Glycogen Turnover 295 



In view of the lack of information concerning the metabolic homo- 

 geneity of glycogen in tissues of the intact animal, it was considered 

 of interest to re-explore the intimate nature of glycogen regeneration. 

 Is glycogen turnover a process in which a molecule is broken down 

 completely to be replaced by a newly formed one, or does it involve 

 removal and replacement of a fraction of the glucosyl residues within 

 a given glycogen molecule? The experiments designed to this end have 

 taken advantage both of the newer knowledge of glycogen structure 

 and of the specific enzyme activities discussed above. Glycogen ex- 

 hibits a certain structural homogeneity, not shared by many of the 

 mammalian macromolecular substances, in that on total hydrolysis 

 all of its carbon is recoverable exclusively in glucose. The question 

 of whether glycogen constituted, metabolically, a perfectly mixed pool 

 of glucose residues wherein, after isotopic enrichment by one or an- 

 other device, glucose residues, regardless of their location in the mole- 

 cule, had an equal probability of being labeled could now be studied. 



The basic experimental design has been simple. Into normal rats 

 and rabbits glucose uniformly labeled with C 14 has been injected. After 

 various intervals of time, animals were sacrificed and glycogen was 

 isolated from livers and carcasses. After purification of these products, 

 their radioactivities were determined. Samples were then subjected to 

 one or more enzyme digestions, and the radioactivity of the products 

 of such degradations were examined. 11 



Enzyme digestions, in the earlier experiments, were limited to treat- 

 ment with barley malt /^-amylase. The commercially available prep- 

 aration of this enzyme, although far from pure, is demonstrably free 

 of a-amylase, amylo-l,6-glucosidase, and maltase activity. Glycogen 

 subjected to exhaustive treatment with /^-amylase gives, in excellent 

 yields, maltose and a limit dextrin, LD (glycogen, /3-amylase), which 

 resists all further attack by this enzyme. In effect this enzyme bisects 

 the glycogen molecule into two approximately equal portions, the 

 peripheral cortex of unbranched termini, recoverable as maltose, and 

 the central medullary portion, recoverable as the limit dextrin. 



When the concentrations of C 14 in these two portions of a given 

 sample of glycogen were compared, they were found, in general, to be 

 unequal. Samples of glycogen were studied, which were obtained from 

 rat livers and rat carcasses, from 3 to 48 hours after intraperitoneal 

 injection of glucose-C 14 . Results are given in Table 1. In the earlier 

 time intervals, the specific activity was in all cases higher in the maltose 

 (periphery) than in the LD (glycogen, /^-amylase). Indeed, despite 

 a relative rise in the specific activity of maltose and a relative fall in 



