LIPIDS PRESENT IN MILK FAT 807 



According to the results of Popjak and co-workers, 576 which were also 

 summarized by Popjak alone, 577 the synthesis of fatty acids from acetate 

 occurs by the addition of the 2-carbon group to the carboxyl group of an 

 acetate molecule. The short-chain acids originate in this manner; further- 

 more, all acids up to and including palmitic acid apparently originate from 

 acetate by this mechanism, although stearic and oleic acid in milk fat appear 

 to arise principally by some other process. 578 Possibly the blood triglyc- 

 erides are the source of stearic and oleic acids. Although the butyric acid 

 in milk fat is known to arise in part from acetate, another possible source 

 would be /3-hydroxybutyrate, from which butyrate could readily be syn- 

 thesized. Shaw and Knodt 547 demonstrated the presence of appreciable 

 quantities of the /3-hydroxybutyrate in the blood of the cow, and showed 

 by the technic of arteriovenous differences that appreciable amounts (2.5 

 milligram per cent) were absorbed by the bovine udder. Butyrate itself 

 is present in the blood of ruminants in too low a concentration to serve as a 

 source of the butyrate in milk fat. 570 



It would thus appear that acetate plays an outstanding role in the metab- 

 olism of the udder; this compound is one source of the short-chain acids 

 while, in part, they originate from /3-hydroxybutyrate formed in other tis- 

 sues. Both acetate formed catabolically and that brought in the blood 

 to the mammary tissue can function in the synthesis of fat. 



d'. Proof of the Synthesis of Milk Fat from Acetate Based upon Per- 

 fusion of the Udder : The in vivo experiments cited in the previous section 

 are always subjected to the criticism that the liver may in some way have 

 contributed to the fat synthesis from acetate, and that the change may not 

 have been completely and independently mediated by the mammary tissue. 

 This criticism is not valid as applied to the experiments of Cowie et al., &79 

 who demonstrated that, when the isolated mammary gland was perfused 

 with blood containing C 14 -acetate, and with labeled bicarbonate, fatty 

 acids were synthesized which contained the isotopic carbon. Similar re- 

 sults were obtained with C 14 -acetate in the lactating cow, 580 as well as in the 

 lactating rabbit. 558 The experiments of McClymont 567,571 likewise support 



576 G. Popjak, T. H. French, G. D. Hunter, and A. J. P. Martin, Biochem. J., 48, 612- 

 618(1951). 



577 G. Popjak, "Fat Synthesis from Small Molecules," in R. T. Williams, Lipid Me- 

 tabolism, Biochem. Soc. Symposia, No. 9, Cambridge Univ. Press, 37-51 (1952). 



578 T. H. French, G. D. Hunter, A. J. P. Martin, and G. Popjak, Biochem. J., 48, vi-vii 

 (1951). 



579 A. T. Cowie, W. G. Duncombe, S. J. Folley, T. H. French, R. F. Glascock, L. Mas- 

 sart, G. J. Peeters, and G. Popjak, Biochem. J., 49, 610-615 (1951). 



580 M. Kleiber, A. H. Smith, A. L. Black, M. A. Brown, and B. M. Tolbert, ./. Biol. 

 Chem., 197, 371-379 (1952). 



