Glycogen Turnover 



DEWITT STETTEN, Jr., and 

 MARJORIE R. STETTEN 



Since the time of Claude Bernard it has been appreciated by bio- 

 chemists that glycogen represents an animal storage form of glucose. 

 Glycogen is well suited to this storage function for several reasons. 

 Its high molecular weight and low intrinsic viscosity permit its accumu- 

 lation in tissue fluids with but slight effect upon the osmotic pressure 

 or the fluidity. The readily reversible phosphorylase reaction permits 

 both the synthesis of glycogen from hexose phosphate and glycogen 

 breakdown to glucose-1-phosphate to proceed with very small energy 

 transfers. The breakdown of liver glycogen, under hormonal regulation 

 by epinephrine and probably also by glucagon, serves as an elegant 

 homeostatic device to sustain the blood glucose concentration under 

 a wide variety of circumstances. 



The storage of glycogen in liver or in muscle is not a dead storage. 

 It is, on the contrary, very much alive in that the constituent atoms 

 of the glycogen molecule, even in the animal maintained in a balanced 

 nutritional state, are continuously being regenerated de novo at the 

 expense of a variety of precursors. Since, under such controlled experi- 

 mental conditions, the quantity of glycogen in a given tissue remains 

 approximately constant, coincident with new glycogen synthesis the 

 occurrence of continuous glycogen breakdown must be postulated. 



The fact that, in the animal in nutritional balance, glycogen of liver 

 and muscle is in a dynamic steady state, undergoing continuous "turn- 

 over," was first observed in the study of the appearance of carbon- 

 bound deuterium in glycogen isolated from animals whose body fluids 

 were enriched with deuterium oxide. 1 In these experiments glycogen 

 was isolated from tissues of rats maintained for varying periods of time 

 with their body water enriched with D 2 0, and a progressive enrichment 

 of glycogen with stably bound deuterium was noted. When glucose 



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