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

A Method to Estimate the Dietary α-Linolenic Acid Requirement Using Nonesterified DHA and Arachidonic Acid Oxylipins and Fatty Acids

BACKGROUND

The dietary requirement for α-linolenic acid (ALA) remains unclear, as evidenced by the absence of a Recommended Dietary Allowance (RDA) for this essential fatty acid (FA). In previous studies, we observed that the amount of dietary ALA required to maximize nonesterified (NE) DHA oxylipins appears to be higher than the amount required to maximize tissue esterified DHA, which have classically been used to estimate the ALA requirement. Further, we observed that dietary ALA reduces esterified arachidonic acid (ARA) and its NE oxylipins.

OBJECTIVES

Since NE oxylipins and FA mediate the biological activities of FA, we examined whether these DHA and ARA pools could be used to determine the dietary ALA requirement.

METHODS

Nine groups of 4-wk-old male Sprague-Dawley rats (n = 5) and 10 groups of male and female CD1 mice (n = 6) were provided 0.1-2.5 g ALA and 2 g of linoleic acid per 100 g of AIN93G-based diets. NE DHA and ARA and their oxylipins in serum, liver, kidney, and brain homogenates underwent solid phase extraction and were quantified by HPLC-MS/MS. Breakpoint analysis of transitions from increase to plateau was conducted using piecewise regression.

RESULTS

In response to increasing dietary ALA, NE DHA oxylipins, and DHA in serum, liver, and kidney (but not the brain) initially increased rapidly and then reached a plateau whereas ARA oxylipins and ARA tended to decrease before reaching a plateau. Thus, breakpoints were calculated for the ratios of DHA/ARA and hydroxy-DHA/hydroxy-ARA (DHAOH/ARAOH), which consisted of oxylipins synthesized via pathways common to both FA. In serum, liver, and kidney, the highest estimated breakpoint indicated an ALA requirement of ∼0.7 g/100 g diet (1.7% energy), approximately twice that of previous estimations.

CONCLUSIONS

This study supports the use of NE DHAOH/ARAOH or DHA/ARA as biochemical indicators of the ALA requirement. Applying this method in rats and mice indicates that the requirement is higher than previously estimated using esterified DHA alone.

Current Insights into the Effects of Dietary α-Linolenic Acid Focusing on Alterations of Polyunsaturated Fatty Acid Profiles in Metabolic Syndrome

The plant-derived α-linolenic acid (ALA) is an essential n-3 acid highly susceptible to oxidation, present in oils of flaxseeds, walnuts, canola, perilla, soy, and chia. After ingestion, it can be incorporated in to body lipid pools (particularly triglycerides and phospholipid membranes), and then endogenously metabolized through desaturation, elongation, and peroxisome oxidation to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), with a very limited efficiency (particularly for DHA), beta-oxidized as an energy source, or directly metabolized to C18-oxilipins. At this moment, data in the literature about the effects of ALA supplementation on metabolic syndrome (MetS) in humans are inconsistent, indicating no effects or some positive effects on all MetS components (abdominal obesity, dyslipidemia, impaired insulin sensitivity and glucoregulation, blood pressure, and liver steatosis). The major effects of ALA on MetS seem to be through its conversion to more potent EPA and DHA, the impact on the n-3/n-6 ratio, and the consecutive effects on the formation of oxylipins and endocannabinoids, inflammation, insulin sensitivity, and insulin secretion, as well as adipocyte and hepatocytes function. It is important to distinguish the direct effects of ALA from the effects of EPA and DHA metabolites. This review summarizes the most recent findings on this topic and discusses the possible mechanisms.

Evaluation of Influencing Factors on Metabolism of Land-Based n-3 Poly Unsaturated Fatty Acids-The KoALA Study

This study aimed to investigate the impact of influencing factors (sex, eicosapentaenoic acid (EPA) status at baseline, linoleic acid (LA) intake, milk fat intake) on the conversion of α-linolenic acid (ALA) obtained from linseed oil into its long-chain metabolites. In addition, the effect of ALA on cardiovascular risk markers was investigated. This study used a parallel design approach by randomly assigning the 134 subjects to one of four diets (high in LA (HLA); low in LA (LLA); high in milk fat (MF); control (Western diet)) each enriched with linseed oil (10 en%, 22-27 mL ≙ 13-16 g ALA). Blood samples were taken at baseline and after 4, 8, and 12 weeks of dietary intervention. The study was fully completed by 105 subjects (57.4 ± 12.1 years; 65.7% female). Results showed that ALA (296-465%), C-20:4n3 (54-140%), and EPA (37-73%) concentrations in erythrocytes increased in all groups (p < 0.01). In contrast, docosahexaenoic acid (19-35%, p < 0.01) and n-3 index (10-21%, p < 0.05) dropped in the HLA, LLA, and control groups. An increase in C-22:5n3 was only observed in the MF (36%) and control groups (11%) (p < 0.05). In addition, an increase in LA (7-27%) was found in the HLA, LLA, and control groups, whereas C-20:3n6 (16-22%), arachidonic acid (10-16%), C-22:4n6 (12-30%), and C-22:5n6 (32-47%) decreased (p < 0.01). The conversion into EPA was higher in men than in women (69 vs. 39%, p = 0.043) and in subjects with low EPA status compared to participants with high EPA status (79 vs. 29%, p < 0.001). A high LA status attenuates the conversion rate. In line with the literature, no clear effects on blood lipids and parameters of glucose metabolism were found in relation to ALA supplementation.

Omega-3 fatty acids accelerate fledging in an avian marine predator: a potential role of cognition

Consuming omega-3 fatty acids (n-3 LCPUFAs) during development improves cognition in mammals, but the effect remains untested in other taxa. In aquatic ecosystems, n-3 LCPUFAs are produced by phytoplankton and bioaccumulate in the food web. Alarmingly, the warming and acidification of aquatic systems caused by climate change impair n-3 LCPUFA production, with an anticipated decrease of 80% by the year 2100. We tested whether n-3 LCPUFA consumption affects the physiology, morphology, behaviour and cognition of the chicks of a top marine predator, the ring-billed gull. Using a colony with little access to n-3 LCPUFAs, we supplemented siblings from 22 fenced nests with contrasting treatments from hatching until fledging; one sibling received n-3 LCPUFA-rich fish oil and the other, a control sucrose solution without n-3 LCPUFAs. Halfway through the nestling period, half the chicks receiving fish oil were switched to the sucrose solution to test whether n-3 LCPUFA intake remains crucial past the main growth phase (chronic versus transient treatments). Upon fledging, n-3 LCPUFAs were elevated in the blood and brains of chicks receiving the chronic treatment, but were comparable to control levels among those receiving the transient treatment. Across the entire sample, chicks with elevated n-3 LCPUFAs in their tissues fledged earlier despite their morphology and activity levels being unrelated to fledging age. Fledging required chicks to escape fences encircling their nest. We therefore interpret fledging age as a possible indicator of cognition, with chicks with improved cognition fledging earlier. These results provide insight into whether declining dietary n-3 LCPUFAs will compromise top predators' problem-solving skills, and thus their ability to survive in a rapidly changing world.

Modulation of colostrum composition and fatty acid status in neonatal calves by maternal supplementation with essential fatty acids and conjugated linoleic acid starting in late lactation

Sufficient maternal supply of essential fatty acids (EFA) to neonatal calves is critical for calf development. In the modern dairy cow, EFA supply has shifted from α-linolenic acid (ALA) to linoleic acid (LA) due to the replacement of pasture feeding by corn silage-based diets. As a consequence of reduced pasture feeding, conjugated linoleic acid (CLA) provision by rumen biohydrogenation was also reduced. The present study investigated the fatty acid (FA) status and performance of neonatal calves descended from dams receiving corn silage-based diets and random supplementation of either 76 g/d coconut oil (CTRL; n = 9), 78 g/d linseed oil and 4 g/d safflower oil (EFA; n-6/n-3 FA ratio = 1:3; n = 9), 38 g/d Lutalin (BASF SE, Ludwigshafen, Germany) providing 27% cis-9,trans-11 and trans-10,cis-12 CLA, respectively (CLA; n = 9), or a combination of EFA and CLA (EFA+CLA; n = 11) in the last 9 wk before parturition and following lactation. The experimental period comprised the first 5 d of life, during which calves received colostrum and transition milk from their own dam. The nutrient compositions of colostrum and transition milk were analyzed. Plasma samples were taken after birth and before first colostrum intake and on d 5 of life for FA analyses of the total plasma fat and lipid fractions. Maternal EFA and CLA supplementation partly affected colostrum and transition milk composition but did not change the body weights of calves. Most EFA in calves were found in the phospholipid (PL) and cholesterol ester (CE) fractions of the plasma fat. Maternal EFA supplementation increased the percentage of ALA in all lipid fractions of EFA and EFA+CLA compared with CTRL and CLA calves on d 1 and 5, and the increase was much greater on d 5 than on d 1. The LA concentration increased from d 1 to 5 in the plasma fat and lipid fractions of all groups. The concentrations of docosapentaenoic acid, docosahexaenoic acid, and arachidonic acid in plasma fat were higher on d 1 than on d 5, and the percentage of n-3 metabolites was mainly increased in PL if dams received EFA. The percentage of cis-9,trans-11 CLA was higher in the plasma fat of EFA+CLA than CTRL calves after birth. By d 5, the percentages of both CLA isomers increased, leading to higher proportions in plasma fat of CLA and EFA+CLA than in CTRL and EFA calves. Elevated cis-9,trans-11 CLA enrichment was observed on d 5 in PL, CE, and triglycerides of CLA-treated calves, whereas trans-10,cis-12 CLA could not be detected in individual plasma fractions. These results suggest that an altered maternal EFA and CLA supply can reach the calf via the placenta and particularly via the intake of colostrum and transition milk, whereas the n-3 and n-6 FA metabolites partly indicated a greater transfer via the placenta. Furthermore, the nutrient supply via colostrum and transition milk might be partly modulated by an altered maternal EFA and CLA supply but without consequences on calf performance during the first 5 d of life.

Dual expression of transgenic delta-5 and delta-6 desaturase in tilapia alters gut microbiota and enhances resistance to Vibrio vulnificus infection

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), exhibit antibacterial and anti-inflammatory activities. Furthermore, diets rich in n-3 PUFAs are known to improve disease resistance and limit pathogen infection in commercial aquaculture fishes. In this study, we examined the effects of transgenic overexpression of n-3 PUFA biosynthesis genes on the physiological response to bacterial infection in tilapia. We first established tilapia strains with single or dual expression of salmon delta-5 desaturase and/or delta-6 desaturase and then challenged the fish with Vibrio vulnificus infection. Interestingly, our data suggest that n-3 PUFA-mediated alterations in gut microbiota may be important in determining disease outcome via effects on immune response of the host. Both liver- and muscle-specific single and dual expression of delta-5 desaturase and delta-6 desaturase resulted in higher n-3 PUFA content in transgenic fish fed with a LO basal diet. The enrichment of n-3 PUFAs in dual-transgenic fish is likely responsible for their improved survival rate and comparatively reduced expression of inflammation- and immune-associated genes after V. vulnificus infection. Gut microbiome analysis further revealed that dual-transgenic tilapia had high gut microbiota diversity, with low levels of inflammation-associated microbiota (i.e., Prevotellaceae). Thus, our findings indicate that dual expression of transgenic delta-5 and delta-6 desaturase in tilapia enhances disease resistance, an effect that is associated with increased levels of n-3 PUFAs and altered gut microbiota composition.

Two alternative pathways for docosahexaenoic acid (DHA, 22:6n-3) biosynthesis are widespread among teleost fish

Docosahexaenoic acid (DHA) plays important physiological roles in vertebrates. Studies in rats and rainbow trout confirmed that DHA biosynthesis proceeds through the so-called “Sprecher pathway”, a biosynthetic process requiring a Δ6 desaturation of 24:5n−3 to 24:6n−3. Alternatively, some teleosts possess fatty acyl desaturases 2 (Fads2) that enable them to biosynthesis DHA through a more direct route termed the “Δ4 pathway”. In order to elucidate the prevalence of both pathways among teleosts, we investigated the Δ6 ability towards C₂₄ substrates of Fads2 from fish with different evolutionary and ecological backgrounds. Subsequently, we retrieved public databases to identify Fads2 containing the YXXN domain responsible for the Δ4 desaturase function, and consequently enabling these species to operate the Δ4 pathway. We demonstrated that, with the exception of Δ4 desaturases, fish Fads2 have the ability to operate as Δ6 desaturases towards C₂₄ PUFA enabling them to synthesise DHA through the Sprecher pathway. Nevertheless, the Δ4 pathway represents an alternative route in some teleosts and we identified the presence of putative Δ4 Fads2 in a further 11 species and confirmed the function as Δ4 desaturases of Fads2 from medaka and Nile tilapia. Our results demonstrated that two alternative pathways for DHA biosynthesis exist in teleosts.

A case-control study between gene polymorphisms of polyunsaturated fatty acid metabolic rate-limiting enzymes and acute coronary syndrome in Chinese Han population

The purpose of this study is to analyze the relationship between the polymorphisms of fatty acid desaturase 1 (FADS1), fatty acid desaturase 2 (FADS2), and elongation of very long-chain fatty acids-like 2 (ELOVL2) and acute coronary syndrome (ACS) in Chinese Han population. Therefore, we selected three single nucleotide polymorphisms (SNPs) from these candidate genes and genotyped them using PCR-based restriction fragment length polymorphism analysis in 249 ACS patients and 240 non-ACS subjects, as were Han Chinese ancestry. The results showed that rs174556 in the FADS1 gene is found to be in allelic association (P = 0.003 ) and genotypic association (P = 0.036) with ACS. The frequencies of rs174556 minor allele (T) in case group were obviously higher than in control group. The trans-phase gene-gene interaction analysis showed that the combined genotype of rs174556 (T/T) and rs3756963 (T/T) was associated with ACS (P = 0.031). And the results suggest that, for rs174556 C>T, the CT/TT genotypes were more likely to lead in ACS in subjects with hypertension after correction of all risk factors (OR = 4.236, 95% CI, 2.216-7.126). These findings suggest that the polymorphisms of rs174556 in the FADS1 gene are very likely to be associated with ACS in Chinese Han population, especially in subjects with hypertension.

Docosahexaenoic acid synthesis from alpha-linolenic acid is inhibited by diets high in polyunsaturated fatty acids

The conversion of the plant-derived omega-3 (n-3) α-linolenic acid (ALA, 18:3n-3) to the long-chain eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) can be increased by ALA sufficient diets compared to ALA deficient diets. Diets containing ALA above an optimal level result in no further increase in DHA levels in animals and humans. The present study evaluates means of maximizing plasma DHA accumulation by systematically varying both linoleic acid (LA, 18:2n-6) and ALA dietary level. Weanling rats were fed one of 54 diets for three weeks. The diets varied in the percentage of energy (en%) of LA (0.07-17.1 en%) and ALA (0.02-12.1 en%) by manipulating both the fat content and the balance of vegetable oils. The peak of plasma phospholipid DHA (>8% total fatty acids) was attained as a result of feeding a narrow dietary range of 1-3 en% ALA and 1-2 en% LA but was suppressed to basal levels (∼2% total fatty acids) at dietary intakes of total polyunsaturated fatty acids (PUFA) above 3 en%. We conclude it is possible to enhance the DHA status of rats fed diets containing ALA as the only source of n-3 fatty acids but only when the level of dietary PUFA is low (<3 en%).

Do FADS genotypes enhance our knowledge about fatty acid related phenotypes?

Several physiological processes, such as visual and cognitive development in early life, are dependent on the availability of long-chain polyunsaturated fatty acids (LC-PUFAs). Furthermore, the concentration of LC-PUFAs in phospholipids has been associated with numerous complex diseases like cardiovascular disease, atopic disease and metabolic syndrome. The level and composition of LC-PUFAs in the human body is mainly dependent on their dietary intake or on the intake of fatty acid precursors, which are endogenously elongated and desaturated to physiologically active LC-PUFAs. The delta-5 and delta-6 desaturase are the most important enzymes in this reaction cascade. In the last few years, several studies have reported an association between single nucleotide polymorphisms (SNPs) in the two desaturase encoding genes (FADS1 and FADS2) and the concentration of omega-6 and omega-3 fatty acids. This shows that beside nutrition, genetic factors play an important role in the regulation of LC-PUFAs as well. This review focuses on current knowledge of the impact of FADS genotypes on LC-PUFA and lipid metabolism and discusses their influence on infant intellectual development, neurological conditions, metabolic disease as well as cardiovascular disease.

Dietary n-6 PUFA deprivation for 15 weeks reduces arachidonic acid concentrations while increasing n-3 PUFA concentrations in organs of post-weaning male rats

Few studies have examined effects of feeding animals a diet deficient in n-6 polyunsaturated fatty acids (PUFAs) but with an adequate amount of n-3 PUFAs. To do this, we fed post-weaning male rats a control n-6 and n-3 PUFA adequate diet and an n-6 deficient diet for 15 weeks, and measured stable lipid and fatty acid concentrations in different organs. The deficient diet contained nutritionally essential linoleic acid (LA,18:2n-6) as 2.3% of total fatty acids (10% of the recommended minimum LA requirement for rodents) but no arachidonic acid (AA, 20:4n-6), and an adequate amount (4.8% of total fatty acids) of alpha-linolenic acid (18:3n-3). The deficient compared with adequate diet did not significantly affect body weight, but decreased testis weight by 10%. AA concentration was decreased significantly in serum (-86%), brain (-27%), liver (-68%), heart (-39%), testis (-25%), and epididymal adipose tissue (-77%). Eicosapentaenoic (20:5n-3) and docosahexaenoic acid (22:6n-3) concentrations were increased in all but adipose tissue, and the total monounsaturated fatty acid concentration was increased in all organs. The concentration of 20:3n-9, a marker of LA deficiency, was increased by the deficient diet, and serum concentrations of triacylglycerol, total cholesterol and total phospholipid were reduced. In summary, 15 weeks of dietary n-6 PUFA deficiency with n-3 PUFA adequacy significantly reduced n-6 PUFA concentrations in different organs of male rats, while increasing n-3 PUFA and monounsaturated fatty acid concentrations. This rat model could be used to study metabolic, functional and behavioral effects of dietary n-6 PUFA deficiency.

Competition between 24:5n-3 and ALA for Δ6 desaturase may limit the accumulation of DHA in HepG2 cell membranes

The use of Delta 6 desaturase (D6D) twice in the conversion of alpha-linolenic acid (ALA; 18:3n-3) to docosahexaenoic acid (DHA; 22:6n-3) suggests that this enzyme may play a key regulatory role in the synthesis and accumulation of DHA from ALA. We examined this using an in vitro model of fatty acid metabolism to measure the accumulation of the long-chain metabolites of ALA in HepG2 cell phospholipids. The accumulation of ALA, eicosapentaenoic acid (20:5n-3), docosapentaenoic acid (22:5n-3), and 24:5n-3 in cell phospholipids was linearly related to the concentration of supplemented ALA over the range tested (1.8-72 microM). The accumulation of the post-D6D products of 22:5n-3, 24:6n-3 and DHA, in cell phospholipids was saturated at concentrations of >18 microM ALA. Supplementation of HepG2 cells with preformed DHA revealed that, although the accumulation of DHA in cell phospholipids approached saturation, the level of DHA in cell phospholipids was significantly greater compared with the accumulation of DHA from ALA, indicating that the accumulation of DHA from ALA was not limited by incorporation. The parallel pattern of accumulation of 24:6n-3 and DHA in response to increasing concentrations of ALA suggests that the competition between 24:5n-3 and ALA for D6D may contribute to the limited accumulation of DHA in cell membranes.

alpha-Linolenic acid, Delta6-desaturase gene polymorphism, and the risk of nonfatal myocardial infarction

BACKGROUND

Delta(6)-Desaturase (FADS2) is the rate-limiting step in the polyunsaturated fatty acid (PUFA) biosynthetic pathway.

OBJECTIVE

The aim was to test whether the common deletion [T/-] in the promoter of FADS2 affects the PUFA biosynthetic pathway and consequently modifies the effect of alpha-linolenic acid (ALA) on myocardial infarction (MI).

DESIGN

Case subjects (n =1694) with a first nonfatal acute MI were matched by age, sex, and area of residence to 1694 population-based control subjects in Costa Rica. PUFAs were quantified by gas-liquid chromatography from plasma and adipose tissue samples. Least-squares means from generalized linear models and odds ratios (ORs) and 95% CIs from multiple conditional logistic regression models were estimated.

RESULTS

The prevalence of the variant T/- allele was 48%. Eicosapentaenoic acid, gamma-linolenic acid, and arachidonic acid decreased in adipose tissue and plasma with increasing number of copies of the variant allele with a monotonic trend (P < 0.05 for all). Fasting plasma triacylglycerols by genotype were 2.08 mmol/L for TT, 2.16 mmol/L for T-, and 2.26 mmol/L for - - [ie, homozygous for the variant (deletion) allele] (P = 0.03). The FADS2 deletion was not associated with MI and did not significantly modify the association between adipose tissue ALA and the risk of MI.

CONCLUSIONS

The FADS2 deletion may prevent the conversion of ALA into very-long-chain PUFAs. However, this metabolic effect is not translated into an attenuated risk between ALA and MI among carriers of the variant. It is possible that, at current intakes of ALA, any potential defect in the transcription of the gene is masked by the availability of substrate. Further research in populations deficient in ALA intake is warranted.

Differential modulation by simvastatin of the metabolic pathways in the n-9, n-6 and n-3 fatty acid series, in human monocytic and hepatocytic cell lines

Statins affect the production of long chain polyunsaturated fatty acids (PUFA), both in vitro and in vivo. Various studies have shown the effects of statins on the pattern of n-6 fatty acids (FA), but limited attention has been paid to the n-3 FA. We investigated, in THP-1 and in HepG2 cells, the effects of simvastatin on the conversion of the 18C FA precursors in the n-3 and n-6 series, [1-(14)C] alpha-linolenic acid (alpha-LNA) and [1-(14)C] linoleic acid (LA) respectively, and on the metabolism of [1-(14)C] stearic acid (SA). THP-1 cells, as in the case of LA, actively converted alpha-LNA to its products, and after simvastatin treatment, the total conversion was significantly increased (from 57.2+/-7.2 to 74.3+/-8.5%, p<0.05). HepG2 cells also converted LA and alpha-LNA, but simvastatin increased significantly only the conversion of LA (9.5+/-1.9% versus 23.8+/-5.1%, p<0.02). SA conversion was similar in untreated cells (about 50%), while statin increased the production of oleic acid in HepG2, but in THP-1 cells there was a decrease. In conclusion, LA, alpha-LNA and SA are differentially metabolized in THP-1 and in HepG2 cells and their increased conversion by simvastatin is lower in HepG2 than in THP-1. These differences may reflect the distinct features of the two cell lines: monocytes, precursors of phagocytic cells, versus hepatocytes with mainly metabolic functions. Substantial differences concern also cellular FA pools: structural in THP-1 cells, and also depot, resulting in sequestering of the substrates, in HepG2. The greater n-3 FA metabolism in THP-1 cells may have favourable functional effects.

Dose effect of α-linolenic acid on PUFA conversion, bioavailability, and storage in the hamster

If an increased consumption of alpha-linolenic acid (ALA) is to be promoted in parallel with that of n-3 long-chain-rich food, it is necessary to consider to what extent dietary ALA can be absorbed, transported, stored, and converted into long-chain derivatives. We investigated these processes in male hamsters, over a broad range of supply as linseed oil (0.37, 3.5, 6.9, and 14.6% energy). Linoleic acid (LA) was kept constant (8.5% energy), and the LA/ALA ratio was varied from 22.5 to 0.6. The apparent absorption of individual FA was very high (>96%), and that of ALA remained almost maximum even at the largest supply (99.5%). The capacity for ALA transport and storage had no limitation over the chosen range of dietary intake. Indeed, ALA intake was significantly correlated with ALA level not only in cholesteryl esters (from 0.3 to 9.7% of total FA) but also in plasma phospholipids and red blood cells (RBC), which makes blood components extremely reliable as biomarkers of ALA consumption. Similarly, ALA storage in adipose tissue increased from 0.85 to 14% of total FA and was highly correlated with ALA intake. As for bioconversion, dietary ALA failed to increase 22:6n-3, decreased 20:4n-6, and efficiently increased 20:5n-3 (EPA) in RBC and cardiomyocytes. EPA accumulation did not tend to plateau, in accordance with identical activities of delta5- and delta6-desaturases in all groups. Dietary supply of ALA was therefore a very efficient means of improving the 20:4n-6 to 20:5n-3 balance.

Effect of the delta6-desaturase inhibitor SC-26196 on PUFA metabolism in human cells

The objective of this study was to determine the effect of 2,2-diphenyl-5-(4-[[(1 E)-pyridin-3-yl-methylidene]amino]piperazin-1-yl)pentanenitrile (SC-26196), a delta6-desaturase inhibitor, on PUFA metabolism in human cells. SC-26196 inhibited the desaturation of 2 microM [1-14C] 18:2n-6 by 87-95% in cultured human skin fibroblasts, coronary artery smooth muscle cells, and astrocytes. By contrast, SC-26196 did not affect the conversion of [1-14C]20:3n-6 to 20:4 in the fibroblasts, demonstrating that it is selective for delta6-desaturase. The IC50 values for inhibition of the desaturation of 2 microM [1-14C] 18:3n-3 and [3-14C]24:5n-3 in the fibroblasts, 0.2-0.4 microM, were similar to those for the inhibition of [1-14C 18:2n-6 desaturation, and the rates of recovery of [1-14C]18:2n-6 and [3-14C]24:5n-3 desaturation after removal of SC-26196 from the culture medium also were similar. SC-26196 reduced the conversion of [3-14C]22:5n-3 and [3-14C]24:5n-3 to DHA by 75 and 84%, respectively, but it had no effect on the retroconversion of [3-14C]24:6n-3 to DHA. These results demonstrate that SC-26196 effectively inhibits the desaturation of 18- and 24-carbon PUFA and, therefore, decreases the synthesis of arachidonic acid, EPA, and DHA in human cells. Furthermore, they provide additional evidence that the conversion of 22:5n-3 to DHA involves delta6-desaturation.

Hormonal modulation of delta6 and delta5 desaturases: case of diabetes

Animal biosynthesis of high polyunsaturated fatty acids from linoleic, alpha-linolenic and oleic acids is mainly modulated by the delta6 and delta5 desaturases through dietary and hormonal stimulated mechanisms. From hormones, only insulin activates both enzymes. In experimental diabetes mellitus type-1, the depressed delta6 desaturase is restored by insulin stimulation of the gene expression of its mRNA. However, cAMP or cycloheximide injection prevents this effect. The depression of delta6 and delta5 desaturases in diabetes is rapidly correlated by lower contents of arachidonic acid and higher contents of linoleic in almost all the tissues except brain. However, docosahexaenoic n-3 acid enhancement, mainly in liver phospholipids, is not explained yet. In experimental non-insulin dependent diabetes, the effect upon the delta6 and delta5 desaturases is not clear. From all other hormones glucagon, adrenaline, glucocorticoids, mineralocorticoids, oestriol, oestradiol, testosterone and ACTH depress both desaturases, and a few hormones: progesterone, cortexolone and pregnanediol are inactive.

Optimizing DHA levels in piglets by lowering the linoleic acid to alpha-linolenic acid ratio

We examined the effect of altering the linoleic acid (LA, 18:2n-6) to alpha-linolenic acid (ALA, 18:3n-3) ratio in the dietary fats of 3 day old piglets fed formula for 3 weeks. The LA-ALA ratios of the experimental formulas were 0.5:1, 1:1, 2:1, 4:1, and 10:1. The level of LA was held constant at 13% of total fats while the level of ALA varied from 1.3% (10:1 group) to 26.8% (0.5:1 group). Incorporation of the n-3 long chain PUFA EPA and 22:5n-3 into erythrocytes, plasma, liver, and brain tissues was linearly related to dietary ALA. Conversely, incorporation of DHA into all tissues was related to dietary ALA in a curvilinear manner, with the maximum incorporation of DHA appearing to be between the LA-ALA ratios of 4:1 and 2:1. Feeding LA-ALA ratios of 10:1 and 0.5:1 resulted in lower and similar proportions of DHA in tissues despite the very different levels of dietary ALA (1.3 vs. 26.8% of total fats, respectively). These results are relevant to term infant studies in that they confirm our earlier findings of the positive effect on DHA status by lowering the LA-ALA ratio from 10:1 to 3:1 or 4:1, and they predict that ratios of LA-ALA below 4:1 would have little further beneficial effect on DHA status.

The same rat Delta6-desaturase not only acts on 18- but also on 24-carbon fatty acids in very-long-chain polyunsaturated fatty acid biosynthesis

The recently cloned Delta6-desaturase is known to catalyse the first step in very-long-chain polyunsaturated fatty acid biosynthesis, i.e. the desaturation of linoleic and alpha-linolenic acids. The hypothesis that this enzyme could also catalyse the terminal desaturation step, i.e. the desaturation of 24-carbon highly unsaturated fatty acids, has never been elucidated. To test this hypothesis, the activity of rat Delta6-desaturase expressed in COS-7 cells was investigated. Recombinant Delta6-desaturase expression was analysed by Western blot, revealing a single band at 45 kDa. The putative involvement of this enzyme in the Delta6-desaturation of C(24:5) n-3 to C(24:6) n-3 was measured by incubating transfected cells with C(22:5) n-3. Whereas both transfected and non-transfected COS-7 cells were able to synthesize C(24:5) n-3 by elongation of C(22:5) n-3, only cells expressing Delta6-desaturase were also able to produce C(24:6) n-3. In addition, Delta6-desaturation of [1-(14)C]C(24:5) n-3 was assayed in vitro in homogenates from COS-7 cells expressing Delta6-desaturase or not, showing that Delta6-desaturase catalyses the conversion of C(24:5) n-3 to C(24:6) n-3. Evidence is therefore presented that the same rat Delta6-desaturase catalyses not only the conversion of C(18:3) n-3 to C(18:4) n-3, but also the conversion of C(24:5) n-3 to C(24:6) n-3. A similar mechanism in the n-6 series is strongly suggested.

Long-term melatonin administration reduces hyperinsulinemia and improves the altered fatty-acid compositions in type 2 diabetic rats via the restoration of Delta-5 desaturase activity

The objective of this study was to investigate the effect of long-term melatonin administration on plasma levels of triglycerides, insulin and leptin, and on the fatty-acid metabolism of plasma and hepatic lipids in type 2 diabetic rats. Otsuka Long-Evans Tokushima Fatty (OLETF) rats, an animal model of type 2 diabetes mellitus, were divided into two groups: one untreated (n=6), and one implanted with time-releasing melatonin pellets (1.1 mg/day for 30 wk) under the abdominal skin (n=6). Age-matched Long-Evans Tokushima Otsuka (LETO) rats (n=6) were used as healthy controls. The untreated diabetic rats had the increased plasma levels of triglycerides, cholesterol, insulin and leptin at 35 wk, as compared with the healthy control rats (n=6). The diabetic rats also had augmented ratios of 20:3n-6/20:4n-6 fatty acids, owing to diminished activity of Delta-5 desaturase, an insulin-permissive enzyme, in the liver. Melatonin administration to OLETF rats reduced the hypertriglyceridemia (-39%, P < 0.05), hyperinsulinemia (-33%, P < 0.01) and hyperleptinemia (-43%, P < 0.01), and restored hepatic Delta-5 desaturase activity (148%, P < 0.005). This resulted in a return to normal ratios of 20:3n-6/20:4n-6 fatty acids in plasma and hepatic lipids. There was a significant correlation (r=0.64, P < 0.005) between plasma levels of insulin and the ratios of 20:3n-6/20:4n-6 in plasma phospholipids of all rats in the three groups. Thus, subcutaneous implantation of a melatonin-releasing pellet thus resulted in improved lipid metabolism in diabetic rats, probably through restored insulin resistance.

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

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

Hepatic microsomal and peroxisomal docosahexaenoate biosynthesis during piglet development

The roles of peroxisomes and microsomes on the biosynthetic pathway for docosahexaenoic acid (DHA) from alpha-linolenic acid (ALA) were investigated. Microsomes and peroxisomes were prepared from livers of fetal and neonatal piglets by a combination of differential and gradient layer centrifugation. Microsomes, peroxisomes, and combined cell fractions were incubated with [13C-U]18:3n-3. The [M] and [M + 18] isotopomers of the fatty acids in the long-chain polyunsaturated fatty acid (LCPUFA) n-3 pathway were detected by gas chromatography-mass spectrometry. The quantity of each fatty acid was determined by gas chromatography, and synthesis of each fatty acid was calculated for a 30-min period. Synthesis of DHA was not detected in combined fetal liver fractions. The data suggest that DHA in the fetus is probably supplied from maternal sources through the placenta. In either singly incubated microsomal or peroxisomal preparations from neonatal livers, no DHA synthesis was detected. After combination of the microsomal and peroxisomal fractions, DHA synthesis was evident and increased rapidly between birth and 2 wk of age. This is the first demonstration of the entire biosynthetic LCPUFA n-3 pathway in subcellular organelles starting from isotopically labeled ALA to the final product, DHA, with all the intermediates present and isotopically labeled. The primary importance of the data is that it unequivocally demonstrates that peroxisomes are required for biosynthesis of DHA from ALA.

Effect of experimental diabetes on the fatty acid composition, molecular species of phosphatidyl-choline and physical properties of hepatic microsomal membranes

Streptozotocin diabetes depresses delta 9, delta 6 and delta 5 fatty acid desaturases, decreasing arachidonic acid and increasing linoleic acid, but also unexpectedly increasing docosahexaenoic acid in the different phospholipids of liver microsomal lipids. 18:0/20:4n-6, 16:0/20:4n-6 and 16:0/18:2n-6 are the predominant phosphatidyl choline (PC) molecular species in control rats, determining mainly PC contribution to the dynamic and biochemical properties of this bilayer. Diabetes decreases 20:4n-6 containing species and increases 18:2n-6 and 22:6n-3 containing species, maintaining the bulk dynamic properties in the hydrophobic interior of the bilayer, but changing its biochemical properties. The different dynamic parameters were measured by fluorometry using the probes 1,6-diphenyl-1,3,5-hexatriene (DPH), (4-trimethylammonium phenyl) 6-phenyl-1,3,5 (TMA-DPH) and 6-lauroyl-2,4-dimethyl aminonaphtalene (Laurdan). In the surrounding of the hydrophobic/hydrophilic interphase lipid molecules were less ordered and tightly packed in the diabetic samples, allowing a higher mobility of incorporated water molecules. The fact that diabetes decreases highly polyunsaturated acid of n-6 family, but increases docosahexaenoic acid, indicates the necessity of re-evaluating its effect in human physiology.

Regulation of the biosynthesis of 22:5n-6 and 22:6n-3: a complex intracellular process

Both 22:4n-6 and 22:5n-3 are synthesized from n-6 and n-3 fatty acid precursors in the endoplasmic reticulum. The synthesis of both 22:5n-6 and 22:6n-3 requires that 22:4n-6 and 22:5n-3 are metabolized, respectively, to 24:5n-6 and 24:6n-3 in the endoplasmic reticulum. These two 24-carbon acids must then move to peroxisomes for partial degradation followed by the movement of 22:5n-6 and 22:6n-3 back to the endoplasmic reticulum for use as substrates in membrane lipid biosynthesis. Clearly an understanding of the control of intracellular fatty acid movement as well as of the reactions carried out by microsomes, peroxisomes, and mitochondria are all required in order to understand not only what regulates the biosynthesis of 22:5n-6 and 22:6n-3 but also why most tissue lipids selectively accumulate 22:6n-3.

The Delta8-desaturase of Euglena gracilis: an alternate pathway for synthesis of 20-carbon polyunsaturated fatty acids

Desaturation of fatty acids is an important metabolic process. In mammals, 20-carbon and longer polyunsaturated fatty acids are not only incorporated into cellular membranes in a tissue-specific manner, but also serve as the precursors to synthesis of eicosanoid metabolic regulators. The processes of desaturation and elongation in human liver are well characterized, but an alternate Delta8 desaturation pathway that may be important in certain tissues or in cancer cells is less well examined. The Delta8-desaturase enzyme introduces a double bond at the 8-position in 20-carbon fatty acids that have an existing Delta11 unsaturation. We have isolated the first fatty acid Delta8-desaturase, from the protist Euglena gracilis, in order to explore this alternate pathway. A full-length cDNA was obtained after reverse transcription of mRNA purified from heterotrophically grown Euglena, followed by PCR amplification with primers degenerate to conserved histidine-rich regions of microsomal desaturases. The protein predicted from the cDNA sequence is highly homologous to Delta5 and Delta6 desaturases of Caenhorabditis elegans. When the cDNA was expressed in Saccharomyces cerevisiae, the yeast cultures readily desaturated appropriate 20-carbon fatty acids by inserting an additional double bond at the Delta8-position. The enzyme demonstrated a preference for substrates of metabolic significance, 20:3 Delta11,14,17 and 20:2 Delta11,14. Cloning of a Delta8 fatty acid desaturase offers the opportunity to examine an alternate pathway of long chain fatty acid biosynthesis.

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

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

An update on the pathways of polyunsaturated fatty acid metabolism

The biosynthesis of 4, 7, 10, 13, 16-22:5 and 4, 7, 10, 13, 16, 19-22:6 from dietary linoleate and linolenate, respectively, does not totally take place in the endoplasmic reticulum but does require the participation of enzymes in the endoplasmic reticulum and peroxisomes. The absence of an endoplasmic reticulum-associated acyl-CoA-dependent delta 4 desaturase also requires the controlled movement of 22- and 24-carbon polyunsaturated fatty acids between the endoplasmic reticulum and peroxisomes.

Molecular cloning and functional characterization of rat delta-6 fatty acid desaturase

Mammalian cDNA fragments putatively encoding amino acid sequences characteristic of the fatty acid desaturase were obtained using expressed sequence tag (EST) sequence informations. These fragments were subsequently used to screen a rat liver cDNA library, yielding a 1573-bp clone. Expression of DNA fragment containing either of two possible open reading frames (nucleotide numbers 97-1431 and 148-1431) of the isolated clone in yeast led to the accumulation of gamma-linolenic acid in the presence of exogenous linoleic acid. In this system, the addition of alpha-linolenic acid also resulted in the accumulation of its Delta-6 desaturated product whereas dihomo-gamma-linolenic acid failed to be a substrate. These results indicate that the protein encoded by the rat cDNA is Delta-6 fatty acid desaturase, and the first 17 amino acids corresponding to the coding region 97-147 of the clone are not required to function in yeast.

Neonatal polyunsaturated fatty acid metabolism

The importance of n-6 and n-3 polyunsaturated fatty acids (PUFA) in neonatal development, particularly with respect to the developing brain and retina, is well known. This review combines recent information from basic science and clinical studies to highlight recent advances in knowledge on PUFA metabolism and areas where research is still needed on infant n-6 and n-3 fatty acid requirements. Animal, cell culture, and infant studies are consistent in demonstrating that synthesis of 22:6n-3 involves C24 PUFA and that the amounts of 18:2n-6 and 18:3n-3 influence PUFA metabolism. Studies to show that addition of n-6 fatty acids beyond delta6-desaturase alters n-6 fatty acid metabolism with no marked increase in tissue 20:4n-6 illustrate the limitations of analyses of tissue fatty acid compositions as an approach to study the effects of diet on fatty acid metabolism. New information to show highly selective pathways for n-6 and n-3 fatty acid uptake in brain, and efficient pathways for conservation of 22:6n-3 in retina emphasizes the differences in PUFA metabolism among different tissues and the unique features which allow the brain and retina to accumulate and maintain high concentrations of n-3 fatty acids. Further elucidation of the delta6-desaturases involved in 24:5n-6 and 22:6n-3 synthesis; the regulation of fatty acid movement between the endoplasmic reticulum and peroxisomes; partitioning to acylation, desaturation and oxidation; and the effects of dietary and hormonal factors on these pathways is needed for greater understanding of neonatal PUFA metabolism.

Isolation and characterization of a Delta 5-fatty acid desaturase from Caenorhabditis elegans

Arachidonic acid and eicosapentaenoic acid are important precursors for the production of prostaglandins and other hormone-like eicosanoid molecules. These fatty acids are synthesized by animals by elongating and desaturating precursor fatty acids such as linoleic acid (18:2Delta9,12) and alpha-linolenic acid (18:3Delta9, 12,15). We have identified a Delta5 fatty acid desaturase gene (fat-4) from the nematode Caenorhabditis elegans. We have expressed this gene product in Saccharomyces cerevisiae and demonstrate that it readily converts di-homo-gamma-linolenic acid (20:3Delta8,11,14) to arachidonic acid (20:4Delta5,8,11,14). The FAT-4 Delta5-desaturase also acts on a number of other substrates, including fatty acids that do not contain a double bond at the Delta8 position.

Fatty acid desaturase activities and polyunsaturated fatty acid composition in human liver between the seventeenth and thirty-sixth gestational weeks

OBJECTIVE

The aim of the study was to characterize n-3 and n-6 fatty acid delta5- and delta6-desaturase activities and their time course variations in human fetal liver between the 17th and 36th gestational week.

STUDY DESIGN

Twenty-one biologic samples were obtained after legally approved medical abortion, according to French law. The desaturase activities were measured in the 21 liver samples by a radiochemical method by means of reverse-phase high-performance liquid chromatography. The fatty acid composition (percentage by weight) of liver phospholipids was assessed in 16 samples by gas-liquid chromatographic analysis.

RESULTS

Both delta5- and delta6-desaturase activities were significantly expressed between the 17th and 36th gestational weeks. During the second trimester n-6 fatty acid delta5- and delta6-desaturase activities showed opposite patterns of variation; both then remained stable between the 25th and 36th weeks. Delta6-desaturation was higher in n-3 than n-6 fatty acids and peaked at the 18th gestational week. The percentages of linoleic and docosahexaenoic acids in liver microsomes were positively correlated with the gestation age (P < .01), whereas arachidonic acid remained stable.

CONCLUSION

Significant n-3 and n-6 delta5- and delta6-desaturase activities are expressed in human fetal liver as early as the 17th gestational week and are stable throughout the third trimester. Their theoretic capacity evaluated from in vitro measurements appears lower than polyunsaturated fatty acid requirements and is not directly related to liver microsomal membrane fatty acid composition.

Effects of docosahexaenoic (22:6n-3), tetracosapentaenoic (24:5n-3) and tetracosahexaenoic (24:6n-3) acids on the desaturation and elongation of n-3 polyunsaturated fatty acids in trout liver microsomes

The effects of long chain n-3 polyunsaturated fatty acids (PUFA) on the desaturation and elongation systems involved in the conversion of 18:3n-3 to 24:6n-3 were investigated. Microsomes were prepared from the livers of rainbow trout and incubated with 14C-labelled 18:3n-3 and cofactors required for elongation and/or desaturation in the presence of 22:6n-3, 24:5n-3 or 24:6n-3. The formation of 24:6n-3 was significantly inhibited in the presence of 50 microM 22:6n-3, 24:5n-3 or 24:6n-3, whereas the amount of radiolabelled 20:5n-3 formed was inhibited by only 24:5n-3 or 24:6n-3 at the same concentration. When malonyl-CoA was omitted from the incubation system to allow the measurement of desaturation in the absence of elongation, the Delta6 desaturation of 14C-18:3n-3 to 14C-18:4n-3 was inhibited by approximately 25% in the presence of 24:5n-3 or 24:6n-3 but was not affected by 22:6n-3. The Delta5 desaturation of 14C-20:4n-3 was not affected by the presence of any of the long chain PUFA and no significant effect of 18:3n-3, 22:6n-3 or 24:6n-3 on the Delta6 desaturation of 24:5n-3 to 24:6n-3 was observed. To permit the measurement of individual elongation reactions, KCN was included in the incubation medium to inhibit desaturation and 14C-labelled 18:3n-3, 18:4n-3, 20:4n-3, 20:5n-3 and 22:5n-3 were examined as substrates. 18:4n-3 and 22:5n-3 were more extensively used for elongation than 18:3n-3, 20:4n-3 and 20:5n-3. The presence of 22:6n-3, 24:5n-3 or 24:6n-3 in the incubation system had no effect on any of the specific elongations of any of the substrates examined. It is concluded that, in the conversion of 18:3n-3 to 24:6n-3 by trout liver microsomes, the Delta6 desaturation of 18:3n-3 may be subjected to direct feedback inhibition and that 24:5n-3 may be preferred over 18:3n-3 as a substrate for Delta6 desaturation.

Adrenic acid delta4 desaturation and fatty acid composition in the liver of marine-oil fed streptozotocin diabetic rats

The aim of the present study was to assess the effect of streptozotocin diabetes and insulin treatment on adrenic acid delta4 desaturation and fatty acid composition of liver microsomes in Wistar rats fed a fat free semi-synthetic basal diet supplemented with 10% EPA-rich marine oil. Results showed that, in liver microsomes of hyperglycemic rats, the 22:6n-3/22:5n-3 ratio in total lipids was elevated and desaturation of adrenic acid to n-6 docosapentaenoic acid was enhanced. Insulin treatment with 2.0 I.U./100 g body weight-1 twice a day for 3 days resulted in hypoglycemia and suppressed both the increased delta4 n-6 desaturation and 22:6n-3/22:5n-3 ratio. It is concluded that the delta4 desaturation enzyme system, which is activated by experimental diabetes, is regulated by mechanisms different from those regulating delta6 and delta5 desaturations.

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

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

Studies to determine if rat liver contains multiple chain elongating enzymes

According to the revised pathways of polyunsaturated fatty acid biosynthesis three, rather than two acids, must be chain elongated for converting linoleate and linolenate, respectively, to 22:5(n-6) and 22:6(n-3) (Sprecher et al. (1995) J. Lipid Res. 36, 2471-2477). The present study was undertaken to determine whether microsomes contained chain-length specific chain-elongating enzymes and, secondly, whether reaction rates for any of these reactions might be rate limiting in the synthesis of 24:5(n-6) and 24:6(n-3), which are the immediate precursors of 22:5(n-6) and 22:6(n-3). Rates of total chain elongation products produced from both 18:4(n-3) and 20:5(n-3) were about 3 nmol/min/mg of microsomal protein while only about 0.5 nmol/min/mg of 24:5(n-3) plus 24:6(n-3) was synthesized from 22:5(n-3). The rate of 24:5(n-3) synthesis was similar to that for the desaturation of 24:5(n-3), at position 6, to yield 24:6(n-3) (Geiger et al. (1993) Biochim. Biophys. Acta 1170, 137-142). The results suggest that the last chain elongation step in unsaturated fatty acid biosynthesis may be equally regulatory in governing the synthesis of fatty acids as is desaturation at position 6. When an enzyme saturating level of [1-(14)C]18:4(n-3) was incubated with increasing amounts of 18:3(n-6) there was a decrease in the production [1-(14)C]20:4(n-3). In a similar way it was observed that 18:4(n-3) inhibited the chain elongation of [1-(14)C]18:3(n-6). Identical cross-over inhibitory studies, using 20:4(n-6) and 20:5(n-3), as well as 22:4(n-6) and 22:5(n-3) also suggested that microsomes contain chain length specific chain-elongating enzymes. This conclusion was further supported by the finding that neither 20:5(n-3) or 22:5(n-3) inhibited the chain elongation of [1-(14)C]18:4(n-3). However, 18:4(n-3), and to a lesser degree, 22:5(n-3) did inhibit the chain elongation of [1-(14)C]20:5(n-3). This latter finding suggests that 18:4(n-3) and 20:5(n-3) might interact with the enzyme that chain elongates 20:5(n-3) to depress its ability to synthesize 22:5(n-3). Our results are most consistent with the presence of multiple chain-elongating enzymes, but a more definitive answer requires the purification of these membrane-bound proteins. In addition our results suggest that the channeling of acids between enzymes in the endoplasmic reticulum may play an important role in regulating the biosynthesis of unsaturated fatty acids.

On the molecular etiology of decreased arachidonic (20:4n-6), docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) acids in Zellweger syndrome and other peroxisomal disorders

Alterations in the metabolism of arachidonic (20:4n-6), docosapentaenoic (22:5n-6), and docosahexaenoic (22:6n-3) acids and other polyunsaturated fatty acids in Zellweger syndrome and other peroxisomal disorders are reviewed. Previous proposals that peroxisomes are necessary for the synthesis of 22:6n-3 and 22:5n-6 are critically examined. The data suggest that 22:6n-3 is biosynthesized in mitochondria via a channelled carnitine-dependent pathway involving an n-3-specific delta-4 desaturase, while 20:4n-6, 20:5n-3 and 22:5n-6 are synthesized by both mitochondrial and microsomal systems; these pathways are postulated to be interregulated as compensatory-redundant systems. Present evidence suggests that 22:6n-3-containing phospholipids may be required for the biochemical events involved in successful neuronal migration and developmental morphogenesis, and as structural cofactors for the functional assembly and integration of a variety of membrane enzymes, receptors, and other proteins in peroxisomes and other subcellular organelles. A defect in the mitochondrial desaturation pathway is proposed to be a primary etiologic factor in the clinicopathology of Zellweger syndrome and other related disorders. Several implications of this proposal are examined relating to effects of pharmacological agents which appear to inhibit steps in this pathway, such as some hypolipidemics (fibrates), neuroleptics (phenothiazines and phenytoin) and prenatal alcohol exposure.

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

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

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

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

New biological and clinical roles for the n-6 and n-3 fatty acids

Four new findings of the biochemistry and biology of the essential n-6 and n-3 fatty acids have recently been demonstrated. These findings will augment current knowledge as to the role of the essential fatty acids in human health.