404 



ADVEXTURES IX EADIOISOTOl'E RESEARCH 



present at every other carbon atom; i.e. at the odd number carbon 

 atoms of the fatty acids. Later work^^^ showed that at least 25 percent 

 of the carbon atoms of the fatty acids are derived from acetate. Evidence 

 was also obtained that acetic acid can furnish every carbon atom of the 

 molecule. 



Recently the rate of turnover of fatty acids has been re-investigated 

 with the aid of acetic acid labelled by ^HJ in the carboxyl group by Pihl 

 et alM^ The percentage renewal of the fatty acid molecules is determined by 



o 

 O 

 u 



u 

 a> 

 a 



10 20 50 40 SO 80 100 120 



180 



21*0 

 Time,minuhes 



Fig. 1 . Change of the specific activity of tlie liver fatty acids with t ime. 



comparing the ^^C content of the fatty acid carbon at the end of lhe 

 experiment with the average value of the ^^C content of the precursor 

 carbon which prevailed during the experiment. To arrive at the last 

 mentioned data, phenyl-DL-aminobutyric acid was fed simultaneously 

 with labelled acetate to adult rats kept on fat free diet, and consecutive 

 samples of the excreted acetyl derivatives were analysed.^^^ Though the 

 labelled acetate content of the diet was kept constant, the isotope con- 

 centration of the acetyl group was found to increase in the course of the 

 30 days period with about 30% of the initial value. This increase was 

 shown to be due to the catabolism of the labelled higher fatty acids 

 I'ormed during the experiment. The metabolic products of the labelled 



^-' K. PoNTECORVO, D. RiTTENBERG and K. Block, J. Biol. Cheiu. 179, 893 

 ( 1949). A. Pihl, K. Bloch and H. S. Anker, J. Biol. Chein. 183, 441 (1950). 



(2^ K. PoNTECORVO, D. RiTTENBERG and K. Bloch, J. Biol. Chem. 179, 893 

 (1949). 



'^Uv. Bloch and D. Rittenberg, J. Biol. Chem. 159, 45 (1945). 



