ACTION OF IONIZING RADIATIONS ON CELL CONSTITUENTS 



Table I. Effects of ionizing radiation on individual enzymes — continued 



one immediately sees that, when irradiated in vivo, most proteins studied 

 so far are very radio-resistant. A same enzyme, in different tissues, may 

 show a variety of responses to irradiation according to the experiment and 

 even — SH enzymes Hke adenosine triphosphatase, hexokinase and ribo- 

 nuclease do not appear to be inactivated by dosages which initiate profound 

 biological damage. 



(2) Energy sources 



If instead of individual enzymes, complete chain reactions like glycolysis, 

 respiration or oxidative phosphorylations are investigated, more promising 

 results are obtained. Classical experiments of Crabtree and Gray^'^ have 

 shown that irradiation of rat retinal tissue with l,000r inhibits the formation 

 of lactic acid without altering respiration. Holmes*^ discovered that lactic 

 dehydrogenase was more radio-sensitive in tissues of young embryos than 

 in more mature organisms. Hickman and Ashwell obtained similar 

 inhibitions in mouse spleen homogenates after total body irradiation**^ ; 

 but triose phosphate dehydrogenase (an — SH enzyme) was not affected. 

 The block seems to result from the alteration of an adenine nucleotide 

 phosphorous acceptor and such inhibitions of glycolysis may be related to the 

 fact that liver glycogen increases after in^adiation, even in fasting animals 

 (McKee^s, Ross and Ely^^). 



96 



