Desaturation Activity Research Articles

Effects of diets containing linseed oil on fatty acid desaturation and oxidation in hepatocytes and intestinal enterocytes in Atlantic salmon (Salmo salar)

We hypothesized that replacing fish oil with 18:3n-3-rich linseed oil may enable salmon to maintain the levels of tissue n-3HUFA levels through a combination of increased desaturation activity and increased substrate fatty acid provision. To this end we investigated desaturation/elongation of [1-14C18:3n-3 in hepatocytes and intestinal enterocytes, and determined the extent to which 18:3n-3 was oxidized and desaturated by measuring both simultaneously in a combined assay. Salmon smolts were stocked randomly into five seawater pens and fed for 40 weeks on diets in which the fish oil was replaced in a graded manner by linseed oil. At the end of the trial, fatty acyl desaturation/elongation and oxidation activities were determined in isolated hepatocytes and intestinal enterocytes using [1-14C]18:3n-3 as substrate, and samples of liver and intestinal tissue were collected for analysis of lipid and fatty acid composition. The results showed that, despite increased desaturation of [1-14C]18:3n-3 in hepatocytes, provision of dietary 18:3n-3 did not prevent the decrease in tissue n-3HUFA in fish fed linseed oil. Intestinal enterocytes were a site of significant fatty acid desaturation but, in contrast to hepatocytes, the activity was not increased by feeding linseed oil and was generally lower in fish fed linseed oil compared to fish fed only fish oil. In contrast, oxidation of [1-14C]18:3n-3 in enterocytes was generally increased in fish fed linseed oil compared to fish fed the diet containing only fish oil. However, oxidation of [1-14C]18:3n-3 in hepatocytes was 4- to 8-fold lower than in enterocytes and was not affected by diet. Furthermore, oxidation of [1-14C]18:3n-3 in enterocytes exceeded desaturation irrespective of dietary treatment, whereas similar amounts of [1-14C]18:3n-3 were desaturated and oxidized in hepatocytes from fish fed only fish oil and desaturation exceeded oxidation by 3-fold in fish fed the diet containing 100% linseed oil. The molecular mechanisms underpinning these results were discussed.

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.

Fat Absorption and Metabolism in Bile Duct Ligated Rats

Background: Bile excretion is obstructed in children with extrahepatic bile duct atresia (EHBA) resulting in fat malabsorption and disturbed lipid metabolism. Aim: Investigate if the bile duct ligated rat exhibits similar deviations as patients with EHBA under different feeding conditions. Methods: 6 bile duct ligated Wistar rats and 12 matched paired controls were randomised over 3 feeding groups. Rats were killed 16 or 30 days postsurgery. Faeces, blood and livers were collected. Fat absorption was evaluated, markers for cholestasis and the fatty acid composition of serum phospholipids (PL) and cholesterol esters (CE) were determined. Fatty acid desaturation activities in liver microsomes were measured. Results: Cholestatic bile duct ligated rats have a lower fat absorption coefficient and a lower fraction of 18:2n–6 and 18:3n–3 in serum triglycerides than their controls. This demonstrates that bile duct ligated rats suffer from fat malabsorption. In contrast to the observations in serum triglycerides, 18:2n–6 and 18:3n–3 were not reduced in serum PL and CE of cholestatic rats. Overflow of 18:2n–6 rich biliary PL in the general circulation could contribute to this observation. In agreement with what was found in man, serum PL of cholestatic rats have a higher 16:0/18:0 ratio, increased monoenes and reduced unsaturated fatty acids. However, no differences were observed in microsomal desaturation activities. Conclusion: Cholestatic bile duct ligated rats exhibit similar deviations in serum fatty acid composition as found in patients with EHBA, therefore they can be used as a model for this human disease.

Hepatocyte fatty acid desaturation and polyunsaturated fatty acid composition of liver in salmonids: effects of dietary vegetable oil

The desaturation and elongation of [1- 14C]18:3n-3 was investigated in hepatocytes from different populations and three different species of salmonids indigenous to Scotland, brown trout, Atlantic salmon and Arctic charr. Two groups of fish were sampled, before and after they were fed two experimental diets, a control diet containing fish oil and a diet containing vegetable oil (a 1:1 blend of linseed and rapeseed oils) for 12 weeks. At each sampling time, fatty acyl desaturation and elongation activity was determined in isolated hepatocytes, and samples of liver were also collected for lipid compositional analysis. At the initiation of the dietary trial, the liver polar-lipid fatty acid compositions of salmon and brown trout were very similar to each other, and the two charr populations were similar to each other, having lower total n-3 polyunsaturated fatty acids (PUFA) and 22:6n-3, but higher 20:5n-3 than the other salmonids. Initially, hepatocyte desaturation activity varied, with the highest activity in brown trout, followed by salmon and then charr. Production of 20:5n-3 was particularly high in brown trout. Desaturation of [1- 14C]18:3n-3 was significantly greater in all fish fed the diet containing vegetable oil compared to fish fed the diet containing fish oil. The increase in activity was less in brown trout compared to the other groups of fish. Feeding the vegetable oil diet increased the levels of 18:2n-6, 20:3n-6, total n-6 PUFA, 18:3n-3, 18:4n-3, 20:3n-3 and 20:4n-3, and decreased 22:6n-3 and the n-3/n-6 ratio in salmon and brown trout. By contrast, in charr fed the vegetable oil diet, there was no increase in 18:3n-3, 18:4n-3, 20:3n-3 or 20:4n-3 in liver polar lipids and the level of 22:6n-3 was not decreased. In addition, there was only a modest increase in the levels of 18:2n-6 and total n-6 PUFA, and so the n-3/n-6 ratio was only slightly decreased. The percentage of 20:4n-6, which was not increased in salmon and brown trout fed vegetable oil, was increased in charr fed the vegetable oil diet. Overall, the results indicated that there were significant differences in liver PUFA metabolism between Arctic charr and the other salmonids, which could have important consequences, both physiologically and in their ability to be successfully cultured on diets containing vegetable oils.

Metabolism of very long chain polyunsaturated fatty acids in isolated rat germ cells

Which cell type is responsible for the high levels of very long chain polyunsaturated fatty acids in testis and whether this fatty acid pattern is a result of a local synthesis are not presently known. In this study, fatty acid conversion from 20:4n-6 to 22:5n-6 and from 20:5n-3 to 22:6n-3 was investigated in isolated rat germ cells incubated with [1-14C]-labeled fatty acids. The germ cells elongated the fatty acids from 20- to 22-carbon atoms and from 22- to 24-carbon atoms but had a low delta6 desaturation activity. Thus, little [14C]22:5n-6 and [14C]22:6n-3 were synthesized. When Sertoli cells were incubated with [1-14C]20:5n-3 for 24 h, an active fatty acid elongation and desaturation were observed. In vivo germ cells normally have a higher content of 22:5n-6 or 22:6n-3 than Sertoli cells. An eventual transport of essential fatty acids from Sertoli cells to germ cells was thus studied. Different co-culture systems were used in which germ cells were on one side of a filter and Sertoli cells on the opposite side. When isolated pachytene spermatocytes or round spermatids were added to the opposite side of a semipermeable filter, approximately 1 nmol [14C]22:6n-3 crossed the filter. Little of this was esterified in the germ cells. Similarly, in using [1-14C]20:4n-6 in identical experiments, very little [14C]22:5n-6 was esterified in germ cells on the opposite side of the filter. Although the very active synthesis of 22:5n-6 and 22:6n-3 observed in Sertoli cells suggests a transport of these compounds to germ cells, this was not experimentally determined.

Effects of Diabetes and Insulin on Hepatic Δ6 Desaturase Gene Expression

It has been recognized that rat liver microsomal Δ6 desaturation activity is defective in experimental diabetes, a fact that may be reverted by means of insulin treatment. In the present study, we used streptozotocin-induced diabetic rats in order to determine the regulatory role of insulin on the expression of hepatic Δ6 desaturase gene. The abundance of hepatic Δ6 desaturase mRNA in the diabetic rats is sevenfold lower than in the control. Insulin administration to diabetic rats induces Δ6 desaturase mRNA eightfold within 24 h. The effect of insulin on the Δ6 desaturase mRNA was inhibited 70% with dibutyryl-cAMP and theophylline administration and 90% by cycloheximide administration. Therefore, our data demonstrate that the activity of hepatic Δ6 desaturase in response to insulin is, at least in part, regulated by pretranslational events that require the synthesis of an unknown protein(s).

Incorporation and Metabolism of Dietary Trans Isomers of Linolenic Acid Alter the Fatty Acid Profile of Rat Tissues

To study the influence on lipid metabolism and platelet aggregation of the fatty acid isomerization that occurs during heat treatment, weanling rats were fed for 8 wk a diet enriched with 5% isomerized (experimental group) or normal (control group) canola oil. Geometrical isomers of alpha-linolenic acid representing 0.2 g/100 g of the experimental diet were incorporated into liver, platelets, aorta and heart, at the expense of their cis homologue and of 18:2(n-6). The major isomer, 9c,12c,15t-18:3, was also metabolized to 5c,8c,11c,14c,17t-20:5 and to an unknown compound, found in liver, platelets and aorta, which has been identified tentatively as 7c, 10c,13c,16c,19t-22:5. The greater 20:4(n-6)/18:2(n-6) ratio in the liver, platelets and heart of the experimental group than the control group indicated an enhancement of desaturation activities. This induced a higher content of long-chain (n-6) fatty acids in the experimental group. Platelet aggregation tended to be slightly higher (P: = 0.065) in the experimental group. We conclude that 0.2 g of trans isomers of alpha-linolenic acid per 100 g of diet was sufficient to be incorporated and metabolized, thus altering the fatty acid profile of rat tissues.

A repeat-batch membrane bioreactor with a phase inversion for the desaturation of isopropyl palmitate by a mutant Rhodococcus strain

A repeat-batch membrane bioreactor was constructed for the novel bioconversion of isopropyl hexadecanoate to isopropyl cis-6-hexadecenoate by a Rhodococcus mutant. The addition of glutamate, thiamine, and MgSO 4 was very effective in improving not only the rate and yield of the bioconversion but also the maintenance of desaturation activity during cell recycling. An oil-in-water (O/W) type emulsion of the reaction medium was inverted to a water-in-oil (W/O) type by discharging the water phase from the reaction mixture. The continuous oil phase containing the product could effectively be recovered through a hydrophobic hollow-fiber module. By decreasing the oil-to-water ratio upon addition of fresh medium, the medium was spontaneously inverted again to an O/W type emulsion to proceed with the next conversion. The batch reaction coupled with the phase inversion could be repeated more than 13 times for over about 300 h operation. Finally, a highly purified product was obtained with high yield by the urea adduct procedure.

Mammalian Δ-6 Fatty Acid Desaturase :

Δ-6 Fatty acid desaturase was noted to catalyze the conversion of linoleic acid to γ-linolenic acid and α-linolenic acid to n-3 octadecatetraenoic acid through the insertion of a double bond at the Δ-6 position of the fatty acid substrate. In mammals and paticularly in humans, this is the first rate-limiting step in the biosynthesis of polyunsaturated fatty acids such as dihomo-γ-linolenic, arachidonic, and eicosapentaenoic acids, all known as precursors of eicosanoids. Δ-6 desaturating activity has been shown associated with physiological and clinical observation findings. Expression cloning of plant and mammalian Δ-6 desaturase genes has recently been achieved and has provided indication of a new group of cytochrome b5-fusion proteins. The key role of this enzyme in lipid metabolism is presently being studied at the molecular level.

Polyunsaturated fatty acid metabolism in Atlantic salmon (Salmo salar) undergoing parr-smolt transformation and the effects of dietary linseed and rapeseed oils.

Duplicate groups of Atlantic salmon parr were fed diets containing either fish oil (FO), rapeseed oil (RO), linseed oil (LO) or linseed oil supplemented with arachidonic acid (20:4n-6; AA) (LOA) from October (week 0) to seawater transfer in March (week 19). From March to July (weeks 20–34) all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation activity in isolated hepatocytes incubated with [1-14C]18:3n-3 increased in all dietary groups, peaking in early March about one month prior to seawater transfer. Desaturation activities at their peak were significantly greater in fish fed the vegetable oils, particularly RO, compared to fish fed FO. Docosahexaenoic acid (22:6n-3:DHA) and AA in liver and gill polar lipids (PL) increased in all dietary groups during the freshwater phase whereas eicosapentaenoic acid (20:5n-3; EPA) increased greatly in all groups after seawater transfer. The AA/EPA ratio in tissue PL increased up to seawater transfer and then decreased after transfer. AA levels and the AA/EPA ratio in gill PL were generally higher in the LOA group. The levels of 18:3n-3 in muscle total lipid were increased significantly in the LO, LOA and, to a lesser extent, RO groups prior to transfer but were reduced to initial levels by the termination of the experiment (week 34). In contrast, 18:2n-6 in muscle total lipid was significantly increased after 18 weeks in fish fed the diets supplemented with RO and LO, and was significantly greater in the FO and RO groups at the termination of the experiment. Gill PGF production showed a large peak about two months after transfer to seawater. The production of total PGF post-transfer was significantly lower in fish previously fed the LOA diet. However, plasma chloride concentrations in fish subjected to a seawater challenge at 18 weeks were all lower in fish fed the diets with vegetable oils. This effect was significant in the case of fish receiving the diet with LOA, compared to those fed the diet containing FO. The present study showed that during parr-smolt transformation in Atlantic salmon there is a pre-adaptive increase in hepatocyte fatty acyl desaturation/elongation activities that is controlled primarily by environmental factors such as photoperiod and temperature but that can also be significantly modulated by diet. Feeding salmon parr diets supplemented with rapeseed or linseed oils prevented inhibition of the desaturase activities that is induced by feeding parr diets with fish oils and thus influenced the smoltification process by altering tissue PL fatty acid compositions and eicosanoid production. These effects, in turn, had a beneficial effect on the ability of the fish to osmoregulate and thus adapt to salinity changes.

Essential fatty acid deficiency in freshwater fish: the effects of linoleic, α-linolenic, γ-linolenic and stearidonic acids on the metabolism of [1-14C]18:3n-3 in a carp cell culture model.

The desaturation of [1-14C]18:3n-3 to 20:5n-3 and 22:6n-3 is enhanced in an essential fatty acid deficient cell line (EPC-EFAD) in comparison with the parent cell line (EPC) from carp. In the present study, the effects of competing, unlabeled C18 polyunsaturated fatty acids (PUFA), linoleic (18:2n-6), α-linolenic (18:3n-3), γ-linolenic (18:3n-6) and stearidonic (18:4n-3) acids, on the metabolism of [1-14C]18:3n-3 were investigated in EPC-EFAD cells in comparison with EPC cells. The incorporation of [1-14C]18:3n-3 in both cell lines was significantly reduced by competing C18 PUFA, with the rank order being 18:4n-3>18:3n-3 = 18:2n-6>18:3n-6. In the absence of competing PUFA, radioactivity from [1-14C]18:3n-3 in EPC cells was predominantly recovered in phosphatidylethanolamine followed by phosphatidylcholine. This pattern was unaffected by competing n-6PUFA, but n-3PUFA reversed this pattern as did essential fatty acid deficiency in the presence of all competing PUFA. The altered lipid class distribution was most pronounced in cells supplemented with 18:4n-3. Competing C18 PUFA significantly decreased the proportions of radioactivity recovered in 22:6n-3, pentaene and tetraene products, with the proportions of radioactivity recovered in 18:3n-3 and 20:3n-3 increased, in both cell lines. However, the inhibitory effect of competing C18 PUFA on the desaturation of [1-14C]18:3n-3 was significantly greater in EPC-EFAD cells. The magnitude of the inhibitory effects of C18 PUFA on [1-14C]18:3n-3 desaturation was dependent upon the specific fatty acid with the rank order being 18:4n-3>18:3n-3>18:2n-6, with 18:3n-6 having little inhibitory effect on the metabolism of [1-14C]18:3n-3 in EPC cells. The differential effects of the C18 PUFA on [1-14C]18:3n-3 metabolism were consistent with mass competition in combination with increased desaturation activity in EPC-EFAD cells and the known substrate fatty acid specificities of desaturase enzymes. However, the mechanism underpinning the greater efficacy with which the unlabeled C18PUFA competed with [1-14C]18:3n-3 in the desaturation pathway in EPC-EFAD cells was unclear.

Fatty acid delta9-desaturation in the Triatoma infestans fat body: response to food and trehalose administrations.

The effects of food intake and carbohydrate administration on fatty acid delta9-desaturation were investigated in isolated microsomes from Triatoma infestans fat body. Fifth instar nymphs, which were solely blood-fed just after the molt and then fasted, were used as controls and for determination of optimal assay conditions. Both [1-14C]palmitic and [1-14C]stearic acids in a medium containing ATP, CoA, MgCl2, NADH, NaF, and O2 were tested. For the control group, optimal conditions were a pH of 6.8-7.2, an incubation temperature of 29 degrees C, and an incubation time of 10 min. delta9-Desaturation depended on the presence of reduced pyridine dinucleotides; NADH and NADPH were equally efficient. Stearic acid showed a higher apparent Vmax than palmitic acid, but the apparent Km were very similar. When fifth instar nymphs were blood-fed weekly, a marked increase of delta9-desaturation was observed for both acids. Higher desaturation activity was also induced by injection of the carbohydrate trehalose into the hemolymph of fasted nymphs. These results indicate that insect delta9-desaturation, like the mammalian counterpart, is sensitive to dietary changes and carbohydrate administration.

Effects of conjugated linoleic acid isomers on the hepatic microsomal desaturation activities in vitro.

The influence of individual conjugated linoleic acid (CLA) isomers on the delta6 desaturation of linoleic and alpha-linolenic acids and on the delta9 desaturation of stearic acid was investigated in vitro, using rat liver microsomes. The delta6 desaturation of 18:2n-6 was decreased from 23 to 38% when the ratio of 9cis,11trans-18:2 to 18:2n-6 increased from 0.5 to 2. The compound 10trans,12cis-18:2 exhibited a similar effect only at the highest concentration. The delta6 desaturation of alpha-linolenic acid was slightly affected by the presence of CLA isomers. The sole isomer to induce an inhibitory effect on the delta9 desaturation of stearic acid was 10trans,12cis-18:2.

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.

Relationship between dietary oil during gestation and lactation and biosynthesis of polyunsaturated fatty acids in control and in malnourished dam and pup rats

Protein-calorie malnutrition during gestation and lactation affects the fatty-acid desaturase activities, and hence the fatty-acid composition of developing tissues. Wistar rats were fed protein–calorie-deficient diets during gestation and lactation, and the influence of different oils (either corn or soy oil) on liver Δ5 and Δ6 desaturase activities was studied. Dam and pup growth was monitored until 18 days’ postpartum, fatty-acid composition was determined in milk, as well as in dam and pup liver phospholipids. It was observed that malnutrition affects milk and liver fatty-acid composition specially in those dams receiving corn oil. Similar results were obtained in liver fatty-acid composition of dams and pups. The rate of liver Δ6 and Δ5 desaturation was measured using [1- 14C]18:2n-6 and [1- 14C]20:3n-6 as substrate, and the conversion of precursor to product was determined by reverse-phase high-pressure liquid chromatography. Protein–calorie malnutrition produced a reduction in liver Δ5 and Δ6 desaturase activities, in both dams and pups. Feeding dam rats with soy oil during gestation and lactation results in an increase of both Δ6 and Δ5 desaturation activities, even in malnourished animals, in comparison with those receiving corn oil. These observations could be helpful to devise a nutritional strategy related to polyunsaturated fatty-acid supply for the normal growth and tissue development.

Highly purified eicosapentaenoic acid and docosahexaenoic acid in humans have similar triacylglycerol-lowering effects but divergent effects on serum fatty acids

To compare the effects of highly purified ethyl ester concentrates of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on serum lipids, apolipoproteins, and serum phospholipid fatty acids in humans, we conducted a double-blind, placebo-controlled, parallel design intervention study. Healthy nonsmoking men (n = 234) aged 36-56 y were randomly assigned to dietary supplementation with 3.8 g EPA/d, 3.6 g DHA/d, or 4.0 g corn oil/d (placebo) for 7 wk. Serum triacylglycerols decreased 26% (P < 0.0001) in the DHA group and 21% (P = 0.0001) in the EPA group compared with the corn oil group. Although not significant, net decreases in serum triacylglycerols were consistently greater in the DHA group across all quartiles of baseline triacylglycerol concentrations. Serum high-density-lipoprotein cholesterol increased 0.06 mmol/L (P = 0.0002) in the DHA group. In the EPA group, serum total cholesterol decreased 0.15 mmol/L (P = 0.02) and apolipoprotein A-I decreased 0.04 g/L (P = 0.0003). In the DHA group, serum phospholipid DHA increased by 69% and EPA increased by 29%, indicating retroconversion of DHA to EPA. In the EPA group, serum phospholipid EPA increased by 297% whereas DHA decreased by 15%, suggesting that EPA is not elongated to DHA in humans. The serum phospholipid ratio of n-3 to n-6 fatty acids increased in both groups, whereas the relative changes in n-6 fatty acids suggested possible alterations in liver desaturation activity in the DHA group. We conclude that both DHA and EPA decrease serum triacylglycerols, but have differential effects on lipoprotein and fatty acid metabolism in humans.

Perfluorodecanoic acid enhances the formation of oleic acid in rat liver.

The feeding of perfluorodecanoic acid (PFDA) to male rats at a dietary concentration of 0.005% (w/w) for 7 days resulted in a marked increase in the activity of microsomal stearoyl-CoA desaturation in the liver. This increase in the overall desaturation activity was due to the induction of terminal desaturase among the components comprising the desaturation system. In contrast, PFDA inhibited desaturation in vitro, seemingly due to interference with electron transport through the desaturation system. Accordingly, PFDA can be an inducer and also an inhibitor of delta9-desaturation. PFDA feeding enhanced the conversion of radioactive stearic acid into oleic acid in the liver in vivo, indicating that the induction of delta9-desaturase by PFDA functions in vivo. PFDA feeding increased the mass of octadecenoic acid (C18:1) in the liver and the proportion of C18:1 in microsomal lipid. A highly significant linear correlation existed between the microsomal desaturase activity and the proportion of C18:1 in microsomal lipid when compared using rats in five different physiological states: control, PFDA-fed, p-chlorophenoxyisobutyric acid (clofibric acid)-fed, starved and starved/refed. These results suggest that the increase in the hepatic level of C18:1 caused by feeding of PFDA to rats can be explained by the common concept of regulation, i.e. the hepatic level of C18:1 is under the control of delta9-desaturase. The dietary administration of PFDA also increased the content of cytochrome P-450 and the activity of 7-ethoxycoumarin O-de-ethylase in the liver.

The effect of dietary lipid on polyunsaturated fatty acid metabolism in Atlantic salmon (Salmo salar) undergoing parr-smolt transformation.

The aim of this study was to measure the changes in lipid metabolism which occur during smoltification and seawater transfer in Atlantic salmon (Salmo salar). Duplicate groups of Atlantic salmon parr were fed diets containing either fish oil (FO) or a blend of linseed and rapeseed oils, vegetable oil (VO), from October (week 0) to seawater transfer in May (week 26). From May to August (weeks 26-43), all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation activity were followed in isolated hepatocytes incubated with radioactive 18:3n-3 and 18:2n-6. Metabolism of 18:3n-3 was consistently around 5-fold greater than metabolism of 18:2n-6, and total metabolism of both substrate polyunsaturated fatty acids (PUFA) was increased in fish fed both VO and FO up to seawater transfer after which desaturation activities were reduced. Desaturation activities with both 18:3n-3 and 18:2n-6 were significantly greater in fish fed VO, compared to fish fed FO, at 22 and 26 wk. Arachidonic acid (20:4n-6; AA) in liver polar lipids (PL) of fish fed VO increased consistently from weeks 0-22 but varied after seawater transfer. In fish fed FO, AA in liver PL remained constant up to week 17 before increasing at seawater transfer and leveling off thereafter. Eicosapentaenoic acid (20:5n-3; EPA) in liver PL of fish fed VO decreased significantly from week 0-22 before rising at seawater transfer and increasing rapidly posttransfer. EPA in liver PL of fish fed FO showed a similar trend except EPA was always greater in the freshwater phase compared to fish fed VO. Docosahexaenoic acid (DHA) levels in liver PL of fish fed VO remained constant in the seawater phase before increasing following seawater transfer. In fish fed FO, DHA in liver PL increased from weeks 0-17 reducing and leveling off postseawater transfer. The levels of PGF(2 alpha) and PGF(3 alpha) were measured in isolated gill cells stimulated with calcium ionophore A23187. PGF(2 alpha) production in fish fed VO increased significantly between 0-7 wk before decreasing toward seawater transfer. After transfer, PGF(2 alpha), production increased to a peak at 35 wk. PGF(2 alpha) production in fish fed FO was not significantly altered during the trial period. The changes in PGF(3 alpha) production were broadly similar to those occurring with PGF(2 alpha), but the latter was always in excess of the former (2- to 4-fold). Plasma chloride concentrations in fish subjected to seawater challenge at 20 wk were significantly lower in fish fed VO compared to those fed FO. This study has provided new information on the changes in lipid metabolism which accompany parr-smolt transformation and suggests that diets which have a fatty acid composition more similar to that in aquatic invertebrates may be beneficial in effecting successful seawater adaptation.

Production of 5,8,11-Eicosatrienoic Acid (Mead Acid) by a (Delta)6 Desaturation Activity-Enhanced Mutant Derived from a (Delta)12 Desaturase-Defective Mutant of an Arachidonic Acid-Producing Fungus, Mortierella alpina 1S-4

Enhanced production of 5,8,11-eicosatrienoic acid (Mead acid, 20:3(omega)9) was attained by a mutant fungus, Mortierella alpina M209-7, derived from (Delta)12 desaturase-defective M. alpina Mut48. The 20:3(omega)9 production by M209-7 was 1.3 times greater than that by its parent strain, Mut48. This is thought to be due to its enhanced (Delta)6 desaturation activity, which was 1.4 times higher than that of Mut48. In both strains, 87 to 88% of the total lipids comprised triacylglycerol (TG) and 85% of 20:3(omega)9 was contained in TG. On optimization of the culture conditions for M209-7, earlier glucose feeding and shifting of the growth temperature from 28 to 19(deg)C on the second day were shown to be effective. Under the optimal conditions with a 10-liter jar fermentor, 20:3(omega)9 production reached 1.65 g/liter of culture medium (corresponding to 118 mg/g of dry mycelia and 28.9% of total fatty acids), which is about twice that reported previously (0.8 g/liter).

Cadmium suppresses Δ9 desaturase activity in rat hepatocytes

The effects of cadmium (Cd) on the fatty acid composition and Δ9 desaturation activity were studied in hepatocytes which had been cultured in serum-free medium. The aim of this study was to determine whether the inhibition of Δ9 desaturase seen in Cd-treated rat liver (Kudo et al. (1991) Toxicology, 68, 133–142) is the direct effect of Cd or not. When hepatocytes were cultured in serum-free medium, increases in the proportions of monounsaturated fatty acids such as oleic acid (18:1) were observed in the phospholipid (PL) and triacylglycerol (TG) fractions of hepatocytes, suggesting that Δ9 desaturase was induced. By contrast, essential fatty acids such as linoleic acid (18:2) and arachidonic acid (20:4) decreased. These changes were similar to those seen in the livers of essential fatty acid (EFA)-deficient rats. When hepatocytes were cultured in the presence of CdCl 2, the changes in the fatty acid composition were suppressed. The effect of CdCl 2 was concentration-dependent. The rate of the conversion of 18:0 to 18:1, which is catalyzed by Δ9 desaturase, was reduced by treatment with Cd, whereas the rate of conversion of 18:2 to 20:4, which is catalyzed by Δ6 desaturase, was slightly increased. Cd did not inhibit liver microsomal Δ9 desaturase activity in vitro. We concluded that Cd suppressed the induction of Δ9 desaturase in cultured hepatocytes. These phenomena can explain the selective inhibition of Δ9 desaturase in the livers of Cd-treated rats.