpy, and entropy per site for 5'-AMP binding were calculated at each temperature. The results show that the major contribution to the negative value of DeltaG0 stems from the value of DeltaH in the range of 25 to 35 degrees C. The enthalpy change of binding is strongly temperature-dependent, arising from a large negative DeltaCp of binding equal to -1.45 +/- 0.02 kJ K-1 (mol of 5'-AMP bound)-1, which suggests significant changes in the polar and apolar surfaces accessible to the solvent.

Allosteric regulation of liver phosphorylase a: revisited under approximated physiological conditions.

Phosphorylase removes glucosyl units from the terminal branches of glycogen through phosphorolysis, forming glucose-1-P. It is present in two interconvertible forms, phosphorylase a and b. The a form is the active form and is rate limiting in glycogen degradation. The activities of phosphorylase a and of total phosphorylase as conventionally measured exceed the activities of glycogen synthase R (active form) and of total synthase by approximately 10- and 20-fold. Thus, unless phosphorylase a is inhibited or compartmentalized or its substrates are exceedingly low in vivo, net glycogen synthesis could not occur. In addition, following an administered dose of glucose, phosphorylase a activity changes little when glycogen is being synthesized, is stable, or is being degraded, suggesting an important role for allosteric effectors in regulation. Therefore, we have determined the effect of potential modifiers of enzyme activity at estimated intracellular concentrations. Purified liver phosphorylase a was used. Activity was measured in the direction of glycogenolysis, at 37 degrees C, pH 7.0, and under initial rate conditions. Both a Km and a near-saturating concentration of inorganic phosphate (substrate) were used in the assays. A physiological concentration of AMP was saturating. It decreased the Km for Pi by approximately 50% and stimulated activity. ADP, ATP, and glucose inhibited activity. Fructose-1-P inhibited activity only at a high and nonphysiological concentration. Glucose-6-P and UDP-glucose were not significant inhibitors. Inhibition of activity by ADP was little affected by the addition of AMP. However, AMP partially abolished the inhibitory effect of ATP and completely abolished the inhibitory effect of glucose. When AMP, ADP, ATP, glucose-6-P, UDP-glucose, glucose, and fructose-1-P were added together, the net effect was no change in phosphorylase a activity compared to the activity without any effectors. In addition, changes in glucose concentration did not affect activity. K glutamine modestly stimulated activity. Numerous other metabolites were tested and were without effect. The present data indicate that the known endogenous allosteric effectors cannot explain the smaller than expected in vivo phosphorylase a activity or the regulation of phosphorylase a activity.

Purification and properties of glycogen phosphorylase from the fat body of larval Manduca sexta.

Glycogen phosphorylase b has been purified to homogeneity from the fat body of larval Manduca sexta. The purification procedure involved ammonium sulfate precipitation, and chromatography of DEAE-cellulose, 5'-AMP-Sepharose and Q-Sepharose. The final product, which showed a single band on SDS-PAGE with a M(r) = 92,500, was purified 50-fold from the original homogenate in a yield of about 3%. The molecular mass of the native purified phosphorylase b was estimated to be 186,000 Da from gel filtration, suggesting that the native enzyme is a dimer. The apparent Km values for glycogen, phosphate and 5'-AMP were 1.4 mM, 82 mM and 1.1 mM, respectively. The enzyme had a pH optimum of 7.05, and was inhibited by ATP, ADP and glucose, but not by trehalose, even at high concentration. Conversion of phosphorylase b into the a form was achieved by incubation with rabbit phosphorylase kinase and Mg(2+)-ATP. The molecular mass of phosphorylase a was estimated to be 250,000 Da by gel filtration chromatography. The specific activity of the a form in the presence of 5'-AMP was 1.6-1.7-fold higher than the specific activity of the b form under the same conditions. Thus, 5'-AMP activates the a form by about 20%, whereas ATP has no effect on the phosphorylase a activity.

Glycogen phosphorylase in the fat body of two cockroach species, Periplaneta americana and Nauphoeta cinerea: isolation, partial characterization of three forms and activation by hypertrehalosaemic hormones.

The presence of endogenous phosphorylase kinase and phosphorylase phosphatase in crude extracts of fat bodies from the cockroaches Nauphoeta cinerea and Periplaneta americana is demonstrated in vitro by activation/inactivation of glycogen phosphorylase under appropriate conditions. Fractionation of fat body extracts of both cockroach species on an anion-exchange medium results in the elution of three peaks with phosphorylase activity. According to their AMP dependency these activity peaks are designated as phosphorylase b (inactive without AMP), phosphorylase ab (active without AMP, but several times stimulated with AMP) and phosphorylase a (active without AMP). It is shown chromatographically that incubating crude extracts of fat bodies from both cockroaches, under conditions where the phosphorylase kinase is active, results in all phosphorylase b being converted to the ab- or a-form, whereas under conditions where the phosphorylase phosphatase is active all phophorylase a is converted to the ab- or b-form. Endogenous phosphorylase kinase of N. cinerea crude fat body extract can convert vertebrate phosphorylase b into the a-form, and, conversely, vertebrate muscle phosphorylase kinase and phosphorylase phosphatase, respectively, are able to convert partially purified N. cinerea phosphorylase ab or b and the ab- und a-form, respectively. In resting cockroaches most of the phosphorylase activity residues in the b-form and only a small fraction (10%) in the a-form, whereas between 26% (N. cinerea) and 35% (P. americana) occurs in the ab-form. Injection of endogenous hypertrehalosaemic peptides into N. cinerae (the decapeptide Bld-HrTH) or P. americana (the two octapeptides Pea-CAH-I and II) causes interconversion of phosphorylase; after injection, mainly (60%) phosphorylase a is present, while 25% and 15% exists in the ab- und b-form, respectively. Purification of the three phosphorylase forms from N. cinerea is achieved by anion-exchange chromatography on DEAE-Sephacel followed by affinity chromatography on AMP-Sepharose. The final specific activities are 2.1, 6.9 and 27.2 U/mg protein for the a-, ab- und b-form. The molecular mass of the active molecules on gel filtration is between 173,000 and 177,000, and SDS gel electrophoresis reveals a subunit mass of 87,100, suggesting a homodimeric structure for all three forms. Kinetic studies show hyperbolic saturation curves for the substrates glycogen and Pi, respectively, with KM-values of 0.021, 0.019 and 0.073% for glycogen and 8.3, 6.3 and 17.9 mM for Pi (a-, ab- and b-form). Phosphorylase a exhibits a more or less hyperbolic response to AMP and needs 70 microM AMP for maximal stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Binary affinity chromatography for the purification of glycogen phosphorylase.

A binary affinity chromatography medium was prepared and found to be useful for the purification and quantitative isolation of glycogen phosphorylase from rabbit skeletal muscle and liver. Glycogen is used as the binary ligand as it has affinity toward both the column matrix and the enzyme. Agarose beads derivatized with concanavalin A bound glycogen to the level of 35 mg/ml. The glycogen-impregnated beads were able to bind 9 mg/ml of phosphorylase a or b. The phosphorylase is tightly bound so that the column can be washed free of contaminants before quantitative elution of the phosphorylase by 2 M glucose, which releases the glycogen-phosphorylase complex. It appears that binary affinity chromatography may have general utility for the isolation and purification of enzymes and other specific binding agents.

Glycogen phosphorylase in fish muscle: demonstration of three interconvertible forms.

White skeletal muscle of crucian carp contains a single isoenzyme of glycogen phosphorylase, which was purified approximately 300-fold to a specific activity of approximately 13 mumol.min-1.mg protein-1 (assayed in the direction of glycogen breakdown at 25 degrees C). Tissue extracts of crucian muscle produced three distinct peaks of phosphorylase activity when separated on DEAE-Sephacel. Peaks 1 and 3 were identified, in terms of kinetic properties and by interconversion experiments, as phosphorylase b and a, respectively. Peak 2 was shown to be a phospho-dephospho hybrid. The three interconvertible forms of phosphorylase were purified and shown to be dimeric molecules at 20 degrees C. At 5 degrees C, a and the hybrid tended to form tetramers. The Mr of the subunit was estimated to be 96,400 from sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hybrid is kinetically homogeneous, and its kinetic properties are intermediate between those of b and a forms. The b, hybrid, and a forms of phosphorylase can be isolated from rapidly frozen muscle of crucian but in different proportions, depending on whether fish were anesthetized or forced to muscular activity for 20 s. Muscle of anesthetized crucian had 36, 36, and 28% of phosphorylase b, hybrid, and a forms, respectively, whereas the corresponding values for exercised fish were 12, 37, and 51%. Results suggest that three interconvertible forms of phosphorylase exist simultaneously in crucian muscle and that hybrid phosphorylase is active in contracting muscle in vivo.

[Glycogen phosphorylase from human leukocytes. Isolation and kinetic properties]. / Glikogenfosforilaza iz leikotsitov cheloveka. Vydelenie i kineticheskie svoistva.

Homogeneous glycogen phosphorylase from human leukocytes has been obtained. A one-step bioluminescent procedure for the enzyme activity assay has been developed. This method is based on a continuous recording of the product of the glycogen phosphorylase-catalyzed reaction using a coimmobilized multienzyme system (phosphoglucomutase, glucose-6-phosphate dehydrogenase, NADH:FMN oxidoreductase and bacterial luciferase). The method sensitivity is 10 times as high compared to earlier described methods. The Km values for glycogen (0.2 mg/ml) and phosphate (3.9 mM) at pH 7.9 were determined. AMP was shown to be the enzyme effector.

The catalytic subunit of phosphatase 2A dephosphorylates phosphoopsin.

Rod cell outer segments were found to contain a protein phosphatase activity toward phosphoopsin with properties very similar to those of protein phosphatase 1 or 2A. The opsin phosphatase activity was stable to ethanol precipitation, had a Mr of 35,000-38,000 as determined by gel filtration, and was not dependent on divalent cations for activity. The chromatographic properties on DEAE-cellulose of the rod outer segment protein phosphatase were also similar to those reported for protein phosphatase 1 or 2A. In order to distinguish between these two protein phosphatases, we tested homogeneous preparations of protein phosphatases 1 and 2A from skeletal muscle for activity toward phosphoopsin. Protein phosphatase 2A dephosphorylated phosphoopsin at approximately 10% of its rate toward phosphorylase a, whereas protein phosphatase 1 had no activity toward phosphoopsin. We conclude that protein phosphatase 2A is present in the rod cell outer segment and that it is a likely candidate to perform the in vivo dephosphorylation of rhodopsin in the visual cycle.

Purification of several proteins involved in glycogen metabolism.

Several well-established procedures for the isolation of enzymes involved in glycogen metabolism have been modified such that all the enzymes can now be isolated from the same muscle preparation. The purified proteins are the catalytic subunit of cyclic AMP-dependent protein kinase, its thermostable inhibitor, glycogen phosphorylases a and b, and phosphorylase kinase. Phosphorylase kinase is separated by acid precipitation of the muscle extract. The other proteins are purified from the acid supernatant by chromatography on DEAE-cellulose. Further purification of each protein to homogeneity is then achieved using previously described methods. The proposed protocol saves sample tissue, and considerably reduces the work involved in obtaining muscle samples.

Insoluble glycogen and its interaction with phosphorylase. A novel method for the purification of liver phosphorylase A.

Purified liver glycogen dissolved in Tris-HCl buffer (pH 6.8) was converted into an insoluble polymer by incubation with phosphorylase and glucose 1-phosphate. Elongation of the outer chains of glycogen did not alter the average molecular weight significantly as judged by sedimentation velocity measurements, but the spectrophotometric analysis of glycogen-iodine complexes showed marked differences. Insoluble glycogen could bind rabbit skeletal muscle and liver phosphorylases. The association of insoluble glycogen with phosphorylase could be treated as a distribution equilibrium between glycogen-bound and unbound phosphorylase. Based on the formation of an insoluble glycogen protein complex sedimentable even by low-speed centrifugation, a novel method has been developed for the purification of liver phosphorylase a in a homogeneous form.

Kinetic properties of glycogen synthase and phosphorylase and structural aspects of glycogen in the db/db mouse liver.

Kinetic studies were carried out on liver glycogen synthase and phosphorylase isolated from genetically diabetic db/db mice. Glycogen synthase a and b enzymes from diabetic mice had Vmax values 30% and 20% lower, respectively, than the enzymes from normal mice. Glycogen synthase b from diabetic mice also had a 30% lower I0.5 for Pi and ATP at physiologic concentrations of UDP-glucose (0.25 mM) compared with the normal enzyme. Kinetic studies of phosphorylase a showed that, at low glycogen concentrations (0.25 mg/ml), the Vmax of the diabetic enzyme was twofold greater than that of the normal enzyme. This was probably related to the diabetic phosphorylase a having a lower apparent Km for glycogen. This enzyme also had a slightly higher I0.5 for ATP compared with the enzyme from normal mice. Structural studies of liver glycogen isolated from these diabetic mice showed differences from normal mouse glycogen. Both the alpha- and beta-amylase limits were lower in the diabetic glycogen, and the average chain lengths, exterior chain lengths, and interior chain lengths calculated from these limits were all shorter in the glycogen from diabetic mice. Although both normal and diabetic glycogen absorbed light maximally at 430 nm when complexed with iodine, the absolute absorbance value was significantly lower for the diabetic glycogen. These data suggest an altered branching pattern of liver glycogen from the diabetic mice and it is suggested that this altered structure may ultimately influence the activities of glycogen-metabolizing enzymes. These results provide further characterization of the db/db mouse and show heretofore undescribed changes in phosphorylase a kinetics and glycogen structure that occur in diabetes.

Purification and crystallization of bovine liver phosphorylase.

We have purified and crystallized bovine liver phosphorylase a. Starting from 2.5 kg of liver, we obtain 250 mg of phosphorylase a, with a specific activity of 90 units/mg, representing 15% recovery. SDS polyacrylamide gels show three bands, a 95 kDa band with the same mobility as muscle phosphorylase, and two smaller bands of 55 kDa and 40 kDa, which are probably proteolytic fragments. These fragments remain associated and have native conformation and catalytic activity. Crystals which diffract to 2.8 A resolution, were prepared by the hanging drop method using polyethylene glycol PEG 4000 as precipitant. The crystals were prepared in the presence of activators maltotriose and phosphite and crack when placed in solutions containing the inhibitors glucose and caffeine. This suggests phosphorylase is present in an active conformation.

Isolation and partial characterization of two forms of rat heart glycogen phosphorylase.

Rat heart glycogen phosphorylase a has been purified to apparent homogeneity by a procedure involving precipitation with ammonium sulfate, DEAE-Sephacel chromatography, and AMP-Sepharose affinity chromatography. In contrast to the skeletal muscle enzyme which appeared as a single peak upon ion-exchange chromatography, heart phosphorylase was separated into two distinct peaks (I and II), both only active in the presence of AMP. The isoelectric points of skeletal muscle phosphorylase b (dephosphorylated form) and heart phosphorylase Ib were both at 6.2, that of heart phosphorylase IIb was 5.2. The Km of phosphorylase IIb for AMP was threefold lower (7 microM) than that of Ib (22 microM). The dissociation constant K8 of phosphorylase Ia (phosphorylated form) was 0.37 mM for glycogen and 5.6 mM for Pi. Increasing the levels of glycogen decreased the apparent Km for Pi and vice versa. No such interaction between substrate binding was observed with phosphorylase IIa, since the Ks values for glycogen (0.27 mM) and Pi (3.8 mM) were not significantly influenced by increasing concentrations of the other substrate. The specific activity of both isoenzymes was 46 units/mg of protein at pH 6.8 and 30 degrees C. The subunit Mr of both forms was 92,500. By incubation with purified phosphorylase kinase and [gamma-32P]ATP both forms were converted to their corresponding forms by 32P incorporation of 1 mol/mol of subunit. This suggests the existence of two native forms of phosphorylase b in rat heart.

Effect of glucose-6-P on the catalytic and structural properties of glycogen phosphorylase a.

Kinetic studies of muscle phosphorylase a in cationic buffer (pH 6.8) demonstrate that glucose-6-P competitively inhibits the binding of the substrate, glucose-1-P, to the enzyme. The inhibitory effect of glucose-6-P is largely overcome by glycerol-2-P. AMP counteracts inhibition of the enzyme by glucose-6-P, while glucose and glucose-6-P can interact to produce a synergistic inhibition of phosphorylase a activity. Preincubation of phosphorylase a with glucose-6-P at 20 degrees C results in approximately 3-fold increase in activity, while ultracentrifugation experiments carried out under the same conditions showed that phosphorylase a can be converted to dimers by glucose-6-P.

Rabbit muscle phosphorylase derivatives with oligosaccharides covalently bound to the glycogen storage site.

Linear maltooligosaccharides, e.g., maltoheptaose or terminal 4-O-methylmaltoheptaose, activated by cyanogen bromide, react covalently with rabbit muscle phosphorylases b and a (EC 2.4.1.1). Site-specific modification prevents further binding to glycogen and shifts the phosphorylase a tetramer-dimer equilibrium in favor of the dimer. Use was made of these properties to separate by affinity chromatography and gel filtration phosphorylase a dimers with specifically bound oligosaccharide from unspecifically modified products. The phosphorylase a-maltoheptaose derivative carries one oligosaccharide residue per monomer and can be distinguished from the native enzyme by its electrophoretic mobility in polyacrylamide gels or by affinity electrophoresis. Phosphorylase a preparations with covalently bound maltooligosaccharides are enzymatically active in the presence of a primer and alpha-D-glucopyranose 1-phosphate (glucose-1-P). Methylation of the nonreducing chain terminus of the bound oligosaccharide has no effect on glycogen synthesis. These findings exclude the participation of bound oligosaccharides in chain elongation. Purified covalent phosphorylase a-maltoheptaose complexes are stable dimers. They are no longer activated by glycogen. The properties of covalently modified phosphorylase-oligosaccharides are consistent with and provide direct evidence for the existence of a glycogen storage site in rabbit muscle phosphorylases. Covalent occupation of the storage site renders the affinity of glucose-1-P to phosphorylase a independent of modulation by glycogen, supporting the assumption that the glycogen storage site is involved in interactions with the catalytic site.