162 J. SOSKA, L. BENES, V. DEASIL, Z. KARPFEL, E. PALECEK AND M. SKALKA 



spots containing deoxyribosidic comiDoiinds increased in the sequence 

 from non-irradiated control to irradiated control, irradiated hepatecto- 

 mized to hepatectomized non-irradiated livers. Since Sugino and 

 Potter (1960) have recently established that the activity of deoxycyti- 

 dylate-deaminase is reduced after irradiation, it seemed probable that 

 the nucleotides accumulating after irradiation are derivatives of de- 

 oxycytidine whereas the others appearing after hepatectomy in non- 

 irradiated animals might perhaps be derivatives of deoxyuridine and 

 thymidine. The nucleotides were dephosphorylated by a snake-venom 

 preparation and subjected then to paper chromatography as nucleo- 

 sides: preliminary results have, however, indicated the presence of 

 deoxyuridine and thymidine derivatives in all groups except in non- 

 irradiated controls, i.e. also in irradiated groups. The interval of 28 hr 

 after hepatectomy was perhaps too long and the appearance of thymi- 

 dine derivatives may be the result of recovery. 



Summing uj), Ave should like to stress some of the results. The effect 

 of deoxynucleotides on mitosis, the possibihty of increasing radiation- 

 inhibited DNA-synthesis by means of deoxynucleotides or by the nucle- 

 otide fraction of embryo -extract, further the changes of the content of 

 free deoxynucleotides and deoxynucleosides in regenerating liver after 

 irradiation. They suggest that the effect of radiation on processes 

 leading to DNA-synthesis in animals may be more important than the 

 effect on the process of polymerization or on the macromolecular DNA 

 itself. 



This is in accordance with the results of Sugino and Potter (1960) 

 who have found that radiation blocks the enzymatic reactions that re- 

 sult in the synthesis of thymidylic acid. Even then there remains the 

 unsolved question why these enzymes are inactivated, since in the case 

 of other enzymes, no inactivation as a result of irradiation in vivo has 

 been so far observed. Quite on the contrary, a raised enzymatic activity 

 has been often noted. Consequently, it appears that the second possi- 

 bility is more probable, that is to say that after irradiation there takes 

 place disintegration of large structural subcellular units or entire 

 specialized cells that are carriers of the respective synthetic activity. 

 Experiments with regenerating liver would support the interpretation 

 that irradiation blocks some inductive process resulting in the synthesis 

 of enzymes that make themselves apparent in the preparation of DNA- 

 SA^nthesis, in particular in the synthesis of some deoxyribotides. 



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Berenbom, M., and Peters, E. R. (1956). Radn iJes. 5, ol."). 



CoHN, W. E., and Carter, C. E. (1950). J. Amer. rhein. Soc 72, 4273. 



