864 ADVENTURES IN RADIOISOTOPE RESEARCH 



One of the most conspicuous exceptions from this correlation is the 

 great radiosensitivity of lymphocytes in spite of the almost total absence 

 of mitotic action. It is, however, of interest to recall the fact mentioned 

 above that the DNA of lymphocytes of the thymus is depolymerised 

 under the action of a very restricted radiation dose. Furthermore erythro- 

 cytes, though very refractory towards exposure to radiation, may be 

 damaged by haemolysing agencies accumulating in the irradiated organ- 

 ism. Such agencies may affect the fragile lymphocytes much more 

 pronouncedly. 



Radiosensitivity of plant seed may differ by a factor of 10 or more, 

 whereas their growth rate per time, unit, and consequently DNA forma- 

 tion, do not much differ. The composition of seeds may strongly differ 

 and in different seeds a very different fraction of radiation may reach 

 the sensitive spots of the mechanism responsible for the synthesis of 

 DNA. 



The radiosensitivities of higher plants show great variation as well. 

 The lethal dose for acute radiation varies from less than 1000 r for 

 Podophyllum peltatum to a dose in excess of 200,000 r for Craptopetalum 

 MacDougallii. Sparrow (1955) suggested that the high radiation tole- 

 rance of certain cruciferae or of Gladiolus might be due to a high content 

 of vitamin C or other reducing substances. Thus, these plants exert 

 self-protection by removing damaging oxidizing radicals and reducing 

 oxygen tension. 



Another example that marked turnover rate of DNA is compatible 

 with high radioresistance is the existence of refractory tumours. Even 

 massive doses cannot interfere wdth the formation of DNA in these. 

 Fig. 8, taken from a paper of Kelly et al. (1955) demonstrates that 

 while ^^P incorporation into lymphosarcoma ceases almost fully for a 

 day or so following an exposure to a dose of 800 r, incorporation into the 

 investigated mammary carcinoma is hardly influenced. 



Numerous types of DNA exist. DNA of a given cell was found by 

 Chargaff (1955) to be composed of a very large number of differently 

 constituted individuals. From the Walker carcinoma two DNA fractions 

 were isolated, the ratio of which markedly changed after exposure to 

 a close of 5000 r (Harbers and Bachmann, 1956). Different types of 

 DNA having different radiosensitivities, refractory tumours may be 

 those in which DNA of small radiosensitivity predominates. The radio- 

 sensitivity of tumour cells may, however, be determined to a large 

 extent by conditions prevailing in their environment. 



That the presence of oxygen strongly enhances radiation damage is 

 a well established fact (Gray, 1953a). In the presence of oxygen more 

 damaging radicals are formed from the irradiated tissue water, which 

 transmit much of the radiation energy. Furthermore the effect of irra- 

 diation depends on the metabolic activity of the cell during irradiation 



