Formation of Fatty Acids 267 



Whatever the origin of the acetic acid its occurrence never- 

 theless served as a useful guide showing that redistribution 

 of the label from the injected acetate did not occur in the 

 animal since ^*C was found only in the carboxyl carbon. 

 The degradation of butyric acid revealed the presence of 

 equal amounts of ^*C in carbon atoms 1 (COOH) and 3, 

 carbons 2 and 4 being totally inactive. This finding indicates 

 that butyric acid has been synthesized in the udder by con- 

 densation of two acetate units of equal isotope content, the 

 methyl carbon of one having been linked with the carboxyl 

 carbon of the other. However, we do not consider acetate 

 to be the only source of butyric acid in the milk of the goat, 

 but it is thought that another precursor, probably a C4 

 compound, is also involved. The necessity for this assumption 

 should be clear from the information obtained by the degra- 

 dation of caproic acid. 



The results of the degradation of caproic acid from the early 

 part of the experiment (cf. Table II) should be considered 

 first. Only carbon atoms 1 (COOH), 3 and 5 contained ^^C. 

 The specific activity of the COOH carbon was 2 • 5 times that 

 of carbons 3 and 5, whose specific activities were equal, just 

 as in carbons 1 and 3 of butyric acid. Further, the specific 

 activities of carbons 3-6 of caproic acid were sufficiently 

 close to the activities of the butyric acid carbons as to leave 

 very little doubt that in fact these carbons were derived 

 from the carbon chain of butyric acid. Therefore the syn- 

 thesis of caproic acid must have occurred by the elongation 

 of this shorter chain, at what corresponds to the carboxyl 

 carbon of butyric acid, by the addition of a C2 unit. It may 

 be inferred from the similarity of the radioactivities of carbons 

 3, 4, 5 and 6 of caproic acid to those of butyric acid that the 

 newly synthesized caproic acid has not been diluted in the 

 udder to a significant extent by pre-existing non-isotopic 

 caproic acid, i.e. the metabolic "pool" of caproic acid in the 

 gland must be very small. 



The difference between the ^*C content of the "acetic" and 

 "butyric" parts of caproic acid in the early part of the 



