40. RADIATION' AND NUCLEIC ACID METABOLISM 533 



interesting feature of the findings was that the depression of DXA specific 

 activity rarely exceeded 50%, even with doses up to 9000 r. The variability 

 of experimental conditions does not allow the drawing of a "retrospect" dose 

 response curve based on these experiments, but the observation that in the 

 dose range of 1000-5000 r. the depression remained of the same order of 

 magnitude presented a new problem. A proposition to solve this "50% 

 depression" problem was that radiation depresses DXA synthesis, but not 

 DXA "turnover." However, in the tissues concerned there is no evidence 

 of DX'A "turnover" and a more likely explanation of the"50%" phenomenon 

 will be discussed later- in this chapter. It is also interesting that in two solid 

 tumors with similar growth rate and mitotic index a significant difference 

 in respect of P 32 incorporation following 800 r.TBR (total body irradiation) 

 was found : about 20 % depression in a mammary carcinoma, and about 50 % 

 in a lymphosarcoma — both in the same strain of mice. 18 Holmes, investigat- 

 ing a "radioresistant" and a "radiosensitive" strain of solid Ehrlich car- 

 cinoma found a similar depression of P 32 uptake in both — about 50% \ l 2 

 hours after 1500-2000 r. 20 



The ascites tumors appear to be more radioresistant in respect of DXA 

 synthesis than the solid tumors mentioned above. Forssberg and Klein 

 found a depression of about 25% (glycine-C 14 incorporation into DXA) 

 within the first 2 hours following 1250 r. in mouse Ehrlich ascites cells, 21 

 Kelly et al. found no significant depression of P 32 incorporation into DXA 

 of their Ehrlich cells in their strain of mice, following 800 r., 22 and Harring- 

 ton and Lavik, investigating the effect in different types of ascites tumors 

 found a depression of about 40% in the Ehrlich tetraploid carcinoma and 

 in the 6C3HED lymphosarcoma, but essentially no effect in a diploid Ehr- 

 lich tumor and in the TA3 adenocarcinoma. 23 Similarly in the L2 lymphoma, 

 no effect of 300 r. was detected on the P 32 incorporation. 3 



The apparent discrepancy between net DXA synthesis and glycine-C 14 

 incorporation into DXA reported by Forssberg and Klein 21 • 24 could be 

 explained perhaps on the basis of timing differences. They found a 66% 

 depression in the rate of net DXA synthesis, but only a 25 % depression of 

 glycine-C 14 incorporation into DX^A (Ehrlich ascites, after 1250 r.). How- 



18 L. S. Kelly, J. D. Hirsch, G. Beach, and A. H. Payne, Radiation Research 2, 490 

 (1955). 



19 R. Backman and E. Harbers, Biochim. et Biophys. Acta 16, 604 (1955). 



20 B. E. Holmes, in "Ciba Foundation Symposium on Ionising Radiations and Cell 

 Metabolism" (G. E. W. Wolstenholme and C. M. O'Connor, eds.) p. 225. Churchill, 

 London, 1956. 



21 A. Forssberg and G. Klein, Exptl. Cell Research 7, 480 (1954). 



22 L. S. Kelly, J. D. Hirsch, G. Beach, and N. K. Petrakis, Proc Soc. Exptl. Biol. 

 Med. 94, 83 (1957). 



23 H. Harrington, D. Rauschkolb, and P. S. Lavik, Cancer Research 17, 34 (1957). 



24 G. Klein and A. Forssberg, Exptl. Cell Research 6, 211 (1954). 



