304 Essays in Biochemistry 



ments, the animal is in approximate nutritional balance and analyt- 

 ically in an approximately steady state, the process of glycogen growth 

 must be offset by a process of glycogen decay. The steps involved 

 in this process are presumed to be the phosphorolytic uncoupling of 

 peripherally situated glucosyl residues operating in conjunction with 

 the hydrolytic cleavage of exposed glucosyl residues in a-1,6' linkage 

 due to the action of the debranching enzyme, amylo-l,6-glucosidase. 



It should be noted that the description just given of glycogen turn- 

 over may also serve to explain glycogen accrual and depletion. If this 

 proves to be the case, it represents an unusual mode of storage in that 

 no change in the number of glycogen molecules is postulated. If 

 glycogen accrual results merely from the action of phosphorylase and 

 branching enzyme, if depletion is the result simply of phosphorylase 

 and debranching enzyme activity, then changes in glycogen content, 

 in contrast to alterations in fat or protein content, are reflections of 

 changes in mean molecular weight of glycogen, the number of molecules 

 remaining constant. This would be a happy arrangement in that it 

 would permit large fluctuations in the magnitude of the glycogen re- 

 serve to occur with no accompanying changes in the colligative proper- 

 ties, notably osmotic pressure, of tissue fluids. 



Regardless of whether the count of glycogen molecules in a given 

 adult tissue is or is not constant, it is clear that during growth the 

 number of glycogen molecules must at some stage increase. Within 

 the limits of our present understanding of the actions of phosphorylase, 

 amylo-l,6-glucosidase and amylo-(l,4 -^ l,6)-transglucosidase, no ex- 

 planation for replication of glycogen molecules exists. Replication, 

 when and if it does occur, may result from the a-amylolytic activity 

 of tissues, generating a plurality of seeds for glycogen synthesis from 

 a single glycogen molecule. 



It appears likely that much of what has been said about the nature 

 of glycogen synthesis in the living animal may also apply, with very 

 minor modifications, to the synthesis of the less highly branched plant 

 polysaccharides, the starches. Certain differences exist, however, which 

 may raise the question of whether polysaccharide synthesis occupies 

 the same crucial position in animal as it does in vegetable economy. 

 If attention is momentarily focused upon glycogen of liver, it will be 

 recalled that this is considered to be a mobile reservoir of glucose and 

 of energy. It is, however, a small reservoir, amounting calorically in 

 the rat to some 3% of the total daily caloric requirement. Further- 

 more, after it has been virtually eliminated, as by fasting or by injec- 



