Plasma palmitoleic acid, a product of stearoyl-coA desaturase activity, is an independent marker of triglyceridemia and abdominal adiposity.

BACKGROUND AND AIM: In an animal model VLDL-triglyceride secretion is highly dependent on stearoyl-coA desaturase (SCD) activity and could explain abdominal fattening. The aim was to assess the relationship of plasma palmitoleic acid content, a product of SCD activity, with triglyceridemia and abdominal adiposity in humans. METHODS: We evaluated 134 healthy men. Plasma palmitoleic acid content was used as an indirect measurement of SCD activity because that enzyme catalyzes the desaturation from saturated to monounsaturated fatty acids and palmitoleic acid intake is very small. RESULTS: Subjects with triglycerides > or =75th percentile had a higher palmitoleic acid content than those with triglycerides <75th percentile (3.8+/-0.8 vs 2.8+/-0.9%, p<0.0001). Triglyceridemia was strongly correlated with palmitoleic acid content (PAC) (r=0.533, p<0.001). Mean triglyceridemia was 114% higher (1.43+/-0.75 vs 0.67+/-0.22 mmol/l) in the fourth quartile than in the first quartile of palmitoleic acid content. In a stepwise logistic regression analysis, palmitoleic acid content was the most strongly and independently associated parameter with triglyceridemia, and also with waist circumference when triglyceridemia was not included in the analysis. CONCLUSION: Plasma palmitoleic acid content, a product of SCD activity, is an independent marker of triglyceridemia and abdominal adiposity in men. This enzyme (SCD) could represent a target for prevention and treatment of these metabolic disorders in particular in subjects at risk of developing a metabolic syndrome.

Dietary myristic acid at physiologically relevant levels increases the tissue content of C20:5 n-3 and C20:3 n-6 in the rat.

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.

Myristic acid increases delta6-desaturase activity in cultured rat hepatocytes.

In order to study the effects of saturated fatty acids on delta6-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 delta6-desaturase activity were as follows: 60 micromol x L(-1) alpha-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 x L(-1) myristic acid during 43 h specifically increased delta6-desaturase activity. This improvement, reproducible for three substrates of delta6-desaturase, i.e. oleic acid (C18:1n-9), linoleic acid (C18:2n-6) and alpha-linoleic acid (C18:3n-3) was dose-dependent in the range 0.1-0.5 mmol x L(-1) myristic acid concentration.

Conversion of hexadecanoic acid to hexadecenoic acid by rat Delta 6-desaturase.

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 Delta 6-desaturase in mammals. To investigate the putative involvement of the known Delta 6-desaturase (FADS2) in this process, COS-7 cells expressing rat Delta 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 Delta 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 Delta 6 -desaturation.

The same rat Delta6-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 linoleic and alpha-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(24:6) n-3 was measured by incubating transfected cells with C(22:5) n-3. Whereas both transfected and non-transfected 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-(14)C]C(24:5) n-3 was assayed in vitro in homogenates from COS-7 cells expressing Delta6-desaturase or not, showing that Delta6-desaturase 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.