862 ADVENTURES IN RADIOISOTOPE RESEARCH 



or DNA protein suspension (Errera, 1947). Recently, Anderson has 

 shown that in fresh rat thymus homogenates containing only 50 figm. 

 of DNA in 1 ml. 1 M NaCl solution viscosity is reduced with time after 

 exposure to as little as 50 r of y or X-rays. With 500 r a marked reduction 

 was apparent at once, with an almost complete loss of viscosity after 

 four hours. The results obtained in fresh homogenates exposed to 150 r 

 much resemble those arrived at when purified preparations of DNA were 

 exposed from 50,000 to 100,000 r, a coincidence which supports the view 

 that the native DNA exists in the form of molecules of extreme length 

 while the purified samples are composed of relatively small molecules. 

 It demonstrated also the very significant fact that large molecules are 

 more easily damaged by irradiation than small ones. 



In experiments with young rats Ord and Stocken (1956) found a dose 

 of 25 r to suffice to reduce ^^P incorporation into DNA of the thymus at 

 two hours by 10 per cent, the percentage damage increasing proportion- 

 ally with the dose, as seen in Fig. 7. A restricted dose thus interferes 

 both with the formation of DNA in the tyhmus nuclei of the exposed 

 animal and with the DNA present in the exposed diluted homogenate. 

 This damage may open the way for interference with the normal enzy- 

 matic pattern with all its far-reaching consequences. 



That a few days after the exposure of the adult rat to about 500 rep 

 the DNA content of the thymus is found to be reduced to half of its 

 initial value (Harrington and Lavik, 1955) may be at least partly 

 due to the suppressed replacement of the lymphocytes. In the liver, 

 as to be expected, no loss of DNA was observed. DNA prepared from 

 livers of rats irradiated with 1000 r after intervals of one day and two 

 days was found to be very similar to that prepared from unirradiated 

 livers (Butler, 1956). 



That enzymes present in radiosensitive organs can be interfered with 

 by applying restricted doses, not, however, such present in less radio- 

 sensitive organs, may be a consequence of the easier vulnerability of 

 the desoxyribosepolynucleotides in the former. It was shown recently 

 by FoRSSBERG (1956) that the concentration of lactate, citrate, aceto- 

 acetate and other cell constituents is strongly influenced by irradiation 

 in the ascites tumour cells of the mouse. As soon as the faculty of the 

 cell to synthesis DNA recovers, the above mentioned concentration 

 changes cease as well. In the liver, which is much less radiosensitive than 

 the thymus, the concentration changes of metabolites were found to be 

 absent. 



In in vitro experiments, Dale (1956) observed that an energy of 32.5 eV 

 absorbed sufficed to oxidize 40 to 60 cysteine molecules to cystine, and 

 the liberation of sulphur from thiourea can reach values of 90. These 

 are large yields due to chain reactions. We do not usually observe similar 

 effects in in vivo experiments as the radiation energy, due to the pre- 



