THE RATE OF RENEWAL FROM RATE OF DISAPPEARANCE OF LAB. .MOLECULES 421 



activity of phosphatide fatty acids was larger than that of the glyceride 

 fatty acids, in experiments of longer duration, however, no significant 

 difference in the ^K' content of th(> two l^^pes of fatty acids could be 

 observed. 



In all our experiments 1 to 4 days after injecting the labelled acetate 

 the specific activity of the total fatty acids of neutral fat of the rat liver 

 was found to be a few per cent higher than the specific activity of phos- 

 phatide fatty acids, while according to Pihl and Block (8) 3 days 

 after feeding labelled acetate to rats kept on lipid free diet, the phos- 

 phatide fatty acids are 6 to 13 per cent more active than the total fatty 

 acids of neutral fat. 



As mentioned above, investigation of the incorporation of i*C into 

 fatty acids of the liver shortly after the administration of labelled acetate 

 to mice revealed the existence of a rapidly metabolising fatty acid fraction. 

 A rapid decrease in the ^^C content of total fat, neutral and phosphatide 

 fatty acids of the liver of the rat is also observed shortly after the ad- 

 ministration of labelled acetate but is less pronounced than in the case 

 of the mouse liver. Some results obtained when injecting several rats 

 each of which being killed at a different time after intraperitoneal 

 injection of the labelled acetate are shown in Tables la and b. 



These and similar results are to be interpreted cautiously, among 

 others because — in contrast to the experiments on mice where pooled 

 livers of a large number of mice where extracted — in the present inves- 

 tigation each point indicates the result obtained when extracting a single 

 liver only. The fatty acid metabolism may strongly vary from animal to 



Table 1. — Percentage injected acetate 

 i''C present in 1 mgm of neutral total 



FATTY ACIDS OF NEUTRAL FAT EXTRACTED FROM 



THE livp:r of 182 — 190 gm rats 



Time after injection 



I'erceiitage injected x 10^ in 

 1 mgm total fatt}- acids 

 of neutral fat 



