Chapter 3 



GLYCOGEN 



G 



lycogen is perhaps the most important storehouse of carbo- 

 hydrates that Ave possess. It is a macro-molecular polysaccharide 

 with a tree-like structure. Though found in all cells, including 

 those of placenta and endometrium, its main storage sites are the 

 liver and muscles. Granules of glycogen are found in the leukocytes, 

 cartilage cells and frequently in fat cells (Cameron, 1952) . Com- 

 binations of protein and glycogen exist in muscle in the form of 

 myosin and myoglobin and with serum globulin. A protein fraction 

 containing glycogen has been obtained from liver. 



Under ordinary circumstances the liver contains 0.5 to 6 g. per 

 100 g. wet tissue of glycogen, varying in amount and in lobular dis- 

 tribution throughout the day and reaching a maximum during the 

 night. Though largely formed from absorbed glucose it is also con- 

 cerned in gluconeogenesis. In this case fatty acids and proteins are 

 transformed to carbohydrates, first by the production of glycogen 

 and ultimately with the formation of glucose. In this way the blood 

 receives an important gift of sugar independently of that derived 

 directly from the glycogen stores and also by absorption from the 

 intestines. Hepatic glycogen and blood sugar are in equilibrium; 

 when the blood glucose level exceeds approx. 120 mg. per 100 ml. 

 the liver releases less glucose and glycogen synthesis increases. When 

 the blood glucose level falls, more glycogen than usual is broken 

 down by glycogenolysis to replenish the glucose in the blood. This 

 mechanism is controlled by the autonomic nervous system, imple- 

 mented by hormones from the anterior pituitary, adrenal cortex, 

 thyroid gland and the islets of Langerhans in the pancreas. 



THE STRUCTURE OF GLYCOGEN 



Since an understanding of glycogen behaviour in the damaged 

 cell depends upon our knowledge of its structure we shall first of all 

 discuss modern ideas on its constitution and then indicate how the 



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