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

Discovery and Biotechnological Exploitation of Glycoside-Phosphorylases

Among carbohydrate active enzymes, glycoside phosphorylases (GPs) are valuable catalysts for white biotechnologies, due to their exquisite capacity to efficiently re-modulate oligo- and poly-saccharides, without the need for costly activated sugars as substrates. The reversibility of the phosphorolysis reaction, indeed, makes them attractive tools for glycodiversification. However, discovery of new GP functions is hindered by the difficulty in identifying them in sequence databases, and, rather, relies on extensive and tedious biochemical characterization studies. Nevertheless, recent advances in automated tools have led to major improvements in GP mining, activity predictions, and functional screening. Implementation of GPs into innovative in vitro and in cellulo bioproduction strategies has also made substantial advances. Herein, we propose to discuss the latest developments in the strategies employed to efficiently discover GPs and make the best use of their exceptional catalytic properties for glycoside bioproduction.

Enzymological investigations on the causes for the PSE-syndrome, II. Comparative studies on glycogen phosphorylase from pig muscles

In order to investigate the cell biological causes for the fast breakdown of glycogen which is observed during the development of the PSE (pale, soft, exudative) syndrome in muscles of stress-susceptible pigs, muscle glycogen phosphorylase (GP) as a key enzyme in two isoforms, a and b, of the energy turnover was isolated from M. longissimus dorsi of normal and PSE-prone pigs of the German Landrace. GP b as well as GP a from normal and PSE-muscles exist in a dimeric form with a molecular weight of 97 000 D per subunit. The tendency for tetramerization of GP b increases in the presence of ATP, whereas the enzyme activity is simultaneously inhibited. The catalytic activities of GP a and GP b from both groups of animals show an optimum at pH 7.0. GP b can be activated to GP a by phosphorylation with the result of a 25% higher optimum specific activity in the case of normal and PSE-muscles. In interaction with glycogen and glucose-1-phosphate GP b follows the characteristics of a Michaelis-Menten kinetic, whereas the binding of AMP and phosphate proves to be allosteric. In comparison of the structural and kinetic characteristics of GP from normal as well as PSE-muscles no significant differences could be determined, indicating that GP does not belong to those factors which are triggering an accelerated energy turnover of ATP in muscles of stress-susceptible pigs.

Identification of a putative collagen-binding protein from chicken skeletal muscle as glycogen phosphorylase

We have purified and generated antisera to a 95 kDa skeletal muscle protein that constitutes the largest mass fraction of gelatin-agarose binding proteins in skeletal muscle. Preliminary results indicated that this 95 kDa chicken skeletal muscle protein bound strongly to gelatin-agarose and type IV collagen-agarose, suggesting a possible function in muscle cell adhesion to collagen. However, N-terminal sequencing of proteolytic fragments of the 95 kDa protein indicates that it is the chicken skeletal muscle form of glycogen phosphorylase, the binding of which to gelatin-agarose is unlikely to be biologically relevant. Further characterization showed that the skeletal muscle form of glycogen phosphorylase is immunologically distinct from the liver and brain forms in the chicken, and suggests that, unlike mammalian skeletal muscle, chicken skeletal muscle may have two phosphorylase isoforms. Furthermore, immunolocalization data and solubility characteristics of glycogen phosphorylase in muscle extraction experiments suggest the enzyme may interact strongly with an unidentified component of the muscle cytoskeleton. Thus, this study yields a novel purification technique for skeletal muscle glycogen phosphorylase, provides new information on the distribution and isoforms of glycogen phosphorylase, and provides a caveat for using gelatin affinity chromatography as a primary step in purifying collagen-binding proteins from skeletal muscle.

An automated assay of glycogen phosphorylase in the direction of phosphorolysis

We have developed a rapid, automated assay for glycogen phosphorylase that measures the degradative reaction. The substrate contains higher concentrations of phosphate and AMP than other methods, and the enzyme is preincubated with the substrate before adding phosphate to start the reaction. These modifications improve the activity and linearity of the assay. The new assay compares favourably with a standard phosphorylase assay in the synthetic direction and is linear to 500 U/L. We have used this assay to measure phosphorylase activity in human tissue samples and muscle biopsy specimens.

[Enzyme activity of cardiac glycogen metabolism: study of an in situ hypoxia protocol in the rat]

Myocardial hypoxia, induced by arrest of the artificial ventilation of anaesthetized open-chest rats, was utilized in order to study some aspects of the regulation of myocardial glycogen metabolism. Atenolol, a cardioselective beta-adrenergic receptor antagonist, and verapamil, an inhibitor of sarcolemmal calcium transfer, were used to determine the respective role of adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium in the activation of the enzymes of glycogen phosphorolysis and synthesis. Glycogen degradation is reduced by atenolol treatment, as a consequence of a reduced activation of glycogen phosphorylase. Verapamil treatment has no significant effect, neither on the enzyme activation nor on the glycogen utilization. The activation of glycogen synthase, expressed by the conversion of the enzyme from the D to the I form, which results from the decrease in glycogen stores during hypoxia, is lowered under the effect of both drugs. However, in the beta-blocker treatment case, this effect results from a lower glycogen depletion while this effect is more specific in hearts from rats treated with verapamil. Under the effect of verapamil, the reduction of synthase activation, for a similar depletion of glycogen stores, was confirmed by experiments using isolated rat hearts submitted to ischaemia. These results show that: 1. the glycogenolysis in the hypoxic myocardium in situ is mainly controlled by a cAMP-dependent enzyme conversion or by metabolic allosteric effectors; 2. the activation of myocardial glycogen synthase, which is essentially correlated to the reduction of glycogen stores, is also calcium-dependent and most probably totally cAMP-independent.

Activation and inactivation of glycogen phosphorylase isoenzymes purified from diabetic rat heart

1. Hearts of diabetic rats gradually accumulate glycogen, although the activities of glycogen synthase and glycogen phosphorylase are altered in favor of a depletion of glycogen. 2. Phosphorylase in diabetic hearts has been reported to be even more activated in response to adrenaline than controls. 3. The situation is further complicated by the fact that in rat heart two isoenzymes of phosphorylase are present. Therefore we have studied the properties of phosphorylases purified from diabetic rat heart in more detail. 4. This investigation revealed that compared to controls: (A) the amount of enzyme protein which could be isolated from diabetic animals is drastically lower; (B) the affinities towards glycogen and inorganic phosphate are decreased; (C) the activation by phosphorylase kinase is delayed; and (D) the inactivation by protein phosphatase-1 is accelerated. 5. We conclude that all of the reported changes in diabetes might contribute to a phosphorylase system less able to catalyze glycogen breakdown effectively.

Rat heart glycogen phosphorylase II is genetically distinct from phosphorylase I

Previous studies in our laboratory have shown that rat heart glycogen phosphorylase (1,4-alpha-D-glucan: orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1) separates into two forms upon ion-exchange chromatography. Both forms could be shown to have the same subunit Mr and to incorporate one molecule of phosphate per subunit. The studies reported here were done to check whether both forms are native isoenzymes and, further, which form might represent the heart-specific phosphorylase. Firstly, the iso-electric points of the purified enzymes are compared with those associated with phosphorylase activity in crude extracts from rat heart. Two out of four major bands coincided with the bands of purified phosphorylase Ib and IIb (isoelectric points: 5.5 and 6.25), indicating apparent identity. Secondly, antibodies to rat skeletal muscle phosphorylase reacted with rat heart phosphorylase I, whereas phosphorylase II was neither inhibited nor precipitated by the antibody. Thirdly, peptide maps obtained after proteolytic digestion of SDS-denatured phosphorylase I and II showed different patterns. In addition to the kinetic differences between these two forms reported earlier, phosphorylase IIa was inhibited by glucose 6-phosphate, whereas phosphorylase Ia was not. These results suggest that phosphorylase II is a heart-specific isoenzyme which is presumably encoded by a different gene.

Isolation of partial cDNAs for rat liver and muscle glycogen phosphorylase isozymes

cDNA clones for the rat liver and muscle glycogen phosphorylase isozymes have been isolated using isozyme-selective antibodies and libraries prepared in the expression vector, lambda gt11. A 1.2 kb cDNA coding for the carboxy-terminal domain of rat liver phosphorylase was found to have 82% homology with the amino acid sequence of rabbit muscle phosphorylase. Limited sequencing of rat muscle phosphorylase cDNA indicated a 95% homology with the rabbit muscle enzyme. The rat liver clone has eight additional amino acid residues at the COOH-terminus compared to the rat muscle clone. Furthermore, 17 of 26 (65%) residues between amino acids 815-840 differ between liver and muscle isozymes. The similarity in enzymatic properties and conservation of structure except at the COOH-terminus suggest that the liver and muscle phosphorylase isozymes do not exist in order to have significant differences in the regulation of glycogen breakdown in the two tissues. Rather, the phosphorylase isozymes probably evolved for tissue-specific transcriptional regulation of the genes in liver and muscle.