A new enzymatic activity participating in the initiation of glycogen biosynthesis in rat brain.

A rat brain extract, able to synthesize from UDP-Glc an alpha-1,4-glucan covalently bound to a protein in the absence of added primer is described. The compound formed is precipitable by dilute trichloroacetic acid (TCA). In the presence of glycogen, added as primer, this molecule is enlarged and is not precipitable by TCA. Unprimed and primed activities differ in several aspects, such as the behavior in the presence of some effectors, and the optimum pH. Umprimed and primed activities presented two pHs optima, both sharing only one. The proteoglucans synthesized under the different pHs gave different patterns after analysis under denaturing PAGE and the oligosaccharides synthesized on the protein backbone differ in the glucosyl length. It is concluded that also in rat brain, the initiation process of glycogen biosynthesis is mediated through the formation of a glycoprotein. Our present results showed that the step of the putative "Glycogen Initiator" proposed by use before, requires two enzymes UDPGlc-transglucosylating activities, Glycogen Initiator 1 and Glycogen Initiator 2, before Glycogen Synthase in the alpha-1,4-glucosidic linkages formation.