X-Rays and Cell Metabolism 121 



Holmes: That is why we try to follow it in the body with tracers. 

 As soon as you take the tissue out I suppose mitosis stops, and I don't 

 think we should have much hope of seeing mitosis effects on tissue slices. 



Brown: With regard to the problem of interference with nucleic acid 

 synthesis, you probably are familiar with the observations by Skipper 

 and co-workers at Southern Research Institute that an anti-folic-acid 

 will decrease the incorporation of formate into the purines of mixed 

 nucleic acids, PNA and DNA. We have done a similar experiment 

 in which, rather than a chronic toxic dose, a single lethal dose of 

 Amethopterin was given. In this case the incorporation of formate into 

 the PNA is decreased on the first day, but if the formate is given 24 

 or 48 hours afterwards, there is a much greater incorporation of that 

 formate. The animal still dies on about the fourth day, but on the 

 third day it has largely recovered the ability to elaborate purines from 

 formate. Another experiment has been done in which the formate was 

 labelled with i*C and the adenine, which was given simultaneously, 

 was labelled with ^^N. This experiment indicates that the anti-folic-acid 

 has apparently greatly decreased the synthesis of purine from formate, 

 but has not correspondingly decreased the synthesis of nucleic acids 

 from preformed adenine. 



The fact that two different precursors of the same final product can 

 yield such different results adds just one more complication to any 

 studies of the effects of a drug or of radiation. 



