RADIOACTIVE TRACERS 875 



apparent until % — I hour after irradiation. The rale of synthesis of DXA was 

 found to depend on two components, S^ and Sj. As to th(> natiuc of the S, compo- 

 nent Lajtha and assoc. propose that it represents the integrity of the desoxyriho- 

 nucleic acid template. The importance of the template for DXA synthesis is most 

 spectacularly brought out by the work of Kornberg showing that the template has 

 to be present to achieve in vitro synthesis of DXA. S^ is interpreted as a part of 

 the biochemical mechanism of precursor synthesis. 



To a very similar conclusion arrive Ord and Stocken studying the effect of 

 exposure to radiation on rat thymus desoxy ribonucleic acid. They suggest that 

 the initial steep portion of the do3e — response curve corresponds to inhibition of 

 nuclear phosphorylation and that the phase of only slowly increasing inhibition 

 can be associated with progressive damage to the template. 



Hagen has recently shown that DXA may be extracted from tissues with 90% 

 phenol in the presence of trichloracetate. He found that after exposure of rats to 

 irradiation with X-rays (2 hr after 200 — 800 r) a higher yield of DXA can be 

 obtained at the extraction with 1% trichloroacetate from the thymus tissue as 

 well as from isolated thymvisnucleohistone. If calcium ions are added to the nucleo- 

 histone of irradiated animals, the raised yield of DXA decreases again to normal 

 values. A dose of 200 r suffices thus to influence the template of DXA synthesis. 



M. G. Obd and L. A. Stocken, Nature 182, 1787 (1958). 



L. G. Lajtha, B. Oliver, R. Berry and W. D. Xoyes, Nature 182, 1790 (l9r)S) 



U. Hagen, Biochem. 76, 56 (1960). 



