302 



Essays in Biochemistry 



specific activity of each derivative was lower than that of the parent 

 substance from which it was derived. In each sample of carcass glyco- 

 gen (Fig. 5), the outermost tier of glucose residues (P-l) had a higher 

 specific activity than did the central core (LD-3) and, in each case, 

 LD-3 was the least radioactive portion of the molecule. With the 

 passage of time, the difference in specific activity between the periph- 

 eral and the central portions of the molecule diminished slowly. 



200 



150 



> 100 



'-3 

 M 



o 50 

 J3 



'o 

 Q. 



(0 



CD 150 



> 

 1 100 



50 

 L 



P-l 



24 HR. 

 P-3 



48 HR. 



P-2 



LD-3 



P-l 



P-2 



P-3 



LD-3 



25 50 75 100 25 50 75 100 

 Per cent glucose residues R 



Fig. 5. Distribution of radioactivity in rat-carcass glycogen. 



Rat-liver glycogen, isolated 6 hours after injection of glucose-C 14 , 

 resembled, in regard to isotope distribution (Table 5), the samples 

 secured from rat carcass. Again each successive limit dextrin was less 

 radioactive than its precursor, and the specific activity decreased reg- 

 ularly as the molecule was entered from the non-reducing termini (Fig. 

 4) . However, with the passage of time, a striking change occurs which 

 is clearly manifested in the 48-hour sample. Here we find a reversal 

 of the sequence of specific activities of the serial polysaccharides, 

 LD-3 > LD-2 > LD-1 > glycogen. This is reflected in the recon- 

 struction of the glycogen molecule in Fig. 6, where it will be seen that 

 the outermost tier (P-l) is now the least radioactive and that the 

 radioactivity systematically increases as the reducing end of the mole- 

 cule is approached. 



From these results it is clear that, in addition to a process which 

 adds glucose residues to the non-reducing ends of preformed glycogen 

 molecules, glycogen regeneration entails a second process which results 

 in the introduction of glucosyl residues into inner tiers of the molecule. 



