418 ADVENTURES IN RADIOISOTOPE RESEARCH 



increasing values of the specific activities of desoxyribonucleic acid 

 with time by a calculation based on decreasing specific activity values 

 with time, we cannot fully eliminate the difficulty arising from the lack of 

 knowledge of the pertinent precursor of desoxyribonucleic acid P. We 

 meet, however, very different conditions when faced with the task to 

 calculate the rate of renewal of a carbon labelled compound from specific 

 activity or similar data. 



i^C of rapidly metaboUzed carbon compounds such as acetate, glucose 

 and so on, is within a comparatively short time exhaled to a very large 

 extent as carbon dioxide. Correspondingly the rate oflossof i*C by fatty 

 acid molecules in a later phase of the experiment in which, for example, 

 labelled acetate was administered to the rat, is no longer a resultant of the 

 disappearance of "old" strongly labelled molecules and the formation 

 of "new" less strongly labelled ones but, at least in the first approxima- 

 tion, the result of the decay of labelled molecules only. By following the 

 rate of decrease of the i^c content of fatty acids extracted from the 

 organs of the rat we can thus calculate the renewal rate of fatty acid 

 molecules in an analogous way to that, in which we calculate the period 

 of decay of a radioactive body from its "decay curve." In the present 

 note the calculation of the renewal rate of the fatty acids of the liver 

 from the rate of decrease of the i^C content of the fatty acids in the 

 rat injected with acetate labelled in the carboxyl group is described. 



EXPERIMENTAL 



44 rats weighing 190 to 244 gm were injected intraperitoneally each 

 with 0.2 ml of phys. sodium chloride solution containing acetate labelled 

 in the carboxyl group of 10—26 fi C activity. The animals were killed by 

 decapitation and the total fat, fatty acids, neutral and phosphatide fatty 

 acids, and also cholesterol of the liver, secured as described previously. 

 (2, 8) The activity of the samples was compared without conversion into 

 barium carbonate. 



RESULTS 



In an early investigation, using deuterium as an indicator, Stetten" 

 and Boxer (12) found the half life-time of fatty acids of the rat liver 

 to amount to 1.9 days. Rittenberg and Block, (11) feeding ^^C label- 

 led acetate, observed a more rapid incorporation of ^^C into saturated 

 than into unsaturated fatty acids of the mouse hver. In a more recent 

 work, PiHL et at. (8, 9) compared the ^^C content of the fatty acid carbon 

 (at the end of the experiment taking many days) with the average value 



