268 G. POPJAK 



experiment would admit two possible explanations. The first 

 one is that the butyric acid synthesized from acetate was 

 diluted 2 • 5 times by pre-existing non-isotopic butyrate. 

 This would imply a fairly large pool of this acid. If this were 

 so, during the second part of the experiment, when the 

 specific activity of the acetate precursor must have been 

 declining rapidly, the "acetic" portion of caproic acid should 

 have a lower specific activity than the "butyric" portion. 



The second possible explanation is that the butyric acid 

 synthesized from acetate is being continuously diluted 2 • 5 

 times by a non-isotopic C4 compound derived from the blood. 

 This compound might be butyric acid itself or another C4 

 compound which is converted into the acid. If this second 

 explanation is the right one, the "butyric" portion of caproic 

 acid in the milk should have a lower isotope content than the 

 "acetic" portion at all times in the experiment. 



The degradation of caproic acid from the later part of the 

 experiment (cf. Table II, 12-48 hr. sample) showed that even 

 here the specific activity of its "acetic" portion was about 

 2-5 times greater than that of its "butyric" portion. There- 

 fore the contribution of the first mechanism as an explanation 

 of the observed diff'erences must be negligible. There is a good 

 deal of evidence to show that the pools of the volatile acids 

 (up to decanoic) within the cells of the mammary gland are 

 very small. 



Therefore, we cannot avoid the conclusion that the butyric 

 acid synthesized from acetate in the udder is continuously 

 being diluted about 2 '5 times by a non-isotopic C4 compound. 

 Thus butyrate in the udder seems to have a dual origin which 

 may be represented as follows: — 



2CH,C**OOH ->CHs.C**H,.CH,C**OOH ■) 



y CH3.C*Hj.CH,C*OOH 

 C4 compound (non-isotopic) — > CH8.CH2.CH2.COOH J 



The synthesis of caproic acid may then be visualized by: — 



CH,.C»H2.CH8.C*OOH + CH3.C**OOH -^ CH8.C*Hj.CH,.C*H2.CHa.C**OOH 



We are proceeding now with the degradation of the higher 

 fatty acids also, but we think that the increasing specific 



