26 II. BIOSYNTHESIS 



to palmitic and stearic acids, while Stevens and Chaikoff,-^ as well as Anker^' 

 proved that myristic (tetradecanoic) acid could likewise be changed to 

 palmitic or stearic acid. In these tests also, it was demonstrated that the 

 new palmitic acid originated by the condensation of the methyl group of 

 acetate with the carboxyl group of the longer-chain acid.^^ Finally, Brady 

 and Gurin^^ demonstrated that carboxyl-labeled butyric, caproic, and 

 caprylic acids are converted by liver slices to long-chain fatty acids. Ap- 

 parently, these acids are largely cleaved into two-carbon fragments which 

 are employed as the building stones. 



Popj4k et al.^'' have presented further evidence that a four-carbon acid 

 can serve as the primary building stone for the acetate condensation to 

 form long-chain acids. It was shown that the specific activity of the 

 carboxyl carbon in caproic acid was 2.5 times that of carbons 3 and 5. 

 This was best explained by the assumption that only 40 per cent of the 

 butyric acid used for caproate synthesis was formed from acetate, while 

 60 per cent was derived from another non-isotopic C4 compound. Although 

 the non-isotopic precursor might be butyric acid itself, which was absorbed 

 from the blood, or a substance converted by mammary tissue into butyric 

 acid, it was suggested that the four-carbon primary building stone was 

 jS-hydroxybutyric acid; it is known that this substance is present in the 

 blood of lactating ruminants in considerable amounts, and that it can be 

 readily absorbed by the udder.'"" 



Further mdication that acetic acid may add to medium-length fatty 

 acid to produce an elongation has been reported by Dauben and co- 

 workers'"^ who used intact mice. "When carboxyl-labeled acetate was in- 

 jected into mice four hours before they were sacrificed, it Avas found that 

 the first and third carbons of the sixteen-carbon acids were about equally 

 labeled whereas, in the eighteen-carbon acids, the carboxyl carbon was 

 labeled to about three times that of the third carbon. It was found that 

 the tail end of the molecule was not equally labeled in this short test. 



Not only is acetate emploj'^ed for the synthesis of the saturated fatty 

 acids, but it may likewise be directly concerned with the sjmthesis of un- 

 saturated acids. Thus, Mead and coworkers'"'*'"* reported that arachi- 

 donic acid containing labeled carbon could be obtained from the liver 

 lipids of rats after they were fed acetate containing C^*. 



99 H. S. Anker, /. Biol. Chem., 194, 177-182 (1952). 



loo J. C. Shaw and C. B. Iviiodt, J. Biol. Chem., 138, 287-292 (1941). 



"1 W. G. Dauben, E. Hoerger, and J. W. Petersen, /. Am. Chem. Sac, 75, 2347-2351 

 (1953). 



102 J. F. Mead, G. Steinberg, and D. R. Howton, Federation Proc, 12, 244 (1953). 



1°' G. Steinberg, D. R. Howton, and J. F. Mead, Univ. Calif. (Los Angeles) School 

 Med., Atomic Energy Project No. 91120-10007, May 27, 1953. 



