Fatty acyl desaturation in isolated hepatocytes from Atlantic salmon (Salmo salar): stimulation by dietary borage oil containing gamma-linolenic acid

Biosynthesis of LC-PUFAs and VLC-PUFAs in Pampus argenteus: Characterization of Elovl4 Elongases and Regulation under Acute Salinity

Salinity has been demonstrated to influence the biosynthesis of long-chain (C_20-24) polyunsaturated fatty acids (LC-PUFAs) in teleost fish. Since LC-PUFAs are essential nutrients for vertebrates, it is central to understand how fish cope with an acute change in salinity associated with natural events. We herein report on the cloning and functional characterization of two elongation of very-long-chain fatty acid (Elovl)4 proteins, namely, Elovl4a and Elovl4b, and study the roles that these enzymes play in the biosynthesis of LC-PUFAs and very-long-chain (>C₂₄) polyunsaturated fatty acids (VLC-PUFAs) in marine teleost Pampus argenteus. The P. argenteus Elovl4 displayed all of the typical features of Elovl-like enzymes and have eyes and brain as major sites through which they exert their functions. Moreover, functional studies showed that the P. argenteus Elovl4 can effectively elongate C_18-22 substrates to C₃₆ VLC-PUFA. Because both P. argenteus Elovl4 are able to produce 24:5n - 3 from shorter precursors, we tested whether the previously reported Δ6 Fads2 from P. argenteus was able to desaturate 24:5n - 3 to 24:6n - 3, a key step for docosahexaenoic acid (DHA) synthesis. Our results showed that P. argenteus can indeed bioconvert 24:5n - 3 into 24:6n - 3, suggesting that P. argenteus has the enzymatic capacity required for DHA biosynthesis through the coordinated action of both Elovl4 and Fads2. Furthermore, an acute salinity test indicated that low-salinity stress (12 ppt) upregulated genes involved in LC-PUFA biosynthesis, with 12 ppt salinity treatment showing the highest hepatic LC-PUFA content. Overall, our results unveiled that the newly characterized Elovl4 enzymes have indispensable functions in LC- and VLC-PUFA biosynthesis. Moreover, acute salinity change influenced the biosynthesis of LC-PUFA in P. argenteus. This study provided new insight into the biosynthesis of LC- and VLC-PUFAs in vertebrates and the physiological responses that teleosts have under acute salinity stress.

Diet-Induced Physiological Responses in the Liver of Atlantic Salmon (Salmo salar) Inferred Using Multiplex PCR Platforms

The simultaneous quantification of several transcripts via multiplex PCR can accelerate research in fish physiological responses to diet and enable the development of superior aquafeeds for farmed fish. We designed two multiplex PCR panels that included assays for 40 biomarker genes representing key aspects of fish physiology (growth, metabolism, oxidative stress, and inflammation) and 3 normalizer genes. We used both panels to assess the physiological effects of replacing fish meal and fish oil by terrestrial alternatives on Atlantic salmon smolts. In a 14-week trial, we tested three diets based on marine ingredients (MAR), animal by-products and vegetable oil (ABP), and plant protein and vegetable oil (VEG). Dietary treatments affected the expression of genes involved in hepatic glucose and lipid metabolism (e.g., srebp1, elovl2), cell redox status (e.g., txna, prdx1b), and inflammation (e.g., pgds, 5loxa). At the multivariate level, gene expression profiles were more divergent between fish fed the marine and terrestrial diets (MAR vs. ABP/VEG) than between the two terrestrial diets (ABP vs. VEG). Liver ARA was inversely related to glucose metabolism (gck)- and growth (igfbp-5b1, htra1b)-related biomarkers and hepatosomatic index. Liver DHA and EPA levels correlated negatively with elovl2, whereas ARA levels correlated positively with fadsd5. Lower hepatic EPA/ARA in ABP-fed fish correlated with the increased expression of biomarkers related to mitochondrial function (fabp3a), oxidative stress (txna, prdx1b), and inflammation (pgds, 5loxa). The analysis of hepatic biomarker gene expression via multiplex PCR revealed potential physiological impacts and nutrient-gene interactions in Atlantic salmon fed lower levels of marine-sourced nutrients.

Are we what we eat? Changes to the feed fatty acid composition of farmed salmon and its effects through the food chain

'Are we what we eat?' Yes and no. Although dietary fat affects body fat, there are many modifying mechanisms. In Atlantic salmon, there is a high level of retention of the n-3 fatty acid (FA) docosahexaenoic acid (DHA, 22:6n-3) relative to the dietary content, whereas saturated FAs never seem to increase above a specified level, which is probably an adaptation to low and fluctuating body temperature. Net production of eicosapentaenoic acid (EPA, 20:5n-3) and especially DHA occurs in salmon when dietary levels are low; however, this synthesis is not sufficient to maintain EPA and DHA at similar tissue levels to those of a traditional fish oil-fed farmed salmon. The commercial diets of farmed salmon have changed over the past 15 years towards a more plant-based diet owing to the limited availability of the marine ingredients fish meal and fish oil, resulting in decreased EPA and DHA and increased n-6 FAs. Salmon is part of the human diet, leading to the question 'Are we what the salmon eats?' Dietary intervention studies using salmon have shown positive effects on FA profiles and health biomarkers in humans; however, most of these studies used salmon that were fed high levels of marine ingredients. Only a few human intervention studies and mouse trials have explored the effects of the changing feed composition of farmed salmon. In conclusion, when evaluating feed ingredients for farmed fish, effects throughout the food chain on fish health, fillet composition and human health need to be considered.

Early nutritional intervention can improve utilisation of vegetable-based diets in diploid and triploid Atlantic salmon (Salmo salar L.)

The present study investigated nutritional programming in Atlantic salmon to improve utilisation of a vegetable-based diet. At first exogenous feeding, fry were fed either a marine-based diet (Diet M^stimulus, 80% fishmeal (FM)/4% fish oil (FO)) or a vegetable-based diet (Diet V^stimulus, 10% FM/0% FO) for 3 weeks. Subsequently, all fish were then fed under the same conditions with a commercial, marine-based, diet for 15 weeks and thereafter challenged with a second V diet (Diet V^challenge, 10% FM/0% FO) for 6 weeks. Diploid and triploid siblings were run in parallel to examine ploidy effects. Growth performance, feed intake, nutrient utilisation and intestinal morphology were monitored. Fish initially given Diet V^stimulus (V-fish) showed 24 % higher growth rate and 23 % better feed efficiency compared with M-fish when later challenged with Diet V^challenge. There was no difference in feed intake between nutritional histories, but increased nutrient retentions highlighted the improved utilisation of a V diet in V-fish. There were generally few significant effects of nutritional history or ploidy on enteritis scores in the distal intestine after the challenge phase as only V-triploids showed a significant increase (P<0·05) in total score. The data highlighted that the positive effects were most likely a result of nutritional programming and the ability to respond better when challenged later in life may be attributed to physiological and/or metabolic changes induced by the stimulus. This novel study showed the potential of nutritional programming to improve the use of plant raw material ingredients in feeds for Atlantic salmon.

EPA, DHA, and Lipoic Acid Differentially Modulate the n-3 Fatty Acid Biosynthetic Pathway in Atlantic Salmon Hepatocytes

The aim of the present study was to investigate how EPA, DHA, and lipoic acid (LA) influence the different metabolic steps in the n-3 fatty acid (FA) biosynthetic pathway in hepatocytes from Atlantic salmon fed four dietary levels (0, 0.5, 1.0 and 2.0%) of EPA, DHA or a 1:1 mixture of these FA. The hepatocytes were incubated with [1-¹⁴C] 18:3n-3 in the presence or absence of LA (0.2 mM). Increased endogenous levels of EPA and/or DHA and LA exposure both led to similar responses in cells with reduced desaturation and elongation of [1-¹⁴C] 18:3n-3 to 18:4n-3, 20:4n-3, and EPA, in agreement with reduced expression of the Δ6 desaturase gene involved in the first step of conversion. DHA production, on the other hand, was maintained even in groups with high endogenous levels of DHA, possibly due to a more complex regulation of this last step in the n-3 metabolic pathway. Inhibition of the Δ6 desaturase pathway led to increased direct elongation to 20:3n-3 by both DHA and LA. Possibly the route by 20:3n-3 and then Δ8 desaturation to 20:4n-3, bypassing the first Δ6 desaturase step, can partly explain the maintained or even increased levels of DHA production. LA increased DHA production in the phospholipid fraction of hepatocytes isolated from fish fed 0 and 0.5% EPA and/or DHA, indicating that LA has the potential to further increase the production of this health-beneficial FA in fish fed diets with low levels of EPA and/or DHA.

Minor lipid metabolic perturbations in the liver of Atlantic salmon (Salmo salar L.) caused by suboptimal dietary content of nutrients from fish oil

The present study was conducted to evaluate the effects on Atlantic salmon hepatic lipid metabolism when fed diets with increasing substitution of fish oil (FO) with a vegetable oil (VO) blend. Four diets with VOs replacing 100, 90, 79 and 65 % of the FO were fed for 5 months. The levels of eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) in the experimental diets ranged from 1.3 to 7.4 % of fatty acids (FAs), while cholesterol levels ranged from 0.6 to 1.2 g kg(-1). In hepatocytes added [1-(14)C] α-linolenic acid (ALA, 18:3n-3), more ALA was desaturated and elongated to EPA and DHA in cells from fish fed 100 % VO, while in fish fed 65 % VO, ALA was elongated to eicosatrienoic acid (ETE; 20:3n-3), indicating reduced Δ6 desaturation activity. Despite increased desaturation activity and activation of the transcription factor Sp1 in fish fed 100 % VO, liver phospholipids contained less EPA and DHA compared with the 65 % VO group. The cholesterol levels in the liver of the 100 % VO group exceeded the levels in fish fed the 65 % VO diet, showing an inverse relationship between cholesterol intake and liver cholesterol content. For the phytosterols, levels in liver were generally low. The area as a proxy of volume of lipid droplets was significantly higher in salmon fed 100 % VO compared with salmon fed 65 % VO. In conclusion, the current study suggests that suboptimal dietary levels of cholesterol in combination with low levels of EPA and DHA (1.3 % of FAs) can result in minor metabolic perturbations in the liver of Atlantic salmon.

Assessment of a land-locked Atlantic salmon (Salmo salar L.) population as a potential genetic resource with a focus on long-chain polyunsaturated fatty acid biosynthesis

The natural food for Atlantic salmon (Salmo salar) in freshwater has relatively lower levels of omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA) than found in prey for post-smolt salmon in seawater. Land-locked salmon such as the Gullspång population feed exclusively on freshwater type lipids during its entire life cycle, a successful adaptation derived from divergent evolution. Studying land-locked populations may provide insights into the molecular and genetic control mechanisms that determine and regulate n-3 LC-PUFA biosynthesis and retention in Atlantic salmon. A two factorial study was performed comparing land-locked and farmed salmon parr fed diets formulated with fish or rapeseed oil for 8 weeks. The land-locked parr had higher capacity to synthesise n-3 LC-PUFA as indicated by higher expression and activity of desaturase and elongase enzymes. The data suggested that the land-locked salmon had reduced sensitivity to dietary fatty acid composition and that dietary docosahexaenoic acid (DHA) did not appear to suppress expression of LC-PUFA biosynthetic genes or activity of the biosynthesis pathway, probably an evolutionary adaptation to a natural diet lower in DHA. Increased biosynthetic activity did not translate to enhanced n-3 LC-PUFA contents in the flesh and diet was the only factor affecting this parameter. Additionally, high lipogenic and glycolytic potentials were found in land-locked salmon, together with decreased lipolysis which in turn could indicate increased use of carbohydrates as an energy source and a sparing of lipid.

A comparative analysis of the response of the hepatic transcriptome to dietary docosahexaenoic acid in Atlantic salmon (Salmo salar) post-smolts

Background

The present study aimed to explore the impact of dietary docosahexaenoic acid (DHA) on aspects of the metabolism of Atlantic salmon (Salmo salar). The effects of diets containing increasing levels of DHA (1 g kg⁻¹, 3 g kg⁻¹, 6 g kg⁻¹, 10 g kg⁻¹ and 13 g kg⁻¹) on the liver transcriptome of post-smolt salmon was examined to elucidate patterns of gene expression and responses of specific metabolic pathways. Total RNA was isolated from the liver of individual fish and analyzed using a custom gene expression 44K feature Atlantic salmon oligo-microarray.

Results

The expression of up to 911 unique annotated genes was significantly affected by dietary DHA inclusion relative to a low DHA reference diet. Analysis of a total of 797 unique genes were found with a significant linear correlation between expression level and dietary DHA. Gene-Set Enrichment Analysis (GSEA) identified a range of pathways that were significantly affected by dietary DHA content.

Conclusions

Pathways that showed a significant response to dietary DHA level included those for long-chain polyunsaturated fatty acid biosynthesis, fatty acid elongation, steroid biosynthesis, glycan biosynthesis, protein export and protein processing in the endoplasmic reticulum. These findings suggest that in addition to clear roles in influencing lipid metabolic pathways, DHA might also have key functional roles in other pathways distinct from lipid metabolism.

Three-year breeding cycle of rainbow trout (Oncorhynchus mykiss) fed a plant-based diet, totally free of marine resources: consequences for reproduction, fatty acid composition and progeny survival

Terrestrial plant resources are increasingly used as substitutes for fish meal and fish oil in fish feed in order to reduce the reliance of aquaculture on marine fishery resources. Although many studies have been conducted to assess the effects of such nutritional transition, no whole breeding cycles of fish fed diets free from marine resources has been reported to date. We therefore studied the reproductive performance of trout after a complete cycle of breeding while consuming a diet totally devoid of marine ingredients and thus of n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFAs) that play a major role in the formation of ova. Two groups of female rainbow trout were fed from first feeding either a commercial diet (C, marine and plant ingredients), or a 100% plant-based diet (V, blend of plant proteins and vegetable oils). Livers, viscera, carcasses and ova were sampled at spawning and analyzed for lipids and fatty acids. Although the V-diet was devoid of n-3 LC-PUFAs, significant amounts of EPA and DHA were found in livers and ova, demonstrating efficient bioconversion of linolenic acid and selective orientation towards the ova. Some ova were fertilized to assess the reproductive performance and offspring survival. We observed for the first time that trout fed a 100% plant-based diet over a 3-year breeding cycle were able to produce ova and viable alevins, although the ova were smaller. The survival of offspring from V-fed females was lower (-22%) at first spawning, but not at the second. Our study showed that, in addition to being able to grow on a plant-based diet, rainbow trout reared entirely on such a diet can successfully produce ova in which neo-synthesized n-3 LC-PUFAs are accumulated, leading to viable offspring. However, further adjustment of the feed formula is still needed to optimize reproductive performance.

Cloning and tissue distribution of a fatty acyl Δ6-desaturase-like gene and effects of dietary lipid levels on its expression in the hepatopancreas of Chinese mitten crab (Eriocheir sinensis)

Fatty acyl Δ6-desaturase is the rate-limiting enzyme in the biosynthetic pathway of highly unsaturated fatty acids (HUFAs) in vertebrates. In this report, a fatty acyl Δ6-desaturase-like cDNA was cloned from the hepatopancreas of Eriocheir sinensis (Chinese mitten crab) and characterized by performing rapid-amplification of cDNA ends. The 2278-bp long full-length cDNA encodes a polypeptide with 442 amino acids. Gene expression analysis via real-time quantitative polymerase chain reaction revealed that the fatty acyl Δ6-desaturase-like transcripts are widely distributed in various tissues, with high expression levels in the hepatopancreas and cranial ganglia. This study focuses on the nutritional regulation of genes involved in the HUFA biosynthetic pathway in Chinese mitten crab. A feeding trial was performed whereby crablets were fed for 238 d with four different diets: control diet without oil lipids (added with 3% basic lipid of the fundamental diets); fish oil diet (FO; added with 3% of the fundamental diets); soybean oil diet (SO; added with 3% of the fundamental diets); and FO/SO diet (1:1; added with 3% of the fundamental diets). The hepatopancreas of crabs sampled at 168 d and 238 d to determine the effects on fatty acyl Δ6-desaturase-like mRNA expression. The results show that the expression of fatty acyl Δ6-desaturase-like is higher in the hepatopancreas of crabs fed with SO diet than those fed with FO diet. Furthermore, gene expression increased by 2.45-fold in the hepatopancreas of crabs fed with SO after 238 d than those fed after 168 d but remained steady for those fed with FO after 238 d.

Effect of salinity on the biosynthesis of n-3 long-chain polyunsaturated fatty acids in silverside Chirostoma estor

The genus Chirostoma (silversides) belongs to the family Atherinopsidae, which contains around 150 species, most of which are marine. However, Mexican silverside (Chirostoma estor) is one of the few representatives of freshwater atherinopsids and is only found in some lakes of the Mexican Central Plateau. However, studies have shown that C. estor has improved survival, growth, and development when cultured in water conditions with increased salinity. In addition, C. estor displays an unusual fatty acid composition for a freshwater fish with high docosahexaenoic acid (DHA)/ eicosapentaenoic acid (EPA) ratios. Freshwater and marine fish species display very different essential fatty acid metabolism and requirements, and so the present study investigated long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis to determine the capacity of C. estor for endogenous production of EPA and DHA, and the effect that salinity has on these pathways. Briefly, C. estor were maintained at three salinities (0, 5, and 15 ppt), and the metabolism of ¹⁴C-labeled 18:3n-3 was determined in isolated hepatocyte and enterocyte cells. The results showed that C. estor has the capacity for endogenous biosynthesis of LC-PUFA from 18-carbon fatty acid precursors, but that the pathway was essentially only active in saline conditions with virtually no activity in cells isolated from fish grown in freshwater. The activity of the LC-PUFA biosynthesis pathway was also higher in cells isolated from fish at 15 ppt compared with fish at 5 ppt. The activity was around fivefold higher in hepatocytes compared with enterocytes; although the majority of 18:3n-3 was converted to 18:4n-3 and 20:4n-3 in hepatocytes, the proportions of 18:3n-3 converted to EPA and DHA were higher in enterocytes. The data were consistent with the hypothesis that conversion of EPA to DHA could contribute, at least in part, to the generally high DHA/EPA ratios observed in the tissue lipids of C. estor.

LC-PUFA biosynthesis in rainbow trout is substrate limited: use of the whole body fatty acid balance method and different 18:3n-3/18:2n-6 ratios

Five experimental diets with constant total C(18) PUFA and varying 18:3n-3/18:2n-6 ratios were fed to rainbow trout over an entire production cycle. The whole-body fatty acid balance method demonstrated a clear trend of progressively reduced fatty acid bioconversion activity along the n-3 and n-6 pathways, up to the production of 20:5n-3 and 20:4n-6, respectively. This suggests that the pathway exhibits a "funnel like" progression of activity rather than the existence of a single rate limiting step. The production of 22:5n-3 and 22:6n-3 was more active than that of 20:5n-3. However, despite this trend in reduced apparent in vivo net enzyme activity, the efficiency of the various bioconversion steps (measured as % of bioconverted substrate) confirmed an opposing trend. A 3.2-fold higher Δ-6 desaturase affinity towards 18:3n-3 over 18:2n-6 and an 8-fold greater Δ-5 desaturase affinity towards 20:4n-3 over 20:3n-6 were recorded. The main results of the study were that (1) rainbow trout are quite efficient at bioconverting 18:3n-3 to 22:6n-3, and (2) the LC-PUFA biosynthetic pathway is substrate limited. Fillet n-3 LC-PUFA concentrations increased with the increasing dietary supply of 18:3n-3. Despite an almost identical dietary supply of n-3 LC-PUFA, originating from the fish meal fraction of the diets, the fillets of trout fed the diet richest in 18:3n-3 were 2-fold higher in n-3 LC-PUFA than fish fed low 18:3n-3 diets. Nevertheless, fillets of trout fed a fish oil control diet contained more than double the amount of n-3 LC-PUFA compared to fish fed the diets richest in 18:3n-3.

Effects of dietary α-linolenic acid (18:3n-3)/linoleic acid (18:2n-6) ratio on fatty acid metabolism in Murray cod (Maccullochella peelii peelii)

Global shortages in fish oil are forcing the aquaculture feed industry to use alternative oil sources, the use of which negatively affects the final fatty acid makeup of cultured fish. Thus, the modulation of fatty acid metabolism in cultured fish is the core of an intensive global research effort. The present study aimed to evaluate the effects of various dietary α-linolenic acid (ALA, 18:3n-3)/linoleic acid (LA, 18:2n-6) ratios in cultured fish. A feeding trial was implemented on the freshwater finfish Murray cod, in which fish were fed either a fish oil-based control diet or one of five fish oil-deprived experimental diets formulated to contain an ALA/LA ratio ranging from 0.3 to 2.9, but with a constant total C₁₈ PUFA (ALA+LA) content. The whole-body fatty acid balance method was used to evaluate fish in vivo fatty acid metabolism. The results indicate that dietary ALA was more actively β-oxidized and bioconverted, whereas LA appears to be more efficiently deposited. LA was β-oxidized at a constant level (~36% of net intake) independent of dietary availability, whereas ALA was oxidized proportionally to dietary supply. The in vivo apparent Δ-6 desaturase activity on n-3 and n-6 PUFA exhibited an increasing and decreasing trend, respectively, in conjunction with the increasing dietary ALA/LA ratio, clearly indicating that this enzymatic activity is substrate dependent. However, the maximum Δ-6 desaturase activity acting on ALA peaked at the substrate level of 3.2186 (μmol g fish⁻¹ day⁻¹), suggesting that additional inclusion of ALA is not only wasteful but counterproductive in terms of n-3 LC-PUFA production. Despite a constant total supply of ALA+LA, the recorded total in vivo apparent Δ-6 desaturase activity on both substrates (ALA and LA) increased in synchrony with the ALA/LA ratio, peaking at 1.54, and a 3.2-fold greater Δ-6 desaturase affinity toward ALA over LA was recorded.

Effect of feeding Atlantic salmon (Salmo salar L.) a diet enriched with stearidonic acid from parr to smolt on growth and n-3 long-chain PUFA biosynthesis

Vegetable oils (VO) have become the predominant substitute for fish oil (FO) in aquafeeds; however, the resultant lower content of n-3 long-chain ( ≥ C20) PUFA (n-3 LC-PUFA) in fish has put their use under scrutiny. The need to investigate new oil sources exists. The present study tested the hypothesis that in Atlantic salmon (Salmo salar L.), a high intake of stearidonic acid (SDA) from Echium oil (EO) would result in increased n-3 LC-PUFA biosynthesis due to a lower requirement for Δ6 desaturase. Comparisons were made with fish fed on diets containing rapeseed oil (RO) and FO in freshwater for 112 d followed by 96 d in seawater. EO fish had higher whole-carcass SDA and eicosatetraenoic acid (ETA) in freshwater and prolonged feeding on the EO diet in seawater resulted in higher SDA, ETA, EPA and docosapentaenoic acid (DPA) compared with RO fish. Fatty acid mass balance of freshwater fish indicated higher biosynthesis of ETA and EPA in EO fish compared with fish fed on the other diets and a twofold increase in n-3 LC-PUFA synthesis compared with RO fish. In seawater, n-3 biosynthetic activity was low, with higher biosynthesis of ETA in EO fish and appearance of all desaturated and elongated products along the n-3 pathway. SDA-enriched VO are more suitable substitutes than conventional VO from a human consumer perspective due to the resulting higher SDA content, higher total n-3 and improved n-3:n-6 ratio obtained in fish, although both VO were not as effective as FO in maintaining EPA and DHA content in Atlantic salmon.

n-3 Oil sources for use in aquaculture--alternatives to the unsustainable harvest of wild fish

The present review examines renewable sources of oils with n-3 long-chain (> or = C20) PUFA (n-3 LC-PUFA) as alternatives to oil from wild-caught fish in aquafeeds. Due to the increased demand for and price of wild-caught marine sources of n-3 LC-PUFA-rich oil, their effective and sustainable replacement in aquafeeds is an industry priority, especially because dietary n-3 LC-PUFA from eating fish are known to have health benefits in human beings. The benefits and challenges involved in changing dietary oil in aquaculture are highlighted and four major potential sources of n-3 LC-PUFA for aquafeeds, other than fish oil, are compared. These sources of oil, which contain n-3 LC-PUFA, specifically EPA (20:5n-3) and DHA (22:6n-3) or precursors to these key essential fatty acids, are: (1) other marine sources of oil; (2) vegetable oils that contain biosynthetic precursors, such as stearidonic acid, which may be used by fish to produce n-3 LC-PUFA; (3) single-cell oil sources of n-3 LC-PUFA; (4) vegetable oils derived from oil-seed crops that have undergone genetic modification to contain n-3 LC-PUFA. The review focuses on Atlantic salmon (Salmo salar L.), because it is the main intensively cultured finfish species and it both uses and stores large amounts of oil, in particular n-3 LC-PUFA, in the flesh.

Phospholipid molecular species, beta-oxidation, desaturation and elongation of fatty acids in Atlantic salmon hepatocytes: effects of temperature and 3-thia fatty acids

We have investigated the effects of a 3-thia fatty acid (TTA) and of temperature on the fatty acid (FA) metabolism of Atlantic salmon (Salmo salar). One experiment investigated the activity of the peroxisomal beta-oxidation enzyme, acyl-CoA oxidase (ACO), and the incorporation of TTA into phospholipid (PL) molecular species. Salmon hepatocytes in culture were incubated either without TTA (control(spades)) or with 0.8 mM TTA (TTA(spades)) in a short term (48 h) temperature study at 5 degrees C and at 12 degrees C. TTA was incorporated into the four PL classes studied: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS). TTA was preferentially esterified with 18:1, 16:1, 20:4 and 22:6 in the PLs. Hepatocytes incubated with TTA had higher ACO activity at 5 degrees C than at 12 degrees C. In a second experiment salmon were fed a diet based on fish meal-fish oil without any TTA added (control) or a fish meal-fish oil diet supplemented with 0.6% TTA for 8 weeks at 12 degrees C and 20 weeks at 5 degrees C. At the end of the feeding trial, hepatocytes from fish acclimated to high or low temperatures were isolated from both dietary groups and incubated with either [1-(14)C]18:1 n-9 or [1-(14)C]20:4 n-3 at 5 degrees C or 12 degrees C. Radiolabelled 18:1 n-9 was mainly esterified into neutral lipids (NL), whereas [1-(14)C]20:4 n-3 was mainly esterified into PL at both temperatures. The rate of elongation of [1-(14)C]18:1 n-9 to 20:1 n-9 was twice as high in hepatocytes from fish fed the control diet than it was in hepatocytes from fish fed the TTA diet, at both temperatures. The amount of [1-(14)C]20:4 n-3 converted to 22:6 n-3 was approximately the same in hepatocytes from the two dietary groups, but there was a tendency to higher production of 22:6 n-3 at the lower temperature. Oxidation of [1-(14)C]18:1 n-9 to acid soluble products (ASP) and CO(2) was approximately 10-fold greater in hepatocytes kept at 5 degrees C than in those kept at 12 degrees C and the main oxidation products formed were acetate, oxaloacetate and malate.

Effect of diets enriched in Δ6 desaturated fatty acids (18:3n−6 and 18:4n−3), on growth, fatty acid composition and highly unsaturated fatty acid synthesis in two populations of Arctic charr (Salvelinus alpinus L.)

This study aimed to test the hypothesis that diets containing relatively high amounts of the Delta6 desaturated fatty acids stearidonic acid (STA, 18:4n-3) and gamma-linolenic acid (GLA, 18:3n-6), may be beneficial in salmonid culture. The rationale being that STA and GLA would be better substrates for highly unsaturated fatty acid (HUFA) synthesis as their conversion does not require the activity of the reputed rate-limiting enzyme, fatty acid Delta6 desaturase. Duplicate groups of two Arctic charr (Salvelinus alpinus L.) populations with different feeding habits, that had been reported previously to show differences in HUFA biosynthetic capacity, were fed for 16 weeks on two fish meal based diets containing 47% protein and 21% lipid differing only in the added lipid component, which was either fish oil (FO) or echium oil (EO). Dietary EO had no detrimental effect on growth performance and feed efficiency, mortalities, or liver and flesh lipid contents in either population. The proportions of 18:2n-6, 18:3n-3, 18:3n-6, 18:4n-3, 20:3n-6 and 20:4n-3 in total lipid in both liver and flesh were increased by dietary EO in both populations. However, the percentages of 20:5n-3 and 22:6n-3 were reduced by EO in both liver and flesh in both strains, whereas 20:4n-6 was only significantly reduced in flesh. In fish fed FO, HUFA synthesis from both [1-(14)C]18:3n-3 and [1-(14)C]20:5n-3 was significantly higher in the planktonivorous Coulin charr compared to the demersal, piscivorous Rannoch charr morph. However, HUFA synthesis was increased by EO in Rannoch charr, but not in Coulin charr. In conclusion, dietary EO had differential effects in the two populations of charr, with HUFA synthesis only stimulated by EO in the piscivorous Rannoch morph, which showed lower activities in fish fed FO. However, the hypothesis was not proved as, irrespective of the activity of the HUFA synthesis pathway in either population, feeding EO resulted in decreased tissue levels of n-3HUFA and 20:4n-6. This has been observed previously in salmonids fed vegetable oils, and thus the increased levels of Delta6 desaturated fatty acids in EO did not effectively compensate for the lack of dietary HUFA.

Fatty acid metabolism in the freshwater fish Murray cod (Maccullochella peelii peelii) deduced by the whole-body fatty acid balance method

The whole-body fatty acid balance method was used to investigate the fatty acid metabolism in Murray cod (Maccullochella peelii peelii) fed diets containing canola (CO) or linseed oil (LO). Murray cod were able to elongate and desaturate both 18:2n-6 and 18:3n-3. In fish fed the CO diet, 54.4% of the 18:2n-6 consumed was accumulated, 38.5% oxidized and 6.4% elongated and desaturated to higher homologs. Fish fed the LO diet accumulated 52.9%, oxidized 37% and elongated and desaturated 8.6% of the consumed 18:3n-3. The overall roles of n-6 fatty acids appeared more important in Murray cod compared to other freshwater species. Murray cod also showed a preferential order of utilization of C18 fatty acid for energy production (18:3n-3 > 18:2n-6 > 18:1n-9). Moreover, it is demonstrated that an increase in dietary 18:3n-3 is directly responsible of increased desaturase activity and augmented saturated fatty acid accumulation in the fish body. The present study also suggests that, in the context of the possible maximization of the natural ability of fish to produce long chain polyunsaturated fatty acids, the whole-body approach can be considered well suited and informative and Murray cod is a suited candidate to fish oil replacement for its diets.

Fatty acid metabolism in Atlantic salmon (Salmo salar L.) hepatocytes and influence of dietary vegetable oil

Isolated hepatocytes from Atlantic salmon (Salmo salar), fed diets containing either 100% fish oil or a vegetable oil blend replacing 75% of the fish oil, were incubated with a range of seven (14)C-labelled fatty acids. The fatty acids were [1-(14)C]16:0, [1-(14)C]18:1n-9, 91-(14)C]18:2n-6, [1-(14)C]18:3n-3, [1-(14)C]20:4n-6, [1-(14)C]20:5n-3, and [1-(14)C]22:6n-3. After 2 h of incubation, the hepatocytes and medium were analysed for acid soluble products, incorporation into lipid classes, and hepatocytes for desaturation and elongation. Uptake into hepatocytes was highest with [1-(14)C]18:2n-6 and [1-(14)C]20:5n-3 and lowest with [1-(14)C]16:0. The highest recovery of radioactivity in the cells was found in triacylglycerols. Of the phospholipids, the highest recovery was found in phosphatidylcholine, with [1-(14)C]16:0 and [1-(14)C]22:6n-3 being the most prominent fatty acids. The rates of beta-oxidation were as follows: 20:4n-6>18:2n-6=16:0>18:1n-9>22:6n-3=18:3n-3=20:5n-3. Of the fatty acids taken up by the hepatocytes, [1-(14)C]16:0 and [1-(14)C]18:1n-9 were subsequently exported the most, with the majority of radioactivity recovered in phospholipids and triacylglycerols, respectively. The major products from desaturation and elongation were generally one cycle of elongation of the fatty acids. Diet had a clear effect on the overall lipid metabolism, with replacing 75% of the fish oil with vegetable oil resulting in decreased uptake of all fatty acids and reduced incorporation of fatty acids into cellular lipids, but increased beta-oxidation activity and higher recovery in products of desaturation and elongation of [1-(14)C]18:2n-6 and [1-(14)C]18:3n-3.

Effects of dietary supplementation of rapeseed oil on metabolism of [1-14C]18∶1n−9, [1-14C]20∶3n−6, and [1-14C]20∶4n−3 in atlantic salmon heaptocytes

Atlantic salmon were fed fish meal-based diets supplemented with either 100% fish oil (FO) or 100% rapeseed oil (RO) from an initial weight of 85 g to a final average weight of 280 g. The effects of these diets on the capacity of Atlantic salmon hepatocytes to elongate, desaturate, and esterify [1-14C] 18:1n-9 and the immediate substrates for the delta5 desaturase, [1-14C] 20:3 n-6 and [1-14C] 20:4n-3, were investigated. Radiolabeled 18:1n-9 was mainly esterified into cellular TAG, whereas the more polyunsaturated FA, [1-14C] 20:3n-6 and [1-14C] 20:4n-3, were primarily esterified into cellular PL. More of the elongation product, [1-14C] 20:1n-9, was produced from 18:1n-9 and more of the desaturation and elongation products, 22:5n-6 and 22:6n-3, were produced from [1-14C]20:3n-6 and [1-14C] 20:4n-3, respectively, in RO hepatocytes than in FO hepatocytes. Further, we studied whether increased addition of [1-14C]18:1n-9 to the hepatocyte culture media would affect the capacity of hepatocytes to oxidize 18:1n-9 to acid-soluble products and CO2. An increase in exogenous concentration of 18:1 n-9 from 7 to 100 microM resulted in a nearly twofold increase in the amount of 18:1n-9 that was oxidized. The conversion of 20:4n-3 and 20:3n-6 to the longer-chain 22:6n-3 and 22:5n-6 was enhanced by RO feeding in Atlantic salmon hepatocytes. The increased capacity of RO hepatocytes to produce 22:6n-3 was, however, not enough to achieve the levels found in FO hepatocytes. Our data further showed that there were no differences in the hepatocyte FA oxidation capacity and the lipid deposition of carcass and liver between the two groups.

Environmental and dietary influences on highly unsaturated fatty acid biosynthesis and expression of fatty acyl desaturase and elongase genes in liver of Atlantic salmon (Salmo salar)

Highly unsaturated fatty acid (HUFA) synthesis in Atlantic salmon (Salmo salar) was known to be influenced by both nutritional and environmental factors. Here we aimed to test the hypothesis that both these effectors involved similar molecular mechanisms. Thus, HUFA biosynthetic activity and the expression of fatty acyl desaturase and elongase genes were determined at various points during an entire 2 year production cycle in salmon fed diets containing either 100% fish oil or diets in which a high proportion (75% and 100%) of fish oil was replaced by C18 polyunsaturated fatty acid-rich vegetable oil. The results showed that HUFA biosynthesis in Atlantic salmon varied during the growth cycle with peak activity around seawater transfer and subsequent low activities in seawater. Consistent with this, the gene expression of Delta6 desaturase, the rate-limiting step in the HUFA biosynthetic pathway, was highest around the point of seawater transfer and lowest during the seawater phase. In addition, the expression of both Delta6 and Delta5 desaturase genes was generally higher in fish fed the vegetable oil-substituted diets compared to fish fed fish oil, particularly in the seawater phase. Again, generally consistent with this, the activity of the HUFA biosynthetic pathway was invariably higher in fish fed diets in which fish oil was substituted by vegetable oil compared to fish fed only fish oil. In conclusion, these studies showed that both nutritional and environmental modulation of HUFA biosynthesis in Atlantic salmon involved the regulation of fatty acid desaturase gene expression.

Molecular cloning and functional characterization of fatty acyl desaturase and elongase cDNAs involved in the production of eicosapentaenoic and docosahexaenoic acids from alpha-linolenic acid in Atlantic salmon (Salmo salar)

Fish are the only major dietary source for humans of omega-3 highly unsaturated fatty acids (HUFAs) and with declining fisheries farmed fish such as Atlantic salmon (Salmo salar) constitute an increasing proportion of the fish in the human diet. However, the current high use of fish oils, derived from wild capture marine fisheries, in aquaculture feeds is not sustainable in the longer term and will constrain continuing growth of aquaculture activities. Greater understanding of how fish metabolize and biosynthesize HUFA may lead to more sustainable aquaculture diets. The study described here contributes to an effort to determine the molecular genetics of the HUFA biosynthetic pathway in salmon, with the overall aim being to determine mechanisms for optimizing the use of vegetable oils in Atlantic salmon culture. In this paper we describe the cloning and functional characterization of 2 genes from salmon involved in the biosynthesis of HUFA. A salmon desaturase complementary DNA, SalDes, was isolated that include an open reading frame of 1362 bp specifying a protein of 454 amino acids. The protein sequence includes all the characteristics of microsomal fatty acid desaturases, including 3 histidine boxes, 2 transmembrane regions, and an N-terminal cytochrome b(5) domain containing a heme-binding motif similar to that of other fatty acid desaturases. Functional expression in the yeast Saccharomyces cerevisiae showed SalDes is predominantly an omega-3 delta5 desaturase, a key enzyme in the synthesis of eicosapentaenoic acid (20:5n-3) from alpha-linolenic acid (18:3n-3). The desaturase showed only low levels of delta6 activity toward C(18) polyunsaturated fatty acids. In addition, a fatty acid elongase cDNA, SalElo, was isolated that included an open reading frame of 888 bp, specifying a protein of 295 amino acids. The protein sequence of SalElo included characteristics of microsomal fatty acid elongases, including a histidine box and a transmembrane region. Upon expression in yeast SalElo showed broad substrate specificity for polyunsaturated fatty acids with a range of chain lengths, with the rank order being C(18) > C(20) > C(22). Thus this one polypeptide product displays all fatty acid elongase activities required for the biosynthesis of docosahexaenoic acid (22:6n-3) from 18:3n-3.

Effects of water temperature and diets containing palm oil on fatty acid desaturation and oxidation in hepatocytes and intestinal enterocytes of rainbow trout (Oncorhynchus mykiss)

Food grade fisheries have reached their sustainable limits while aquaculture production has increased to meet consumer demands. However, for growth in aquaculture to continue and utilise sustainable, feeding ingredients, alternatives to fish oil (FO), the predominant lipid component of fish diets, must be developed. Therefore, there is currently considerable interest in the regulation of fatty acid metabolism in fish in order to determine strategies for the best use of plant oils in diets for commercially important cultured fish species. Plant oils are characteristically rich in C18 polyunsaturated fatty acids (PUFA) but devoid of C20 and C22 highly unsaturated fatty acids (HUFA) found in FO. The fatty acyl desaturase enzyme activities involved in the biosynthesis of HUFA from PUFA are known to be under nutritional regulation and can be increased in fish fed diets rich in plant oils. However, fatty acid desaturase activity is also known to be modulated by water temperature in fish. The present study aimed to investigate the interaction between water temperature and diet in the regulation of fatty acid metabolism in rainbow trout. Trout, acclimatized to 7, 11 or 15 degrees C, were fed for 4 weeks on diets in which the FO was replaced in a graded manner by palm oil. At the end of the trial, fatty acyl desaturation/elongation and beta-oxidation activities were determined in isolated hepatocytes and intestinal enterocytes using [1-14C]18:3n-3 as substrate, and samples of liver were collected for analysis of lipid and fatty acid composition. The most obvious effect of temperature was that fatty acid desaturation/elongation and beta-oxidation were reduced in both hepatocytes and intestinal enterocytes from fish maintained at the highest water temperature (15 degrees C). There were differences between the two tissues with the highest desaturation/elongation and beta-oxidation activities tending to be in fish held at 11 degrees C in the case of hepatocytes, but 7 degrees C in enterocytes. Correlations between fatty acid metabolism and dietary palm oil were most clearly observed in desaturation/elongation activities in both hepatocytes and enterocytes at 11 degrees C. The highest beta-oxidation activities were generally observed in fish fed FO alone in both hepatocytes and enterocytes with palm oil having differential effects in the two cell types.

Influence of temperature and high dietary linoleic acid content on esterification, elongation, and desaturation of PUFA in Atlantic salmon hepatocytes

The esterification, desaturation, and elongation of [1-14C]18:3n-3, [1-14C]18:2n-6, and [1-14C]20:5n-3 at 5 and at 12 degrees C were studied using cultivated hepatocytes from Atlantic salmon. The salmon were fed diets, in which 0, 50, or 100% of the supplementary fish oil had been replaced by soybean oil, for 950 day-degrees at 5 and 12 degrees C. The endogenous percentage of 18:2n-6 in hepatocyte lipids was 2% in cells from fish fed a diet with 100% of the supplemental lipid from fish oil, and it was slightly less than 25% in cells from fish fed the diet with 100% of the supplemental lipid from soybean oil. Furthermore, the percentages of 20:3n-6 and 20:4n-6 were significantly higher in hepatocytes from fish fed on soybean oil than they were in those of fish fed on fish oil. The percentages of 20:5n-3 and 22:6n-3, on the other hand, were lower. The endogenous levels of n-6 FA were not significantly correlated with the total amounts of radiolabeled FA esterified in hepatocyte lipids. The main radiolabeled products formed from 18:2n-6 were 20:2n-6 and 20:3n-6. The level of the important eicosanoid precursor 20:4n-6 was twice as high in hepatocyte phospholipids from fish fed the 100% soybean oil diet as it was in hepatocytes from fish fed the diet with 100% of supplemental lipid from fish oil. The main products formed from 18:3n-3 were 20:4n-3, 20:5n-3, and 22:6n-3. High levels of dietary 18:2n-6 do allow, or even seem to increase, the production of 22:6n-3 from 18:3n-3 in hepatocytes. The main products formed from 20:5n-3 were 22:5n-3 and 22:6n-3. The production of 22:6n-3 from 20:5n-3 was higher at 5 degrees C than at 12 degrees C. The percentage of 24:5n-3 was higher at 5 degrees C than it was at 12 degrees C, as was the ratio of 24:5 to 22:5. These results suggest that the elongation rate of 22:5n-3 to 24:5n-3 is higher at the lower temperature.

β-Oxidation of 18∶3n−3 in atlantic salmon (Salmo salarL.) hepatocytes treated with different fatty acids

To study whether Atlantic salmon beta-oxidation was affected by dietary FA composition, an in vitro study with primary hepatocytes was undertaken. Isolated hepatocyte cultures were stimulated with either 16:0, 18:1n-9, 18:2n-6, 18:3n-3, 20:5n-3, or 22:6n-3 in triplicate for 24 h. In addition, a control was included where no FA stimulation was performed, also in triplicate. After stimulation, radiolabeled [1-14C] 18:3n-3 was added and the cells were incubated for 2 h at 20 degrees C. The cells were then harvested, and radioactivity was determined in the acid-soluble part of the cells and medium, i.e., the end products of the beta-oxidation pathway. Specific beta-oxidation activity was significantly higher in hepatocytes stimulated with 18:3n-3. Further, when taking into account the amount of radiolabeled [1-14C]18:3n-3 taken up by the cells--the relative amount of beta-oxidized [1-14C]18:3n-3 of the total FA taken up by the hepatocytes-no significant differences were found. Thus, the regulation of beta-oxidation activity in the primary Atlantic salmon hepatocytes seems to be at the level of FA uptake and transport into the cell. This in vitro study shows that the catabolism processes in salmon hepatocytes are affected by the FA available and probably already regulated at the level of FA uptake.

Effects of dietary vegetable oil on atlantic salmon hepatocyte fatty acid desaturation and liver fatty acid compositions

Fatty acyl desaturase activities, involved in the conversion of the C18 EFA 18:2n-6 and 18:3n-3 to the highly unsaturated fatty acids (HUFA) 20:4n-6, 20:5n-3, and 22:6n-3, are known to be under nutritional regulation. Specifically, the activity of the desaturation/elongation pathway is depressed when animals, including fish, are fed fish oils rich in n-3 HUFA compared to animals fed vegetable oils rich in C18 EFA. The primary aims of the present study were (i) to establish the relative importance of product inhibition (n-3 HUFA) vs. increased substrate concentration (C18 EFA) and (ii) to determine whether 18:2n-6 and 18:3n-3 differ in their effects on the hepatic fatty acyl desaturation/elongation pathway in Atlantic salmon (Salmo salar). Smolts were fed 10 experimental diets containing blends of two vegetable oils, linseed (LO) and rapeseed oil (RO), and fish oil (FO) in a triangular mixture design for 50 wk. Fish were sampled after 32 and 50 wk, lipid and FA composition of liver determined, fatty acyl desaturation/elongation activity estimated in hepatocytes using [1-14C]18:3n-3 as substrate, and the data subjected to regression analyses. Dietary 18:2n-6 was positively correlated, and n-3 HUFA negatively correlated, with lipid content of liver. Dietary 20:5n-3 and 22:6n-3 were positively correlated with liver FA with a slope greater than unity suggesting relative retention and deposition of these HUFA. In contrast, dietary 18:2n-6 and 18:3n-3 were positively correlated with liver FA with a slope of less than unity suggesting metabolism via beta-oxidation and/or desaturation/elongation. Consistent with this, fatty acyl desaturation/elongation in hepatocytes was significantly increased by feeding diets containing vegetable oils. Dietary 20:5n-3 and 22:6n-3 levels were negatively correlated with hepatocyte fatty acyl desaturation. At 32 wk, 18:2n-6 but not 18:3n-3 was positively correlated with hepatocyte fatty acyl desaturation, whereas the reverse was true at 50 wk. The data indicate that both feedback inhibition through increased n-3 HUFA and decreased C18 fatty acyl substrate concentration are probably important in determining the level of hepatocyte fatty acyl desaturation and that 18:2n-6 and 18:3n-3 may differ in their effects on this pathway.

The effects of dietary lipid and strain difference on polyunsaturated fatty acid composition and conversion in anadromous and landlocked salmon (Salmo salar L.) parr

Five experimental diets containing different proportions of olive, sunflower and linseed oils were used in a 55-day feeding trial on both anadromous and landlocked parr of Atlantic salmon (Salmo salar) of the same age, in order to study the effects of diet and strain on growth and fatty acid composition and absolute gains in fish whole body triacylglycerols (TAG) and phospholipids (PL). Growth rate was higher in landlocked than in anadromous parr, but not between the different diets. By contrast, the effect of diet on whole body fatty acid composition was much more pronounced than that of strain difference. The fatty acids deposition results establish significant (P<0.05) positive correlations and linear relationships between the percentage of several fatty acids (18:1n-9, 18:2n-6, 18:3n-3) in dietary lipids and their absolute gains in whole body TAG and PL of both stocks. They also indicate the selective deposition of 18:1n-9 compared with linoleic acid (LLA) and linolenic acid (LNA). Finally, the results suggest the occurrence of the conversion of LLA and LNA to long-chain polyunsaturated fatty acids, its stimulation by increased substrate availability, a significantly higher n-3 and n-6 polyunsaturated fatty acids conversion capacity in landlocked than in anadromous parr and a strong genetic influence on docosahexaenoic acid content in salmon parr PL.

The esterification and modification of n-3 and n-6 polyunsaturated fatty acids by hepatocytes and liver microsomes of turbot (Scophthalmus maximus)

The present study was undertaken to establish whether the formation of 22:6n-3 from 18:3n-3 and/or 20:5n-3 can occur in turbot liver and if this conversion is consistent with the operation of a Delta4 desaturase-independent pathway. At the same, time the effects of feeding a diet devoid of long chain polyunsaturated fatty acids (PUFA) on the patterns of esterification and modification of 18:3n-3, 20:5n-3 and 18:2n-6 by turbot hepatocytes and liver microsomes were examined. For this purpose, two groups of fish (25-30 g) were employed: one was fed a commercial diet containing fish oil (FO) and thus rich in long chain n-3 PUFA and the other was fed an experimental diet based on olive oil (OO). After 5 months of feeding, hepatocytes and liver microsomes isolated from individuals in the two groups of fish were incubated with [1-(14)C]-PUFA [either 18:3n-3, 20:5n-3 or 18:2n-6]. After 3 h of incubation, most radioactivity from all three radiolabelled substrates incorporated into lipids by hepatocytes and microsomes was recovered in the original substrate. The formation of desaturation products of n-3 radiolabelled substrates was higher in hepatocytes isolated from OO-fed than FO-fed fish. Small amounts of radiolabelled 22:6n-3 were formed from [1-(14)C]18:3n-3 and [1-(14)C]20:5n-3, but only by hepatocytes from fish fed OO, which also synthesised a small amount of radiolabelled 24:6n-3 from 14C-20:5n-3. Elongation products predominated over desaturation products in hepatic microsomes from both groups of fish studied, particularly in microsomes from fish fed FO. The results confirm that regardless of the long chain PUFA content of the diet, the production of 22:6n-3 in turbot liver from 18:3n-3 and/or 20:5n-3, and of 20:4n-6 from 18:2n-6, is very limited. The presence of radiolabelled 24:6n-3 in microsomes coupled with the absence of radiolabelled 22:6n-3 suggests that the formation of 22:6n-3 that does occur in turbot liver cells, may involve C24 intermediates and peroxisomal beta-oxidation.

Substituting fish oil with crude palm oil in the diet of Atlantic salmon (Salmo salar) affects muscle fatty acid composition and hepatic fatty acid metabolism

Supplies of marine fish oils (FO) are limited and continued growth in aquaculture production dictates that substitutes must be found that do not compromise fish health and product quality. In this study the suitability of crude palm oil (PO) as a replacement for FO in diets of Atlantic salmon was investigated. Duplicate groups of Atlantic salmon post-smolts were fed four practical-type diets in which the added lipid was either 100% FO and 0% crude PO (0% PO); 75% FO and 25% PO (25% PO); 50% FO and 50% PO (50% PO); and 100% PO, for 30 wk. There were no effects of diet on growth rate or feed conversion ratio nor were any histopathological lesions found in liver, heart or muscle. Lipid deposition was greatest in fish fed 0% PO and was significantly greater than in fish fed 50% and 100% PO. Fatty acid compositions of muscle total lipid were correlated with dietary PO inclusion such that the concentrations of 16:0, 18:1(n-9), 18:2(n-6), total saturated fatty acids and total monoenoic fatty acids increased linearly with increasing dietary PO. The concentration of eicosapentaenoic acid [20:5(n-3)] was reduced significantly with increasing levels of dietary PO but the concentration of docosahexaenoic acid [22:6(n-3)] was significantly reduced only in fish fed 100% PO, compared with the other three treatments. Similar diet-induced changes were seen in liver total lipid fatty acid compositions. Hepatic fatty acid desaturation and elongation activities were approximately 10-fold greater in fish fed 100% PO than in those fed 0% PO. This study suggests that PO can be used successfully as a substitute for FO in the culture of Atlantic salmon in sea water. However, at levels of PO inclusion above 50% of dietary lipid, significant reductions in muscle 20:5(n-3), 22:6(n-3) and the (n-3):(n-6) PUFA ratio occur, resulting in reduced availability of these essential (n-3) highly unsaturated fatty acids to the consumer.

Biosynthesis and tissue deposition of docosahexaenoic acid (22:6n-3) in rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) weighing ca. 5 g and previously acclimated for 8 wk on a diet comprising vegetable oil (11%), fish meal (5%), and casein (48%) as the major constituents were fed a pulse of diet containing deuterated (D5) (17,17,18,18,18)-18:3n-3 ethyl ester. The synthesis and tissue distribution of D5-22:6n-3 was determined 3, 7, 14, 24, and 35 d after the pulse. The whole-body accumulation of D5-22:6n-3 was linear over the first 7 d, corresponding to a rate of 0.54 +/- 0.12 microg D5-22:6n-3/g fish/mg D5-18:3n-3 eaten/d. Maximal accretion of D5-22:6n-3 was 4.3 +/- 1.2 microg/g fish/mg of D5-18:3n-3 eaten after 14 d. The amount of D5-22:6n-3 peaked in liver at day 7, in brain and eyes at day 24, and plateaued after day 14 in visceral and eye socket adipose tissue and in the whole fish. The majority of D5-22:6n-3 was found in the carcass (remaining tissues minus the above tissues analyzed separately) at all times. On a per milligram lipid basis, liver and eyes had the highest concentration of D5-22:6n-3. The experimental diet also contained 21:4n-6 ethyl ester as a marker to estimate the amount of food eaten by individual fish. From such estimates it was calculated that the great majority of the D5-tracer was catabolized, with the combined recovery of D5-18:3n-3 plus D5-22:6n-3 being 2.6%. The recovery of 21:4n-6 was 57.6%. The concentration of 22:6n-3 in the fish decreased during the 13-wk period, and the amount of 22:6n-3 synthesized from 18:3n-3 was only about 5% of that obtained directly from the fish meal in the diet.

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

The desaturation and elongation of [1-(14)C]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-(14)C]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.

Replacement of fish oil with rapeseed oil in diets of Atlantic salmon (Salmo salar) affects tissue lipid compositions and hepatocyte fatty acid metabolism

Duplicate groups of Atlantic salmon post-smolts were fed five practical-type diets in which the added lipid was 100% fish oil [FO; 0% rapeseed oil (0% RO)], 90% FO + 10% RO (10% RO), 75% FO + 25% RO (25% RO), 50% FO + 50% RO (50% RO) or 100% RO, for a period of 17 wk. There were no effects of diet on growth rate or feed conversion nor were any histopathological lesions found in liver, heart, muscle or kidney. The greatest accumulation of muscle lipid was in fish fed 0% RO, which corresponded to significantly lower muscle protein in this group. The highest lipid levels in liver were found in fish fed 100% RO. Fatty acid compositions of muscle lipid correlated with RO inclusion in that the proportions of 18:1(n-9), 18:2(n-6) and 18:3(n-3) all increased with increasing dietary RO (r = 0.98-1.00, P < 0.013). The concentrations of eicosapentaenoic acid [20:5(n-3)] and docosahexaenoic acid [22:6(n-3)] in muscle lipid were significantly reduced (P < 0.05), along with total saturated fatty acids, with increasing dietary RO. Diet-induced changes in liver fatty acid compositions were broadly similar to those in muscle. Hepatic fatty acid desaturation and elongation activities, measured using [1-(14)C] 18:3(n-3), were increased with increasing dietary RO. Limited supplies of marine fish oils require that substitutes be found if growth in aquaculture is to be maintained such that fish health and product quality are not compromised. Thus, RO can be used successfully as a substitute for fish oil in the culture of Atlantic salmon in sea water although at levels of RO >50% of dietary lipid, substantial reductions occur in muscle 20:5(n-3), 22:6(n-3) and the (n-3)/(n-6) polyunsaturated fatty acid (PUFA) ratio, which will result in reduced availability of the (n-3) highly unsaturated fatty acids that are beneficial for human health.

In vivo and in vitro effects of prolactin and growth hormone on lipid metabolism in a teleost, Anabas testudineus (Bloch)

Prolactin (PRL) has an important role in the regulation of water and electrolyte homeostasis in teleosts. The present study was designed to evaluate the role of PRL and GH on malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH) and isocitrate dehydrogenase (ICDH) in Anabas testudineus. Ovine prolactin significantly inhibited ME, G6PDH and ICDH activities when administered in vivo compared to vehicle treated controls. In vivo administration of PRL reversed the action of bromocryptine on enzyme activities. Ovine growth hormone in vivo also modified the effect of bromocryptine but not to the level of prolactin. Combined action of PRL+GH in vivo was most effective in keeping the enzyme activities at normal level after bromocryptine treatment. Prolactin in vitro also reversed the action of bromocryptine on enzyme activities, while GH in vitro failed to do so. Hence, prolactin seems to have an inhibitory effect on lipid metabolism in this teleost. Combined action of PRL+GH is more prominent in in vivo conditions at low PRL levels. Dopaminergic pathways may be involved in the control of prolactin and to some extent on growth hormone secretion.

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%.

Thyroid hormones regulate lipid peroxidation and antioxidant enzyme activities in Anabas testudineus (Bloch)

The role of thyroid hormones in metabolic pathways are well known. However, their involvement in lipid peroxidation and antioxidant enzyme activities is not known. In this study, the in vivo injection of 6-propylthiouracil (6-PTU) did not alter the concentration of malondialdehyde (MDA) and conjugated dienes in liver. The administration of triiodothyronine (T3) or diiodothyronine (T2) increased the peroxidation rate in hypothyroid fish. However, in normal fish, only a high dose of T2 caused increased malondialdehyde (MDA) production, rather than T3. SOD activity was higher in T2-treated groups in both experiments. Glutathione peroxidase (GPx) activity was also high in hypothyroid fish treated with T2. In normal specimens, injections of T3 and T2 had no effect on GPx activity. Glutathione reductase (GR) activity was not altered by hypothyroidism while T3 (1 microg) and T2 (1 microg) increased it. Glutathione content was low in 6-PTU treated fish and high in both T3- and T2-treated groups. Thus it can be concluded that not only T3 but also T2, formed by sequential monodeiodination of T4, is also effective in influencing lipid peroxidation and antioxidant enzyme activities in Anabas. Furthermore, hypothyroidism as well as hyperthyroidism affects lipid peroxidation in this teleost.

Long-term feeding of dietary oils alters lipid metabolism, lipid peroxidation, and antioxidant enzyme activities in a teleost (Anabas testudineus Bloch)

Anabas testudineus (climbing perch), average body weight 21+/-1 g, were maintained in culture tanks and fed a 35% protein feed plus an additional supplementation of three dietary oils (20% each of coconut oil, palm oil, or cod liver oil). Body weight gain was similar among all groups. However, several hepatic lipogenic enzymes such as malic enzyme (ME), NADP-isocitrate dehydrogenase (ICDH), glucose 6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and beta-hydroxy-1-methyl glutaryl CoA reductase (HMG CoA reductase) were assayed, and they responded differently. Hepatic ME and G6PDH activities showed a significant decrease in the coconut oil and palm oil groups, but there was no significant change in ICDH activity. The 6PGDH activities were reduced, whereas HMG CoA reductase activity was increased in the palm oil-treated group. Cholesterol synthesis in the liver and muscle increased in the palm oil-treated group, but liver phospholipids did not show any significant change in fish supplemented with oils rich in saturated fatty acids. Triacylglycerol and free fatty acid concentrations were high in the coconut oil- and palm oil-supplemented groups. Lipid peroxidation products such as thiobarbituric acid-reactive substances and conjugated dienes decreased in the same two groups. Antioxidant potential was high in all groups as evidenced by increased activity of superoxide dismutase, glutathione peroxidase, and glutathione content. The results of this study indicate that in fish, dietary lipids depress hepatic lipogenic activity as well as lipid peroxidation products by maintaining high levels of antioxidant enzymes.

Thyroid hormones regulate lipid metabolism in a teleost Anabas testudineus (Bloch)

We compared the long-term action of 3,5,3'-triiodo-L-thyronine (T3) and 3,5-diiodo-L-thyronine (T2) on lipid metabolism in a teleost Anabas testudineus. Among the six groups of animals used in this experiment, except for the control group, all received 6-propylthiouracil (6-PTU) to create a hypothyroid state in order to analyse the action of iodothyronines on lipid metabolism. Injections of 6-PTU reduced T3 concentration in the circulation by 79.6% and injections of iodothyronines enhanced the level of T3 in the plasma, and a maximum increase was observed in T3 (500 ng)-treated specimens. Analysis of lipogenic enzymes in liver and heart showed that a tissue-specific variation exists in the action of thyroid hormones and, in many cases, activity is higher in T2-treated groups. Analysis of various lipid classes showed that long-term administration of T2 is also effective in producing a comparable effect with that of T3 on lipid metabolism.

Studies on the effect of growth hormone in vivo and in vitro on lipogenic enzymes and transaminases in a teleost Anabas testudineus (BLOCH)

The specific activities of three lipogenic enzymes, malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH) and isocitrate dehydrogenase (ICDH), in liver and heart and two transaminases (AST & ALT) in liver and muscle, were studied in response to the in vivo and in vitro administration of growth hormone (GH) in a teleost Anabas testudineus. Ovine growth hormone (oGH) in vivo significantly reduced the activities of lipogenic enzymes, except for heart G6PDH, which showed an increase at the highest dose of hormone. Transaminase activity either increased or decreased depending on the dose of GH. The lowest dose of hormone employed (0.1 microg/gm b/w) exhibited a stimulatory effect and the highest dose (0.5 microg/gm. b/w) an inhibitory effect on transaminase activity. Both ovine GH and carp GH (oGH and cGH) in vitro significantly reduced the activities of ME, G6PDH and ICDH. Activities of AST and ALT were increased by oGH and cGH in vitro. The present study reveals that irrespective of origin, GH in vitro has a direct inhibitory effect on lipogenic enzymes ME, G6PDH, ICDH and a stimulatory effect on transaminases AST and ALT in A. testudineus, thus favoring gluconeogenesis.