AFTER-EFFECTS OF IRRADIATION OF DNA 



J. A. V. Butler and (Miss) P. Simson 

 Chester Beatty Research Institute, Royal Cancer Hospital, London 



'After-effects' of irradiation are not unusual. The irradiation initiates 

 a sequence of eflfects which continues for some time. This is particularly 

 the case with living organisms, which are dynamic systems, in which inter- 

 ference with one part may produce a long sequence of changes, ending in 

 death. After-effects can be observed in some cases in purely chemical 

 systems such as DNA. Their interest is mainly in the opportunity they 

 give of determining the mechanism of the processes involved. 



We should like to add some new observations to those previously given 

 on the subject. The slow decrease of the viscosity of DNA which occurs 

 after irradiation in the presence of oxygen has been ascribed^-^ to (1) the 

 formation of substances of a peroxidic nature which are formed by processes 

 such as 



RH + OH-> R- + H2O ; R* + O^ -> ROa* ; RO/ + H -> RO^H. 



It would follow that such peroxides would undergo a slow decomposition. 

 (2) the formation of labile phosphate owing to the oxidation of the C4 atom 

 of the sugar, giving a keto-sugar from which the phosphate is capable of 

 slow hydrolysis^. 



We have made some further experiments in order to define the circum- 

 stances under which the latter process occurs. Weiss and Scholes have 

 stated that although only small amounts of phosphate are hberated in the 

 action of X-rays, considerably greater amounts are formed by hydrolysis 

 with acids after such treatment. 



We have used as a hydrolysing agent 0-5 N, H2SO4 at 70°. This produces 

 practically no phosphate in 1 hour with unirradiated DNA, but in longer 

 times the amount of phosphate liberated increases rapidly {Figure 1). Now 

 acid treatment of this nature is known to liberate purines from the DNA 

 and the initial lag period may be ascribed to the time taken for an appreci- 

 able amount of depurination to occur since the intact nucleotide chain is 

 apparently not susceptible to hydrolysis until the purines have been removed. 

 Another factor is the fact, already discussed^, that two adjacent phosphate- 

 sugar hnks have to be broken before inorganic phosphate is liberated. 

 That is, the breakage of one phosphate-sugar link, although it breaks the 

 nucleotide chain, can only result in the formation of a terminal phosphate 

 group. The free phosphate produced by hydrolysis of X-ray treated DNA 

 might then be the result of the acid hydrolysis of the terminal phosphate 

 groups. It is found {Figure 1) that the effect of irradiation with X-rays is 

 to shorten the initial lag period. Comparatively large doses are required 

 to produce a marked effect. It is difficult to give an unambiguous explana- 

 tion of this, as it might be due to {a) an increase in the susceptibility of the 

 nucleotide chain to hydrolysis owing to loss of purines ; {b) the partial 



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