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The recently cloned Δ6-desaturase is known to catalyse the first step in very-long-chain polyunsaturated fatty acid biosynthesis, i.e. the desaturation of linoleic and α-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 Δ6-desaturase expressed in COS-7 cells was investigated. Recombinant Δ6-desaturase expression was analysed by Western blot, revealing a single band at 45kDa. The putative involvement of this enzyme in the Δ6-desaturation of C24:5n-3 to C24:6n-3 was measured by incubating transfected cells with C22:5n-3. Whereas both transfected and non-transfected COS-7 cells were able to synthesize C24:5n-3 by elongation of C22:5n-3, only cells expressing Δ6-desaturase were also able to produce C24:6n-3. In addition, Δ6-desaturation of [1-14C]C24:5n-3 was assayed invitro in homogenates from COS-7 cells expressing Δ6-desaturase or not, showing that Δ6-desaturase catalyses the conversion of C24:5n-3 to C24:6n-3. Evidence is therefore presented that the same rat Δ6-desaturase catalyses not only the conversion of C18:3n-3 to C18:4n-3, but also the conversion of C24:5n-3 to C24:6n-3. A similar mechanism in the n-6 series is strongly suggested.

-This study was designed to investigate the effect of myristic acid on the biosynthesis and metabolism of highly unsaturated fatty acids, when it is supplied in a narrow physiological range in the diet of the rat (0.2-1.2% of total dietary energy). Three experimental diets were designed, containing 22% of total dietary energy as lipids and increasing doses of myristic acid (0.71, 3.00 and 5.57% of total fatty acids). Saturated fat did not exceed 31% of total fat and the C18:3 n-3 amount in each diet was strictly equal (1.6% of total fatty acids). After 7 weeks, the diets had no effect on plasma cholesterol level but greatly modified the liver, plasma and adipose tissue saturated, monounsaturated and polyunsaturated fatty acid profiles. Firstly, daily intakes of myristic acid resulted in a dose-dependent tissue accumulation of myristic acid itself. Palmitic acid was significantly increased in the tissues of the rats fed the higher dose of myristic acid. A dose-response accumulation of tissue C16:1 n-7 as a function of dietary C14:0 was also shown. Secondly, a main finding was that, among n-3 and n-6 polyunsaturated fatty acids, a dose-response accumulation of liver and plasma C20:5 n-3 and C20:3 n-6 (two precursors of eicosanoids) as a function of dietary C14:0 was shown. This result suggests that dietary myristic acid may participate in the regulation of highly unsaturated fatty acid biosynthesis and metabolism.dietary myristic acid / highly unsaturated FA biosynthesis and metabolism / rat

-In order to study the effects of saturated fatty acids on ∆6-desaturase activity, rat hepatocytes in primary culture were incubated with lauric (C12:0), myristic (C14:0) or palmitic (C16:0) acids. After optimization, the standard in vitro conditions for the measurement of ∆6-desaturase activity were as follows: 60 µmol·L -1 α-linolenic acid (C18:3n-3), reaction time of 20 min and protein content of 0.4 mg. Data showed that cell treatment with 0.5 mmol·L -1 myristic acid during 43 h specifically increased ∆6-desaturase activity. This improvement, reproducible for three substrates of ∆6-desaturase, i.e. oleic acid (C18:1n-9), linoleic acid (C18:2n-6) and α-linoleic acid (C18:3n-3) was dose-dependent in the range 0.1-0.5 mmol·L -1 myristic acid concentration. myristic acid / ∆6-desaturase / saturated fatty acids / cultured rat hepatocytes

A higher content of C16:1 n-10 has recently been reported in the preputial gland of mice with a targeted disruption of the gene encoding stearoyl-CoA desaturase 1 (SCD1 ؊ / ؊ mice) when compared with wild-type mice. This result has provided the first physiological evidence for the presence and regulation of a palmitoyl-CoA ⌬ 6-desaturase in mammals. To investigate the putative involvement of the known ⌬ 6-desaturase (FADS2) in this process, COS-7 cells expressing rat ⌬ 6-desaturase were incubated with C16:0. Transfected cells were able to synthesize C16:1 n-10, while nontransfected cells did not produce any C16:1 n-10. Evidence is therefore presented that the rat ⌬ 6-desaturase, which acts on the 18-and 24-carbon fatty acids of the n-6 and n-3 series, is also able to catalyze palmitic acid ⌬ 6-desaturation. The presence of a palmitoyl-CoA ⌬ 6-desaturase has recently been described in mice (1). This unexpected ⌬ 6-desaturase activity was found in mice deficient for stearoyl-CoA desaturase 1 (SCD1 Ϫ / Ϫ mice). However, this palmitoyl-CoA ⌬ 6-desaturase activity has not yet been ascribed to any known desaturase gene.The ⌬ 6-desaturase (FADS2) has been cloned in several mammalian species (2, 3). In a previous work (4), we reported that recombinant rat ⌬ 6-desaturase expressed in COS-7 cells acts on 18-and 24-carbon fatty acids of the n-3 and n-6 series in polyunsaturated fatty acid (PUFA) biosynthesis. This result, consistent with experiments performed in yeast (5) and the PUFA biosynthetic pathway initially proposed by Sprecher and coworkers (6), illustrated the broad chain length specificity of the ⌬ 6-desaturase.In the present work, we investigated the putative involvement of FADS2 protein in the ⌬ 6-desaturation of palmitic acid. We established that C16:0 is desaturated to C16:1 n-10 by COS-7 cells when expressing rat ⌬ 6-desaturase. Moreover, coexpression of both ⌬ 6-desaturase and SCD1 resulted in both ⌬ 6-and ⌬ 9-desaturation of palmitic acid. Therefore, this study reports for the first time that a single gene encodes a mammalian ⌬ 6-desaturase that acts on one saturated fatty acid in addition to act on PUFAs. MATERIALS AND METHODS ChemicalsFetal calf serum (FCS) was purchased from Perbio (Bezons, France). Solvents were purchased from Fischer Scientific (Elancourt, France). Fatty acids and other reagents were from Sigma (St Quentin Fallavier, France). Radiolabeled [1-14 C]palmitic acid was purchased from Perkin Elmer Life Sciences (Paris, France). The anti-rat liver ⌬ 9-desaturase serum was a generous gift from Dr J. Ozols (7). Plasmid construction for expression of rat ⌬ 6-and ⌬ 9-desaturasesThe plasmid constructed for expression of rat ⌬ 6-desaturase in mammalian cells (referred to as pCMV/ ⌬ 6) has already been described (4). A plasmid coding for rat ⌬ 9-desaturase (SCD1) was constructed for expression in mammalian cells and is referred to as pcDNA3/ ⌬ 9. From the published (8) rat SCD1 sequence (GenBank accession number J02585), oligonucleotide primers were designed to PCR amplify the entire codi...