264 G. PopjAK 



assume that the tracer acetate was mixed completely with 

 the body acetate, the calculation implies that in the lactating 

 goat 50 per cent of the body acetate which is not oxidized 

 is converted into milk fat in six hours. 



Another important point emerged from the relationship 

 between the specific activities of the short-chain volatile 

 acids (water-soluble and water-insoluble steam volatile acids) 

 and those of the non-volatile (solid and liquid) acids. At 

 the time of their maximum isotope content the former con- 

 tained several times more ^C than the latter, and therefore 

 the short-chain acids must have originated by synthesis from 

 acetate and not by the degradation of oleogiycerides, con- 

 firming our findings on non-lactating pregnant rabbits (Popjak, 

 Folley and French, 1949; Popjak and Beeckmans, 1950). 



In casting for an explanation for the differences in the iso- 

 tope contents of the milk fatty acid fractions we were im- 

 pressed by two points: (1) although the fractions were not 

 homogeneous, their specific activities decayed with almost 

 identical half- lives of about four hours and (2) that the 

 specific activities did not bear a simple relationship to the 

 average chain length of the fractions. The steam-volatile 

 fraction insoluble in water, and which had the highest ^^C 

 content, consists mainly of Cg to C12 acids and the water- 

 soluble fraction of C4 to Cg acids. It seemed to us probable 

 therefore that the differences were connected in some way 

 with the biochemical mechanisms of fatty acid synthesis rather 

 than with any other factor, e.g. differences in turn-over rates 

 or dilutions by blood fat of low isotope content. 



In order to interpret more fully the role of acetate in milk 

 fat synthesis, we have resolved the crude fractions into 

 individual acids (Popjak, French, Hunter and Martin, 1951). 

 Since it would have been an enormous task to carry out this 

 type of work on all the 22 milk samples separately, we decided 

 to pool the material from various parts of the experiment. 

 If the specific activities of the four crude fatty acid fractions 

 had maintained the same order relative to one another 

 throughout the experiment, it would have been justifiable to 



