SATURATED FATTY ACIDS 25 



It would thus appear that the relationship between specific activity and 

 chain length of the fatty acids is in some way related to the mechanism of 

 fatty acid synthesis. On analysis of the butyrate, caproate, and caprylate 

 formed in the udder of the goat after the injection of CHa-C'^OONa, it 

 was found^'^ that the C^"* was present exclusively in carbons 1, 3, and 5, 

 and that the isotope was completely absent from carbons 2, 4, 6, and 8. 

 These results are especially convincing in indicating that the synthesis of 

 the long-chain acid results from successive additions of the methyl groups 

 of acetic acid fragments to the carboxyl group of the partly synthesized 

 acids. 



The results of Popjak, French, and Folley^^ are likewise of interest in 

 this connection. The milk fatty acids obtained four hours after the in- 

 jection of 5 mc. of C'^-carboxyl-labeled acetate were separated and isolated 

 into four fractions, namely: (1) steam volatile and soluble, (2) steam vola- 

 tile and insoluble, (3) nonvolatile and liquid, and (4) non-volatile and 

 solid. Although all four fractions show^ed maximum specific activities 

 three to four hours after the injection, those of the two volatile fractions 

 were much higher than were those of the non-volatile fractions. This ex- 

 periment proves that the formation of new milk fat proceeds from the 

 shorter acids to longer-chain acids rather than in the reverse direction. 

 Further support for this hypothesis is afforded by the demonstration that 

 acetate and butyrate are normal components of animal fat; James and 

 Popjak^^ were able to demonstrate the presence of acetic and butyric acids 

 in small amounts in the distillate of glyceride fatty acids of rabbit liver . 



c. The Elongation of the Fatty Acid Chains with Acetate. The fact 

 that long-chain fatty acids can be elongated or shortened by two-carbon 

 fragments was first demonstrated by the classical experiments of the 

 Schoenheimer group. Thus, Stetten and Schoenheimer,^^ using deuterio- 

 palmitic acid, proved that it could be converted to deuteriostearic acid m 

 rats, as well as into deuteriopalmitoleic acid. On the other hand, Schoen- 

 heimer and Rittenberg,^^ using mice, had proved earlier with the deuterium 

 technic that stearic acid could be shortened by two carbons to yield palmitic 

 acid. A similar phenomenon was demonstrated somewhat later by Zabin" 

 in both in vivo and in vitro tests. In the latter experiments, it was proved 

 that the elongation of palmitic acid to stearic acid resulted from the addi- 

 tion of an acetate unit to the carboxyl group of the palmitic acid. 



Klem" demonstrated that lauric (dodecanoic) acid can be lengthened 



'« A. T. James and G. Popjdk, Unpublished observations, 1952; cited by G. Popjak 

 in R. T. Williams, Lipid Metabolism, Biochem. Sec. Symposia, No. 9, Cambridge Univ. 

 Press, 1952, p. 39. 



