Glycogen Metabolism in Quail Embryo Muscle

Abstract

Cultured quail embryo muscle has proven to be an excellent model system for studying the synthesis of macromolecular glycogen from, and its degradation to, glycogenin, the autocatalytic, self-glucosylating primer for glycogen synthesis. We recently demonstrated that proglycogen, a low-M, form of glycogen, is an intermediate in the synthesis. Here we show that proglycogen also functions as an intermediate in macroglycogen degradation and, in one set of circumstances, represents an arrest point in glycogen breakdown, which does not continue to glycogenin. We suggest that in the nutritionally dependent turnover of glycogen in tissues, the molecules cycle between proglycogen and macromolecular glycogen and are not normalIy degraded to glycogenin. Nevertheless, when this does happen, the released glycogenin is active, capable of re-initiating glycogen synthesis. Under culture conditions where the conversion of proglycogen into glycogenin does take place, the intermediates lying between form a discrete rather than a continuous series, suggestive of a cluster structure for proglycogen and indicating that breakdown is stepwise. Evidence of post-translational modification of glycogenin was obtained by the finding that, in glycogen from cultured muscle, glycogenin is phosphorylated.Keywords: glycogen ; glycogenin ; proglycogen ; macroglycogen ; glycogen synthase.Glycogenin is a protein having a molecular mass of 38 kDa, to which glycogen is covalently attached via an a-glucosidic bond between the terminal reducing glucose residue of the polysaccharide and the hydroxyl group of Tyrl94 of the protein (for reviews see [ 1, 21). When obtained in a glycogen-free state from muscle, heart, liver or kidney, glycogenin shows enzymic activity. It transfers glucose residues from UDP-Glc to itself in an autocatalytic manner, stimulated by Mn7+ [3, 41, building a maltosaccharide chain varying in length over 5 -l l glucose units [5] which can now prime glycogen synthesis by glycogen synthase and branching enzyme. The glucotransferase activity is preserved even when self-glucosylation is abolished by recombinant DNA replacement of Tyrl94 with Phe or Thr. This was demonstrated by the use of external acceptors for glucose transfer, dodecyl p-maltoside [ 61 and p-nitrophenyl maltosaccharides 171. The prediction by Krisman and Barengo [Sj that a protein can serve as a primer for glycogen synthesis and remains covalently bound within the glycogen, has been fully validated by the above findings, with the exceptions that only one molecule of glycogen is formed on the protein, while 'glycogen initiator synthase', postulated to glucosylate glycogenin, is glycogenin itself.In normal conditions, glycogenin in skeletal muscle is an integral part of glycogen. Free glycogenin cannot be detected in a fresh extract by SDSPAGE and radioautography after incubation with UDP-['4C]Glc or by immunoblotting [Y]. When the Correspondence to W. J.

Analysis of proglycogen and macroglycogen content in muscle biopsy specimens obtained from horses

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