POLYUNSATURATED FATTY ACIDS 793 



or linolenic acids in cod-liver oil, it is practically impossible to correct for 

 the presence of the highly unsaturated acids. The best results, with this 

 procedure, are obtained with plant fats in which these highly unsaturated 

 acids are not present. This method can be employed in studies of animal 

 fat when the highly unsaturated acids are absent, or when they are present 

 in a relatively insignificant amount as compared with the dienoic and tri- 

 enoic acids. However, since the procedure is empirical, and the extinction 

 coefficients are obtained from the literature, it is imperative that all condi- 

 tions of the analysis be rigidly observed. The degree of alkali isomeriza- 

 tion varies with the degree of alkalinity, the solvent, the temperature, and 

 the time. When all conditions are scrupulously fulfilled, the agreement 

 between different determinations is good. However, Deuel, Greenberg, 

 Anisfeld, and Melnick^^ showed that, although there is usually agreement 

 l)etween the EF A content determined by the alkali-isomerization method and 

 by the growth bioassay, this is not invariably true. In the case of a shorten- 

 ing prepared by non-selective hydrogenation, the bioassay method gave a 

 value approximately twice that obtained by the spectrophotometric pro- 

 cedure. It is suggested that there may be isomers of unsaturated fatty 

 acid which are biologically active but in which conjugation cannot be effected 

 by the alkali treatment. It would therefore appear to the authors that the 

 absolute index of EFA content can be obtained only when the sample is 

 tested by bioassay procedure. 



(4) Synthesis of Polyunsaturated Acids 



Although the polyunsaturated acids appear superficially to be rather 

 simple organic compounds, only the dienoic acid, linoleic acid, has been sat- 

 isfactorily prepared synthetically. The synthesis of this acid was recently 

 reported almost simultaneously by workers in three different laboratories, 

 i.e., Walborsky et al.,'''^ Gensler and Thomas,^^ and Raphael and Sond- 

 heimer.^^ The procedures employed were essentially the same and de- 

 pended upon the synthesis of a long-chain acetylenic acid (9,12 octadecadiy- 

 noic acid) by the method of Ahmad, Bumpus, and Strong,^^" followed by 

 partial hydrogenation to produce the corresponding C2s-ethylenic acid. 



'^ H. J. Deuel, Jr., S. M. Greenberg, L. Anisfeld, and D. Melnick, /. Nutrition, j^5, 

 535-549(1951). 



" H. M. Walborsky, R. H. Davis, and D. R. Howton, /. Am. Chem. Soc, 73, 2590- 

 2594(1951). 



'8 W. J. Gensler and G. R. Thomas, /. Am. Chem. Soc, 73, 4601-4604 (1951). 



" R. A. Raphael and F. Sondheimer, /. Chem. Soc, 1950, 2100-2103. 



"=■ K. Ahmad, F. M. Bumpus, and S. M. Strong, /. Am. Chem. Soc, 70, 3391-3394 

 (1948). 



