Glycogen Turnover 



303 



The presumed source of a glucosyl residue in inner tiers is a glucosyl 

 residue in an outer tier, which hypothesis requires that new branch 

 points be continuously established. A mechanism whereby this may 

 happen has recently been elucidated by Larner, 14 who has given the 

 name amylo-(l,4 — » l,6)-transglucosidase to a branching enzyme which 

 generates a-1,6' glucosidic bonds at the expense of a-1,4' bonds. He 

 has demonstrated an effect in vitro which places a labeled glucosyl 

 residue, initially in the outermost tier, into an inner tier of the glycogen 



6HR. 



150 

 100 

 50 



L_ 

 150 



100 



50 







12 HR. 



P-l 



P-2 



^P-3 



^P-3 

 LD-3 



24 HR. 

 P-3. 



P-l 



P-L' 



LD-3 



48 HR. 

 P-3 



P-l 



P-2 



LD-3 



25 50 75 100 25 50 75 100 

 Per cent glucose residues R 



Fig. 6. Distribution of radioactivity in rat-liver glycogen. 



molecule. It must be supposed that it is this or some similar mecha- 

 nism which is responsible for the effect which we have observed, 

 namely, the gradual transfer of labeled glucose residues, initially most 

 abundant at the periphery of glycogen, into more and more centrally 

 situated laminae. 



From these studies a picture, or rather a motion picture, is presented 

 of at least a part of the process of glycogen regeneration. It is quite 

 clear that this is not a process wherein a newly synthesized glycogen 

 molecule is created in place of another one which is destroyed. Rather, 

 new glucose residues are introduced into the glycogen reservoir by a 

 process of continuous accretion at the periphery of pre-existing glyco- 

 gen molecules. By a second process, involving establishment of new 

 branch points, glucose residues in the glycogen molecule become pro- 

 gressively more remote from the outermost tier of glucosyl residues. 

 Since, under most circumstances, including those of the present experi- 



