The absence of an 8-desaturases in rat liver: a reevaluation of optional pathways for the metabolism of linoleic and linolenic acids.

11-[3- -14 C] Octadecenoic acid, 11,14-[1- -14C] nonadecadienoic acid, 11,14-[1- -14 C] eicosadienoic acid, 11,14-[1- -14 C] heneicosadienoic acid and 11,14,17-[3- -14 C] eicosatrienoic acid were fed and injected into tail veins of rats raised on balanced and fat-free diets. Analysis of the total liver lipids showed that 11-ated to 5,11,14-19:3; 11,14-20:2 was desaturated to 5,11,14-20:3; 11,14-21:2 was desaturated to 5,11,14-21:3 and 11,14,17-20:3 was desaturated to 5,11,14,17-20:4. In rats raised on balanced diets 11,14-19:2 and 11,14-21:2 were desaturated at very slow rates. The 11,14-20:2 and 11,14,17-20:3 were desaturated to 5,11,14-20:3 and 5,11,14,17-20:4but not as rapidly as in rats raised on fat-free diets. These findings strongly suggest that rat liver does not have a desaturase capable of introducing a double bond in the 8-position during polyunsaturated fatty acids biosynthesis. From these studies it can be concluded that linoleate is only converted to arachidonate via the following pathway: 9,12-18:2 yields 6,9,12-18:3 yields 8,11,14-20:3 yields 5,8,11,14-20:4. Linolenate is only metabolized as follows: 9,12,15-19:3 yields 6,9,12,15-18:4 yields 8,11,14,17-20:4yields 5,8,11,14,17-20:5 yields 7,10,13,16,19-22:5 yields 4,7,10,13,16,19-22:6. Optional pathways such as 9,12-18:2 yields 11,14-20:2 yields 8,11,14-20:3 yields etc. and 9,12,15-18:3 yields 11,14,17-20:3 yields 8,11,14,17-20:4 yields etc. are inoperative because 11,14-20:2 and 11,14,17-20:3 are probably not produced in vivo and if produced may well be converted to 9,12-18:2 and 9,12,15-19:3, respectively, by retroconversion.