Measurement of the metabolic interconversion of deuterium-labeled fatty acids by gas chromatography/mass spectrometry

Effect of different levels of docosahexaenoic acid supply on fatty acid status and linoleic and α-linolenic acid conversion in preterm infants

OBJECTIVES

Long-chain polyunsaturated fatty acid (LC-PUFA) enrichment of preterm infant formulas is recommended to meet high demands. Dietary LC-PUFA may inhibit endogenous LC-PUFA synthesis, thus limiting their benefit. We investigated effects of different docosahexaenoic acid (DHA) intakes on plasma and erythrocyte fatty acids and endogenous LC-PUFA synthesis in preterm infants.

METHODS

Forty-two preterm infants (birth weight 1000-2200 g) were randomized double-blind to preterm formulas with γ-linolenic acid (0.4%) and arachidonic acid (AA, 0.1%) but different DHA contents (A: 0.04%, B: 0.33%, C: 0.52%); 24 received human milk (HM: 0.51% AA, 0.38% DHA, nonrandomized). Blood was sampled on study days 0, 14, and 28. Uniformly C-labeled linoleic acid (2 mg/kg) and α-linolenic acid (1 mg/kg) were applied orally on day 26 and blood samples collected 48  hours later.

RESULTS

On day 28, group A had the lowest and group C the highest plasma phospholipid concentrations of eicosapentaenoic acid and DHA. Erythrocyte phospholipid DHA was lowest in group A, but comparable in groups B, C, and HM. Plasma and erythrocyte AA were lower in formula groups than in HM. DHA intake had no effect on DHA synthesis. LC-PUFA synthesis was lower in HM infants.

CONCLUSIONS

DHA supply dose dependently increased plasma DHA. Formula DHA levels of 0.33% matched plasma DHA status of infants fed HM. LC-PUFA synthesis was lower in infants fed HM than formulas with different DHA and low AA contents. With the LC-PUFA supplementation used, DHA in formulas did not inhibit AA or DHA synthesis.

De novo arachidonic acid synthesis in Perkinsus marinus, a protozoan parasite of the eastern oyster Crassostrea virginica

The capability of synthesizing fatty acids de novo in the meront stage of the oyster protozoan parasite, Perkinsus marinus, was investigated employing stable-isotope-labeled precursors (1,2 13C-acetate and palmitic-d(31) acid). Fatty acid methyl esters derived from 1,2 13C-acetate and palmitic-d(31) acid were analyzed using gas chromatography/mass spectrometry and gas chromatography/flame ionization detection. Results revealed that in vitro cultured P. marinus meronts utilized 13C-acetate to synthesize a range of saturated and unsaturated fatty acids. The saturated fatty acids 14:0, 16:0, 18:0, 20:0, 22:0, 24:0 and the unsaturated fatty acids, 18:1(n-9), 18:2(n-6), 20:1(n-9), 20:2(n-6), 20:2(n-9), 20:3(n-6), 20:4(n-6) were found to contain 13C, after 7, 14, and 21 days incubation with the precursor. This indicates that meronts can synthesize fatty acid de novo using acetate as a substrate. Meronts efficiently elongated 16:0-d(31) to 18:0, 20:0, 22:0, 24:0, but desaturation activity was limited, after 7 and 14 days cultivation. Only a small quantity of 18:1-d(29) was detected. This suggests that meronts cannot directly convert exogenous palmitic acid or its products of elongation to unsaturated counterparts. The ability to synthesize 20:4(n-6) from acetate is particularly interesting. No parasitic protozoan has been reported to be capable of synthesizing long chain essential fatty acids, such as 20:4(n-6) de novo. Future study will be directed to determine whether the observed in vitro activities indeed reflect the in vivo activities, when meronts are associated with the host.

Comparison of bolus versus fractionated oral applications of [13C]-linoleic acid in humans

BACKGROUND

The endogenous conversion of linoleic acid into long-chain polyunsaturated fatty acids is of potential importance for meeting substrate requirements, particularly in young infants. After application of [13C]-linoleic acid, we estimated its conversion to dihomo-gamma-linolenic and arachidonic acids from only two blood samples.

DESIGN

Oral tracer doses were given to five healthy adults as a single bolus. In four subjects the tracer was given in nine equal portions over 3 days. Concentration and 13C content of fatty acids from serum phospholipids were analysed by gas chromatography combustion isotope ratio-mass spectrometry. Areas under the tracer-concentration curves were calculated, and fractional transfer and turnover rates estimated from compartmental models.

RESULTS

The median fractional turnover of linoleic acid was 93.7% per day (interquartile range 25.3) in the bolus group and 80. 0% per day (6.3) in the fraction group (NS). Fractional conversion of linoleic to dihomo-gamma-linolenic acid was 1.5% (0.9) vs. 2.1% (0.7) (bolus vs. fraction, P < 0.05), and fractional conversion of linoleic to arachidonic acid was 0.3% (0.3) vs. 0.6% (0.3) (bolus vs. fraction, NS). In the fraction group conversion was significantly higher based on areas under the curve. The ratio of tracer concentration in conversion products to linoleic acid 48 h after dosing correlated very well (r >/= 0.94, P < 0.05) with the ratio of areas under the curve.

CONCLUSIONS

Using areas under the curve overestimates the conversion, because different residence times are not considered. Estimation of conversion intensity appears possible with only one blood sample obtained after tracer application.

Fatty acid metabolism in marine fish: low activity of fatty acyl delta5 desaturation in gilthead sea bream (Sparus aurata) cells

Marine fish have an absolute dietary requirement for C20 and C22 highly unsaturated fatty acids. Previous studies using cultured cell lines indicated that underlying this requirement in marine fish was either a deficiency in fatty acyl delta5 desaturase or C18-20 elongase activity. Recent research in turbot cells found low C18-20 elongase but high delta5 desaturase activity. In the present study, the fatty acid desaturase/elongase pathway was investigated in a cell line (SAF-1) from another carnivorous marine fish, sea bream. The metabolic conversions of a range of radiolabeled polyunsaturated fatty acids that comprised the direct substrates for delta6 desaturase ([1-14C]18:2n-6 and [1-14C]18:3n-3), C18-20 elongase ([U-14C]18:4n-3), delta5 desaturase ([1-14C]20:3n-6 and [U-14C]20:4n-3), and C20-22 elongase ([1-14C]20:4n-6 and [1-14C]20:5n-3) were utilized. The results showed that fatty acyl delta6 desaturase in SAF-1 cells was highly active and that C18-20 elongase and C20-22 elongase activities were substantial. A deficiency in the desaturation/elongation pathway was clearly identified at the level of the fatty acyl delta5 desaturase, which was very low, particularly with 20:4n-3 as substrate. In comparison, the apparent activities of delta6 desaturase, C18-20 elongase, and C20-22 elongase were approximately 94-, 27-, and 16-fold greater than that for delta5 desaturase toward their respective n-3 polyunsaturated fatty acid substrates. The evidence obtained in the SAF-1 cell line is consistent with the dietary requirement for C20 and C22 highly unsaturated fatty acids in the marine fish the sea bream, being primarily due to a deficiency in fatty acid delta5 desaturase activity.

Low C18 to C20 fatty acid elongase activity and limited conversion of stearidonic acid, 18:4(n-3), to eicosapentaenoic acid, 20:5(n-3), in a cell line from the turbot, Scophthalmus maximus

The TF cell line, derived from a top predatory, carnivorous marine teleost, the turbot (Scophthalmus maximus), is known to have a limited conversion of C18 to C20 polyunsaturated fatty acids (PUFA). To illuminate the underlying processes, we studied the conversions of stearidonic acid, 18:4(n-3), and its elongation product, 20:4(n-3), in TF cells and also in a cell line, AS, derived from Atlantic salmon (Salmo salar), by adding unlabelled (25 microM), U-14C (1 microM) or deuterated (d5; 25 microM) fatty acids. Stearidonic acid, 18:4(n-3), was metabolised to 20:5(n-3) in both cells lines, but more so in AS than in TF cells. Delta5 desaturation was more active in TF cells than in AS cells, whereas C18 to C20 elongation was much reduced in TF as compared to AS cells. Only small amounts of docosahexaenoic acid (22:6(n-3)) were produced by both cell lines, although there was significant production of 22:5(n-3) in both cultures, especially when 20:4(n-3) was supplemented. We conclude that limited elongation of C18 to C20 fatty acids rather than limited fatty acyl Delta5 desaturation accounts for the limited rate of conversion of 18:3(n-3) to 20:5(n-3) in the turbot cell line, as compared to the Atlantic salmon cell line. The results can account for the known differences in conversions of C18 to C20 PUFA by the turbot and the Atlantic salmon in vivo.

Delta 8 desaturation in vivo of deuterated eicosatrienoic acid by mouse liver

In vitro evidence has been reported for an alternate pathway that involves delta 8 desaturation of n-6 and n-3 polyunsaturated fatty acids (PUFA). The present study was designed to allow detection of delta 8 desaturation in vivo and to provide an estimation of the relative contribution of delta 8 desaturation to the in vivo synthesis of n-3 fatty acids. Male adult ICR mice were fed a semisynthetic fat-free diet for eight days, and then the diets were supplemented for three days with deuterated 11,14,17-eicosatrienoic acid (20:3-d8) labeled at the 3,3,4,4,8,8,9,9 carbon positions. Analysis of liver total lipid by gas chromatography/mass spectroscopy indicates that the total deuterated fatty acids contained 22.3% 20:3n-3-d8 and 28.9% of metabolites formed by elongation and delta 5 desaturation of 20:3n-3-d8. Deuterated metabolites resulting from retroconversion to 18:3-d6 and subsequent metabolism by classical pathways represented 35.3% of the total deuterated fatty acids. The retroconversion product (18:4n-3-d6) of 20:4n-3-d6 and/or -d8 was 9.0% of the total. A minor percentage (4.4%) of the products identified (20:4n-3-d6, 20:5n-3-d6, 22:5n-3-d6, 22:6n-3-d5 and 24:6n-3-d5) were formed by delta 8 desaturation. This study provides the first clear evidence of delta 8 desaturation in vivo in the mouse liver. Whether delta 8 desaturation would have a greater importance in vivo when the delta 6 desaturase pathway is disrupted remains to be determined.

Dietary linoleic acid influences desaturation and acylation of deuterium-labeled linoleic and linolenic acids in young adult males

The purpose of this study was to investigate the effect of dietary linoleic acid (18:2(n - 6)) on the conversion of 18:2(n - 6) and 18:3(n - 3) to their respective n - 6 and n - 3 metabolites; to compare the incorporation of these fatty acids into human plasma lipids; to evaluate the importance of dietary 18:3(n - 3) as a precursor for the biosynthesis of long-chain length n - 3 fatty acids. The approach used was to feed young adult male subjects (n = 7) diets containing 2 levels of linoleic acid (SAT diet, 15 g/day; PUFA diet, 30 g/day) for 12 days. A mixture of triacylglycerols containing deuterated linolenic (18:3(n - 3)) and linoleic (18:2(n - 6)) acids was fed and blood samples were drawn over a 48 h period. Concentrations of deuterated 18:3(n - 3) in plasma total lipid ranged from 309.2 to 606.4 microgram/ml and concentrations of 18:2(n - 6) ranged from 949.2 to 1743.3 micrograms/ml. The sum of the deuterated n - 3 long-chain length fatty acid metabolites in plasma total lipid were 116 +/- 4.3 micrograms/ml (SAT diet) and 41.6 +/- 12.4 micrograms/ml (PUFA diet). The total deuterated n - 6 fatty acid metabolites were 34.6 +/- 12.2 micrograms/ml (SAT diet) and 9.8 +/- 5.9 micrograms/ml (PUFA diet). The total percent conversion of deuterated 18:3(n - 3) to n - 3 fatty acid metabolites and deuterated 18:2(n - 6) to n - 6 fatty acid metabolites were 11-18.5% and 1.0-2.2%, respectively. The percentages for deuterated 20:5(n - 3), 22:5(n - 3) and 22:6(n - 3) (6.0%, 3.5%, and 3.8%) were much higher than for 20:3(n - 6) and 20:4(n - 6) (0.9% and 0.5%). Overall, conversion of deuterated 18:3(n - 3) and 18:2(n - 6) was reduced by 40-54% when dietary intake of 18:2(n - 6) was increased from 15 to 30 g/day. Comparison of the deuterated 18:3(n - 3) and 18:2(n - 6) data for plasma triacylglycerol and phosphatidylcholine (PC) indicated that 18:2(n - 6) was preferentially incorporated into PC. Dietary 18:2(n - 6) intake did not alter acyltransferase selectivity but activity was reduced when 18:2(n - 6) intake was increased. Based on these results, conversion of the 18:3(n - 3) in the US diet (2 g) is estimated to provide 75-85% of the long-chain length n - 3 fatty acids needed to meet daily requirements for some (but not all) adults.