406 ADVENTUKES IN RADIOISOTOPE RESEARCH 



DISCUSSION 



The relative specific activity of the total fatty acids extracted from 

 the liver of mice killed at different times after intraperitoneal injection 

 of acetate labelled in the carboxyl group is plotted in Fig. 1. In view 

 of the fact that the five experiments, the results of which are recorded, 

 were carried out with different strains of mice, we plotted the results 

 of each experiment by taking the value obtained after 60 min experi- 

 ment to be 100. Each point indicates a value obtained by extracting 

 the fatty acids of to 12 pooled livers. The total number of mice involved 

 in these experiments amounted to 250. 1 mgm of fatty acid of the liver of 

 a 20 gm mouse contains 8.2 X 10^^ p^rt of the ^^0 administered. This figure 

 indicate thes value corresponding to the highest peak of the curves. 



It takes several minutes until the injected labelled acetate or its 

 decomposition products penetrate into the liver and are incorporated 

 into fatty acid molecules. Correspondingly, the specific activity of the 

 fatty acids increases for the first minutes. This time was determined 

 for the rat to be 15 min. <^) The increase in the specific activity of the 

 fatty acid precursors with time is soon followed by a decrease. While 

 the endogenous inactive acetate is constantly being produced, about 1"2 

 mgm are formed daily per 100 mgm of rat tissue^-), the injected labelled 

 acetate is not replaced. ( -orrespondingl}^ after a while fatty acid mole- 

 cules w^ill be synthesized in the liver from almost inactive precursors. 

 The time will also arrive when the labelled fatty acid molecules present 

 will be renewed a second time from less active precursors than the first 

 time. Active fatty acid molecules will thus be replaced by less active 

 ones, and the specific activity of the average fatty acid molecule shall 

 now decrease with time. The rate of decrease will be determined by 

 the half-life time of the liver fatty acid molecules. Let us assume that 

 after the lapse of 30 min. no further active fatty acid molecules are 

 formed in the liver, the precursors being no longer active, and corre- 

 spondingly the specific activity of the fatty acid of the liver Avill decrease 

 with time according to the formula 



In 2 t 



S^ = 8,e- ^~ 



where Sq denotes the specific activity after 30 min., S that at any later 

 time t, while T = half-life time of the fatty acid molecules in the liver. 

 Assuming T — \ day as found by different workers in feeding experi- 

 ments, than in the interval between 30 and 6<) min. the decline in the 



^1^ R.G.Gould, F. M. Sinex, I. N. Rosenberg, A. K. Solomon and A. B. 

 Hastings, J. Biol. Chem. 177, 295 (1949). 



(2) K. Block and D. Rittenber(;, J. Biol. Chem., 159, 45 (1945); A. Pihl 

 K. Block and H. S. Anker, J. Biol. Chem. 183, 441 (1950). 



