TRIGLYCERIDES AND FATTY ACIDS 185 



chidonic acid is usually a component of animal tissues, and may substitute 

 for linoleate in the nutrition of different animals. These findings are 

 interpreted as a priori evidence of the linoleate -^ arachidonate conversion. 



(6) Transformation to Polyunsaturated Acids Other Than Arachidonic 

 Acid. Experimental evidence has been adduced that, in addition to tetra- 

 enoic acids, other polyunsat\u-ated acids may originate from linoleate. 

 Although the data are still fragmentary, there appears to be a species 

 difference in the transformations which can be effected with linoleic acid. 



In the case of the lajdng hen. Reiser et aZ.^--'^^^ found that linoleate was 

 changed to pentaenoic acid as well as to tetraenoic acid. However, no 

 hexaenoic acids originate in this species. In contradistinction to these 

 results, the Holman group^^'^'^^^ reported that, in the case of the rat, penta- 

 enoic and hexaenoic acids arise from linolenic acid, and arachidonic (tetra- 

 enoic acid) from linoleic acid. Although these workers failed to note the 

 formation of pentaenoic or hexaenoic acid when selected tissues were 

 used, the evidence for the synthesis of both of these types of acids was 

 obtained by analysis of the whole animal. 



b. Interconversions of Linolenic Acid. The metabolism of linolenic acid 

 poses certain interesting variations as compared with that of linolenic acid. 

 One of the earliest observations was that this trienoic acid was present in 

 the tissues to only an exceedingly small extent, even when there was a 

 considerable amount present in the diet. This led Bloor^^g ^q postulate 

 that linolenic acid is preferentially utilized and deposited. However, it now 

 appears that the deposition of linolenic acid in the tissue fat is a function of 

 species. Thus, on the one hand, Ellis and Isbell^^^-^^^ observed that only 

 small amounts of trienoic acids were deposited in pig fat, even when the 

 swine were fed diets high in soybeans, which are relatively rich in lino- 

 lenate. However, Beadle et aU'^^ did note that as much as 11.4% of trienoic 

 acid was present in the "yellow" fat of hogs. On the other hand, rats 

 which had been fed on a diet containing linseed oil were found to have as 

 much as 25.6% of linolenate in their fat depots. According to Brooker 

 and Shorland,*^^ linolenic acid may comprise as much as 20% of the depot 

 fat of pasture-fed horses. In general, however, linolenic acid is prac- 

 tically absent from depot fats. 



The linolenate content of fats is influenced in an interesting manner by 

 dietary conditions. The most striking fact is that the proportion of 



^i's W. R. Bloor, Biochemistry of the Fatty Acids, Reinhold, New York, 1943. 

 6M B. W. Beadle, O. H. M. Wilder, and H. R. Kraybill, /. Biol. Chem., 175, 221-229 

 (1948). 



"1 E. G. Brooker and F. B. Shorland, Biochem. J., 46, 80-85 (1950). 



