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

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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 .alpha.-amylase. Therefore, preformed protein-bound .alpha.-1,4-glucans were acting as primers. The glucoprotein acceptor may be a protein strongly associated with glycogen synthease, or alternatively, the enzyme itself.

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