IMMEDIATE EFFECTS OF X-RAYS ON LIVING CELLS 133 



Stocken (1900), showed tliat the real effect was an inhibition of the 

 formation of the enzymes needed to produce the synthesis of the 

 nucleic acid. If this effect on the formation of adaptive enzymes is 

 likely to be a usual point of attack by X-rays, it must be further 

 studied. It is unfortimately not yet known whether, when such en- 

 zymes appear in the tissue in response to a new need for them, they 

 represent a new or much increased formation or actually an uncovering 

 of enzymes already present. That this type of uncovering of enzymes or 

 removal of enzyme inhibitors can occur in tissues has been demonstrated 

 in various cases. Schneider (1960), however, has been able to cause an 

 inhibition of nucleic acid synthesis in regenerating liver by giving in- 

 hibitors of protein synthesis at this early stage. It may be that, in his 

 experiments also, the appearance of new enzyme is being prevented, 

 and therefore, that this new appearance is indeed a new formation of 

 protein. 



Another system of synthesis, this time actually in the nucleus is the 

 phosphorylation of nucleosides demonstrated by Osawa et al. (1951) in 

 isolated thymus nuclei and shown in Dr. Stocken's department to be 

 extremely easily inhibited by previous irradiation of the animal. It was 

 difficult to demonstrate the existence of the phosphorylating enzyme 

 in various other tissue nuclei but recent work by Crathorne and Shooter 

 (1960) on the whole animal suggests that a phosphorylating enzyme 

 exists in the nuclei of ascites tumour cells and of regenerating liver cells 

 at some early stages of regeneration. When thymidine is injected into 

 the animal, mono-, di- and triphosphate derivatives are found in the 

 nucleus in much greater amounts than in the cytoplasm. It will l)e most 

 interesting to know whether this system is also a primary target of 

 X-ray action. Osawa et al. found that, as would be expected, the exis- 

 tence of a phosphorylating system is necessary for the formation of 

 protein in the isolated nuclei. 



We have lately been following the uptake of amino acid into one 

 fraction of the nuclear protein and attempting to trace the effect of 

 irradiation upon this and upon nucleic acid synthesis at the same time. 

 For this we have done experiments in vivo with rat liver in the condition 

 of regeneration after partial hepatectomy. 



The animals were taken at a time after hepatectomy when nucleic 

 acid synthesis was already occurring actively and when a large X-ray 

 dose (2,000 to 3,000 r) was necessary to reduce it to 50 per cent of the 

 normal rate. In our laboratory, Looney et al. (1960) demonstrated that 

 this inhil)ition represented a slowing of synthesis only and that the 

 irradiated cells eventually formed their full complement of DNA. To 

 do this, they needed about 13 hr instead of the usual 8. 



