790 XTII. ESSENTIAL FATTY ACIDS 



exactly to a 1% value of EFA. Thomasson^^ reported numerous bio- 

 assays Avith this new procedure which are, in general, consistent with other 

 available data. This procedure has the distinct advantage that the total 

 period required is only nine weeks. 



4. Comparative Biopotencies of the Essential Fatty Acids 



(1) Introduction 



Linoleic, linolenic, and arachidonic acids have been recognized as the 

 chief polyunsaturated fatty acids which possess an appreciable biopotency 

 in counteracting the fat-deficiency symptoms in rats, and in allowing a 

 normal growth and metabolism in animals on a fat-free diet. Of these 

 acids, linoleic and linolenic occur as components of many vegetable oils, 

 whereas arachidonic acid is present exclusively in animal fats. Linoleic 

 acid and, in some cases, linolenic acid, are likewise present in the blood and 

 tissue fat of some animals, but it is usually considered that arachidonic 

 acid represents the active biologic form of the EFA. There is excellent 

 evidence that linoleic acid can be transformed to arachidonic acid in the 

 animal body. In the past it was generally considered that the linolenate 

 molecule was convertible into arachidonate, but more recent work shows 

 that if this takes place at all it does so at a much lower order of efficiency 

 than is the case with the linoleate molecule. 



(2) Proof of the Tnahility of Animals to Effect a Biosjjnthesis of the Polijnn- 



satvrated Acids 



The necessity of polyunsaturated fatty acids as dietary components has 

 been based upon the observation that they appeared not to be synthesized 

 by the animal de novo. If they are absent from the diet, tissues in which 

 they play an integral role and enzyme systems in which they may partici- 

 pate become depleted. They then function less effectively, or they may 

 disappear entirely. 



It has long been recognized that the animal can synthesize satiu'ated 

 fatty acids from carbohydrate ; it is now known that two-carbon intermedi- 

 ates, which originate from carbohydrates, participate in this biosynthesis, 

 together with Coenzyme A. Moreover, it was convincingly demonstrated 

 by Schoenheimer and Rittenberg,^^ by the use of tagged stearate molecules, 

 that the mouse is able to bring about the desaturation of stearic acid into 

 oleic acid. Palmitoleic acid has likewise been shown to originate from pal- 



»* R. Schoenheimer and D. Rittenberg, /. Biol. Chem., 113, 505-510 (1936). 



