290 Essays in Biochemistry- 



Table 1. Distribution of Radioactivity in Rat Liver and Carcass 

 Glycogen after Intraperitoneal Injection of Glucose-C 14 



Relative Specific Activity * 



* Expressed as per cent of specific activity of glycogen. 



f The limit dextrin here referred to is LD (glycogen, ^-amylase). 



that of the limit dextrin, even 48 hours after the injection of glucose-C 14 

 the periphery of the carcass glycogen molecule was more radioactive 

 than was the center. Liver glycogen was somewhat different in this 

 regard in that by about the twelfth hour the radioactivity was found 

 to be approximately uniformly distributed between maltose and LD 

 (glycogen, /2-amylase), whereas in the experiments of longer duration 

 the central core of the molecule (limit dextrin) was consistently more 

 radioactive than the peripheral (maltose) tier. The distribution of 

 isotope between the periphery and limit dextrin is plotted in Fig. 2 

 as a function of time after glucose-C 14 injection. 



In the interpretation of these results it must be borne in mind that 

 the glucose-C 14 was all injected at the outset of the experiment and 

 that therefore the specific activity of circulating glucose was constantly 

 declining throughout the period of observation. On the basis of results 

 of Feller et al. 12 it may be estimated that the rate of this decline was 

 such as to give a half-time of about 1 hour. Undoubtedly by the 

 twelfth hour the specific radioactivity of circulating glucose available 

 for glycogen synthesis was lower than the mean specific activity of 

 the glycogen into which it was being incorporated. Whereas the earlier 

 data reflect the incorporation of glucose-C 14 into glycogen, the later 

 data are a consequence of introduction of glucose-C 12 into glycogen-C 14 . 

 The data strongly suggest that, whereas circulating glucose serves as a 

 fairly immediate precursor of the glucosidic residues situated periph- 



