Glycogen Turnover 299 



To explore this process more fully, it was deemed advisable not 

 merely to bisect the glycogen molecule by treatment with /3-amylase 

 but to dissect it, layer by layer, by the alternating exposure of glycogen 

 to phosphorylase and amylo-l,6-glucosidase (debranching enzyme). 

 To this end the methods devised in Cori's laboratory 3> 8 ' 10 were repro- 

 duced and applied to glycogen samples which were obtained from 

 animals after injection of glucose-C 14 . 13 



A preliminary experiment was conducted upon a sample of glycogen 

 secured from rabbit muscle. This animal was killed 6 hours after 

 intravenous injection of glucose-C 14 , and glycogen was isolated from 

 muscle and analyzed for C 14 . Treatment of this polysaccharide with 

 crystalline rabbit-muscle phosphorylase in a phosphate buffer yielded 

 glucose-1-phosphate and a limit dextrin, LD-1- (glycogen, phospho- 

 rylase). This product was assayed for radioactivity and was then 

 treated with a crude rabbit-muscle amylo-l,6-glucosidase solution in 

 the absence of inorganic phosphate. The enzyme was destroyed, and 

 phosphorylase and inorganic phosphate were added, to yield, ulti- 

 mately, a second limit dextrin, LD-2. Repetition of this procedure 

 yielded a third product, LD-3. The extent of each digestion was 

 measured by the sum of the glucose-6-phosphate plus glucose liberated 

 at each step. Radioactivity measurements were performed upon each 

 polysaccharide and, where feasible, also upon phenylglucosazone sam- 

 ples prepared from the products of each successive degradation step. 

 Excellent recoveries of isotope were secured from these combined 

 analyses, so that in later studies only the polysaccharides were 

 analyzed. 



The results secured are given in Table 3. Included also are the 

 results of a single-step /^-amylase degradation performed upon the 



Table 3. Distribution of Radioactivity in Rabbit-Muscle Glycogen 

 6 Hours after Intravenous Injection of Glucose-C 14 



Relative Specific Per Cent of Glycogen 



Sample Activity Digested, Cumulative 



Glycogen 100 



LD-1 (phosphorylase) 69.2 37.9 



LD-2 47.3 63.3 



LD-3 29.9 77.7 



LD (/3-amylase) 61.3 44 . 



same glycogen sample. From the values in this table, the specific 

 activities of the glucose residues eliminated by each degradation step 



