The questionable role of a microsomal Delta 8 acyl-CoA-dependent desaturase in the biosynthesis of polyunsaturated fatty acids

Several experimental approaches were used to determine whether rat liver an d testes express an acyl-CoA-dependent Delta 8 desaturase. When [1-C-14]5,1 1,14-eicosatrienoic acid was injected via the tail vein, or directly into t estes, it was incorporated into liver and testes phospholipids, but it was not metabolized to other labeled fatty acids. When [1-C-14]11,14-eicosadien oic acid was injected, vb the tail vein or directly into testes, or incubat ed with microsomes from both tissues, it was only metabolized to 5,11,14-ei cosatrienoic acid. When ethyl 5,5,11,11,14,14-d(6)-5,11,14-eicosatrienoate was fed to rats maintained on a diet devoid of far, it primarily replaced e sterified 5,8,11 -eicosatrienoic acid, but not arachidonic acid. No labeled linoleate or arachidonate were detected. Dietary ethyl linoleate and ethyl 19,19,20,20-d(4)-1,2-C-13-11,14-eicosadienoate were about equally effectiv e as precursors of esterified arachidonate. The doubly labeled 11,14-eicosa dienoate was metabolized primarily by conversion to 17,17,18,18-d(4),-9,12- ocatdecadienoic acid, followed by its conversion to yield esterified arachi donate, with a mass four units greater than endogenous arachidonate. In add ition, the doubly labeled substrate gave rise to a small amount of arachido nate, six mass units greater than endogenous arachidonate. No Evidence was obtained, with the radiolabeled substrates, for the presence of a Delta 8 d esaturase. However, the presence of an ion, six mass units greater than end ogenous arachidonate when doubly labeled 11,14-eicosadienoate was fed, sugg ests tt,at a small amount of the substrate may have been metabolized by the sequential use of Delta 8 and Delta 5 desaturases.