RADIOACTIVE INDICATORS IN BIOCHEMISTRY 995 



controls. Urethane, which is a metabolic depressor, acts in an opposite 

 way57. jn experiments taking 100 minutes, thus in the descending region 

 of the curve (Fig. 14), the phosphatide fractions or the total fat (Fig. 16) 

 extracted from various organs of urethane- injected mice take up more 

 i^C than those of the controls. A similar, but less pronounced effect is 

 found (Fig. 17) when comparing the incorporation of i4(j into the pro- 

 teins of organs of urethane-injected mice with the incorporation into 

 corresponding fractions of controls. 



In a similar way, acetate labelled in the carboxyl group with i4(j ^as 

 applied in the study of the biochemical effects of ionizing radiation^^s) 

 These investigations revealed an interference, not only, as mentioned 

 previously with the rate of incorporation of i^Q into purines of nucleic 

 acids, but also of other tissue constituents. In the non-fasting mouse, 

 for example, irradiation produced a similar effect on the incorporation 

 of i*C into fats and proteins of the brain to that of administration of 

 urethane. 



While the disentanglement of the numerous, often competing, meta- 

 bolic steps involving ^^(j incorporation and the determination of the 

 extent to which this incorporation is influenced by metabolic depressors 

 and accelerators is a very difficult task, the method outlined above may 

 prove to be a promising approach to the study of metabolic interferences. 



The application of radioactive indicators in biochemistry covers a 

 large field and I had to restrict myself to the discussion of a few exam- 

 ples only. 



Possible interference of ionizing radiation with the normal biochemical 

 pattern is often considered a disadvantage of the method of radioactive 

 indicators. Such an interference can, however, be avoided by using 

 radioactive tracers of restricted activity. A much more dangerous source 

 of error in the application of radioactive indicators is the non-identity 

 of the chemical properties of isotopes. Tritium, for example, having 

 an atomic mass three times as high as hydrogen, differs from hydrogen 

 to a non-negligible extent in many of its chemical properties. We are 

 witnessing the establishment of a new branch of chemistry which in the 

 course of time may become a very important implement for classical 

 chemistry, namely, that of the rare isotopic constituents. In the long 

 run the claim for accuracy in the biochemical application of isotopic 

 indicators is bound to increase and it may become necessary to make use 

 of the advances in this new branche of chemistry. 



The application of isotopic; indicators in biochemistry oi)ened new 

 lines of approach, not only to the solution of known problems, but also 



(57)Hevesy, Nature 164, 1007 (1949); Hevesy, Ruyssen and Beeckmans, 

 Experientia (1951). 



(58) Hevesy, Nature 163, 869 (1949). 



63* 



