298 Essays in Biochemistry- 



Table 2. Distribution of Radioactivity in Glycogen from Fasted and 

 Fed Rats Killed 3 Hours after Administration of Glueose-C 14 



* Specific activity reported as c.p.m. per milliatom C. 



rat appeared to be labeled nearly uniformly. This equality of labeling 

 of the maltose and limit dextrin shows that, in the liver glycogen 

 recovered when glucose is given after a suitable period of fasting, 

 virtually the entire molecule, center as well as periphery, has arisen 

 from blood glucose. This is clearly a consequence of the depletion 

 of hepatic glycogen which results from fasting. The specific activity 

 of carcass glycogen was far higher when derived from the fasted than 

 from the fed rat, probably because of less dilution of injected glu- 

 cose-C 14 by non-isotopic glucose in the fasted rat. Nevertheless, it is 

 noteworthy that the distribution of isotope within this glycogen be- 

 tween maltose and LD (glycogen, /3-amylase) was not greatly different 

 from that in the corresponding sample from the fed animal. This latter 

 finding reflects the well-established observation that muscle glycogen, 

 in contrast to liver glycogen, is not depleted by fasting. 



It would appear, from these results that glycogen regeneration in 

 tissues involves primarily the addition of glucoside residues derived 

 from blood glucose to non-reducing ends of pre-existing polysaccharide 

 molecules. The reactions whereby this occurs probably include those 

 catalyzed by hexokinase, phosphoglucomutase, and phosphorylase. 

 These reactions alone would account for the appearance, after glu- 

 cose-C 14 administration, of radioactivity in the maltose liberated from 

 glycogen by the action of /3-amylase. The introduction of isotope 

 into the limit dextrin (glycogen, ^-amylase) , although a slower process, 

 takes place both in liver and in muscle and indicates the occurrence 

 of another distinct process involving the establishment of new points 

 of branching in the glycogen molecule. 



