MAURICE ERRERA 



cytoplasmic cell particles. If the cells are made to divide in the absence of 

 the adaptive substrate Spiegelman^^ has calculated that these particles 

 would be diluted between the daughter cells and eventually lost. It would 

 be of interest to investigate if the synthesis of adaptive enzymes is more 

 sensitive to irradiation after a culture of micro-organisms has gone through 

 rapid cell divisions : it would furthermore give an experimental check to 

 Spiegelman's attractive hypothesis. 



Concerning fatty acids synthesis, one has not found any inhibition by 

 radiation. As this biochemical activity is performed by mitochondria, one 

 must conclude that their sites, responsible for fatty acid synthesis, are not 

 radio-sensitive. It would be of interest, as we have seen that oxidative 

 phosphorylation which is also a mitochondrial activity, may be inhibited, 

 to study different enzymes bound to mitochondria and see if the ones pre- 

 sumed to be bound to the same class of particles undergo inactivation 

 simultaneously or not. 



Experiments on the synthesis of nucleic acids have given more encouraging 

 results and an inhibition of the synthesis of deoxyribonucleic acid (DNA) 

 has constantly been found in irradiated cells, whereas inhibition of ribo- 

 nucleic acid synthesis or turnover is somewhat less marked or synthesis 

 may even be increased. This is apparent from the work of Hevesy^', 

 Holmes*^, Abrams^, Forssberg^^% Kelly*'' *^, BAcq^ and others and from 

 some of the work performed on bacterial viruses (Latarjet^*') . 



As we have pointed out earlier, the inhibition of DNA synthesis may be 

 the result of one of the following causes : 



(a) Inhibition of the synthesis of DNA precursors : this mechanism seems ■ 

 improbable because the synthesis of mononucleotides in yeast or liver cells 

 is not inhibited (Sherman and Forssberg^'^) . Similar results obtained with 

 E. coli with ultra-violet light point to the same conclusion : various low 

 molecular weight derivatives of DNA begin to accumulate immediately 

 after irradiation when the synthesis of DNA is blocked (Kanazir*'*). These 

 data indicate that the block in DNA synthesis probably operates at later 

 stages of this chain reaction. 



{b) DNA metabolism could become abnormal as a consequence of the 

 incorporation into the normal chain of reactions of an unusual precursor 

 which could be formed by the effects of irradiation on some building block. 

 Many instances of the incorporation of chemical analogues of nitrogen bases 

 have been described in the case of RNA metabolism in mice, bacteria or 

 viruses, and one knows also that abnormal ribosides may be formed by 

 exchange enzymatic reactions (Friedkin^^). 



{c) Several authors have looked for immediate physical or chemical 

 effects of radiations on DNA irradiated in vivo or in situ. The earliest of 

 these attempts was made on nucleated erythrocytes : the gels obtained with 

 the nuclei of irradiated cells are less rigid than those of controls and one 

 knows that the rigidity of these gels depends upon the presence of ' intact ' 

 DNA'-^i' 22. More recently Limperos and Mosher^^ found that the DNA 

 extracted from thymus of irradiated mice, immediately after irradiation, 

 contained a decreased purine to pyrimidine ratio, ' depolymerization ' being 

 observable only 24 hours later. Harrington and Koza^* found that the 

 affinity for methyl green of the nuclei of irradiated embryos was diminished 



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