INITIAL RADIATION DAMAGE AT SUB-CELLULAR LEVEL !l 



variation in tlie radiosensitivity of cells, in ])articiilar tliosr of" micro- 

 organisms. One as])ect which we have been investigating is the |)resence 

 Avithin the cell of some substances, e.g. snlphydryl componmls or 

 bacterial pigments, which can act as jirotective agents and lednce the 

 magnitude of the initial chemical lesion by one of the energy transfer 

 or re])air processes discussed in our other pa])er in this symposium. We 

 must stress that the jn'esence of intracellular agents can at best be only 

 one of the factors making for high i-adio-resistance. We have tested this 

 hypothesis by comi)aring the dose needed to damage the same "marker" 

 molecule Avithin different cells having varying radiosensitivities. In 

 these ex])eriments large doses have to be used since a radiochemical 

 reaction unmagnified by metabolism is being studied. The cells are 

 irradiated at 0*^0 and then immediately broken u]) in a Hughes press at 

 — 25°C. The homogenate is kept frozen until the actual assay occurs. 

 The dose needed to inactivate the enzymes of the Krebs cycle in Micro- 

 coccus sodensis with an LD37 of 33,000 rads (220 kV X-rays) was 

 approximately five times that necessary for the much more radio- 

 sensitive Psewc^omo^irts yZworescews (LD37, 1,800 rads). The protection of 

 the enzymes in the cytoplasm of the micrococcus may be related to 

 the presence of a carotenoid pigment since this is much more sensitive to 

 irradiation within the cell than in isolation. Snlphydryl protection does 

 not appear to be operative since the concentration of SH groups is less 

 in the micrococcus than in the pseudomonas. The DNx4 of the cells does 

 not appear to be protected since approximately similar doses of radia- 

 tion (100,000 to 200,000 rads)^ suffice to reduce the molecular weight 

 to one half (i.e. to jiroduce one "double" break, see p. 14) in pseudo- 

 monas, micrococcus, rat thymocytes and mouse leukaemia cells. These 

 measurements were made on DNA extracted immediately after irradia- 

 tion with X-rays at lO^r/min at 0°C. The amount of DNA that could be 

 extracted from the irradiated cells was, however, less than that obtained 

 from non -irradiated controls and this introduces an ambiguity in the 

 interpretation. 



Removal of the intracellular protective agents should lead to radio- 

 sensitization. lodoacetate, which has been shown to enhance radiation 

 damage in mice, readily combines with suljohydryl compounds that 

 could be physiological protectors. Leukaemia cells in tissue culture 

 exposed to iodoacetate at a concentration of 10-5 m for 30 min prior to 

 irradiation (but not when given after irradiation) are 30 per cent more 

 sensitive to X-rays than those that have not been treated : 300 r produce 

 an effect for which 375r would otherwise be necessary. To make this 

 experiment easily interpretable the treatment with iodoacetate was 

 sufficiently mild so as not to interfere with the growth of the cells in any 



