812 



XIII. ESSENTIAL FATTY ACIDS 



of (possibly) two methylenes, at which point acetate is added. An example 

 of this type of reaction is proposed below for linoleate. The two famihes 

 are not readily interconvertible, and compounds derived from linolenic 

 acid are not highly active as essential fatty acids. 



Indirect evidence would indicate a conversion of linoleate to arachidonate 

 by the following scheme: 



CH3— (CH2)4— CH=CH— CH,— CH=CH— (CH,)7— COOH 



I Linoleic acid (I) 



CH3— ( CH2 )4— CH=CH— CH2— CH=CH— CH2— CH=CH— ( CH. )4— CUOH 

 7-Linolenic acid (II) 



CH3— (CH2)4— CH=CH— CHo— CH==CH— (CH2)9— COOH 

 Homolinoleic acid (III) 



CH3— (CH2)4— CH=CH— CHo— CH=CH— CH2— CH=CH— CH2— CH=CH— CH2— COOH 



i ""' 



CH3— (CH2)4— CH=CH— CH2— CH=CH— CH2— CH=CH— (CH2)6— COOH 



(V) 



CHs— (CH2)4— CH=CH— CH,— CH=CH— CH2— CH=CH— CHo— CH=CH— (CH2)3— COOH 



Arachidonic acid (VI) 



This scheme would explain Thomasson's^^ observations on the require- 

 ment that the first double bond from the methyl group should be at posi- 

 tion 4 to ensure essential fatty acid activity. 



In a search for intermediates in this scheme, it was found that the tri- 

 enoic acid which accumulates during fat deficiency is not linolenic acid but 

 5,8,11-eicosatrienoic acid, which appears to be a hydrogenation product of 

 arachidonic acid."' Why this reaction should take place during fat defi- 

 ciency is a mystery, unless this acid is very active as essential fatty acid, 

 which is doubtful. 



In a separate study by Mead et al. '^^ it was shown that the fat-deficiency 

 state is generally characterized by increased fatty acid oxidation, and that 

 there is no sparing of linoleate by fat-deficient mice. Linoleate was con- 

 verted to carbon dioxide at least as rapidly as were oleate and stearate. 



1" J. F. Mead and W. H. Slaton, Jr., /. Biol. Chem., 219, 705-709 (1956). 

 "2 J. F. Mead, W. H. Slaton, Jr., and A. B. Decker, /. Biol. Chem., 218, 401-407 

 (195G). 



