The same rat Δ6-desaturase not only acts on 18- but also on 24-carbon fatty acids in very-long-chain polyunsaturated fatty acid biosynthesis

The recently cloned Delta6-desaturase is known to catalyse the first step in very-long-chain polyunsaturated fatty acid biosynthesis, i.e. the desaturation of linolcic and a-linolenic acids. The hypothesis that this enzyme could also catalyse the terminal desaturation step, i.e. the desaturation of 24-carbon highly unsaturated fatty acids, has never been elucidated. To test this hypothesis, the activity of rat Delta6-desaturase expressed in COS-7 cells was investigated, Recombinant Delta6-desaturase expression was analysed by Western blot, revealing a single band at 45 kDa. The putative involvement of this enzyme in the Delta6-desaturation of C-24:5 n - 3 to C-24.0 n - 3 was measured by incubating transfected cells with C-22.5 n - 3. Whereas both transfected and nontransfected COS-7 cells were able to synthesize C-24.5 n - 3 by elongation of C-22:5 n-3, only cells expressing Delta6-desaturase were also able to produce C-24:6 n - 3. In addition, Delta6-desaturation of [1 -C-14]C-24:5 n - 3 was assayed in vitro in homogenates from COS-7 cells expressing Delta6-desaturase or not, showing that Delta6desaturase catalyses the conversion of C-24:5 n - 3 to C-24:6 n - 3. Evidence is therefore presented that the same rat Delta6-desaturase catalyses not only the conversion of C-18:3 n - 3 to C-18:4 n - 3, but also the conversion of C-24:5 n-3 to C-24:6 n-3. A similar mechanism in the n - 6 series is strongly suggested.