300 



Essays in Biochemistry 



have been evaluated and the results have been plotted in Fig. 3. The 

 ordinates, in this and succeeding figures, represent counts per minute 

 per milliatom carbon, corrected to 100 c.p.m. per milliatom C in the 

 initial glycogen sample. The abscissas represent the percentages of 

 glucosyl residues, initially present in glycogen, which have been elimi- 

 nated by the successive digestions. With reference to the structure 

 of glycogen (Fig. 1), R is the sole reducing end of the molecule which 

 is approached from the non-reducing ends by the repeated enzyme 



150 i- 



y 100 



p-1 



i — Amylase 



P-2 



^Average 



P-3 



LD-3 



o 



2b 



50 



75 



100 



Per cent glucose residues 



Fig. 3. Distribution of radioactivity in rabbit-muscle glycogen 6 hours after in- 

 jection of glucose-C 14 . 



treatments. The area in each block represents the total c.p.m. con- 

 tained in that segment of the molecule. 



From the data in Table 3 it will be seen that each successive limit 

 dextrin obtained was less radioactive than the parent substance from 

 which it was secured. This finding confirms the metabolic inhomoge- 

 neity of glycogen noted previously. From the schematic representation 

 (Fig. 4) of the tierwise degradation of a glycogen molecule, together 

 with the data plotted in Fig. 3, a clearer picture of the distribution 

 of isotope in this sample of glycogen is obtained. Here it will at once 

 be seen that the outer layer of the glycogen molecule (P-1) is more 

 radioactive and the central core (LD-3) is less radioactive than the 

 average of all the glucoside residues in the molecule. As one progresses 

 from the non-reducing ends of this 6-hour glycogen sample centrally 

 toward the reducing end, R, the enrichment of isotope diminishes in a 

 fairly smooth fashion. 



