KNUT MIKAELSEX 



ends so that restitution of broken chromosomes is favoured ? A combina- 

 tion of both is an alternative possibiUty. Prevention of breakage is con- 

 sidered to be tlie most likely explanation. What arguments exist in favour 

 of such an assumption ? 



Barron and Flood'"- presume that such oxidizing agents are responsible 

 for the oxidation of aqueous solution of certain thiols by ionizing radiation, 

 as they have demonstrated. Therefore, it seems probable that the mechan- 

 ism of the sulphydryl compounds, glutathione, cysteine and thiourea, may 

 be due to their reactions with oxidizing agents produced by irradiation in 

 the cell nuclei. These oxidizing agents may be inorganic or organic free 

 radicals or peroxides. Thus, the sulphydryl compounds compete with the 

 chromosomes for free radicals or other oxidizing reactants and less breaks 

 will be produced. 



It must also be considered that absorption of the sulphydryl compounds, 

 as w^ell as the other chemicals tested, may change the metabolic state of 

 the cell. Patt'^ has reported that his data suggest perhaps that cysteine 

 action is related to the availability of intracellular oxygen. It may be justi- 

 fiable, at least, to conclude that the sulphydryl compounds exert their effects 

 through their — SH groups since the results are so similar with the three 

 compounds. It is reasonable to assume that sodium hyposulphite, which 

 is a strong reducing agent and combines rapidly with molecular oxygen in 

 aqueous solution, decreases the amount of dissolved oxygen in the tissue. 

 It is assumed that by removing oxygen from the tissue, the production of 

 the secondary formation of O2H and H2O2 or other reactive products is 

 prevented. King et al^^ claim, however, that in large measure oxygen 

 exerts its action by altering the biological processes of the cell. 



The protective ability of sodium cyanide may be more complicated and 

 obscure than was the case with the other compounds tested. It appears, 

 especially as regards the two strongest concentrations, that cyanide reduces 

 the mitotic rate. Considerably fewer anaphases were found in these roots 

 and the reduction in fragment frequencies was small compared with the 

 concurrent experiments with the other chemical agents. These facts indicate 

 that metabolic inhibition or metabolic changes may be the most reason- 

 able explanation of the protective effect of sodium cyanide. Such metabolic 

 alterations, involving complicated enzyme systems, initiated by KCN and 

 CO are discussed by King et al^^. The complicated effect of cyanide is 

 demonstrated by D'Amato and Gustafsson'^ who showed that KCN 

 treatment of seeds prior to X-irradiation increased visible mutations in 

 barley with low concentrations. A stronger concentration of 10~-M de- 

 creased the mutation rate, although the rate of chromosome breakage 

 increased. BAcq'** found that cyanide decreased the mortality in X- 

 irradiated mice. 



By comparing the effect of cysteine in acid and neutral medium, a marked 

 difTerence in protection and tolerance is noticed. A similar response in the 

 effect of cysteine, applied at different pH-values, on survival of mice after 

 whole body X-irradiation was obtained by Patt et al'^'^ , ^^ and Goldie 

 et al^^. They state that the probable explanation of this difference may be 

 the fairly rapid oxidation of cysteine to cystine in a neutral medium before 

 its administration. Marshak^" has shown that changes in intracellular 



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