42 II. BIOSYNTHESIS 



3. The Biosynthesis of Unsaturated Fatty Acids 



Although the theory of Leathes and Meyer- WedelP^^ that desaturation 

 is the first step in fatty acid catabolism has been widely accepted since it 

 was propounded in 1909, it remained for Schoenheimer and Rittenberg^^^ 

 to offer the j&rst experimental proof for this reaction in the animal body. 

 In their classical experiments, it was shown that unsaturated fatty acids 

 could originate in the mouse from tagged saturated fatty acids. Although 

 it was not proved in the early work that only monoethenoid acids are in- 

 volved in the synthesis, Bernhard and Schoenheimer^^^ convincingly 

 demonstrated that the rat was unable to synthesize either the diethenoid 

 acid, hnoleic, or the triethenoid acid, hnolenic, from carbohydrate, although 

 the synthesis of large amounts of palmitic and stearic acids could be shown. 

 Stetten and Schoenheimer-^ demonstrated the synthesis of palmitoleic acid 

 from palmitic acid in rats. Anker^^ reported that, after carboxyl-labeled- 

 C^*-myristic acid was fed to rats, myristoleic acid, CH3-(CH2)3-CH:CH- 

 (CH2)7-C^*OOH, could be isolated from the unsaturated fatty acid fraction 

 of the carcass. Reiser and co-workers ^^^ suggest that, although the hen 

 can synthesize minimum amounts of polyethenoid fatty acids, oleic acid 

 replaces the polyunsaturated acids in the neutral fat of the eggs produced 

 by hens on a fat-free ration. This must indicate the synthesis of oleic 

 acid from saturated acids by the hen under these conditions. Although 

 a number of polyunsaturated acids are sjmthesized by hens on a fat-free 

 regimen, Reiser^^^ produced unsaturated fatty acid deficiency in this 

 species. His results are further discussed by Holman.^^^ 



Essential fatty acids cannot be produced de novo in any common species 

 of animal; however, there is evidence that one essential acid can be trans- 

 formed to another. Thus, Mead and associates, ^"^^ ^"^ using rats, demon- 

 strated the synthesis of arachidonic acid which resulted from the addition 

 of labeled acetate to the carboxyl group of linoleic acid, followed by re- 

 moval of hydrogen from specific linkages. 



Polyunsaturated acids can be synthesized by the plant. Thus, Painter^^^ 

 noted that, although unsaturated acids may be synthesized by desatu- 

 ration during oil deposition in the flaxseed, most of it is preformed, and 

 hence is directly synthesized. 



1" J. B. Leathes and L. Meyer-Wedell, /. Physiol, 38, xxxviii-xl (1909). 

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

 189 K. Bernhard and R. Schoenheimer, /. Biol. Chem., 133, 707-712 (1940). 

 "0 R. Reiser, B. Gibson, M. J. Carr, and B. G. Lamp, /. Nutrition, U, 159-175 (1951). 

 1" R. Reiser, J. Nutrition, 42, 319-323 (1950). 



i'2 R. T. Holman, Proc. Third Con}. Research, Council on Research, Am. Meat Inst., 

 Chicago, March, 1951, pp. 1-10. 



»"E. P. Painter, Arch. Biochem., 5, 337-348 (1944). 



