444 ANIMAL BIOCHEMISTRY 



most completely depletes the liver of glycogen, but restoration is rapid 

 as soon as a suitable diet is ingested. Even with average footl intakes 

 the glycogen is continually broken down and replaced, although the 

 day-to-day levels change little. A system of this kind is called a dy- 

 namic equilibrium or steady state. In rats on normal rations, gly- 

 cogen has a half-life of about 1 day, meaning that half the initial 

 glycogen is utilized daily and replaced from glycogenic materials of 

 the diet. 



Glycogenic substances are those potentially leading to the formation 

 of glycogen and include all materials that directly and indirectly 

 yield glucose- 1 -phosphate. Once formed, this last compound is con- 

 verted to liver glycogen by the enzymatic reactions summarized on 

 page 220. In studying glycogenesis, the compounds to be tested are 

 fed to rats previously starved until their livers are depleted of gly- 

 cogen. If a compound can be converted to glycogen, the latter is 

 found upon analysis of the liver. Thus it is possible to discover the 

 compounds that can lead to glycogen after starvation. 



However, not all these compounds will necessarily yield glycogen 

 when they are added to otherwise normal diets. Glycogen will be 

 formed, of course, but since the level is relatively constant and may 

 be maintained from other dietary components, a simple glycogen 

 analysis cannot identify the source. For such studies, radioactive 

 compounds must be used, glycogen isolated, and its radioactivity ex- 

 amined. Much less information of this type is available, but a num- 

 ber of compounds probably contribute to the glycogen supply even 

 under normal dietary conditions. 



The glycogen content of liver varies in any given individual with 

 the composition and quantity of the diet, physical activity, lack of 

 oxygen, acidosis (abnormally low pH of body fluids), and hormones. 

 Administration of epinephrine, an adrenal secretion, stimulates the 

 conversion of glycogen to glucose and rapidly depletes the liver. 

 Other hormones also alter glycogen levels, some promoting increases, 

 others decreases. Insulin with its pronounced effects on the concen- 

 tration of glucose in blood might be expected to affect liver glycogen 

 also. However, there is not a marked influence by insulin, and in 

 some animal species none at all. The situation is often quite different 

 in muscle from that in liver. 



Glycogen is digested by hydrolytic reactions in the intestinal tract 

 just as other polysaccharides are. Inside cells the endogenous glycogen 

 is degraded by phosphorolytic processes instead, and breakdown takes 

 place by the reversal of the synthetic steps (page 220). Thus glucose- 1- 

 phosphate is the primary product rather than glucose. In this way 



