Enzymes in Irradiated Tissues 39 



ing the 24 hours following X-radiation. Kleczkowski (1954) 

 has found that if chymotrypsin is irradiated with ultraviolet 

 light and then kept at 2°, no further activity is lost, but if 

 kept at 37° for 48 hours the irradiated sample loses more 

 activity than a parallel control sample. Irradiation has made 

 the enzyme abnormally susceptible to body temperature. We 

 cannot therefore draw any hard and fast line between the 

 cause of enzyme change detected as soon after radiation as we 

 are able to make the estimation, and enzyme change that 

 develops gradually. 



The time that must elapse between irradiation and bio- 

 chemical testing is necessarily long compared to the time 

 taken in metabolic reactions; a 10-second illumination of 

 green algae in the presence of carbon dioxide is sufficient 

 for the synthesis of a very large number of compounds 

 (Bassham et al., 1954). The time that elapses between irradi- 

 ation and metabolic examination is ample for a long 

 sequence of changes to interpose between direct and indirect 

 effects of radiation and the change we measure. 



It does, however, seem worth while to concentrate on what 

 Errera (1955) has termed initial effects. The fact that few, if 

 any, such effects have been found may be an indication of our 

 ignorance of large parts of tissue metabolism, and the fact 

 that even the known parts have not yet been thoroughly 

 surveyed in relation to radiation. But it could also be ex- 

 plained if the only effect of radiation is to upset the molecules 

 of DNA and to upset its synthesis. This may well be an 

 enzymic effect since one could expect that DNA molecules 

 may have enzymic activities and will be synthesized by 

 enzymic processes, but at least it restricts the area of investi- 

 gation. I do not think any evidence we have at present can 

 decide. Certainly radiation affects cells other than those 

 capable of division. Patt (1955) has pointed out that there are 

 many departures from the simple condition relating radio- 

 sensitivity to growth and differentiation. The metabolic 

 activity of the cell can determine radiosensitivity, e.g. 

 chilling lymphocytes to 2° either shortly before or shortly 



