804 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



transform carbohydrate into fat has been by the use of C 14 -labeled glucose. 

 French and Popjak 558 were able to show that glucose so labeled is as satis- 

 factory a source of fatty acids in the lactating rabbit as is acetate. In fact, 

 Popjak 559 reported that the glucose -*- fat reaction proceeds more effectively 

 in the mammary tissue than in the liver itself. Balmain, Folley and Glas- 

 cock 560 reported that slices of lactating mammary gland of rats actively 

 incorporate C 14 from C 14 -labeled glucose into their fatty acids. These 

 results, carried out by a variety of procedures, are completely convincing 

 in demonstrating that the lactating mammary tissue is able to synthesize 

 some of the fat required for its secretion from carbohydrate. Kleiber 

 et al. 561 found that 2% of the C 14 -carbon injected as isotopic sodium bicar- 

 bonate into cows appeared in the lipid fraction of the milk, while 10% was 

 present in the lactose. The mechanism of CCVnxation is uncertain. 



(b) Acetate as a Source of Milk Fat. It is logical to consider that acetate 

 or some 2-carbon fragment will act as the intermediate in the synthesis of 

 fat. Thus, acetate has been shown to play an essential role in the inter- 

 mediary metabolism of fat. 562 Rittenberg and Bloch, 563 in 1945, indicated 

 that acetate or some closely related compound was utilized for the synthesis 

 of fatty acid chains in the animal body. This behavior on the part of 

 acetate is not unexpected, in view of the fact that it has been shown to be a 

 building stone for other more complicated compounds, including a com- 

 plex molecule such as that of cholesterol. 564 



A number of facts support the hypothesis that acetate may serve as a 

 source of a considerable amount of the milk fat in the lactating ruminant. 

 The most important of these is the recognition that cellulose and other plant 

 polysaccharides are degraded by the microorganisms in the rumen to lower 

 fatty acids, including acetic acid. Proof for this statement has been re- 

 viewed by Elsden and Phillipson, 565 and it appears to be especially convinc- 

 ing. Another proof which supports the theory that acetate is the mother 

 substance of milk fat is the fact that acetate is available in the peripheral 

 blood of cattle and sheep in amounts of 2 to 14 milligram per cent, depend- 



668 T. H. French and G. Popjak, Biochem. J., 49, iii-iv (1951). 



569 G. Popjak, personal communication to S. J. Folley, 1952; cited by S. J. Folley, 

 "Aspects of Fat Metabolism in the Ruminant," in R. T. Williams, Lipid Metabolism, 

 Biochem. Soc. Symposia, No. 9, Cambridge Univ Press, 52-65 (1952), p. 55. 



860 J. H. Balmain, S. J. Folley, and R. F. Glascock, Nature, 169, 447-449 (1952). 



661 M. Kleiber, A. H. Smith, and A. L. Black, J. Biol Chem., 195, 707-714 (1952). 



662 K. Bloch, Physiol. Revs., 27, 574-620 (1947). 



683 D. Rittenberg and K. Bloch J. Biol. Chem., 160, 417-424 (1945). 

 564 K. Bloch and D. Rittenberg, J. Biol. Chem., 143, 297-298 (1942); 10, 625-636 

 (1942); 159, 45-58(1945); 160, 417-424 (1945). 



668 S. R. Elsden and A. T. Phillipson, Ann. Rev. Biochem., 17, 705-726 (1948). 



