270 ESSENTIAL FATTY ACIDS 



of several previous reviews to which the reader is ref erred. ^^"* In the present 

 treatment, the discussion of the essential fatty acids and the deficiency 

 phenomenon caused by their absence from the diet of animals will be 

 general, reference being made only to selected papers from which the 

 reader can obtain more detailed reference. 



The essential fatty acids are a group of naturally occurring polyunsat- 

 urated fatty acids, some of whose structures are well established. They 

 possess the common structure of methylene-interrupted polyunsaturation. 

 The group is generally regarded as consisting of three members, linoleic 

 acid (9,12-octadecadienoic acid), linolenic acid (9,12,15-octadecatrienoic 

 acid), and arachidonic acid (probably 5,8,11 ,14-eicosatetraenoic acid). 

 However, the higher polyunsaturated acids, clupanodonic acid and docosa- 

 hexaenoic acid, are closely related to the essential fatty acids and should be 

 included with them in the more general sense of the classification. Molecu- 

 lar models and conventional structural formulas for the first three acids 

 are shown in Fig. 1. Linoleic and linolenic acids are of vegetable origin and 

 are probably the dietary precursors of the animal polyiansaturated acids. 

 Arachidonic acid is found only in animal lipids and is probably a functional 

 essential fatty acid. As will be mentioned later, linoleic and linolenic acids 

 are not entirely equivalent in action, and linoleic acid is the only vegetable 

 acid which meets all the requirements of the animal for essential fatty acid. 

 Thus, it is apparent that the term essential fatty acids, as ordinarily used, 

 is generic, including many members not equivalent in metabolic action. 

 As more information is gained regarding the roles of the various acids, best 

 usage will require their individual names. 



A. ISOLATION 



Linoleic acid is a major constituent of a variety of vegetable oils. Excel- 

 lent sources of the acid for preparative purposes are corn oil, cottonseed oil, 

 and safflower seed oil, in which little or no conjugated or more highly 

 unsaturated acids are found. The problem in isolation is the separation of 

 linoleic acid from saturated and unsaturated acids of various chain lengths. 

 The various acids present in such mixtures have such slight differences in 

 chemical and physical properties that separation is a difficult procedure, 

 and the product gained is rarely pure. 



The classical procedure for linoleic acid preparation is the bromination- 



ib G. 0. Burr, Federation Proc. 1, 224 (1942). 



2 A. E. Hansen and G. O. Burr, /. Am. Med. Assoc. 132, 855 (1946). 



^ R. T. Holman, Proc. 3rd. Conf. on Research, Am. Meat Inst. Univ. Chicago p. 1 



(1951); Fette u. Seifen 53, 332 (1951). 

 ^ R. T. Holman, Symposium on Biol. Significance of Lipids, Robert Gould Research 



Foundation, 1950. 



