Comparative catalytic properties of liver glycogen synthases from adult and newborn rats

Liver glycogen synthases have been isolated from adult and newborn rats. Comparative catalytic properties show that adult synthase, which undergoes covalent modification resulting in an altered enzyme activity related to the need for glycogen synthesis, has properties consistent with the hypothesis that the alpha (dephosphorylated) enzyme is that which is physiologically active in vivo when liver glycogen is synthesized. Synthase from the newborn, which does not show an endogenous b into a transformation, has catalytic characteristics similar to the adult a-type synthase. However, the newborn synthase is absolutely dependent on Glc-6-P for activity, as is the adult-type synthase b. A comparison of the effects of Glc-6-P and UDP-Glc on Pi and ATP inhibition of the adult and newborn synthases suggest the "apparent" b-type synthase from the newborn is more subject to metabolite regulation of activity than the adult-type b enzyme.