INCORPORATION OF "C INTO TISSUE 805 



rapid decrease in the specific activity of the body acetate will take 

 place and correspondingly the active fatty acid molecules will be more 

 rapidly replaced by less active ones than in the absence of a metabolic 

 accelerator. Thus, w^hen a maximum fatty acid activity is reached the 

 specific activity figures will decline more rapidly than they do in the 

 case of the normally metabolizing liver and vice versa. 



The rapid decline a few minutes only after the administration of the 

 labelled acetate in the activity of the liver fats due to a large extent to 

 the activity of fatty acids, is demonstrated in Figs. 2 and 3. These are 

 also showing the very different behaviour of the liver proteins and also 

 that the change in the activity of the total tissue with time much resem- 

 bles the change in the activity of fats. The data were obtained by investi- 

 gating mice of different race and age and were plotted with the sole 

 reason of demonstrating the spectacular decline of the activity of liver 

 fat and total tissue in contrast to that of proteins in the first hours of 

 the experiment. 



Administration of cyclopentylhexane, which is a metabolic accelerator, 

 was found to have the above described effect on the time-specific activity 

 curves of liver fats^i). 



Metabolic depressors can be expected to act in the opposite direction. 

 Their presence may slow down the rate of incorporation of i*C into 

 fatty acids, but the activity level of the fatty acids will now be longer 

 maintained, since the replacement of the active fatty acid molecules 

 by less active ones takes place at a slower rate. Furthermore, a decreased 

 catabolic rate of glucose, for example, will lead to a reduced formation 

 of fatty acids, which results (these fatty acids being almost inactive) 

 in a reduced dilution of the labelled acetate leading in turn to a decrease 

 in the sensitivity of i*C as an indicator of fatty acids. 



A comparison of the i*C content of the total fat or of the phosphatides 

 of mice injected with urethane with that of the controls, brings out 

 these effects of a metabolic drepressor^^). 



Metabolic activators and depressors have — as seen above — a marked 

 influence on the incorporation of i^Cinto tissue fractions. If such effects 

 were produced by irradiation with ionizing radiation, we should observe 

 significant changes in the rate of incorporation of i*C into tissue, fractions 

 and possibly in the specific activity and even in the amount of exhaled 

 CO2. 



When making the above remarks, w^e have to envisage the existence 

 of several competitive paths which acetate metabolism follows, that two 



(^) M. L. Beeckmans, M. Ca.sier and G. Hevesy, Arch. Int. Pharmacodyn _ 

 80, 33 (1950). 



■2)G. Hevesy, iVafwre 164, 1007 (1949); G. Hevesy, R. Ruyssen and M. L. 

 Beeckmans, Experimentia 7, 317 (1951); H. A. Skipper, Texas Rep. Biol, and 

 Med. 8, 543 (1950). 



