A Molecular Order in the Synthesis and Degradation of Glycogen in the Liver

Abstract

When fasted rats or mice were refed, liver glycogen was synthesised at a linear rate of approximately 1%/h for a period of about 6 h. The administration of a radioactive precursor, usually [14C]galactose, allowed pulse labelling of the glycogen formed at various times after the initiation of refeeding. Soon after the administration of the label, the radioactive glucosyl units were preferentially located on the outer chains of glycogen. An almost equal distribution between the inner and outer chains was attained after a time which increased with increasing amounts of preexisting glycogen. Later on, the radioactive molecules were not further enlarged although the mass of glycogen increased threefold.The degradation of the glycogen that had been labelled at various times after initiation of refeeding was induced either in vivo in anesthetized rats, or in vitro in isolated hepatocytes and in a cell‐free system made up of the enzyme‐glycogen complex as isolated by concanavalin A. Under all of these conditions, the radioactive units that were incorporated last were liberated first and vice versa. This ordered degradation could not be explained by the initial removal of outer chains. In the concanavalin‐A‐bound glycogen, the ordered degradation was much less apparent when the preparation had been submitted to high‐speed centrifugation, vigorous homogenization, ultrasonic treatment or exposure to detergents.