TRIGLYCERIDES AND FATTY ACIDS 183 



an inert metabolite but one which is actively concerned in many inter- 

 mediary reactions. 



(5) The Metabolism of Polyunsaturated Acids 



a. Interconversions of Linoleic Acid, (a) Conversion to Arachidonic 

 Acid. Although there are decisive data to the effect that the animal can- 

 not synthesize the so-called essential fatty acids (EFA) de novo, nevertheless 

 considerable evidence indicates that a certain degree of interconvertibility 

 exists between several members of the group. 



Circumstantial proof of this interconversion consists in the fact that one 

 essential fatty acid may prevent the deficiency caused by the exclusion of 

 others from the diet. Thus, Turpeinen"^ and Smedley-MacLean and 

 Nunn^^^ are of the opinion that linoleic acid is a precursor of arachidonic 

 acid. The higher biopotency of arachidonic acid as compared with linoleic 

 acid (see Chapter XI) is considered to be due to the fact that linoleate is 

 somewhat inefficiently converted to arachidonate. 



The most convincing type of proof of the linoleate -^ arachidonate 

 raction is based upon the use of balance experiments; thus, when linoleate 

 is fed, increased amounts of arachidonate are found in the tissues of the 

 animals, as compared with the level in control animals which have received 

 no supplementation with linoleate. In 1926, Ellis and IsbelP^*-^^^ pre- 

 sented evidence of the appearance of arachidonic acid in the pig following 

 the ingestion of peanuts, which contain linoleic acid. Nunn and Smedley- 

 MacLean^^^ demonstrated that the fat-deficient rat was able to convert 

 linoleate to arachidonate. Widmer and Holman^^'^ obtained similar results 

 in the case of rats. Smedley-MacLean and Hume"^ suggested that lino- 

 leic acid is transformed to clupanodonic acid. Rieckehoff and co-workers^^^ 

 also reported that corn oil (which contains linoleate but no arachidonate) 

 markedly increases the content of the tetraenoic acid (presimiably largely 

 or entirely arachidonic acid) in the liver, heart, kidney, and brain of fat- 

 deficient rats. 



In addition to the pig and rat tests, evidence for the conversion of lino- 

 leate to arachidonate has been adduced from tests on chickens. Thus, 



"2 0. Tiirpeinen, /. Nutrition, 15, 351-366 (1938). 



«•' I. Smedlev-MacLean and L. C. A. Nunn, Biochem. J., 34, 884-902 (1940). 



«i^ X. R. Ellis and H. S. Isbell, /. Biol. Chetn., 69, 219-238 (1926). 



"5 X. R. Ellis and H. S. Isbell, /. Biol. Chem., 69, 239-248 (1926). 



«»« L. C. A. Xunn and I. Smedlev-MacLean, Biochem. J., 32, 2178-2184 (1938). 



*" C. Widmer, Jr., and R. T. Holman, Arch. Biochem., 25, 1-12 (1950). 



618 1. Smedlev-MacLean and E. M. Hume, Biochem. J., 35, 996-1002 (1941). 



«" L G. Rieckehoff, R. T. Holman, and G. O. Burr, Arch. Biochem., 20, 331-340 (1949). 



