Glycogen 31 



the substrate for the brancher enzyme. The brancher acts on a chain 

 of that length only when it is an outer chain of glycogen. It does not 

 branch short dextrin chains. 



Degradation of glycogen is largely brought about by reversed 

 action of these enzymes. Phosphorylase leads to the loss by the chain 

 of one glucose residue and the formation of glucose- 1 -phosphate. 

 Phosphorylysis stops as the outermost tier of branch points of each 

 molecule is approached since phosphorylase can neither break nor 

 by-pass the branch points. The polysaccharide thus obtained is 

 called "phosphorylase limit dextrin." 



A debranching enzyme (amylo - 1, 6 - glucosidase) then removes 

 free glucose from "limit dextrin." Glucose and "limit dextrin" less 

 the outermost tier of branch points are the reaction products of this 

 process. Phosphorylase in the presence of inorganic phosphate can 

 act again on this polysaccharide. Alternating one with the other in 

 this way, the two enzymes degrade the entire glycogen molecule, 

 taking it to pieces tier by tier. Ultimately the digestion mixture 

 contains only glucose and glucose- 1 -phosphate. In the body tissues 

 the latter is transferred to glucose-6-phosphate through a reversible 

 reaction mediated by phosphoglucomutase. These processes of 

 build up and degradation are depicted in the scheme prepared by 

 Cori (1952-3) . 



This summary should make clear how the liver maintains blood 

 glucose at normal levels. Through the process of glycogenosis 

 glycogen is broken down, by the coordinated action of specific 

 enzymes, through glucose-6-phosphate to glucose which is then 

 made available to the blood stream. Glycogen stores are replenished 

 by phosphorylating blood glucose to glucose-6-phosphate which is 

 then transformed to glycogen by hexokinase (Sant' Agnese, 1959). 



Somewhere in this scheme must be fitted in the action of hor- 

 mones. At present we can do no more than offer conjectures based 

 upon experimental studies and await the outcome of future in- 

 vestigations. The pituitary gland exerts an important controlling 

 action on glycogen production in the liver, for that organ almost 

 completely loses its glycogen store when the pituitary is removed. 

 Gluconeogenesis and increased fatty acid synthesis are the main 

 disturbances. Corticotropin (ACTH) and growth hormones de- 



