The initiation of glycogen biosynthesis in rat heart alpha-1,4 glucans tightly associated with glycogen synthase

Abstract

A partially purified glycogen synthase from rat cardiac muscle transferred glucosyl residues from UDP- [14C]glucose to an endogenous protein acceptor in the absence of added primer. After native gel electrophoresis of the enzyme preparation, unprimed activity was detected. Primer-dependent and independent activities were found in the same position. After denaturing gel electrophoresis of the reaction products, radioactivity comigrated with protein. Pulse-chase experiments showed that the size of the reaction products increased as a function of time. These products were degraded by amyloglucosidase, thus suggesting that glycogen-like molecules had grown on the protein acceptor.The activity of the enzyme was markedly reduced upon preincubation with a-amylase. Therefore, preformed protein-bound a-1 ,Cglucans were acting as primers. The glucoprotein acceptor may be a protein strongly associated with glycogen synthease, or alternatively, the enzyme itself.Since the discovery of the enzymes responsible for the formation of a-1 ,Cglucosidic bonds (glycogen synthase) and a-1 ,6-branched linkages (branching enzyme) of glycogen, the biosynthesis of this polysaccharide has presented the unique problem of how primer molecules, which are necessary in order to initiate glycogen synthesis [l], originate.We have previously reported the in vitro synthesis of glycogen apparently attached to protein in crude enzymes systems from rat liver [2-41 and heart [5] and from Escherichiu coli [6, 71, as well as in partially purified preparations from rat heart [8]. Similar evidence has been produced by other laboratories concerning glycogen synthesis in Neurospora crassa [9, 101 and bovine retina [ll]. Presumably, the polysaccharides synthesized in vitro correspond to the protein-bound glycogen reported to exist in vivo in rat liver [12] and heart [13], rabbit muscle [14] and bovine retina [ll].Polysaccharides other than glycogen, such as potato starch [15], paramylon (B-1,3-glucan) from EugZenu grucilis [16], P-1,2-glucans from Agrobacterium [17], dextran from Streptococcus mutans and Leuconostoc mesenteroides [18, 191 and glucosaminoglycan from cartilage [20] were also postulated to be synthesized on protein primers.