806 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



transformation of acetate to fatty acid in the mammary tissue of non- 

 ruminants in the same way as it has been shown to accelerate the forma- 

 tion of fat from acetate in liver slices. 4 



b'. Proof of the Synthesis of Milk Fat from Acetate Based upon Ex- 

 periments with Acetate Containing C 14 : Direct proof of the transforma- 

 tion of acetate to fatty acids has been afforded by the use of acetate labeled 

 with C 14 as a substrate for slices of mammary tissue. 572 Considerable 

 amounts of C 14 were found to be incorporated into the fatty acids. 



c'. Proof of the Synthesis of Milk Fat from Acetate Based upon in vivo 

 Experiments: The most convincing proof of the conversion of acetate to 

 fatty acids by the mammary tissue has been obtained by the use of the 

 intact animal. Thus, Popjak and Beeckmans 573 found that fatty acids 

 could be synthesized in the mammary tissue from short-chain acids; in 

 fact, this synthesis could be demonstrated in the twenty-eight-day pregnant 

 rabbit. It takes place primarily in the mammary gland rather than in the 

 liver, as is proved by the fact that the concentration of C 14 -labeled fat is 

 much greater in the udder than in the liver after the administration of 

 C 14 -acetate. Moreover, the synthesis from small carbon compounds is 

 indicated by the fact that the volatile (short-chain) fatty acids were found 

 to contain much more radioactive carbon than did the long-chain acids. 573574 

 These facts disprove the concept that the short-chain acids arise from the 

 long-chain acids; moreover, they contradict the theory that the primary 

 site of fat synthesis is in the liver, and that the milk fat represents fat which 

 has been previously synthesized in the liver and carried to the mammary 

 gland via the blood stream. 



Further proof of the primary synthesis of the short-chain acids from ace- 

 tate has been obtained by experiments in which C 14 -acetate was injected 

 into goats. The specific radioactivity time curves of the soluble and in- 

 soluble volatile fatty acids, as well as of the liquid and solid non-volatile 

 fatty acids, showed maxima four hours after the injection. However, the 

 highest activities were noted in the volatile fractions; this characterizes 

 them as the primary products. Moreover, a higher specific activity was 

 observed in the fatty acids of milk than in the fatty acid fraction in the 

 blood. This result furnishes convincing evidence that the conversion of 

 acetate into fatty acids is not exclusively a function of the liver or of other 

 non-mammary tissues; rather it indicates that fat synthesis from acetate 

 proceeds better in mammary tissue than in other tissues. 568 ' 575 



674 J. H. Balmain, S. J. Folley, and R. F. Glascock, Biochem. J., 52, 301-306 (1952). 

 673 G. Popjak and M. L. Beeckmans, Biochem. J., 46, 547-558 (1950). 

 574 G. Popjak, S. J. Folley, and T. H. French, Arch. Biochem., 23, 508-510 (1949). 

 676 G. Popjak, T. H. French, and S. J. Folley, Biochem. J., 46, xxviii-xxix (1950) 



