Metabolism of essential fatty acids by human epidermal enzyme preparations: evidence of chain elongation

The present studies were undertaken in order to delineate the source of human epidermal arachidonic acid, 20:4(n-6). Epidermal microsomal preparations from normal (N) and diseased epidermis (clinically uninvolved (PU) and involved psoriatic (PI) epidermis) were incubated in vitro with either [14C]18:2(n-6), [14C]20:3(n-6) or [14C]malonyl CoA to determine the activities of the delta 6, delta 5 desaturases and elongate, respectively. Experiments were performed in parallel with rat liver microsomal preparations where enzyme activities are well documented. Data derived from the enzymatic assays were compared to fatty acid composition data derived from epidermal total lipids. The enzymatic conversion rates were determined after methylation and separation of the 14C-labeled fatty acid methyl esters by argentation thin-layer chromatography and reverse phase high-performance liquid chromatography. Our data demonstrated: that N, PU, and PI epidermis were all capable of elongating 18:3(n-6) into 14C-labeled 20:3(n-6) via the addition of [14C]malonyl CoA, and this activity was markedly elevated (fivefold) in PI preparations; that N, PU, and PI epidermal preparations lacked the capacity to desaturate 18:2(n-6) and 20:3(n-6); and striking alterations in the individual fatty acids (as weight percent) in the total fatty acids of the PI epidermal extracts when compared to the PU and N extracts. These findings indicate that epidermal arachidonic acid is not biosynthesized locally from tissue linoleic acid and must, therefore, depend on contribution from another endogenous source.     

Inability of skin enzyme preparations to biosynthesize arachidonic acid from linoleic acid

The lack of any information as to the origin of epidermal arachidonic acid, an important precursor of eicosanoids in the epidermis, prompted us to determine in vitro whether or not microsomal preparations from rat and guinea pig epidermis possess the delta 6 and delta 5 desaturase activities. The incubations were performed in parallel with microsomal preparations from liver of these animals where activities for these enzymes have previously been reported. The conversions of radioactive fatty acids were determined after methylation and separation of the 14C-fatty acid methyl esters by argentation thin layer chromatography. Data from these studies demonstrated that delta 5 desaturase activity is markedly lower in guinea pig liver than in rat liver. Interestingly, preparations from rat and guinea pig epidermis at all concentrations tested lacked the capacity to transform either linoleic acid into gammalinolenic acid or dihomogammalinolenic acid into arachidonic acid. This observation implies that arachidonic acid that is present in the epidermal phospholipids is biosynthesized elsewhere endogenously and transported to the epidermis for esterification into the phospholipids. The site of this biosynthesis is presumably the liver and the mode of transport to the epidermis remains to be determined. These studies indicate arachidonic acid per se as an essential fatty acid for the epidermis.     

Oxidative metabolism of dihomogammalinolenic acid by guinea pig epidermis: evidence of generation of anti-inflammatory products

Reports that vegetable oils which contain gamma-linolenic acid (18:3n-6) may exert beneficial effects on cutaneous disorders prompted us to investigate whether epidermis possesses the ability to transform dihomogammalinolenic acid (20:3n-6), the epidermal elongase product of 18:3n-6, into oxidative metabolites with anti-inflammatory potential. Incubations of [1-14C]20:3n-6 with the 105,000 g particulate (microsomal) fraction from guinea pig epidermal homogenate resulted in the formation of the 1-series prostaglandin PGE1. The identity of this product was confirmed by argentation thin-layer chromatography (TLC), reverse phase-HPLC, and conversion with alkali treatment to PGB1. Incubations of [1-14C]20:3n-6 with the 105,000 g supernatant (cytosolic) fraction from guinea pig epidermal homogenate resulted in the formation of the 15-lipoxygenase product 15-hydroxy-8, 11, 13-eicosatrienoic acid (15-OH-20:3n6). The identity of this product was confirmed by normal phase-HPLC and gas chromatography/mass spectrometry (GC/MS). Thus, data from these studies indicate the capacity of enzymes in the microsomal and cytosolic fractions of guinea pig epidermal homogenates to transform 20:3n-6 to the eicosanoids PGE1 and 15-OH 20:3n-6, products which reportedly have anti-inflammatory properties. The in vivo significance of these findings remains to be explored.     

Dolichos biflorus agglutinin (DBA) reveals a similar basal cell differentiation in normal and psoriatic epidermis

Binding of N-acetyl galactosamine (GalNAc)-specific Dolichos biflorus agglutinin (DBA) conjugates to frozen sections of normal epidermis and of psoriatic uninvolved and lesional skin was studied in fluorescence microscopy. The DBA conjugates bound only to single basal cell layer in normal and uninvolved psoriatic epidermis from patients with different blood group status. In the lesional area of psoriatic skin a similar reaction with a single basal cell layer was revealed. Other lectin-conjugates applied, presenting also GalNAc specificity, reacted with most cell layers of normal and both uninvolved and lesional psoriatic epidermis and gave an attenuated reaction with the middle epidermal layers. The results show that the basal cell characteristics are confined only to the cells along the basal membrane also in psoriatic epidermis, although cells in three lowest layers may be able to proliferate.     

Dolichos biflorus agglutinin (DBA) reveals a similar basal cell differentiation in normal and psoriatic epidermis

Summary Binding of N-acetyl galactosamine (GalNAc)-specific Dolichos biflorus agglutinin (DBA) conjugates to frozen sections of normal epidermis and of psoriatic uninvolved and lesional skin was studied in fluorescence microscopy. The DBA conjugates bound only to single basal cell layer in normal and uninvolved psoriatic epidermis from patients with different blood group status. In the lesional area of psoriatic skin a similar reaction with a single basal cell layer was revealed. Other lectin-conjugates applied, presenting also GalNAc specificity, reacted with most cell layers of normal and both uninvolved and lesional psoriatic epidermis and gave an attenuated reaction with the middle epidermal layers. The results show that the basal cell characteristics are confined only to the cells along the basal membrane also in psoriatic epidermis, although cells in three lowest layers may be able to proliferate.     

Oxidative metabolism of dihomogammalinolenic acid by guinea pig epidermis: Evidence of generation of anti-inflammatory products

Reports that vegetable oils which contain gammalinolenic acid :3n-6) may exert beneficial effects on cutaneous disorders prompted us to investigate whether epidermis possesses the ability to transform dihomogammalinolenic acid (20 : 3n-6), the epidermal elongase product of 18 : 3n-6, into oxidative metabolites with anti-inflammatory potential. Incubations of [1–14C] 20:3n-6 with the 105, 000 g particulate (microsomal) fraction from guinea pig epidermal homogenate resulted in the formation of the 1-series prostaglandin PGE₁. The identity of this product was confirmed by argentation thin-layer chromatography (TLC), reverse phase-HPLC, and conversion with alkali treatment to PGB₁. Incubations of [1–14C] 20:3n-6 with the 105,000 g supernatant (cytosolic) fraction from guinea pig epidermal homogenate resulted in the formation of the 15-lipoxygenase product 15-hydroxy-8, 11, 13-eicosatrienoic acid (15-OH-20:3n6). The identity of this product was confirmed by normal phase-HPLC and gas chromatography/mass spectrometry (GC/MS). Thus, data from these studies indicate the capacity of enzymes in the microsomal and cytosolic fractions of guinea pig epidermal homogenates to transform 20:3n-6 to the eicosanoids PGE and 15-OH 20:3n-6, products which reportedly have anti-¹ inflammatory properties. The significance of these findings remains to be explored.     

Regional differences in lipid composition and incorporation of saturated and unsaturated fatty acids into microsomal membranes of rat small intestine

Incorporation of [1-14C]palmitic (16:0) and [1-14C]linoleic (18:2 omega 6) acids into microsomal membranes of proximal (jejunum) and distal (ileum) regions of rat small intestine was investigated, and the lipid composition, including fatty acid profiles of membrane phospholipids, was determined. Jejunal microsomes contained significantly higher amounts of total phospholipids, phosphatidylcholine, and phosphatidylinositol, and lower amounts of cholesterol and sphingomyelin when compared with ileal microsomes. Jejunal microsomal phospholipids contained higher levels of stearic (18:0), 18:2 omega 6, and eicosapentaenoic (20:5 omega 3) acids followed by reduced levels of oleic (18:1 omega 9), arachidonic (20:4 omega 6), and docosahexaenoic (22:6 omega 3) acids when compared with those from the ileum, except for phosphatidylinositol where no significant difference between 20:4 omega 6 content of each site was observed. In both jejunal and ileal microsomes, incorporation of [1-14C]18:2 omega 6 was significantly higher than that of [1-14C]16:0. Incorporation of both [1-14C]16:0 and [1-14C]18:2 omega 6 was significantly higher in jejunal microsomal lipid fractions (phospholipids, diacylglycerols, triacylglycerols) when compared with the ileal microsomal fraction. These data suggest that (1) jejunal and ileal microsomal membranes differ from each other in terms of lipid composition and lipid synthesis, (2) site variations in the specificity of acyltransferases for different fatty acids exist, and (3) higher delta 9-, delta 6-, delta 5-, and delta 4-desaturase activities exist in ileal compared with jejunal enterocytes.     

The effect of chronic ethanol consumption on the fatty acid composition of phosphatidylinositol in rat liver microsomes as determined by gas chromatography and 1H-NMR.

Cell membranes and vesicles composed of extracted phospholipids isolated from rats chronically-fed ethanol develop a resistance to disordering by ethanol in vitro (membrane tolerance) and a decreased partitioning of ethanol into the membranes. The anionic lipid phosphatidylinositol (PtdIns) is the only microsomal phospholipid from the ethanol-fed rats that confers tolerance to vesicles of microsomal phospholipids from control rats in a paradigm where phospholipid classes are sequentially swapped. To investigate the molecular basis of this adaptation, the fatty acid content of microsomal PtdIns extracted from the livers of rats chronically fed ethanol for 5 weeks and their calorically-matched controls was analyzed by gas-liquid chromatography (GLC) and 1H-NMR spectroscopy. Chronic ethanol consumption caused an 8.4% decrease in arachidonic acid [20:4(n - 6)], a 20.0% increase in oleic acid [18: 1(n - 9)] and a 47.1% increase in the quantitatively minor fatty acid [20:3(n - 6)]. 1H-NMR was used to quantitatively assay compositional changes in the delta 5 olefinic moiety of the acyl chains in PtdIns, an approach that should be broadly applicable to other lipid systems. After chronic ethanol feeding PtdIns had decreased delta 5 unsaturates (-7.9% NMR, -8.2% GLC) and a corresponding increase in delta 5 saturates (+5.4% NMR, +5.3% GLC). In the other phospholipids, chronic ethanol feeding caused alterations in the fatty acid compositions specific for each phospholipid. PtdIns was the only microsomal phospholipid that exhibited a significant decrease in both the polyunsaturate pool and the ratio of the total olefinic content to the saturated fatty acid content. The major adaptive response in rat liver microsomal PtdIns to chronic ethanol administration involves a decrease in arachidonic acid [20:4 (n - 6)], which is partly compensated for by increases in oleic acid [18:1(n - 9)] and eicosatrienoic acid [20:3 (n - 6)], resulting in a depressed unsaturation and polyunsaturation index. The decreased unsaturation at the delta 5 position may have special functional relevance, due to the proximity of this position to the membrane surface, where ethanol is believed to reside. Whether these acyl changes are merely coincident with, or causative of, membrane tolerance requires further elucidation.     

Activity of human Delta5 and Delta6 desaturases on multiple n-3 and n-6 polyunsaturated fatty acids.

Yeast co-expressing human elongase and desaturase genes were used to investigate whether the same desaturase gene encodes an enzyme able to desaturate n-3 and n-6 fatty acids with the same or different carbon chain length. The results clearly demonstrated that a single human Delta5 desaturase is active on 20:3n-6 and 20:4n-3. Endogenous Delta6 desaturase substrates were generated by providing to the yeast radiolabelled 20:4n-6 or 20:5n-3 which, through two sequential elongations, produced 24:4n-6 and 24:5n-3, respectively. Overall, our data suggest that a single human Delta6 desaturase is active on 18:2n-6, 18:3n-3, 24:4n-6 and 24:5n-3.     

Microsomal fatty acid desaturation and elongation in a human lung carcinoma grown in nude mice

Since tumor cells show abnormal fatty acid composition, it is likely that their desaturase systems were affected to some extent. Although desaturase activities in experimental tumors have been evaluated, to our knowledge, fatty acid desaturases in human neoplasms and particularly in human tumors grown in nude mice have not been assessed yet. We have therefore, chosen a rapidly growing human lung mucoepidermoid carcinoma (HLMC) grown in nude mice to study microsomal fatty acid desaturation and chain elongation activities. Tumor microsomal proteins were incubated with unlabeled malonyl-CoA and one of the following fatty acids: [1-14C]palmitic (16:0), [1-14C]linoleic (18:2), alpha-[1-14C]linolenic (alpha-18:3), and unlabeled gamma-linolenic (gamma-18:3) plus [2-14C]malonyl-CoA. Data show that HLMC microsomes were capable to desaturate 16:0, alpha-18:3, and dihomogammalinolenic acids (20:3) by delta 9, delta 6 and delta 5 desaturase, respectively; however, delta 6 desaturase activity on [14C]18:2 was not detected. The microsomal elongation system was active in all fatty acid series tested except for 18:2. These findings show that the undetectable activity for 18:2 desaturation is not exclusively found in experimental tumors.     

Inability of murine peritoneal macrophages to convert linoleic acid into arachidonic acid. Evidence of chain elongation

Various murine macrophage populations synthesize and secrete large amounts of arachidonic acid (20:4n-6) derived eicosanoids (cyclo-oxygenase and lipoxygenase products). These metabolites are known to possess a wide variety of functions with regard to the initiation and regulation of inflammation and tumorigenesis. Because the dietary intake of 20:4n-6 is usually low, tissues are largely dependent upon dietary linoleic acid (18:2n-6) as an initial unsaturated precursor for the biosynthesis of 20:4n-6. The purpose of these experiments was to determine whether resident or responsive murine macrophages possess desaturase and elongase activities capable of in vitro conversion of 18:2n-6 into 20:4n-6. Peritoneal exudate macrophages were purified by adherence and incubated in serum-free medium containing fatty acid-free BSA with [1-14C] 18:2n-6. Approximately 90 to 98% of the [14C]18:2n-6 at 4 and 16 h was recovered in phosphatidylcholine and phosphatidylethanolamine. The metabolism of [14C]18:2n-6 was determined after transesterification and separation of the 14C-fatty acid methyl esters by argentation TLC, reverse phase HPLC, and electron impact gas chromatography/mass spectrometry. Resident and responsive macrophages lacked the capacity to transform [14C]18:2n-6 into 20:4n-6. In addition, prelabeled macrophages incubated with soluble, calcium ionophore A23187 or phorbol myristate, or particulate, zymosan, membrane perturbing agents also lacked delta 6 desaturase activity. All macrophages tested were capable of elongating [14C]18:2n-6 into [14C]20:2n-6. These observations suggest that 20:4n-6, present in macrophage phospholipids, is biosynthesized elsewhere and transported to the macrophage for esterification into the phospholipids. In addition, these findings demonstrate that elongase activity is present in both the resident and responsive peritoneal macrophage.     

Conversion of linoleic acid into arachidonic acid by cultured murine and human keratinocytes

The origin of arachidonic acid (AA) found in the epidermis is not known. Two possibilities exist: either de novo synthesis within the epidermal keratinocyte, or transport of AA formed at distant tissue sites. The current study examined the ability of cultured murine and human keratinocytes to metabolize exogenously added linoleic acid (LA). Conversion of radiolabeled substrate (14C-LA) into 18:3(n-6), 20:2(n-6), 20:3(n-6), and 20:4(n-6) (AA) was noted. The conversion of non-radiolabeled 18:3(n-6) or 20:2(n-6) was also examined and the pattern of metabolites synthesized suggests that the preferred metabolic pathway for conversion of linoleic acid into arachidonic acid is via the classically described pathway in which a delta 6 desaturase constitutes the initial reaction. Although cultured skin fibroblasts are known to convert linoleic acid into arachidonic acid, the current study demonstrates that cultured epidermal keratinocytes can also avidly metabolize exogenous linoleic acid. The ability of cultured keratinocytes, and not of whole epidermis in vivo, to convert linoleic acid into arachidonic acid suggests that specific enzymatic activities may be induced by the tissue culture system itself. Hence, findings of metabolic capabilities in cultured cells may not necessarily be extrapolated to the in vivo situation.     

The identification of hydroxy fatty acids in psoriatic skin

Psoriasis is a common chronic inflammatory and proliferative skin disease characterised by epidermal neutrophil infiltration which may be induced by chemotactic substances in the involved epidermis. Superficial psoriatic scale was shown to contain biologically active amounts of leukotriene B₄ and monohydroxy-eicosatetraenoic acid (HETE)- like material as determined by assay for chemokinetic activity in high performance liquid chromatography (HPLC) fractions of scale extracts. Extracts of scale and chamber fluid from abraded lesional and uninvolved psoriatic skin were purified by HPLC and appropriate fractions were analysed by gas chromatography - mass spectrometry (GC-MS). The following monohydroxy metabolites of arachidonic, linoleic and 11,14-eicosadienoic acids were identified : 15-HETE, 12-HETE, 11-HETE, 9-HETE, 8-HETE, 5-HETE, 13-hydroxy-octadecadienoic acid (13-HODD), 9-HODD and 15-hydroxy-eicosadienoic acid (15-HEDE). The results suggested that 12-HETE, 13-HODD and 9-HODD are the most abundant monohydroxy fatty acids in the psoriatic skin extracts described above. Assays of 13-HODD, 9-HODD and 15-HEDE for chemokinetic activity were negative with concentrations up to 10^?4M. The biological significance of these three compounds in not known, but some of the hydroxylated metabolites of arachidonic acid may, by virtue of their chemotactic properties, be relevant to the pathogenesis of the psoriatic neutrophil infiltrate.     

Selective effects of isomeric cis and trans fatty acids on fatty acyl delta 9 and delta 6 desaturation by human skin fibroblasts

Human skin fibroblasts incorporate and actively desaturate long-chain fatty acids. Growth of these cells in lipid-free medium can be used to enhance delta 9 and delta 6 desaturation of [14C]stearate and [14C]linoleate, respectively. Medium supplementation with cis fatty acids inhibits delta 9 desaturation; effectiveness as inhibitors is linoleate (9c,12c-18:2) greater than oleate (9c-18:1) greater than vaccenate (11c-18:1). Linoelaidate (9t,12t-18:2), trans-vaccenate (11t-18:1) and saturated fatty acids are without effect; elaidate (9t-18:1) appears stimulatory. By contrast, the trans fatty acids elaidate and linoelaidate are potent inhibitors of delta 6 desaturation; inhibition by trans-vaccenate is 50% of that of elaidate. Desaturation of [14C]linoleate is only slightly inhibited by oleate, cis-vaccenate, or (6c,9c,12c)-linolenate. The relative effectiveness of isomeric cis- and trans-octadecenoic acids as inhibitors of delta 9 and delta 6 desaturation in intact human cells is different from that found in microsomal studies. The cell culture system can thus be important in evaluating physiological effects of isomeric fatty acids on cellular metabolic processes.     

Chronologic studies of the effects of a hypoproteinic diet followed by an equilibrated diet on delta-6 and delta-5 desaturations of linoleic acid in liver microsomes in the rat

A low protein diet affects amounts of linoleic and arachidonic acids in hepatic microsomal phospholipids of growing rats. Are the changes related to modifications in microsomal delta 6- and delta 5- linoleic acid desaturase activities? Two groups of Wistar rats weighing 80 +/- 5 g at the beginning of the experiment were used: Control group (T) was fed on a 16% gluten + 4% casein diet for 53 days; Experimental group (E) was fed on a 4% gluten + 1% casein diet for 26 days (MP) then Control diet for 27 days (RE). After 2, 14 and 26 days of MP and 2, 15 and 27 days of RE, rats of each group were sacrificed. Protein and water contents of liver, quantitative fatty acid, composition of total lipids in liver and hepatic microsomes were determined. delta 6- and delta 5- linoleic acid desaturase activities were estimated from incubation of liver microsomes with [1-14C] C 18: 2 n-6 or [2(14)C] C 20: 3 n-6 respectively. The low protein diet stops practically ponderal growth. The fatty-acid compositions of microsomal total lipids of E rats were affected in comparison with values of T rats. These modifications persist after 27 days of RE. The C 20: 4 n-6/C 18: 2 n-6 ratio in microsomal total lipids was slightly different between T and E rats but increased strongly during refeeding. Same modifications take place in the fatty-acid composition of hepatic total lipids. After two days of MP, delta 6- and delta 5- desaturase activities were depressed, phenomenon that not persist in the course of MP. These enzyme activities increase to higher values than those of the T after two days of RE.     

Phospholipid and triacylglycerol fatty acid content and pattern in the cardiac endothelium of rats depleted in long-chain polyunsaturated omega 3 fatty acids

Rats depleted in long-chain polyunsaturated omega3 fatty acids (omega3-depleted rats) display several features of the metabolic syndrome including hypertension and cardiac hypertrophy. This coincides with alteration of the cardiac muscle phospholipid and triacylglycerol fatty acid content and/or pattern. In the present study, the latter variables were measured in the cardiac endothelium of normal and omega3-depleted rats. Samples derived from four rats each were obtained from 16 female normal fed rats and three groups of 36-40 female fed omega3-depleted rats each aged 8-9, 15-16 and 22-23 weeks. At comparable mean age, the ratio between the square root of the total fatty acid content of phospholipids and cubic root of the total fatty acid content of triacylglycerols was lower in omega3-depleted rats than in control animals. The total fatty acid content of triacylglycerols was inversely related to their relative content in C20:4omega6. Other differences between omega3-depleted rats and control animals consisted in a lower content of long-chain polyunsaturated omega3 fatty acids in both phospholipids and triacylglycerols, higher content of long-chain polyunsaturated omega6 fatty acids in phospholipids, higher activity of delta9-desaturase (C16:0/C16:1omega7 and C18:0/C18:1omega9 ratios) and elongase [(C16:0 + C16:1omega7)/(C18:0 + C18:1omega9) and C20:4omega6/C22:4omega6 ratios], but impaired generation of C22:6omega3 from C22:5omega3 in the former rats. These findings support the view that cardiovascular perturbations previously documented in the omega3-depleted rats may involve impaired heart endothelial function.     

The effects of unsaturated fatty acids on the synthesis of arachidonic acid in rat kidney cells

Cultured rat kidney cells absorbed exogenous linoleic acid (cic, cis-18:2n-6) and esterified it mostly into glycerophospholipids. As the concentration of 18:2 was increased (5-200 microM) the quantity absorbed increased linearly and the amount esterified in the triacylglycerol increased. The cells possessed active acyl delta 6-desaturase and elongase which facilely converted 18:2n-6 to 20:4n-6. At low intracellular concentrations of 18:2n-6 other unsaturated fatty acids, i.e., gamma-linolenic (18:3n-6), alpha-linolenic (18:3n-3), dihomo-gamma-linolenic (20:3n-6), and especially trans, trans-linoleic acid (trans, trans-18:2n- -6) at concentrations ranging from 25 to 200 microM depressed delta 6-desaturase activity. However, suppression of 20:4 synthesis even by trans, trans-18:2 was readily overcome by increasing the concentration of available cis, cis-18:2n-6.     

Interactions of saturated, n-6 and n-3 polyunsaturated fatty acids to modulate arachidonic acid metabolism

Anti-thrombotic effects of omega-3 (n-3) fatty acids are believed to be due to their ability to reduce arachidonic acid levels. Therefore, weanling rats were fed n-3 acids in the form of linseed oil (18:3n-3) or fish oil (containing 20:5n-3 and 22:6n-3) in diets containing high levels of either saturated fatty acids (hydrogenated beef tallow) or high levels of linoleic acid (safflower oil) for 4 weeks. The effect of diet on the rate-limiting enzyme of arachidonic acid biosynthesis (delta 6-desaturase) and on the lipid composition of hepatic microsomal membrane was determined. Both linseed oil- or fish oil-containing diets inhibited conversion of linoleic acid to gamma-linolenic acid. Inhibition was greater with fish oil than with linseed oil, only when fed with saturated fat. delta 6-Desaturase activity was not affected when n-3 fatty acids were fed with high levels of n-6 fatty acids. Arachidonic acid content of serum lipids and hepatic microsomal phospholipids was lower when n-3 fatty acids were fed in combination with beef tallow but not when fed with safflower oil. Similarly, n-3 fatty acids (18:3n-3, 20:5n-3, 22:5n-3, and 22:6n-3) accumulated to a greater extent when n-3 fatty acids were fed with beef tallow than with safflower oil. These observations indicate that the efficacy of n-3 fatty acids in reducing arachidonic acid level is dependent on the linoleic acid to saturated fatty acid ratio of the diet consumed.     

Effect of dietary alpha- and gamma-linolenic acid on tissue fatty acids in guinea pigs

Guinea pigs were fed regular chow diets supplemented with 5% (by weight) safflower oil, evening primrose oil, or linseed oil for 6 weeks. The unsaturated fatty acid content of these oils was 78.9% of 18:2n6, 74.1% of 18:2n6, and 9.2% of 18:3n6, or 21.5% of 18:2n6 and 46.9% of 18:3n3, respectively. In comparison with 18:2n6, dietary supplementation with 18:3n6 significantly increased the tissue levels of 18:3n6 and 20:3n6, whereas dietary 18:3n3 significantly elevated the levels of 18:3n3 in plasma and liver lipids. Dietary 18:3n3 also significantly increased 22:5n3 and 22:6n3 in total phospholipids. The tissue levels of 20:4n6, on the other hand, were not affected by either treatment. These data suggest that both delta 6- and delta 5 desaturation of n-6 fatty acids in guinea pigs are low, and that the metabolism of n-3 and n-6 fatty acids may be regulated by two different enzyme systems.     

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.