Formation of Fatty Acids 265 



pool all the samples for the isolation of the individual acids. 

 However, as can be seen from Fig. 2, the relative position of 

 the specific-activity time curves changed at 12 hr. after the 

 injection. For this reason it was decided to divide the acids, 

 into two main groups: (a) those obtained up to 12 hr. and 

 (b) those obtained between 12 and 48 hr. of the experiment. 

 Thus, during the first period the fatty acids had their highest 

 specific activities, and during the second period the specific 

 activities of all the fractions were already declining. In 

 addition to these two main parts of the experiment some 

 observations were also made on two small samples of the 

 water-soluble volatile acids, one representing 0-7 hr. and the 

 other approximately the entire experimental period. 



Fig. 3 shows graphically the specific activities of the 

 individual fatty acids which, with the exception of acetic 

 acid, were chromatographically pure (for technical details 

 of isolation of the individual acids see Popjak, French, 

 Hunter and Martin, 1951). The acetic acid was contaminated 

 with butyric acid to the extent of about 10 per cent. But as 

 only small quantities (5--10 mg.) of this acid were obtained, 

 no further purification was attempted. It is seen that there 

 is a relationship between isotope content and chain length 

 of the fatty acids. During the first 12 hr. of the experiment 

 the specific activities increased from butyric acid up to and 

 including capric (decanoic) acid, whereas during the second 

 part of the experiment the specific activities increased up to 

 myristic (tetradecanoic) and palmitic (hexadecanoic) acids. 



The finding of acetic acid in milk fat requires some comment. 

 Its presence was quite unexpected, but we have demonstrated 

 it quite consistently among the water-soluble steam-volatile 

 fatty acids. We are uncertaih whether it is present as 

 glyceride or is esterified with some other substance. We have 

 now found it also in the milk fat of rabbits and there can be 

 no doubt as to its genuine occurrence. 



The meaning of the results shown in Fig. 3 became apparent 

 from the information obtained from the chemical degradation 

 of acetic, butyric and caproic acids shown in Table II. 



