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The mechanism of glycogen biosynthesis in the absence of added primers takes place at least in two different steps. The first step could be initiated by a new enzyme named glycogen intiator synthase that catalyzes the transfer of glucose from UDP-glucose to an acceptor protein. This step takes place in vitro only in the presence of some salts at high concentration. The complex product from this reaction has been isolated and demonstrated to act as a precursor for the synthesis of glycogen which takes place in the second step and is catalyzed by the already known glycogen synthase.Since glycogen synthase was discovered by Leloir and Cardini [l] a great deal of research has been done on the biosynthesis of glycogen but very little is known about the mechanism by which the molecules start to grow. It is generally accepted that glycogen synthases function only in the presence of primers that can be glycogen or oligosaccharides [2-41. On the other hand, we have recently shown that glycogen synthetase isolated from rat liver catalyzes the transfer of labelled glucose from UDP-[U-'4C]glucose to protein or protein-bound oligosaccharides when no primer is added [5,6]. The product formed is completely solubilized by treatment with pronase or amylase. It is also alkali-stable but acid-labile and contains a-1 ,Clinked glucosyl residues.In this paper we present results that demonstrate that the initial acceptor for the synthesis of glycogen is therefore a protein. MATERIALS AND METHODS UDP-glucose was obtained from Sigma ChemicalCompany. UDP-[U-'4C]-glucose was prepared according to the method of Wright and Robbins [7].The rat liver enzyme preparation and the determination of enzyme activity was carried out as described in previous papers [5,6].Liver branching enzyme was purified as described previously [8] but the livers were thoroughly perfused Enzymes. Glycogen synthase (EC 2.4.1.11) ; 1 ,Ca-glucan branching enzyme (EC 2.4.1.18).with 250mM sucrose, 5mM EDTA and 10mM mercaptoethanol.Paper chromatography of oligosaccharides was performed with butanol-pyridine -water (4 : 3 : 4)as solvent [9]. Sugar and polyalcohol standard spots were developed with the silver nitrate reagent [lo]. Glycogen was estimated by the iodine-CaC1, method [ll]. Borohydride Reduction and Total Hydrolysis of Glycogen PrecursorTo 1 pmol oligosaccharides 17 pmol sodium borohydride was added in 0.3 ml water. After 20 h at room temperature, the reaction was terminated by the addition of 100 pl 5 N HC1, and boric acid was removed by repeated addition of methanol and evaporation.

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