X-Rays and Cell Metabolism 119 



struggled. In these earlier experiments we always appeared 

 to show a temporary effect of X-rays on ribonucleic acid 

 synthesis and sometimes on the uptake of phosphate into the 

 cell, as well as on deoxyribonucleic synthesis. Hevesy also 

 had found that, if a long, slow irradiation was given, the 

 fastening down of the animal caused a diminution in 

 the amount of phosphate with ^^P arriving in the cell. The 

 contrast of such results with our later specific ones caused a 

 suspicion that they may have been due partly to "shock", 

 and Professor Green's observations (Bullough and Green, 

 1949) that mitotic inhibition occurred in the skin of the ear 

 after release of a ligatured limb made it seem much more 

 probable. Professor Green very kindly carried out one of his 

 experiments on rats bearing the Jensen sarcoma and gave me 

 a chance of following nucleic synthesis with ^^P at the same 

 time. 



The chief effect visible in the first hour of shock, was 

 inhibition of ribonucleic turnover. Later, the inhibition of 

 uptake of ^^p into the cell [a mechanism which is now being 

 shown by Popjak (1950), Sacks (1948) and others to be a 

 phosphorylation process] became so marked that the radio- 

 active counts in the nucleic acid fractions were reduced to a 

 very low level. This effect is not, apparently, due to cir- 

 culatory collapse, since we have not seen it in animals at the 

 point of death from nembutal or chloroform anaesthesia. 

 Inhibition of deoxyribonucleic synthesis did occur, but not 

 to a very marked extent. This more generalized type of 

 phosphorylation inhibition is being examined further, but I 

 should like to return for a minute to the specialized effect of 

 X-rays. 



When such a definite effect on the life of the cell is caused 

 by the small amount of ionization produced by a clinical dose, 

 one is forced to suppose that the destruction or inhibition of 

 an enzyme is involved. In the synthesis of a substance like 

 deoxyribonucleic acid, necessitating phosphorylation and 

 many other processes, a very large number of enzymes must be 

 concerned, the loss of any one of which would put the system 



