Differences in the interconversion between 20- and 22-carbon (n - 3) and (n - 6) polyunsaturated fatty acids in rat liver

TRPV1: Structure, Endogenous Agonists, and Mechanisms

The Transient Receptor Potential Vanilloid 1 (TRPV1) channel is a polymodal protein with functions widely linked to the generation of pain. Several agonists of exogenous and endogenous nature have been described for this ion channel. Nonetheless, detailed mechanisms and description of binding sites have been resolved only for a few endogenous agonists. This review focuses on summarizing discoveries made in this particular field of study and highlighting the fact that studying the molecular details of activation of the channel by different agonists can shed light on biophysical traits that had not been previously demonstrated.

20:5n-3 but not 22:6n-3 is a preferred substrate for synthesis of n-3 very-long- chain fatty acids (C24-C36) in retina

The objective of this study was to determine if 20:5n-3 or 22:6n-3 is the primary precursor of very-long-chain fatty acids (VLCFAs; C24-C36) synthesized in retina. Rats were fed semisynthetic, nutritionally complete diet containing 20% (w/w) fat with 3% (w/w) of 22:6n-3. After 6 weeks feeding, the vitreal fluid of each eye was injected with [3H]20:5n-3 or [3H]22:6n-3. Rats were then maintained under constant light (330 lux) or dark conditions for 48 hr. After 48 hr in vivo metabolism, the amount of label present in individual fatty acids was determined in major phospholipids in retina. For [3H]22:6n-3, 90% of total incorporation remained in 22:6n-3, whereas for [3H]20:5n-3 the label was actively incorporated into pentaenoic and hexaenoic VLCFAs up to 34 carbon chain length. 22:5n-3 derived from [3H]20:5n-3 was among the most highly labeled fatty acids. These observations suggest that 22:6n-3 is incorporated directly into retinal phospholipids without further metabolism, whereas 20:5n-3 and 22:5n-3 are metabolically active precursors for synthesis of VLCFAs.

Differences in the conversion of the polyunsaturated fatty acids [1-(14)C]22:4(n-6) and [1-(14)C]22:5(n-3) to [(14)C]22:5(n-6) and [(14)C]22:6(n-3) in isolated rat hepatocytes

The reasons why most cellular lipids preferentially accumulate 22:6(n-3) rather than 22:5(n-6) are poorly understood. In the present work the metabolisms of the precursor fatty acids, [1-(14)C]20:4(n-6), [1-(14)C]22:4(n-6) versus [1-(14)C]20:5(n-3), [1-(14)C]22:5(n-3) in isolated rat hepatocytes were compared. The addition of lactate and L-decanoylcarnitine increased the formation of [(14)C]24 fatty acid intermediates and the final products, [(14)C]22:5(n-6) and [(14)C]22:6(n-3). In the absence of lactate and L-decanoylcarnitine, no [(14)C]24 fatty acids and [(14)C]22:5(n-6) were detected when [1-(14)C]22:4(n-6) was the substrate, whereas small amounts of the added [1-(14)C]22:5(n-3) was converted to [(14)C]22:6(n-3). Lactate reduced the oxidation of [1-(14)C]22:4(n-6) and [1-(14)C]22:5(n-3) while L-decanoylcarnitine did not. No significant differences between the total oxidation or esterification of the two substrates were observed. By fasting and fructose refeeding the amounts of [(14)C]24:4(n-6) and [(14)C]24:5(n-3) were increased by 2.5- and 4-fold, respectively. However, the levels of [(14)C]22:5(n-6) and [(14)C]22:6(n-3) were similar in hepatocytes from fasted and refed versus fed rats. With hepatocytes from rats fed a fat free diet the levels of [(14)C]24 fatty acid intermediates were low while the further conversion of the n-6 and n-3 substrates was high and more equal, approx. 33% of [1-(14)C]22:4(n-6) was converted to [(14)C]22:5(n-6) and 43% of [1-(14)C]22:5(n-3) was converted to [(14)C]22:6(n-3). The moderate differences found in the conversion of [1-(14)C]22:4(n-6) versus [1-(14)C]22:5(n-3) to [(14)C]22:5(n-6) and [(14)C]22:6(n-3), respectively, and the equal rates of oxidation of the two substrates could thus not explain the abundance of 22:6(n-3) versus the near absence of 22:5(n-6) in cellular membranes.

Cultured fish cells metabolize octadecapentaenoic acid (all-cis delta3,6,9,12,15-18:5) to octadecatetraenoic acid (all-cis delta6,9,12,15-18:4) via its 2-trans intermediate (trans delta2, all-cis delta6,9,12,15-18:5)

Octadecapentaenoic acid (all-cis delta3,6,9,12,15-18:5; 18:5n-3) is an unusual fatty acid found in marine dinophytes, haptophytes, and prasinophytes. It is not present at higher trophic levels in the marine food web, but its metabolism by animals ingesting algae is unknown. Here we studied the metabolism of 18:5n-3 in cell lines derived from turbot (Scophthalmus maximus), gilthead sea bream (Sparus aurata), and Atlantic salmon (Salmo salar). Cells were incubated in the presence of approximately 1 microM [U-14C]18:5n-3 methyl ester or [U-14C]18:4n-3 (octadecatetraenoic acid; all-cis delta6,9,12,15-18:4) methyl ester, both derived from the alga Isochrysis galbana grown in H14CO3-, and also with 25 microM unlabeled 18:5n-3 or 18:4n-3. Cells were also incubated with 25 microM trans delta2, all-cis delta6,9,12,15-18:5 (2-trans 18:5n-3) produced by alkaline isomerization of 18:5n-3 chemically synthesized from docosahexaenoic acid (all-cis delta4,7,10,13,16,19-22:6). Radioisotope and mass analyses of total fatty acids extracted from cells incubated with 18:5n-3 were consistent with this fatty acid being rapidly metabolized to 18:4n-3 which was then elongated and further desaturated to eicosatetraenoic acid (all-cis delta8,11,14,17,19-20:4) and eicosapentaenoic acid (all-cis delta5,8,11,14,17-20:5). Similar mass increases of 18:4n-3 and its elongation and further desaturation products occurred in cells incubated with 18:5n-3 or 2-trans 18:5n-3. We conclude that 18:5n-3 is readily converted biochemically to 18:4n-3 via a 2-trans 18:5n-3 intermediate generated by a delta3,delta2-enoyl-CoA-isomerase acting on 18:5n-3. Thus, 2-trans 18:5n-3 is implicated as a common intermediate in the beta-oxidation of both 18:5n-3 and 18:4n-3.

Chain separation of monounsaturated fatty acid methyl esters by argentation thin-layer chromatography

A technique for separating methyl esters of monounsaturated fatty acids by argentation chromatography using silver nitrate-impregnated TLC plates is described. Monounsaturated fatty acid methyl esters are separated from polyunsaturated and saturated fatty acid methyl esters and the monounsaturated fatty methyl esters are resolved according to chain length. cis isomers are well resolved from the corresponding trans isomers. R(F) values for individual monounsaturated fatty acids are very reproducible. The potential of the technique in metabolic studies is demonstrated in the chain elongation of [14C]-18:1(n-9) and delta-9 desaturation of [14C]-18:0 by human skin fibroblasts. Recoveries of individual [14C]-fatty acids for scintillation counting exceed 94%.

Modulation of cholesterol concentration in Caco-2 cells by incubation with different n-6 fatty acids

Incorporation of exogenous cholesterol was compared in human adenocarcinoma colon cells (Caco-2) after incubation with 100 microM of either linoleic acid (LA, 18:2n-6), gamma-linolenic acid (GLA, 18:3n-6), arachidonic acid (AA, 20:4n-6) or adrenic acid (or n-6 docosatetraenoic acid, DTA, 22:4n-6). In both cells 7 days after seeding and 14 days after confluency, incubation with LA significantly raised the proportion of 18:2n-6 but not its long-chain metabolites in cellular phospholipid. Incubation with GLA increased the levels of 18:3n-6, 20:3n-6, and 20:4n-6. Incubation with AA increased the levels of 20:4n-6 and 22:4n-6, and incubation with DTA increased the levels of 22:4n-6 as well as its retro-conversion metabolite, 20:4n-6. A subsequent addition of cholesterol (180 microM) to the medium significantly raised the cellular cholesterol level but less so in the cells 7 days after seeding incubated with GLA. The increase in cellular cholesterol level was generally greater in the cells of 7 days after seeding, particularly those incubated with long-chain highly unsaturated n-6 fatty acids, than in those of 14 days after confluency. These findings suggest that the cell growth and the extent of unsaturation in cell membrane phospholipid fatty acids modulate the incorporation of the exogenous cholesterol into the Caco-2 cells.

The pathway from arachidonic to docosapentaenoic acid (20:4n-6 to 22:5n-6) and from eicosapentaenoic to docosahexaenoic acid (20:5n-3 to 22:6n-3) studied in testicular cells from immature rats

The concentration-dependent metabolism of 1-(14)C-labelled precursors of 22:5n-6 and 22:6n-3 was compared in rat testis cells. The amounts of [(14)C]22- and 24-carbon metabolites were measured by HPLC. The conversion of [1-(14)C]20:5n-3 to [3-(14)C]22:6n-3 was more efficient than that of [1-(14)C]20:4n-6 to [3-(14)C]22:5n-6. At low substrate concentration (4 microM) it was 3.4 times more efficient, reduced to 2.3 times at high substrate concentration (40 microM). The conversion of [1-(14)C]22:5n-3 to [1-(14)C]22:6n-3 was 1.7 times more efficient than that of [1-(14)C]22:4n-6 to [1-(14)C]22:5n-6 using a low, but almost equally efficient using a high substrate concentration. When unlabelled 20:5n-3 was added to a cell suspension incubated with [1-(14)C]20:4n-6 or unlabelled 22:5n-3 to a cell suspension incubated with [1-(14)C]22:4n-6, the unlabelled n-3 fatty acids strongly inhibited the conversion of [1-(14)C]20:4n-6 or [1-(14)C]22:4n-6 to [(14)C]22:5n-6. In the reciprocal experiment, unlabelled 20:4n-6 and 22:4n-6 only weakly inhibited the conversion of [1-(14)C]20:5n-3 and [1-(14)C]22:5n-3 to [(14)C]22:6n-3. The results indicate that if both n-6 and n-3 fatty acids are present, the n-3 fatty acids are preferred over the n-6 fatty acids in the elongation from 20- to 22- and from 22- to 24-carbon atom fatty acids. In vivo the demand for 22-carbon fatty acids for spermatogenesis in the rat may exceed the supply of n-3 precursors and thus facilitate the formation of 22:5n-6 from the more abundant n-6 precursors.

Long-chain polyunsaturated fatty acids, pregnancy, and pregnancy outcome

During pregnancy, essential long-chain polyunsaturated fatty acids (LCPUFAs) play important roles as precursors of prostaglandins and as structural elements of cell membranes. Throughout gestation, accretion of maternal, placental, and fetal tissue occurs and consequently the LCPUFA requirements of pregnant women and their developing fetuses are high. This is particularly true for docosahexaenoic acid (DHA; 22:6n-3). The ratio of DHA to its status marker, docosapentaenoic acid (22:5n-6), in maternal plasma phospholipids decreases significantly during pregnancy. This suggests that pregnancy is associated with maternal difficulty in coping with the high demand for DHA. The DHA status of newborn multiplets is significantly lower than that of singletons; the same is true for infants of multigravidas as compared with those of primigravidas and for preterm compared with term neonates. Because the LCPUFA status at birth seems to have a long-term effect, the fetus should receive an adequate supply of LCPUFAs. Data from an international comparative study indicated that, especially for n-3 LCPUFAs, the fetus is dependent on maternal fatty acid intake; maternal supplementation with LCPUFAs, their precursors, or both increased LCPUFA concentrations in maternal and umbilical plasma phospholipids. However, significant competition between the 2 LCPUFA families was observed, which implies that effective supplementation requires a mixture of n-6 and n-3 fatty acids. Further research is needed to determine whether higher LCPUFA concentrations in plasma phospholipid will have functional benefits for mothers and children.

A comparison of the metabolism of [3-14C]-labeled 22- and 24-carbon (n-3) and (n-6) unsaturated fatty acids by rat testes and liver

The unsaturated fatty acid composition of phospholipids from different tissues frequently varies. Rat liver phospholipids contain esterified 22:6(n-3) while 22:5(n-6) is the major esterified 22-carbon acid in testes phospholipids. Both testes and liver synthesize polyunsaturated fatty acids. Microsomes, particularly from liver, have been used extensively to measure reaction rates as they relate to polyunsaturated fatty acid and phospholipid biosynthesis. None of these rate studies explain why specific acids are synthesized and subsequently esterified. In this study we compared the metabolism of [3-14C]-labeled (n-3) and (n-6) acids when injected via the tail vein, as a measure of hepatic metabolism, versus when they were injected directly into the testes. Liver preferentially metabolizes [3-14C]-labeled 24:5(n-3) and 24:6(n-3) to yield esterified 22:6(n-3), when compared with the conversion of [3-14C]-labeled 24:4(n-6) and 24:5(n-6) to yield 22:5(n-6). Both 24-carbon (n-3) acids were also converted to 22:5(n-3) but no labeled 22:4(n-6) was detected after injecting the two 24-carbon (n-6) acids. Differences in the hepatic metabolism of 24-carbon (n-3) and (n-6) acids to 22:6(n-3) and 22:5(n-6), versus their partial beta-oxidation to 22:5(n-3) and 22:4(n-6), are important in vivo controls. Surprisingly, in testes a higher percentage of radioactivity was found in esterified 22:6(n-3) versus 22:5(n-6) following injections, respectively, of [3-14C]-labeled 22:5(n-3) versus 22:4(n-6), which is the corresponding metabolic analog. Corresponding pairs of 24-carbon (n-3) and (n-6) acids, as they relate to metabolism, were processed in similar ways by testes. The relative absence of esterified 22-carbon (n-3) fatty acids, versus the abundance of 22- and 24-carbon (n-6) acids in testes phospholipids, does not appear per se to be due to differences in the ability of testes to metabolize (n-3) and (n-6) fatty acids. It remains to be determined if there is selective uptake of specific fatty acids by testes for use as precursors to synthesize polyunsaturated fatty acids.

Modulation of eosinophil chemotactic activities to leukotriene B4 by n-3 polyunsaturated fatty acids

Eosinophil accumulation induced by leukotriene B4 appears to be involved in the pathogenesis of allergic diseases. We evaluated the effects of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) on chemotaxis to leukotriene B4 in guinea pig peritoneal eosinophils. Guinea pigs that were sensitized to polymyxin B were administered an intraperitoneal injection of polymyxin B (1 mg/animal) alone or combined with DHA (15 or 50 mg/kg, i.p.), EPA (50 or 100 mg/kg, i.p.), or with linoleic acid (LA) (100 mg/kg, i.p.). Forty hours later, eosinophils were obtained from the intraperitoneal lavage fluid and purified. The chemotactic and chemokinetic responses of eosinophils to leukotriene B4 were measured using a 96-well microchemotaxis chamber. DHA significantly decreased the chemotactic and chemokinetic responses of eosinophils in a dose-dependent fashion. A higher dose of EPA also significantly inhibited both of those responses, whereas LA had no effect. Our results suggested a possible mechanism for the improvement of allergic diseases by dietary supplementation with n-3 PUFA.

Low-dose eicosapentaenoic or docosahexaenoic acid administration modifies fatty acid composition and does not affect susceptibility to oxidative stress in rat erythrocytes and tissues

In view of the promising future for use of n-3 polyunsaturated fatty acids (PUFA) in the prevention of cancer and cardiovascular diseases, it is necessary to ensure that their consumption does not result in detrimental oxidative effects. The aim of the present work was to test a hypothesis that low doses of eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) do not induce harmful modifications of oxidative cell metabolism, as modifications of membrane fatty acid composition occur. Wistar rats received by gavage oleic acid, EPA, or DHA (360 mg/kg body weight/day) for a period of 1 or 4 wk. Fatty acid composition and alpha-tocopherol content were determined for plasma, red blood cell (RBC) membranes, and liver, kidney, lung, and heart microsomal membranes. Susceptibility to oxidative stress induced by tert-butylhydroperoxide was measured in RBC. EPA treatment increased EPA and docosapentaenoic acid (DPA) content in plasma and in all the membranes studied. DHA treatment mainly increased DHA content. Both treatments decreased arachidonic acid content and n-6/n-3 PUFA ratio in the membranes, without modifying the Unsaturation Index. No changes in tissue alpha-tocopherol content and in RBC susceptibility to oxidative stress were induced by either EPA or DHA treatment. The data suggest that EPA and DHA treatments can substantially modify membrane fatty acids, without increasing susceptibility to oxidative stress, when administered at low doses. This opens the possibility for use of low doses of n-3 PUFA for chemoprevention without risk of detrimental secondary effects.

Regulation of the biosynthesis of 4,7,10,13,16-docosapentaenoic acid

It is now established that fatty acid 7,10,13,16-22:4 is metabolized into 4,7,10,13,16-22:5 as follows: 7,10,13,16-22:4-->9,12,15, 18-24:4-->6,9,12,15,18-24:5-->4,7,10,13,16-22:5. Neither C24 fatty acid was esterified to 1-acyl-sn-glycero-3-phosphocholine (1-acyl-GPC) by microsomes, whereas the rates of esterification of 4, 7,10,13,16-22:5, 7,10,13,16-22:4 and 5,8,11,14-20:4 were respectively 135, 18 and 160 nmol/min per mg of microsomal protein. About four times as much acid-soluble radioactivity was produced when peroxisomes were incubated with [3-14C]9,12,15,18-24:4 compared with 6,9,12,15,18-24:5. Only [1-14C]7,10,13,16-22:4 accumulated when [3-14C]9,12,15,18-24:4 was the substrate, but both 4,7,10,13,16-22:5 and 2-trans-4,7,10,13,16-22:6 were produced from [3-14C]6,9,12,15, 18-24:5. When the two C24 fatty acids were incubated with peroxisomes, microsomes and 1-acyl-GPC there was a decrease in the production of acid-soluble radioactivity from [3-14C]6,9,12,15, 18-24:5, but not from [3-14C]9,12,15,18-24:4. The preferential fate of [1-14C]4,7,10,13,16-22:5, when it was produced, was to move out of peroxisomes for esterification into the acceptor, whereas only small amounts of 7,10,13,16-22:4 were esterified. By using 2H-labelled 9,12,15,18-24:4 it was shown that, when 7,10,13,16-22:4 was produced, its primary metabolic fate was degradation to yield esterified arachidonate. Collectively, the results show that an inverse relationship exists between rates of peroxisomal beta-oxidation and of esterification into 1-acyl-GPC by microsomes. Most importantly, when a fatty acid is produced with its first double bond at position 4, it preferentially moves out of peroxisomes for esterification to 1-acyl-GPC by microsomes, rather than being degraded further via a cycle of beta-oxidation that requires NADPH-dependent 2,4-dienoyl-CoA reductase.

Biosynthesis of docosahexaenoic acid in trout hepatocytes proceeds via 24-carbon intermediates

The role of 24:5n-3 and 24:6n-3 as intermediate in the formation of 22:6n-3 in trout liver was examined. Microsomes prepared from trout liver converted [1-14C]-eicosapentaenoic acid (20:5n-3) to 24: 5n-3 and 24:6n-3 but not docosahexaenoic acid (22:6n-3). The radiolabeled 24:5n-3 and 24:6n-3 were isolated from the microsomal incubations by argentation chromatography and used as substrates in incubations with hepatocytes isolated from trout liver. Both 14C-labelled 24:6n-3 and 22:6n-3-were produced by hepatocytes incubated with radiolabelled 24:5n-3. When hepatocytes were incubated with radiolabelled 24:6n-3, the amount of radioactivity recovered in 22:6n-3 over 6 hr increased in direct relation to the decrease observed in the amount of radioactivity recovered in 24:6n-3. The results suggest that the formation of 22:6n-3 in trout liver does not involve delta 4 desaturation of 22:5n-3 but rather proceeds via the delta 6 desaturation of 24:5n-3 with the subsequent chain shortening of the 24:6n-3 produced.

The metabolism of 22:5(-6) and of docosahexaenoic acid [22:6(-3)] compared in rat hepatocytes

Elevated levels of 22:5(-6), which is the elongated and desaturated product of arachidonic acid, is induced by selective n-3 fatty acid deficiency, especially in brain cortex. Less elongation and desaturation of 20:4(-6) than of 20:5(-3) has been found in intact rat liver cells in previous studies and is probably the main reason why so little 22:5(-6) is found under adequate nutritional conditions. The present study compares the metabolism of 22:5(-6) with the metabolism of 22:6(-3), the main n-3 fatty acid in mammals. Freshly isolated rat liver cells were incubated with [1-14C]22:5(-6) and [1-14C]22:6(-3). Oxidation and esterification in triacylglycerols, diacylglycerols and phospholipids were studied. The phospholipid classes were separated and the different molecular species identified. Rats with essential fatty acid deficiency were compared with control rats. 22:5(-6) was found to be a good substrate for membrane phospholipid biosynthesis and was conserved well in the phospholipid fraction of the rat liver cells for more than 3 h of incubation. More 22:5(-6) was esterified in the total phospholipid fraction and less was incorporated in triacylglycerols than observed with 22:6(-3) in hepatocytes from control animals. This was not the case in animals with essential fatty acid deficiency. 22:5(-6) was esterified to a greater extent in phosphatidylcholine than 22:6(-3) in control cells but not in essential fatty acid deficiency cells. More 22:5(-6) was coupled with 18.0 in the sn-1 position of the phospholipid molecular species than 22:6(-3) was in control cells.

Regulation of the biosynthesis of 4,7,10,13,16,19-docosahexaenoic acid

The synthesis of 4,7,10,13,16,19-docosahexaenoic acid (22:6(n-3)) requires that when 6,9,12,15,18,21-tetracosahexaenoic acid (24:6(n-3)) is produced in the endoplasmic reticulum, it preferentially moves to peroxisomes for one cycle of beta-oxidation rather than serving as a substrate for membrane lipid synthesis. Both 24:6(n-3) and its precursor, 9,12,15,18,21-tetracosapentaenoic acid (24:5(n-3)), were poor substrates for acylation into 1-acyl-sn-glycero-3-phosphocholine (1-acyl-GPC) by rat liver microsomes. When peroxisomes were incubated with 1-14C- or 3-14C-labeled 7,10,13,16,19-docosapentaenoic acid (22:5(n-3)), [1-14C]22:6(n-3), [3-14C]24:5(n-3), or [3-14C]24:6(n-3), only small amounts of acid-soluble radioactivity were produced when double bond removal at positions 4 and 5 was required. When microsomes and 1-acyl-GPC were included in incubations, the preferred metabolic fate of acids, with their first double bond at either positions 4 or 5, was to move out of peroxisomes for esterification into the acceptor rather than serving as substrates for continued beta-oxidation. When [1-14C]22:6(n-3) or [3-14C]24:6(n-3) was incubated with peroxisomes, 2-trans-4,7,10,13,16,19-22:7 accumulated. The first cycle of 20:5(n-3) beta-oxidation proceeds through 2-trans-4,8,11,14,17-20:6 and thus requires both Delta3,5,Delta2, 4-dienoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. The accumulation of the substrate for 2,4-dienoyl-CoA reductase, as generated from 22:6(n-3), but not from 20:5(n-3), suggests that this enzyme distinguishes between subtle structural differences. When 22:6(n-3) is produced from 24:6(n-3), its continued degradation is impaired because of low 2,4-dienoyl-CoA reductase activity. This slow reaction rate likely contributes to the transport of 22:6(n-3) out of peroxisomes for rapid acylation into 1-acyl-GPC by microsomes.

Biosynthesis of docosahexaenoic acid in human cells: evidence that two different delta 6-desaturase activities may exist

It has been proposed that synthesis of docosahexaenoic acid (22:6(n-3) in rat hepatocytes occurs by a route independent of delta 4-desaturase, which involves delta 6-desaturation and retroconversion (Voss A., Reinhart M., Sankarappa S. and Sprecher H. (1991) J. Biol. Chem. 266, 19995-20000). However, most cells exhibit these enzymatic activities and nevertheless synthesize low to undectectable amounts of 22:6(n-3). Moreover, there are few data on the occurrence of this pathway in human cells. In the present work, we have analysed the biosynthetic pathway of 22:6(n-3) in human Y-79 retinoblastoma and Jurkat T-cells. Y-79 cells were supplemented with 18:3(n-3) and 20:5(n-3) or incubated with [1-14C]18:3(n-3) and [1-14C]20:5(n-3) and lipids analysed by argentation TLC, reverse-phase TLC and GLC-mass spectrometry. Pulse-chase experiments revealed that synthesis of 22:6(n-3) from 20:5(n-3) in Y-79 cells occurred through two successive elongations, followed by a delta 6-desaturation of 24:5(n-3) to 24:6(n-3) and retroconversion to 22:6(n-3). Incubation of Y-79 cells with [1-14C]18:3(n-3) in medium containing 50 microM trans-9,12-18:2, a potent inhibitor of delta 6-desaturase, caused a reduction of 22:6(n-3) synthesis mainly by interfering with the desaturation of 18:3(n-3). However, when [1-14C]20:5(n-3) was used as precursor, synthesis of 22:6(n-3) was depressed to a lesser extent and mainly by reduction of 24:6(n-3) retroconversion. Neuronal differentiation of Y-79 cells caused a great increase in delta 6-desaturase activity on 18:3(n-3), though the amount of 22:6(n-3) synthesized did not change or diminish, suggesting the existence of a particular delta 6-desaturase involved in the synthesis of 22:6(n-3). The existence of a distinctive delta 6-desaturase activity could also explain why Jurkat cells growing in serum-free medium showed a near 3-fold increase in the synthesis of pentaenes from 18:3(n-3) and, at the same time, a large decrease in the synthesis of 22:6(n-3). The verification of the involvement of two delta 6-desaturase activities in 22:6(n-3) synthesis would have important implications for the formulation of the nutritional requirements of this fatty acid during development.

Blood lipid docosahexaenoic and arachidonic acid in term gestation infants fed formulas with high docosahexaenoic acid, low eicosapentaenoic acid fish oil

The effect of fish oil high in docosahexaenoic acid (22:6n-3) and low in eicosapentaenoic acid (20:5n-3) in formula on blood lipids and growth of full-term infants was studied. Infants were fed formula with about 15% oleic acid (18:1), 32% linoleic acid (18:2n-6), 4.9% linolenic acid (18:3n-3) and 0, 0.10, or 0.22% 22:6n-3, or 35% 18:1, 20% 18:2n-6, 2.1% 18:3n-3 and 0, 0.11, or 0.24% 22:6n-3 from 3 d to 16 wk of age (n = 16,18,17,21,17,16, respectively). The formulae had < 0.1% 20:5n-3 and no arachidonic acid (20:4n-6). Breast-fed infants (n = 26) were also studied. Plasma phospholipid and red blood cell (RBC) phosphatidylcholine (PC) and phosphatidylethanolamine (PE) fatty acids were determined at 3 d and 4, 8, and 16 wk of age. These longitudinal analyses showed differences in blood lipid 22:6n-3 between breast-fed and formula-fed infants depending on the feeding duration. At 16 wk, infants fed formula with 0.10, 0.11% 22:6n-3, or 0.22% 22:6n-3 had similar 22:6n-3 levels in the plasma phospholipid and RBC PC and PE compared with breast-fed infants, and higher 22:6n-3 than infants fed formula without 22:6n-3. Formula with 0.24% 22:6n-3, however, resulted in higher plasma phospholipid 22:6n-3 than in breast-fed infants at 16, but not 4 or 8 wk of age. Plasma and RBC phospholipid 20:4n-6 was lower in formula-fed than breast-fed infants, but no differences in growth were found. Higher blood lipid C20 and C22 n-6 and n-3 fatty acids in infants fed formula with 20% 18:2n-6 and 2.4% 18:3n-3 compared with 32% 18:2n-6 and 4.9% 18:3n-3 show the increase in blood lipid 22:6n-3 in response to dietary 22:6n-3 depending on other fatty acids in the formula.

The desaturation and elongation of linolenic acid and eicosapentaenoic acid by hepatocytes and liver microsomes from rainbow trout (Oncorhynchus mykiss) fed diets containing fish oil or olive oil

The products of desaturation and elongation of [1-14C]18:3(n - 3) and [1-14C]20:5(n - 3) were studied using hepatocytes and microsomes prepared from livers of trout maintained on diets containing either olive oil or fish oil, to establish the extent to which the formation of 22:6(n - 3) was enhanced in the absence of dietary 22:6(n - 3) and to investigate the pathway(s) of conversion of 18:3(n - 3) and 20:5(n - 3) to 22:6(n - 3). Levels of 20:5(n - 3) and 22:6(n - 3) in the total lipid of hepatocytes from trout fed olive oil were 20-fold and 10-fold, respectively, lower than in cells from trout fed fish oil. For both dietary groups, [1-14C]18:3(n - 3) was incorporated into hepatocyte lipid to a greater extent than [1-14C] 20:5(n - 3). Almost 70% of the total radioactivity from [1-14C]18:3(n - 3) was recovered in hepatocyte triacylglycerols, whereas radioactivity from [1-14C]20:5(n - 3) was recovered almost equally in neutral lipids (52%) and polar lipids (48%). The products of desaturation and elongation from both labelled substrates were esterified mainly into hepatocyte polar lipids, whereas elongation products of [1-14C]18:3(n - 3) were preferentially incorporated into neutral lipids. Radioactivity recovered in the 22:6(n - 3) of polar lipids of hepatocytes from trout fed olive oil, from both 14C substrates, was approximately double that in hepatocytes from trout fed fish oil. No radioactivity from either [1-14C]18:3(n - 3) or [1-14C]20:5(n - 3) was incorporated into 22:6(n - 3) by microsomes isolated from livers from either group of fish and incubated in the presence of acetyl-CoA, malonyl-CoA, NADH, NADPH, ATP and coenzyme A. However, significant radioactivity was recovered in 24:5(n - 3) and 24:6(n - 3) from [1-14C]20:5(n - 3) and more radioactive 24:6(n - 3) accumulated in microsomes from trout fed olive oil than from trout fed fish oil. The results establish that the formation of 22:6(n - 3) from both 18:3(n - 3) and 20:5(n - 3) in hepatocytes of rainbow trout is stimulated by omitting 22:6(n - 3) from the diet and are consistent with the biosynthesis of 22:6(n - 3) in trout liver cells proceeding via 24:5(n - 3) and 24:6(n - 3) intermediates.

Maternal essential fatty acid patterns during normal pregnancy and their relationship to the neonatal essential fatty acid status

Although essential fatty acids (EFA) and their longer chain, more unsaturated derivatives play a major role during pregnancy, hardly any information is available with respect to the course of the maternal EFA status during an uncomplicated pregnancy and its relationship to the neonatal EFA status. Therefore, a longitudinal study was started in which 110 pregnant women gave repeated blood samples from the 10th week of gestation until delivery. After birth a blood sample from the umbilical vein and a maternal venous blood sample were collected as well, and 6 months after delivery a final blood sample from the mother was taken. The absolute (mg/l) and relative (% total fatty acids) amounts of the fatty acids in plasma phospholipids were determined. The total amounts of fatty acids increased significantly during pregnancy. This pattern was similar for the individual fatty acids and fatty acid families. The relative amount of linoleic acid (18:2n-6) did not change during pregnancy, whereas the relative amount of arachidonic acid (20:4n-6) decreased. Despite maternal mobilization of docosahexaenoic acid (22:6n-3, DHA), suggested by a temporary increase in the DHA status until 18 weeks gestation, the DHA status steadily declined thereafter. This pattern was associated with a progressive increase in the DHA deficiency index in maternal blood throughout pregnancy and resulted in a sub-optimal neonatal DHA status. The overall maternal EFA status also declined steadily during pregnancy. Therefore, the question arises whether the mother, under the prevailing dietary conditions, is able to meet the high fetal requirement for EFA.

Peroxisomal-microsomal communication in unsaturated fatty acid metabolism

The addition of 1-acyl-sn-glycero-3-phosphocholine (1-acyl-GPC) to peroxisomes decreased the production of acid-soluble radioactivity formed by beta-oxidation of [1-(14)C]arachidonate due to substrate removal by esterification into the acceptor. This peroxisomal-associated acyl-CoA:1-acyl-GPC acyltransferase activity was due to microsomal contamination. The production of acid-soluble radioactivity from [1-(14)C]7,10,13,16-22:4, but not from [3-(14)C]7,10,13,16-22:4 was independent of 1-acyl-GPC, with and without microsomes. By comparing rates of peroxisomal beta-oxidation with those for microsomal acylation, it was shown that the preferred metabolic fate of arachidonate, when added directly to incubations, or generated via beta-oxidation, was esterification by microsomal 1-acyl-GPC acyltransferase, rather than continued peroxisomal beta-oxidation.

Occurrence of 22:3n-9 and 22:4n-9 in the lipids of the topminnow (Poeciliopsis lucida) hepatic tumor cell line, PLHC-1

The lipids of the hepatic tumor cell line, PLHC-1, from the topminnow (Poeciliopsis lucida), were found to contain considerable amounts of a range of n-9 polyunsaturated fatty acids despite culture in serum containing significant amounts of essential fatty acids. The structural identity of all the n-9 polyunsaturated fatty acids was confirmed by gas chromatography/mass spectrometry. Of particular interest, PLHC-1 cell total lipid contained 1.9% of 22:3n-9 and 3.3% of 22:4n-9. As the culture medium contained virtually no n-9 polyunsaturated fatty acids, these fatty acids are all formed by the PLHC-1 cells, presumably from 18:1n-9. The 22:3n-9 and 22:4n-9 are presumably formed by processes of elongation and "delta 4" desaturation of Mead acid, 20:3n-9, present at over 11% in fatty acids of total lipid. Both 22:3n-9 and 22:4n-9 were primarily located in phosphatidylserine (4.1 and 8.5%, respectively) and, to a lesser extent, in phosphatidylethanolamine (2.2 and 6.5%, respectively), in common with the C22 derivatives of the n-3 and n-6 series, whereas 20:3n-9 was preferentially located in phosphatidylinositol (31.2%). The results establish that long-chain polyunsaturated fatty acids of the n-9 series can be formed in vertebrate tissue other than in conditions of classical essential fatty acid deficiency.

Interconversions and distinct metabolic fate of eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in bovine aortic endothelial cells

The anti-aggregatory activity of endothelial cells being affected by eicosapentaenoic (EPA, 20:5(n-3)) and docosahexaenoic (DHA, 22:6(n-3)) acids, the two main polyunsaturated fatty acids of fish oil, these fatty acids, as well as their intermediary, docosapentaenoic acid (DPA, 22:5(n-3)), were investigated with respect to their metabolism. Primary cultured bovine aortic endothelial cells were supplemented for 22 h at 37 degrees C with either n-3 fatty acid, and the fatty acids of cell media, of cell lipid classes, and of choline and ethanolamine glycerophospholipids (PC and PE) were quantified. Endothelial cells converted each of the three fatty acids into the two others. They were found esterified in cell lipids and partly released in cell media, the respective parts varying according to the fatty acid. For instance, half of the DPA formed from EPA and two third of the EPA formed from DPA were released in the media. Moreover, the DHA formed from EPA and DPA was not esterified but released in media. In addition, the esterified counterparts were found in either PC or PE, depending on whether they were added or formed by conversions. It is concluded that EPA, DPA and DHA are actively interconverted each others, and differ substantially in terms of distribution between media and cells, and within phospholipid classes.

Effect of three intravenously administered fat emulsions containing different concentrations of fatty acids on the plasma fatty acid composition of premature infants

The effects of an intravenously administered lipid emulsion supplemented with gamma-linolenic acid on the fatty acid profile of premature infants were compared with those of two conventional lipid emulsions. Fifty-nine premature neonates receiving total parenteral nutrition were randomly assigned to receive either fat emulsion containing gamma-linolenic acid and long-chain triglycerides (LCT), an LCT emulsion, or a 50% (wt/wt) mixture of medium-chain triglycerides and LCT emulsion. Forty-nine infants completed the study. During the 6-day study there was a significant tenfold increase in the plasma levels of gamma-linoleic acid in the supplemented group versus the other two groups. A significant threefold to fivefold increase in the omega 6 long-chain polyunsaturated fatty acids was observed in all groups. These changes seemed to be attributable mostly to linoleic acid from the lipid emulsion, despite the 50% lower dose in the medium- and long-chain triglycerides group. The increase in the omega 3 long-chain polyunsaturated fatty acids also was mainly caused by a similar increase in the level of alpha-linolenic acid. No differences were recorded in the linoleic/alpha-linolenic acid ratio among the groups. Plasma levels of some of the semiessential fatty acids were significantly higher in the medium- and long-chain triglycerides group than in the LCT group. This may be related to slower elimination of LCT, to the difference between emulsions, or to less substrate inhibition on delta-6-desaturase, which seems to be less of a rate-limiting enzyme than previously considered. Further intravenous feeding trials are needed to identify the optimal balance of fatty acids for nutrition of these premature infants.

In vivo metabolism of [1-14C]linolenic acid (18:3(n-3)) and [1-14C]eicosapentaenoic acid (20:5(n-3)) in a marine fish: time-course of the desaturation/elongation pathway

The metabolism (via the desaturation/elongation pathways) of [1-14C]18:3(n-3) and [1-14C]20:5(n-3) in a marine fish, gilthead sea bream (Sparus aurata L.), were investigated over 8 days to determine the time-courses for the production of delta 6 and delta 5-desaturase products and 22:6(n-3). Fish were starved for 1 week prior to, and during, the period of the experiment. The recovery of radioactivity from [1-14C]20:5(n-3) in tissue lipids exceeded that of [1-14C]18:3(n-3) at all time points. The recoveries of both fatty acids decreased by 85-89% between days 2 and 8, indicating that substantial loss of radioactivity due to beta-oxidation occurred. Incorporation of 18:3(n-3) and 20:5(n-3) was predominantly into triacylglycerol but during the time-course of the experiment there were decreased percentages of radioactivity from both labelled fatty acids recovered in triacylglycerol with concomitant increased percentages recovered in phospholipids indicating preferential oxidation of fatty acids in triacylglycerol and/or redistribution of incorporated fatty acids. Recovery of radioactivity in 22:6(n-3) was 10-fold greater with [1-14C]20:5(n-3) than with [1-14C]18:3(n-3). However, there were few consistently significant trends in the levels of components of the desaturation/elongation pathways during the time-course of the experiment. In particular, the relative recovery of radioactivity in 22:6(n-3) did not increase during the experiment with either substrate. Substantial amounts of radioactivity were found in 24:5(n-3) and 24:6(n-3), particularly after injection with [1-14C]20:5(n-3), indicating that the conversion of 20:5(n-3) to 22:6(n-3) in sea bream may occur by a pathway utilizing delta 6-desaturase activity rather than by a delta 4-desaturation.

Studies to determine if rat liver contains chain-length-specific acyl-CoA 6-desaturases

According to the revised pathway for 22:6(n - 3) biosynthesis in liver (Voss et al. (1991) J. Biol. Chem. 266, 19995-20000) both 18:3(n - 3) and 24:5(n - 3) serve as substrates for desaturation at position-6. The present study was undertaken to determine whether microsomes contain chain-length-specific 6-desaturases. Addition of [1-14C]20:3(n - 6), a substrate for desaturation at position-5, did not depress desaturation of either [1-14C]18:3(n - 3) or [3-14C]24:5(n - 3). An unexplained observation was that both 18:3(n - 3) and 24:5(n - 3) inhibited the metabolism of 20:3(n - 6) to 20:4(n - 6). When an enzyme-saturating level of [3-14C]24:5(n - 3) was now incubated alone and with 40, 80 and 120 nmol of [1-14C]18:3(n - 3), the production of 24:6(n - 3) was inhibited by 43, 67 and 81%. Conversely, when [1-14C]18:3(n - 3) was incubated with 40, 80 or 120 nmol of [3-14C]24:5(n - 3), the synthesis of 18:4(n - 3) was inhibited by only 15, 20 and 27%. These and other competitive studies showed that there was always preferential desaturation of 18:3(n - 3) rather than 24:5(n - 3). In addition, competitive studies between 18:2(n - 6) and 18:3(n - 3), as well as with 24:4(n - 6) and 24:5(n - 3) showed that there was always preferential desaturation of the (n - 3) acid. Although our results are consistent with a single 6-desaturase, further studies, including the isolation of the 6-desaturases(s), is obviously required to determine whether multiple forms of the 6-desaturase exist.