THE EATE OF RENEWAL I'JUJ.M KATE OF DISAPrEAKAXCE OF LAii. .MOLECULES 419 



of the i^C content of the precursor carbon prev aihng during the experi- 

 ment. The saturated fatty acids of the liver were found to reach half of 

 their maximum isotope concentration in less than one day, the unsatu- 

 rated acids in about two days. 



When investigating the effect of muscular exercise on the incorporation 

 of i^C into liver fats, the present writer (5) found after the lapse of 4I/2 hr 

 the i^C content to be only one third of that detected 20 min after injecting 

 labelled acetate into the mouse; however, according to the above men- 

 tioned data, in the course of two hr. the loss of i*C by liver fatty acids 

 should be less than 10 per cent. Obviously, a fatty fraction is present in the 

 liver which has a much shorter half-life time than one day. Ruyssen, 

 Beeckmans and the author (6) investigated the renewal rate of fatty 

 acids in the liver of the mouse shortly after administration of labelled 

 acetate. They obtained the result that the i*C content of the liver fatty 

 acids increase rapidly during about the first 30 minutes, this increase 

 being followed by a rapid decrease. Furthermore, Beeckmans and 

 Elliott (2) could demonstrate that in both the saturated and unsatura- 

 ted fatty acid fractions this early increase is followed by a rapid decline 

 in the ^^C content. The liver must thus contain one or more rapidly 

 renewed fatty acid fractions. To discard the effect of this rapidly meta- 

 bolizing fatty acid fraction we studied the decrease in the ^^C content 

 of liver fatty acids one or 1 | days after administration of labelled 

 acetate only and followed it till the lapse of 4 or 4 | days. As we in our 

 determination of the renewal rate disregard those labelled fatty acid 

 molecules which are formed during the experiment we have obviously 

 quite apart from the above considerations to wait for about 1 day after 

 injecting labelled acetate before securing the samples. 



Gould et al. (4) found that after the lapse of three hr. only 85 per cent 

 of the acetate ^^C injected into the rat is already exhaled as ^^COg. In the 

 later phase of the experiment the rapid loss of the exogenous acetate 

 i*C may to some extent be compensated by formation of labelled endo- 

 genous acetate. Pihl et al. (9) kept the activity of body acetate of the 

 rat at a constant level by feeding labelled acetate; in these experiments 

 an increase in the body acetate activity could be observed after the lapse 

 of ten days which was due to the formation of endogenous acetate of 

 appreciable C^* content. Under our experimental conditions (injection 

 of the labelled acetate at the start of the experiment) the activity of 

 endogenous acetate formed during the experiment was, however, negli- 

 gible. 



In Fig. 1 the decrease in the specific activity of fatty acid carbon 

 with time l| f o 4 ^ days after injection of labelled acetate is plotted; 

 also data obtained by Pihl et al. (9) for the increase in the percentage 

 of labelled fatty acids with time are seen. The figure contains thus 

 both "rise curves" and "decay curves". From the data of Pihl et al. 



27* 



