INTERCONVERSIONS OF POLYUxXSATUUATED PWTTY ACIDS 809 



fat diet caused a marked depression in the growth of the rats, and also a 

 mortahty rate as high as 83% was observed. 



One explanation for the protective effect of fats in thyrotoxicosis is that 

 they furnish the EFA. However, oleic acid and linoJeic acid were found 

 by Keeser^" to prevent the increase in metabohsm following the adminis- 

 tration of thyroid powder. On the other hand, Zain^^^'^^^ reported that 

 linoleic acid, but not stearic acid, prevented the loss of liver glycogen after 

 massive doses of the thyroid hormone. It is of course possible that the 

 failure of stearic acid to prevent thyrotoxicosis may be related to the fact 

 that the acid cannot be absorbed. Since a practically fat-free liver prepara- 

 tion has been shown to have a beneficial effect in the fat-deficiency syn- 

 drome, Ershoff ^^^ is of the opinion that the EFA are not necessarily the spe- 

 cific agents in counteracting thyrotoxicosis. On the other hand, Green- 

 ^gj.gi6i reported that both methyl linoleate and cottonseed oil have a pro- 

 tective effect against thyrotoxicosis. 



6. Intercon versions of the Polyunsaturated Fatty Acids 



(1) Inter conversions of Linoleic Acid 



a. Conversion to Arachidonic Acid, (a) Experimental Evidence of the 

 Reaction. Although there are abundant data to mdicate that the animal 

 cannot synthesize significant amounts of the so-called EFA de novo, there 

 is also considerable evidence that a certain degree of interconversion is 

 possible between the several members of the EFA. 



One indication of the interconversion of the polyunsaturated acids is the 

 fact that one acid may prevent the deficiency caused by the exclusion of 

 other essential fatty acids from the diet. Thus, Turpeinen^^ and Smedley- 

 MacLean and Nunn^^^ are of the opinion that linoleic acid is the precursor 

 of arachidonic acid. The higher biopotency of arachidonic acid as com- 

 pared with linoleic acid (see Section 4(5)) is explained as due to the fact 

 that linoleate is only partially converted to arachidonic acid ; the compara- 

 tive biopotency of linoleate is a reflection of the amount of arachidonate 

 formed from given amounts of linoleate. 



Convincing proof of the linoleate ->- arachidonate conversion is found in 

 balance experiments involving the content of arachidonate in the tissues 



'5«H. Zain, Klin. Wochschr., 15, 1722 (1936). 



''9H. Zain, Arch, exptl. Pathol. PharmakoL, 1S7, 302-323 (1937). 



ISO B. H. Ershoff, J. Nidrilion, 39, 259-281 (1949). 



1" S. M. Greenberg, J. Nutrition, 47, 31-39 (1952). 



i"I. Smedley-AIacLean and L. C. A. Nunn, Biochem. J., 34, 884-902 (1940). 



