Radiation and Regenerating Rat Liver 229 



body irradiation on the incorporation of adenine in the 

 nucleic acids of various tissues. About GO per cent depression 

 of incorporation into DNA was found 1-3 hours after irradia- 

 tion while the effect on RNA was slight. In most tissues, the 

 depression of DNA synthesis after 48 hours was very large, 

 whereas in the intestine the synthesis is actually much above 

 normal at this time. 



On the whole, the DNA synthesis is most affected in all 

 tissues, which made it reasonable to consider the possibility 

 that the synthesis of thymine or the thymine nucleotides of 

 DNA might be particularly sensitive to irradiation. An 

 attempt to show this was made by Mee (1956), who used 

 [ i^C] formate to follow the synthesis of the bases in RNA and 

 DNA and obtained a regeneration curve which followed the 

 32P uptake curve almost exactly. The bases were separated 

 and the activity was estimated. Three hours after a dose of 

 2000 r, the specific activity of the DNA was depressed to 

 half of the control value; the adenine, guanine and thymine 

 were, however, equally affected. The possibility still remains, of 

 course, that lack of thymine has prevented the appearance of 

 half the control amount of new DNA. Ord and Stocken (1956), 

 however, comparing the separated nucleotides of DNA in 

 tissues irradiated in vitro, have given data which suggest that 

 the addition of phosphorus to the purine nucleotides is more 

 easily inhibited than the uptake of pyrimidine nucleotide 

 phosphorus. 



So far, this work had been concerned with changes in the 

 nucleic acid metabolism only, but it was plainly of interest to 

 relate changes in the general metabolism of the liver to these 

 special ones. An investigation of the changes in metabolism 

 accompanying regeneration and the effect of irradiation upon 

 them has been carried out by Dr. Itzhaki of our laboratory and 

 will be described in the following pages. 



It has been known for some time (Ludwig, 1939; Gursch, 

 Vars and Rardin, 1948) that the neutral fat content of 

 regenerating liver was considerably above the normal level, 

 but this rise in neutral fat had not been correlated with any 



