856 



ADVENTURES IN RADIOISOTOPE RESEARCH 



Investigation of the Effect of Exposure to Radiation on Unicellular 



Organisms 



Investigation of the effect of irradiation on enzyme activity in vivo 

 led to the result that damage to the cell presumably mainly precedes 

 inactivation of its enzyme. Enzymes which are found to be refractory 

 in the radioresistant liver can easily be damaged in the radiosensitive 

 spleen. Oxidizing phosphorylation of mitochondria of the thymus, which 

 can be affected by a dose of 50 r, remains unaffected in the liver even 

 after exposure to massive doses (Van Bekkum, 1956). In view of these 



/tt/i-gper 

 cell 



N/Ceii 



PNA/Cell 



DNA/Cell 



Fig. 3. Increase of the average amount of PNA, DNA and total 

 nitrogen per ascites tumour cell after irradiation of the mouse 



with 1250 r. 



facts one may be tempted to interpret interference with DNA synthesis 

 to be a sequel of general cell damage as well. This is presumably often 

 the case. In bone marrow cultures, where incorporation of ^^p and ^^C 

 into DNA is immediately stopped after a 5000 r exposure taking 15 minu- 

 tes (Lajtha and Suit, 1955), cell damage presumably preceded inter- 

 ference with DNA formation. The same applies to the observations made 

 by Holmes (1954) who found that exposure of the rat to 450 r only 

 slightly depressed ^^p incorporation in DNA of regenerating rat-liver, 

 while a massive dose of 2000 r produces an immediate 50 per cent depres- 

 sion. That 32p incorporation into the DNA of the spleen of rats exposed 

 to heavy doses, a radiosensitive organ in which cell destruction due to 

 irradiation is very pronounced, is almost entirely absent is demonstrated 

 by Fig. 2 (Kelley, et al., 1955). Studying the effect of exposure of mice 

 to 1250 r on the incorporation of ^^C of glycine or adenine into DNA of 

 ascites tumour cells, Foessberg and Klein (1953), however, observed 

 a marked interference of '^Kl incorporation into DNA in these unicellular 



