Trans n-3 eicosapentaenoic and docosahexaenoic acid isomers exhibit different inhibitory effects on arachidonic acid metabolism in human platelets compared to the respective cis fatty acids

A convenient route to mono-trans polyunsaturated free fatty acids

Trans unsaturated fatty acids in humans may be originated both from dietary supplementation and from an endogenous free-radical-catalyzed cis−trans isomerization of fatty acid residues in naturally occurring cis lipids. The latter process affords geometrical isomers and the polyunsaturated fatty acid mono-trans isomers were demonstrated to be connected with stress conditions in living organisms. Synthesis of mono-trans polyunsaturated fatty acid is useful for analytical and biological research, and in this case, the availability of free fatty acids is needed as well as the possibility of mg scale of the synthetic protocol. Herein, we report a simple synthetic route to mono-trans isomers of arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid, which includes thiyl radical-catalyzed isomerization reaction of polyunsaturated fatty acid methyl esters and fraction isolation of mono-trans mixture isomers followed by optimization of hydrolysis condition to free fatty acids and purification of each mono-trans polyunsaturated fatty acid. Our approach to mono-trans polyunsaturated fatty acids as free acids can reach the mg scale, thus fostering more applications to biochemical and biological studies.

Trans fatty acids and atopic eczema/dermatitis syndrome: the relationship with a free radical cis-trans isomerization of membrane lipids

The formation of trans FA residues in membrane phospholipids may be due to a free radical-catalyzed isomerization process occurring to the cis unsaturated FA moieties. Radical stress is well documented in inflammatory processes of atopic diseases, but no data are yet available about a possible association with trans FA detected in these patients. We investigated the presence of trans lipid isomers in the erythrocyte and T-lymphocyte membranes of 26 children affected by atopic eczema/dermatitis syndrome (AEDS). Trans lipid isomers were found in both cell membranes, up to a total content of 2.7 and 4.9% of the FA composition, respectively. By using the geometrical trans lipid library derived from in vitro models of thiyl radical-catalyzed isomerization, oleic and arachidonic acid isomers were detected. The statistical significance was evaluated by comparison with an age-matched control group. These results suggest the role of an endogenous free radical isomerization path occurring to membrane unsaturated lipids, complementary to the dietary contribution, which can be involved in the lipid impairment in AEDS. This study contributes to lipidomic research regarding the double bond structure and the influence of a geometrical change of membrane lipids in physiology and diseases.

Docosahexaenoic acid and arachidonic acid release in rat brain astrocytes is mediated by two separate isoforms of phospholipase A2 and is differently regulated by cyclic AMP and Ca2+

1. Docosahexaenoic acid (DHA) and arachidonic acid (AA), polyunsaturated fatty acids (PUFAs), are important for central nervous system function during development and in various pathological states. Astrocytes are involved in the biosynthesis of PUFAs in neuronal tissue. Here, we investigated the mechanism of DHA and AA release in cultured rat brain astrocytes. 2. Primary astrocytes were cultured under standard conditions and prelabeled with [(14)C]DHA or with [(3)H]AA. Adenosine 5'-triphosphate (ATP) (20 micro M applied for 15 min), the P2Y receptor agonist, stimulates release of both DHA (289% of control) and AA (266% of control) from astrocytes. DHA release stimulated by ATP is mediated by Ca(2+)-independent phospholipase A(2) (iPLA(2)), since it is blocked by the selective iPLA(2) inhibitor 4-bromoenol lactone (BEL, 5 micro M) and is not affected either by removal of Ca(2+) from extracellular medium or by suppression of intracellular Ca(2+) release through PLC inhibitor (U73122, 5 micro M). 3. AA release, on the other hand, which is stimulated by ATP, is attributed to Ca(2+)-dependent cytosolic PLA(2) (cPLA(2)). AA release is abolished by U73122 and, by removal of extracellular Ca(2+), is insensitive to BEL and can be selectively suppressed by methyl arachidonyl fluorophosphonate (3 micro M), a general inhibitor of intracellular PLA(2) s. 4. Western blot analysis confirms the presence in rat brain astrocytes of 85 kDa cPLA(2) and 40 kDa protein reactive to iPLA(2) antibodies. 5. The influence of cAMP on regulation of PUFA release was investigated. Release of DHA is strongly amplified by the adenylyl cyclase activator forskolin (10 micro M), and by the protein kinase A (PKA) activator dibutyryl-cAMP (1 mM). In contrast, release of AA is not affected by forskolin or dibutyryl-cAMP, but is almost completely blocked by 2,3-dideoxyadenosine (20 micro M) and inhibited by 34% by H89 (10 micro M), inhibitors of adenylyl cyclase and PKA, respectively. 6. Other neuromediators, such as bradykinin, glutamate and thrombin, stimulate release of DHA and AA, which is comparable to the release stimulated by ATP. 7. Different sensitivities of iPLA(2) and cPLA(2) to Ca(2+) and cAMP reveal new pathways for the regulation of fatty acid release and reflect the significance of astrocytes in control of DHA and AA metabolism under normal and pathological conditions in brain.

Alpha- and beta- alkyl-substituted eicosapentaenoic acids: incorporation into phospholipids and effects on prostaglandin H synthase and 5-lipoxygenase

Alpha-ethyl-, alpha-methyl- and beta-methyl eicosapentaenoic acid (EPA) were prepared and their incorporation into cell lipids and effects on eicosanoid synthesis compared with EPA and docosahexaenoic acid (DHA). alpha- and beta-methyl EPA were incorporated into hepatocyte triacylglycerols as efficiently as EPA, whereas lesser amounts were found in phospholipids. alpha-ethyl EPA was not incorporated into phospholipids but small amounts were detected in triacylglycerol. All derivatives inhibited the synthesis of arachidonic acid, although less efficiently than EPA and DHA. The derivatives were poor substrates of prostaglandin H (PGH) synthase and 5-lipoxygenase, and they all inactivated PGH synthase. In isolated platelets, alpha-methyl EPA was a stronger inhibitor of TxB2 production than EPA, alpha-ethyl- and beta-methyl EPA. All derivatives were stronger inducers of peroxisomal beta-oxidation than EPA and DHA. This increased induction probably is a consequence of the blocked mitochondrial beta-oxidation of the derivatives.

Essential fatty acids and their trans geometrical isomers in powdered and liquid infant formulas sold in Canada

BACKGROUND

Animal and human studies have suggested that trans fatty acids might alter some physiological functions and adversely affect the growth and essential fatty acid balance of infants. In this context it is important to know the fatty acid composition, including the levels of trans isomers of oleic, linoleic and alpha-linolenic acids in infant formulas.

METHODS

Ten liquid and fourteen powdered formulas for term infants were purchased from retail stores in Canada. The fatty acid composition of each formula was determined by capillary gas-liquid chromatography.

RESULTS

All the formulas met the minimum content of 500 mg of linoleic acid/100 kcal formula (equivalent to 4.5% of energy) specified under current Canadian regulations. The formulas all met the minimum energy levels of 3% as linoleic acid and 0.7% as alpha-linolenic acid recommended recently by an ad hoc committee of Health Canada. However, in nine formulas, the proportion of linoleic acid was more than 20% of total fatty acids, and consequently, in five of them, the ratio of linoleic acid to alpha-linolenic acid exceeded the maximum ratio of 16:1 recommended by the ad hoc committee. Trans fatty acids were present in all the samples, and generally the liquid formulas displayed a higher total trans content (mean 1.9%, range 0.9-3.1%) than powdered formulas (mean 1.4%, range 0.6-2.5%). The amounts of trans isomers of linoleic and alpha-linolenic acids and the degree of isomerization of these fatty acids were also higher in liquid formulas than in powdered formulas.

CONCLUSIONS

A few of the Canadian infant formulas would provide one-third of alpha-linolenic acid as trans geometric isomers.

Conversion of 18:3 delta 9cis, 12cis, 15trans in rat liver microsomes

Several years ago, it was established that the delta 15 trans isomer of alpha-linolenic acid is converted in vivo into fatty acids containing 20 and 22 carbons (geometrical isomers of eicosapentaenoic and docosahexaenoic acids). The present study focused on the in vitro delta 6 desaturation, the first step of the biosynthesis of the n-3 long-chain polyunsaturated fatty acids from 18:3n-3. For that purpose, rat liver microsomes were prepared and incubated with radiolabeled 18:3 delta 9cis,12cis,15cis (18:3c,c,c) or 18:3 delta 9cis, 12cis, 15trans (18:3c,c,t) under desaturation conditions. The data show that 18:3c,c,t is converted at a lower rate compared with alpha-linolenic acid. The product of conversion of 18:3c,c,t may be 18:4 delta 6cis, 9cis, 12cis, 15trans resulting from a delta 6 desaturation of the trans substrate. Moreover, the conversion of radiolabeled 18:3c,c,t was strongly decreased by the presence of 18:3c,c,c (up to 48%) while the 18:3c,c,t only slightly decreased the conversion of radiolabeled 18:3c,c,c. Thus, the desaturation enzyme presented a higher affinity for the native all-cis n-3 substrate.

Fish oil fatty acids and human platelets: dose-dependent decrease in dienoic and increase in trienoic thromboxane generation

Dietary enrichment of membrane phospholipids with n-3 (fish-oil-derived) fatty acids has attracted attention as a putative therapeutic regimen for suppression of inflammatory and coagulatory events. Use of n-3 fatty-acid-enriched lipid infusions for parenteral nutrition results in micromolar concentrations of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DCHA) in the plasma-free fatty acid fraction. We investigated the influence of free EPA and DCHA on platelet thromboxane (Tx) A2 and A3 formation by using a recently developed high performance liquid chromatography-ELISA technique for separate quantification of the stable hydrolysis products TxB2 and TxB3. Washed human thrombocytes were incubated with free arachidonic acid (AA; 1 microM), A23187 (0.1 microM) or thrombin (5 U/mL) for stimulation; all regimens provoked large quantities of TxA2 in the absence of TxA3. Simultaneous admixture of free EPA or free DCHA to the incubation medium (concentration range, 0.01-50 microM) largely suppressed platelet TxA2 generation in response to all stimuli used in a dose-dependent manner. The effective concentration with 50% influence of arachidonic acid was 4.2 microM, whereas the inhibitory concentration with 50% effect of EPA and DCHA were both in the same order of magnitude but differed with the nature of the agonist (0.2-7 microM). Platelet (co-)incubation with EPA, but not DCHA, provoked dose-dependent synthesis of n-3-lipid-derived thromboxane: kinetics of formation and absolute quantities of TxA3 approximated 20% of the respective TxA2 data upon stimulation with AA. Both EPA and DCHA dose-dependently suppressed U46619-provoked platelet aggregation. We conclude that EPA and DCHA are potent competitive inhibitors of TxA2 generation by intact platelets, with EPA acting as poor substrate and DCHA being no substrate for the cyclooxygenase/thromboxane synthase complex. Enrichment of the plasma-free fatty acid fraction with n-3 lipids may offer a therapeutic regimen to suppress the synthesis of the potent proaggregatory and vasoconstrictory agent TxA2.

Identification of novel trans isomers of 20:5n-3 in liver lipids of rats fed a heated oil

Trans polyunsaturated n-3 fatty acids are formed as a result of the heat treatment of vegetable oils. It was demonstrated previously that the 18:3 delta 9cis, 12 cis, 15 trans containing a cis delta 9 ethylenic bond was converted to a geometrical isomer of 20:5n-3, the 20:5 delta 5 cis, 8 cis, 11 cis, 14 cis, 17 trans. In the present study, we have identified two new isomers of eicosapentaenoic acid, the delta 11 monotrans and the delta 11, 17 ditrans isomers in liver of rats fed a heated oil. These are formed as a result of the conversion of two of the main isomers of linolenic acid which are present in refined and frying oils, the 18:3 delta 9 trans, 12 cis, 15 cis and the 18:3 delta 9 trans, 12 cis, 15 trans.

Incorporation of trans long-chain n-3 polyunsaturated fatty acids in rat brain structures and retina

During heat treatment, polyunsaturated fatty acids and specifically 18:3n-3 can undergo geometrical isomerization. In rat tissues, 18:3 delta 9c,12c,15t, one of the trans isomers of linolenic acid, can be desaturated and elongated to give trans isomers of eicosapentaenoic and docosahexaenoic acids. The present study was undertaken to determine whether such compounds are incorporated into brain structures that are rich in n-3 long-chain polyunsaturated fatty acids. Two fractions enriched in trans isomers of alpha-linolenic acid were prepared and fed to female adult rats during gestation and lactation. The pups were killed at weaning. Synaptosomes, brain microvessels and retina were shown to contain the highest levels (about 0.5% of total fatty acids) of the trans isomer of docosahexaenoic acid (22:6 delta 4c,7c,10c,13c,16c,19t). This compound was also observed in myelin and sciatic nerve, but to a lesser extent (0.1% of total fatty acids). However, the ratios of 22:6 trans to 22:6 cis were similar in all the tissues studied. When the diet was deficient in alpha-linolenic acid, the incorporation of trans isomers was apparently doubled. However, comparison of the ratios of trans 18:3n-3 to cis 18:3n-3 in the diet revealed that the cis n-3 fatty acids were more easily desaturated and elongated to 22:6n-3 than the corresponding trans n-3 fatty acids. An increase in 22:5n-6 was thus observed, as has previously been described in n-3 fatty acid deficiency. These results encourage further studies to determine whether or not incorporations of such trans isomers into tissues may have physiological implications.

Preferential incorporation of dietary cis-9,cis-12,trans-15 18:3 acid into rat cardiolipins

Cardiolipins from mitochondria of different rat organs (heart; liver and kidney) appear to be privileged targets for the incorporation of cis-9,cis-12,trans-15 18:3 acid, a compound commonly found in deodorized edible linolenic acid-containing oils. When this acid (together with other linolenic acid geometrical isomers (LAGI)) is fed at high load to rats that had been reared on a fat-free diet since weaned for a few days, it replaces the endogenously synthesized monoenoic acids that had accumulated in cardiolipin during fat deficiency. Although there is no discrimination in deposition of any LAGI in adipose tissue triacylglycerols, a high selectivity of incorporation of the cis-9,cis-12,trans-15 18:3 acid over other isomers (including the all-cis 18:3(n-3) acid) is observed either in diradylphospholipids or in cardiolipins. However, cis-9,cis-12,trans-15 18:3 acid accumulates in cardiolipins at a considerably higher level than in other phospholipids (11 times in liver, 5-7 times in heart and kidney). It reaches 22-24% of total fatty acids in cardiolipins from heart and liver, and 13-14% in kidney. The cis-9,cis-12,trans-15 18:3 acid is esterified to both the 1(1")- and 2(2")-positions of liver mitochondria cardiolipin, with a well-marked selectivity for positions 1(1"). Its 1(1")/2(2") selectivity ratio is about the same as that of 18:2(n-6) acid: 2.1 vs 2.2. It is concluded that the trans-15 ethylenic bond is probably perceived as a single bond by enzymic systems that ensure acylation of cardiolipins. The cis-9,cis-12,trans-15 isomer is able to reverse the fatty acid modifications induced in cardiolipins by a diet devoid of essential fatty acids, in a way similar to that of 18:2(n-6) acid supplementation.

Trans-fatty acid patterns in patients with angiographically documented coronary artery disease

The plasma trans-fatty acids of 47 patients with angiographically documented coronary artery disease were compared with those of 56 reference subjects using high-resolution capillary column gas-liquid chromatography to test the hypothesis that trans-fatty acid intake is a risk factor for cardiovascular disease. Individual and total trans-fatty acids were higher in patients than in reference subjects (1.38 vs 1.11% for total trans-fatty acids, p < 0.003; 0.40 vs 0.31% for palmitoleic acid trans, p < 0.001; and 0.28 vs 0.22% for linoleic acid trans, p < 0.007). High-density lipoprotein (HDL) cholesterol and HDL cholesterol/total cholesterol were negatively correlated (r = -0.29, p < 0.004; and r = -0.35, p < 0.001, respectively), whereas triglycerides, total cholesterol and low-density lipoprotein cholesterol were positively correlated (r = 0.47, p < 0.001; r = 0.22, p < 0.03; r = 0.20, p < 0.05, respectively) with palmitoleic acid trans. The correlations were similar and significant for linoleic acid trans, but less strong for total trans-fatty acids (which is more difficult to measure and has greater variability). Saturated and trans-fatty acids and total cholesterol are positively associated, whereas HDL/total cholesterol and polyunsaturated fatty acids are negatively associated with coronary artery disease. These results are consistent with the hypothesis that dietary trans-fatty acids are a cardiovascular risk factor.

Saturated and hydrogenated fats in food in relation to health

The report of the National Cholesterol Education Program (NCEP) expert panel on detection, evaluation, and treatment of high blood cholesterol in adults published in 1988 evoked great interest from the medical profession, the public, and food manufacturers. The merits of different dietary interventions to lower plasma cholesterol are debated in advertising, popular publications, and research publications. The present paper is a review of the contributions of saturated and trans fatty acids (FA) to the US diet, their metabolism, and effects upon plasma cholesterol. Saturated (SFA) and trans FA are metabolized to yield energy. They are not dietary essentials; SFA can be biosynthesized, and trans FA are not naturally occurring in plants, with only very small amounts in animals. Trans FA are produced in hydrogenation of liquid vegetable oils and are estimated to contribute 3-7% of the fat consumed. Most of the SFA in the US diet (35% of total fat consumed) is obtained from meat, poultry, fish, and dairy products (approximately 60%). The fats and oils consumed directly or as components of food products, mostly baked goods, contribute approximately 20% of the SFA. More than 30 years of research led the NCEP to conclude that SFA was the most potent dietary component in increasing plasma cholesterol, and that no more than 10% of the energy (en%) of the diet should be SFA. Trans FA are metabolized similarly to SFA, but no recommendation has been made about their consumption. Reduction of consumption of SFA should be practiced in a prudent manner, by reducing consumption of foods high in SFA, and not by eliminating classes of foods. Some changes in formulations of foods or preparation practices (type of frying fat) can be made. These modifications may decrease the palatability of the food, thereby presenting a challenge to the food industry. Substitution of fats hydrogenated to contain trans FA for fats high in SFA may not be beneficial to health. Labeling of foods would improve the ability of the consumer to make appropriate choices.