262 



G. POPJAK 



In order to obtain evidence on the question whether or not 

 milk fatty acids are derived primarily from the plasma lipids 

 or are synthesized in significant amounts in the udder, the 

 i*C contents of plasma fatty acids were compared with those 

 of four crude fatty acid fractions of the milk glycerides. 



The results are shown in Figs. 1 and 2. It should be pointed 

 out that the scale of the ordinate in Fig. 1 is one hundred 



u 

 (J 



I 



o 



X 



>^ 



*> 



u 

 rJ 



to 



0-5 



0-4 

 0-3 



^.■^--^ 





•-£i 



10 20 30 40 



Time after injection (hr.) 



Fig. 1. Change of specific activities of plasma fatty acids and 



of plasma and milk cholesterol after the injection of 5 mc. 



CHgi^COONa into a lactating goat. 



Plasma total fatty acid. 



© Plasma phospholipid fatty acids. 



Q Plasma non-phospholipid fatty acids. 



• Milk cholesterol. 



A Plasma cholesterol. 



(Popjak, Folley and French, 1951.) 



times larger than in Fig. 2. The comparison shows that the 

 milk fatty acids behaved quite differently from the plasma 

 fatty acids. The specific activity of the latter reached a pla- 

 teau 24 hr. after the injection but continued to rise slightly 

 up to the end of the experiment, whereas the milk fatty acids 

 reached their maximum ^^C content 3-4 hr. after the injection, 

 when they contained several hundred times more isotope 

 than the plasma fatty acids did at any time. 



