280 ESSENTIAL FATTY ACIDS 



from arachidonic acid than from linoleic acid. Very recent tests in which 

 natural methyl arachidonate, prepared by physical means, was compared 

 with bromination-debromination linoleic acid, seem to indicate that the 

 arachidonate was six times as effective as the linoleic acid in promoting 

 growth (see Greenberg et al}) . The proper comparison between comparable 

 natural linoleate and arachidonate esters has not yet been made. 



Linolenic acid and docosahexaenoic acid restore growth but do not 

 improve the animal's skin condition, indicating that these acids are not 

 able to meet all the metabolic requirements for polyunsaturated fatty acid. 

 Many unsaturated fatty acids and compounds related to them have been 

 tested for potency. The results of such tests are gathered in Table 11. 

 From the table it is apparent that only linoleic and arachidonic acids are 

 fully effective in restoring fat-deficient rats to normal, and that linolenic 

 and docosahexaenoic acids allow growth without meeting the requirement 

 for normal skin condition. This points to the dual character of the essential 

 fatty acid deficiency. 



V. Biogenesis and Metabolism 



RALPH T. HOLMAN 



The formation of linoleic and linolenic acids in the plant is generally 

 considered to be a conversion from carbohydrate. The mechanism has not 

 been worked out beyond the observation that in a ripening seed the fat 

 content increases whereas the carbohydrate decreases. The reverse is true 

 during germination,^ a time when linoleic and linolenic acids are oxidized 

 preferentially, lipoxidase activity is initially high, and catalase activity is 

 temporarily high. 



The synthesis of the essential fatty acids in the animal body appears to 

 be a slow and inadequate process. Although many reports indicate that 

 essential fatty acid is not synthesized by the animal,^' ^ more recent reports 

 indicate that synthesis can take place to a limited degree.*- * The polyun- 

 saturated fatty acids are dietary essentials because the animal cannot 

 synthesize them in sufficient quantity for its needs. The animal, however, 

 is able to make interconversions in the polyunsaturated fatty acids. Smed- 



iR. T. Holman, Arch. Biochem. 17, 459 (1948). 



2 K. Bernhard, H. Steinhauser, and F. Bullet, Helv. Chim. Acta 25, 83 (1942). 



' R. Schonheimer, The Dynamic State of Bod}^ Constituents, p. 18. Harvard Univer- 

 sity Press, 1949. 



*G. Medes and D. C. Keller, Arch. Biochem. 15, 19 (1947). 



^V. H. Barki, R. A. Collins, E. B. Hart, and C. A. Elvehjem, Proc. Soc. Exptl. 

 Biol. Med. 71, 694 (1949). 



