Delta 5-desaturation of dihomogammalinolenic acid (20:3(n-6)) into arachidonic acid (20:4(n-6)) by rat liver microsomes and incorporation of fatty acids in microsome phospholipids

Gamma-linolenic acid, Dihommo-gamma linolenic, Eicosanoids and Inflammatory Processes

Gamma-linolenic acid (GLA, 18:3n-6) is an omega-6 (n-6), 18 carbon (18C-) polyunsaturated fatty acid (PUFA) found in human milk and several botanical seed oils and is typically consumed as part of a dietary supplement. While there have been numerous in vitro and in vivo animal models which illustrate that GLA-supplemented diets attenuate inflammatory responses, clinical studies utilizing GLA or GLA in combination with omega-3 (n-3) PUFAs have been much less conclusive. A central premise of this review is that there are critical metabolic and genetic factors that affect the conversion of GLA to dihommo-gamma linolenic acid (DGLA, 20:3n-6) and arachidonic acid (AA, 20:4n-6), which consequently affects the balance of DGLA- and AA- derived metabolites. As a result, these factors impact the clinical effectiveness of GLA or GLA/(n-3) PUFA supplementations in treating inflammatory conditions. Specifically, these factors include: 1) the capacity for different human cells and tissues to convert GLA to DGLA and AA and to metabolize DGLA and AA to bioactive metabolites; 2) the opposing effects of DGLA and AA metabolites on inflammatory processes and diseases; and 3) the impact of genetic variations within the fatty acid desaturase (FADS) gene cluster, in particular, on AA/DGLA ratios and bioactive metabolites. We postulate that these factors influence the heterogeneity of results observed in GLA supplement-based clinical trials and suggest that "one-size fits all" approaches utilizing PUFA-based supplements may no longer be appropriate for the prevention and treatment of complex human diseases.

A mathematical model of fatty acid metabolism and VLDL assembly in human liver

The lipid composition of very-low-density lipoprotein (VLDL) in plasma is crucial for human health. A pre-requisite for the alteration of VLDL composition is a co-ordinated understanding of the complex interactions in VLDL assembly. In order to determine the potential effects of changes in substrate availability on VLDL lipid composition, we constructed, parameterized and evaluated a mechanistic mathematical model of the biosynthesis of triglycerides, phospholipids, and cholesterol esters and the assembly of VLDL in human hepatocytes. Using published data on human liver metabolism, the model was also used to provide insight into the complex process of lipid metabolism and to estimate the affinities of different liver enzymes for different fatty acids (FA). For example, we found that Delta6-desaturase is 19 times more selective for C18:3n-3 than C18:2n-6, stearoyl-CoA-desaturase is 2.7 times more selective for C18:0 than C16:0, Delta5-desaturase desaturates C20:4n-3 preferentially over C20:3n-6 and FA elongase preferentially elongates C18:3n-6. The model was also used to predict the plasma free fatty acid (FFA) composition required to generate a prescribed change in plasma lipoprotein FA composition. Furthermore, the model was tested against a published human feeding trial that investigated the effect of changes in dietary FA composition on human plasma lipid FA composition. The model is a useful tool for predicting the effect of changes in plasma FFA composition on plasma lipoprotein lipid FA composition.

Essential fatty acids and the brain: possible health implications

Linoleic and alpha-linolenic acid are essential for normal cellular function, and act as precursors for the synthesis of longer chained polyunsaturated fatty acids (PUFAs) such as arachidonic (AA), eicosapentaenoic (EPA) and docosahexaenoic acids (DHA), which have been shown to partake in numerous cellular functions affecting membrane fluidity, membrane enzyme activities and eicosanoid synthesis. The brain is particularly rich in PUFAs such as DHA, and changes in tissue membrane composition of these PUFAs reflect that of the dietary source. The decline in structural and functional integrity of this tissue appears to correlate with loss in membrane DHA concentrations. Arachidonic acid, also predominant in this tissue, is a major precursor for the synthesis of eicosanoids, that serve as intracellular or extracellular signals. With aging comes a likely increase in reactive oxygen species and hence a concomitant decline in membrane PUFA concentrations, and with it, cognitive impairment. Neurodegenerative disorders such as Parkinson's and Alzheimer's disease also appear to exhibit membrane loss of PUFAs. Thus it may be that an optimal diet with a balance of n-6 and n-3 fatty acids may help to delay their onset or reduce the insult to brain functions which these diseases elicit.

Differential effect of N-ethyl maleimide on delta6-desaturase activity in human fetal liver toward fatty acids of the n-6 and n-3 series

The effect of N-ethyl-maleimide (NEM) on delta5- and delta6-desaturase activities and the incorporation of substrates and products into different microsomal lipid classes and phospholipid (PL) subclasses were studied in human fetal liver microsomes, obtained after legally approved therapeutic abortion. Desaturase activities were measured by a radiochemical method using reversed-phase high-performance liquid chromatography (HPLC). After nonphospholipid (NPL) and PL separation on silica cartridges, the radioactivity in different lipids of the NPL group was assessed by two-dimensional thin-layer chromatography, and their fatty acid (FA) composition by gas-liquid chromatography. The PL subclasses were separated, and the distribution of radioactivity between products and substrates was determined in PL subclasses. NEM inhibited the delta5- and delta6-desaturase activities in the n-6 series of FA but not the delta6-desaturase activity in the n-3 series, which suggests the existence of two distinct delta6-desaturases, one for the n-6 series and another for the n-3 series. Whether NEM was present or absent, most of the radioactivity was recovered in the free FA form (about 80%). The desaturation products, obtained in the presence or absence of NEM, were preferentially incorporated into PL, suggesting a channeling of the newly synthesized FA toward microsomal PL. The comparison of the distribution of substrates and products incorporated into the different PL classes showed that most of the labeled FA were incorporated into phosphatidylcholine and to a lesser degree into phosphatidylethanolamine.

In vitro desaturation or elongation of monotrans isomers of linoleic acid by rat liver microsomes

Several nutritional studies have shown the in vivo conversion of the 9c, 12t-18:2 and 9t, 12c-18:2 into long chain polyunsaturated fatty acids (PUFA) containing 20 carbons (geometrical isomers of eicosadienoic and eicosatetraenoic acids). In the present work, some in vitro studies were carried out in order to have precise information on the conversion of these two isomers. In a first set of experiments, studies were focused on the in vitro delta6 desaturation, the first regulatory step of the biosynthesis of n-6 long chain PUFA, from 9c, 12c-18:2. Rat liver microsomes were prepared and incubated under desaturation conditions with [1-14C]-9c, 12c-18:2 in presence of unlabelled 9c, 12t-, 9t, 12c- or 9t, 12t-18:2. The data show that each trans isomer induced a decrease of the delta6 desaturation of the [1-14C]-9c, 12c-18:2, but the 9c, 12t-18:2 was the most potent inhibitor (up to 63%). Rat liver microsomes were also incubated with [1-14C]-9c, 12c-18:2, [1-14C]-9c, 12t-18:2 or [1-14C]-9t, 12c-18:2 under desaturation conditions. The results indicated that 18:2 delta9c, 12t is a much better substrate for desaturase than 9t, 12c-18:2. Moreover, the conversion levels of [1-14C]-9c, 12t-18:2 was similar to what was observed for its all cis homologue, at low substrate concentration only. In a second set of experiments, in vitro elongation studies of each mono-trans 18:2 isomer and 9c, 12c-18:2 were carried out. For that purpose, rat liver microsomes were incubated with [1-14C]-9c, 12c-18:2, [1-14C]-9c, 12t-18:2 or [1-14C]-9t, 12c-18:2 underelongation conditions. The data show that [1-14C]-9t, 12c-18:2 is betterelongated than 9c, 12c-18:2 while the amount of product formed from [1-14C]-9c, 12t-18:2 was lower than was produced from the 9c, 12c-18:2. Thus, the desaturation enzymes presented a higher affinity for the 9c, 12t-18:2 whereas the elongation enzyme presented a higher affinity for the 9t, 12c-18:2.

Incorporation of delta 5 desaturase substrate (dihomogammalinolenic acid, 20:3 n-6) and product (arachidonic acid 20:4 n-6) into rat liver cell nuclei

The incorporation of [1-(14)C]20:3 n-6 and its desaturation product, [1-(14)C]20:4 n-6 into nuclear lipids from rat liver cells were investigated during in vitro delta5 desaturation. [1-(14)C]20:3 n-6 activated as 20:3 n-6-CoA by nuclear long chain acyl-CoA synthetase was: (1) incorporated into nuclear lipids mainly esterified to phospholipids and in a lesser proportion, to triglycerides and diglycerides; and (2) desaturated to 20:4 n-6-CoA by the nuclear delta5 desaturase. The amount of [1-(14)C]20:4 n-6 acid synthesized in cell nuclei increased along with time and was stimulated by the cytosol fraction. The major proportion of 20:4 n-6 was found in phospholipids and in a lesser proportion it remained as free fatty acids and was esterified to triglycerides and diglycerides. 20:4 n-6-CoA was incorporated into nuclear lipids and hydrolyzed to free fatty acid. These results indicate that liver cell nuclei possess the necessary enzymes to incorporate the delta5 desaturase substrate (20:3 n-6) as well as the product of desaturation (20:4 n-6) into nuclear TG, DG and PL following an acyl-CoA dependent pathway.

Dietary supplementation with gamma-linolenic acid alters fatty acid content and eicosanoid production in healthy humans

To understand the in vivo metabolism of dietary gamma-linolenic acid (GLA), we supplemented the diets of 29 volunteers with GLA in doses of 1.5-6.0 g/d. Twenty-four subjects ate controlled eucaloric diets consisting of 25% fat; the remaining subjects maintained their typical Western diets. GLA and dihomo-gamma-linolenic acid (DGLA) increased in serum lipids of subjects supplemented with 3.0 and 6.0 g/d; serum arachidonic acid increased in all subjects. GLA supplementation with 3.0 and 6.0 g/d also resulted in an enrichment of DGLA in neutrophil phospholipids but no change in GLA or AA levels. Before supplementation, DGLA was associated primarily with phosphatidylethanolamine (PE) of neutrophil glycerolipids, and DGLA increased significantly in PE and neutral lipids after GLA supplementation. Extending the supplementation to 12 wk did not consistently change the magnitude of increase in either serum or neutrophil lipids in subjects receiving 3.0 g/d. After GLA supplementation, A23187-stimulated neutrophils released significantly more DGLA, but AA release did not change. Neutrophils obtained from subjects after 3 wk of supplementation with 3.0 g/d GLA synthesized less leukotriene B4 (P < 0.05) and platelet-activating factor. Together, these data reveal that DGLA, the elongase product of GLA, but not AA accumulates in neutrophil glycerolipids after GLA supplementation. The increase in DGLA relative to AA within inflammatory cells such as the neutrophil may attenuate the biosynthesis of AA metabolites and may represent a mechanism by which dietary GLA exerts an anti-inflammatory effect.

Delta-6-desaturase and delta-5-desaturase in human Hep G2 cells are both fatty acid interconversion rate limiting and are upregulated under essential fatty acid deficient conditions

Essential fatty acids are interconverted by desaturases and elongases to eicosanoid precursors. In essential fatty acid deficiency (EFAD) an increased hepatic interconversion of linoleic acid (18:2) to arachidonic acid (20:4n-6) has been demonstrated in vivo. We now cultured Hep G2 cells under EFAD conditions. 20:3n-6 appeared in EFAD cells, but also in controls. After adding 14C-18:2 to the medium, interconversion products and their distribution in different lipids were studied by HPLC. When trace amounts 18:2 were incubated, 38% were converted by the EFAD cells after 21 h, vs 6% by controls. 20% was converted to 20:4 by EFAD cells vs 14% by controls. EFAD cells preferentially distributed more 18:2 and conversion products to neutral fats and to phosphatidyl ethanolamine, but less to cardiolipin than controls did, when incubated with trace amount 18:2, but not with 1 mM 18:2. A relative accumulation of radioactivity in 20:3 was observed. In conclusion; in EFAD Hep G2 cells delta-6- and delta-5-desaturase both were found to be upregulated and eicosanoid precursors were distributed more into phosphatidyl ethanolamine. Delta-5-desaturase had a rate limiting property as well as delta-6-desaturase.

Effect of low levels of dietary fish oil on fatty acid desaturation and tissue fatty acids in obese and lean rats

The effect of very low levels of dietary long-chain n-3 fatty acids on delta 6 desaturation of linoleic acid (18:2n-6) and alpha-linolenic acid (18:3n-3), and on delta 5 desaturation of dihomo-gamma-linolenic acid (20:3n-6), in liver microsomes and its influence on tissue fatty acids were examined in obese and lean Zucker rats and in Wistar rats. Animals fed for 12 wk a balanced diet containing ca. 200 mg of long-chain polyunsaturated n-3 fatty acids per 100 g of diet were compared to those fed the same amount of alpha-linolenic acid. Low amounts of long-chain n-3 fatty acids greatly inhibited delta 6 desaturation of 18:2n-6 and delta 5 desaturation of 20:3n-6, while delta 6 desaturation of 18:3n-3 was not inhibited in Zucker rats and was even stimulated in Wistar rats. Inhibition of the biosynthesis of long-chain n-6 fatty acids was reflected in a decrease in arachidonic acid (20:4n-6) content of serum lipids when fasting, and also in the phospholipid fatty acids of liver microsomes. On the contrary, heart and kidney phospholipids did not develop any decrease in 20:4n-6 during fish oil ingestion. Docosahexaenoic acid (22:6n-3), present in the dietary fish oil, was increased in serum lipids and in liver microsome, heart, and kidney phospholipids.

Incorporation of delta 6- and delta 5-desaturation fatty acids in liver microsomal lipid classes of obese Zucker rats fed n - 6 or n - 3 fatty acids

The aim of this work was to study the effect of dietary n - 6 (as borage oil) and of n - 3 (as fish oil) fatty acids on the incorporation--in liver microsomal lipid classes--of fatty acids involved in delta 6- and delta 5-desaturations in obese Zucker rats compared with their lean littermates and with Wistar control rats. We observed that body and liver weights were decreased when obese Zucker rats were fed the fish oil diet. The major part of the radioactivity was recovered, in the obese Zucker rats, into the neutral lipids and especially into the triacylglycerols, while it was recovered into the phospholipid classes, especially into phosphatidylcholine, in the two other strains. Results show, in all phenotypes, an increased alpha-linolenic acid delta 6-desaturation in PL classes when the rats were fed the fish oil diet. However, a decreased linoleic acid delta 6- and delta 5-desaturation was observed in obese Zucker rats fed the fish oil diet. The fish oil diet favours the n - 3 fatty acid biosynthesis and incorporation into liver microsomal lipid classes to the prejudice of the n - 6 fatty acid series. The fatty acid incorporation is simultaneously regulated by the genetical phenotype and dietary fatty acids.

Delta 5 desaturase activity in rat kidney microsomes

Rat kidney microsomal fraction is able to catalyze the enzymatic desaturation of eicosatrienoic acid (20:3n-6) to arachidonic acid (20:4n-6) by the delta 5 desaturase pathway, in the presence of reduced nicotinamide adenine dinucleotide (NADH), adenosinetriphosphate (ATP) and coenzyme A (CoA). The substrate of the reaction [1-14C]eicosa-8,11,14-trienoic acid (20:3n-6), was separated from the product [1-14C]eicosa-5,8,11,14-tetraenoic acid (20:4n-6) by reverse phase high-pressure liquid chromatography (RP-HPLC). These fatty acids were individually collected by monitoring the eluent at 205 nm and their radioactivity was measured by liquid scintillation counting. The delta 5 desaturase activity in kidney microsomes increased linearly with the substrate concentration up to 20 microM. Enzymatic activity was sensitive to pH with the maximum at 7.0 and was proportional with incubation time up to 10 min. The apparent Km and Vmax of delta 5 desaturase were 56 microM and 60 pmoles.min-1.mg-1 microsomal protein, respectively. Neither the cytosolic renal fraction nor the cytosolic liver fraction enhanced the delta 5 desaturase activity. Contrary to a report but in accordance to others, the present results suggest that rat kidneys can synthesize arachidonic acid at least to satisfy partially their needs for eicosanoid production.

Inhibition of fatty acid delta 6- and delta 5-desaturation by cyclopropene fatty acids in rat liver microsomes

delta 6-Desaturation of linoleic acid and delta 5-desaturation of dihomo-gamma-linolenic acid were measured in liver microsomes from rats fed fresh Baobab seed oil containing cyclopropene fatty acids (malvalic acid and sterculic acid) or heated Baobab seed oil practically devoid of these fatty acids or control oil. The presence of cyclopropene fatty acids in the fresh Baobab oil diet highly depressed both desaturations, but delta 6- more than delta 5-desaturation. The decreased capacity of microsomes to desaturate was reflected in the lower arachidonic acid content in microsomal phospholipids from rats fed this oil. However it was also lower in rats fed heated oil although in vitro delta 6- and delta 5-desaturation were not depressed. When liver microsomes prepared from rats fed the control diet were used for the desaturation assays, the presence of free malvalic or sterculic acid in the medium, also highly depressed delta 6- and delta 5-desaturation. The incorporation of arachidonic acid, the product of delta 5-desaturation, into phospholipids was also highly depressed, while that of the precursor dihomo-gamma-linolenic acid was not. This suggests that cyclopropene fatty acids specifically inhibit incorporation of the delta 5-desaturation product into phospholipids or that they specifically inhibit desaturation of the substrate previously incorporated into a membrane phospholipid.

Incorporation into liver microsomal lipids of linoleic and stearic acids and of their respective products of delta 6 and delta 9 desaturation, gamma-linolenic and oleic acids: effect of age and of blackcurrant seed oil

The incorporation of [1-14C]linoleic and [1-14C]stearic acid and of their delta 6 and delta 9 desaturation products (gamma-linolenic and oleic acids, respectively) into different classes of lipids was studied in liver microsomes of rats in function of the diet (blackcurrant seed oil diet, containing gamma-linolenic acid, versus control diet) and in function of age (3, 6 and 9 months). After delta 6 desaturation, total radioactivity was distributed between phospholipids, especially phosphatidylcholine, and neutral lipids. The desaturation product, gamma-linolenic acid, was totally recovered in the phospholipid fraction. Blackcurrant seed oil, which decreased the rate of delta 6 desaturation in 6- and 9-month-old rats, also decreased the incorporation of radioactivity in total phospholipids, especially in phosphatidylcholine. At 6 months of age, after delta 9 desaturation, the majority of radioactivity was recovered in neutral lipids principally as oleic acid, the desaturation product. The precursor, stearic acid, was highly incorporated into phospholipids, especially in rats on a diet of blackcurrant seed oil.