980 ADVENTURES IN EADIOISOTOPE KESEAECH 



In experiments in which the effect of irradiation on the incorporation 

 of i*C into the deoxyribonucleic acid was studied, acetate labelled with 

 ^*C in the carboxyl group was injected into mice. In Table 8, in addition 

 to the effect of irradiation on ^^C incorporation into purine carbon of 

 deoxyribonucleic acid, the corresponding effect on i*C incorporation 

 into protein of the liver is seen as well. Whereas incorporation into 

 purines is markedly depressed, that into proteins is enhanced under 

 the effect of irradiation. 



CHANGE OF SENSITIVITY OF THE RADIOACTIVE INDICATOR IN THE 



COURSE OF THE EXPERIMENT 



When applying isotopic indicators in the study of animal metabolism, 

 two main lines of technique can be followed. We can keep the tracer in 

 the body at a constant level. Deuterium for example, being used as an 

 indicator, dilute heavy water is given to the animal throughout an expe- 

 riment taking days or weeks. The body fluids soon reach a constant heavy - 

 water content and, by comparing the deuterium content of the body 

 water with that of fatty acids extracted from an organ, we can state 

 what percentage of the fatty acid deuterium of that organ reached the 

 body-water deuterium level and thus find the percentage fatty acid 

 renewal during the experiment or, more correctly, the lower limit of that 

 percentage. This method was much applied by Schoenheimer and 

 RiTTENBERG in their classical experiments and also by Stetten, Block, 

 Bernhard, and others, who obtained important information^^*\ 



When studying acetate metabolism, acetate labelled, for example, 

 in the carboxyl group with i3(j or i^C can be added to the daily food and 

 an almost constant ^^C or i*C content of the acetate of the body fluid 

 will thus be obtained. As shown by Rittenberg and Bloch, and also 

 by others^^^a)^ i^ppp again a comparison of the i^C or I'^C content of the 

 Ingested acetate with that of fatty acid or cholesterol, etc., yields the 

 lower limit of percentage renewal of these compounds. 



In the work with radioactive indicators we mostly choose, however, 

 ■a, different technique. We administer the labelled precursor at the start 

 of the experiment only, so the specific activity of the indicator decreases 

 with time. On injection of labelled phosphate of negligible weight, 

 having an activity of 1 mc, into the circulation of a human subject 

 containing 130 mgm. of plasma inorganic P this mc. will first indicate 

 the presence of 130 mgm. of inorganic phosphorus. Soon, however, as the 

 ^2p of the plasma inorganic P rapidly passes the capillary wall, it inter- 



(43a) Rittenberg and Bloch, J. Biol. Chern. 160,417(1946); Pihl, Block 

 and Anker, Ibid. 183, 441 (1950). 



