422 ADVENTURES IN RADIOISOTOPE RESEARCH 



animal, even when experimenting with animals of the same race, age and 

 weight. The reason of these variations is inherent partly in the constitu- 

 tion of the livers compared, and is presumably partly the result of a 

 difference in the amount and time of food taken in by the rat. A diffe- 

 rence of minutes only in the time when the animal last took in food may 

 influence the renewal rate of the fatty acids of the liver. 



In spite of the great variations in the rate of i^C incorporation into 

 liver fatty acids the results shown in Table 1 and further similar results 

 indicate a more rapid loss of fatty acid i*C in the first hours of the ex- 

 periment than in the later phase, the maximum ^^C incorporation being 

 observed 1 — 2 hr after injecting the acetate. One may be inclined to 

 interpret the rapid decrease in the ^^C content of the fatty acids during 

 the first hours of the experiment as a consequence of the presence of 

 an impurity in the fatty acid fraction. After purification of the fatty 

 acids or the phosphatides according to Folch and van Slyke's method 

 (3) the decrease was still observable. For the fatty acids of the mouse 

 liver the i^C content of the isolated lead salts has furthermore shown 

 a similar time dependency to that of fatty acid mixture (2). Lehninger's 

 copper-calcium method of purification, while increasing the activity 

 figures with 10—20 per cent through removal of less active fractions, 

 did not influence significantly the time dependency of activity figures 

 obtained. The same applies to the already mentioned purification with 

 colloidal iron hydroxyde. The specific activity of phosphatide fatty 

 acids, for example, purified by this method, was increased up to 40 

 per cent ; however, the percentage change in i^C content of the fatty 

 acids secured at different times after injecting labelled acetate to the 

 rat was not significantly influenced by the purification process. 



Summary 



Since most of the i*C injected as acetate into the rat is exhaled within a day, 

 incorporation of "C into fatty acids of the hver after that date can be neglected 

 in the first approximation. From the rate of loss of "C by the fatty acids of the 

 hver of rats, to which one or more days previously labelled acetate was admi- 

 nistered, the rate of renewal of the fatty acids can thus be calculated. The half- 

 hfe of the saturated fatty acid mixture was found to be 0.8 days, that of the unsa- 

 turated fatty acid mixture (1 to 4 days after injection of the labelled acetate) 

 to amount to 2.2 days. 



