KADIOACTIVE IXDICATOES IN BIOCHEMISTRY 977 



this indirect radiation effect due to more or less short-lived radicals, 

 the application of labelled phosphate in radiation studies revealed the 

 existence of another type of indirect effect^^^) possibly due to products 

 of radiation. 



In animals bearing two distant tumours, one of which was irradiated 

 while the other was effectively shielded with lead, incorporation of ^^F 

 into the deoxyribonucleic acid of not only the irradiated but also the 

 shielded sarcoma was observed. Irradiation with a dose of 280—2000 r 

 had an effect on the deoxyribonucleic acid formation of the shielded 

 tumour which lagged behind with only 20 — 40% of that observed in 

 the irradiated sarcoma. 



Kelly and Hardin Jones^^^) extended these studies and found that 

 even local irradiation of the liver of the rat with a dose of 170 r leads 

 to a 34% reduction of labelled deoxyribonucleic acid formation in the 

 tumour, while local irradiation of the muscles with 230 r reduces the 

 labelled deoxyribonucleic acid formation in the liver by 26%. 



On introduction of the blood of a strongly irradiated rabbit into the 

 circulation of another rabbit, the rate of incorporation of ^sp into deoxy- 

 rilwnucleic acid of the kidneys of the second rabbit was found to be 

 reduced, an observation which suggests the presence of a substance in 

 the circulation of the irradiated rabbit which has a blocking effect on 

 nucleic acid format ion(^">. In view of more recent results obtained when 

 using i^C as an indicator in radiation studies, which I shall discuss later, 

 such indirect effects appear less puzzling now than at the time of their 

 observation. 



The availability of the long-lived (half-life = 5700 years) ^^c for 

 tracer research was an event of great importance. It immensely increased 

 the stimulus already provided by the applicability of the short-lived 

 iiQ (half-life = 20 minutes) and the stable ^^C. To mention only a single 

 example, it elucidated to a remarkable extent many steps involved in 

 glycogen synthesis. i^C was also applied in the study of the biochemical 

 effects of X-rays. Into a great number of rapidly growing rats, i^C 

 incorporated with the carboxyl group of sodium acetate was injected. 

 Before the injection, half of the rats were irradiated with a dose of 900 r. 

 The silver purines of deoxyribonucleic acid isolated from some of the 

 organs were then obtained, and their radioactivity was compared^*^\ 

 On irradiating new^-born mice with about 900 r, the incorporation of 

 i^C into the purines of the deoxyribonucleic acid was found to be depressed 



^38) Ahlstbom, v. Euler and Hevesy, Arkiv Kemi 19, 4 Nr. 13 (1945); Holmes, 

 Brit. J. Radiol. 22, 487 (1949). 



'-^^^Proc. Soc. Exp. Biol. Med. 74, 493 (1950). 



^*"^ Ahlstrom, v. Euler, Hevesy and K. Zerahn, Arkiv Kemi 23, A Nr. 11 

 (1946). 



^") Hevesy, Nature 163, 869 (1949). 



■32 Hevesv 



