Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid. In physiological literature, it is given the name 20:5(n-3). It also has the trivial name timnodonic acid. In chemical structure, EPA is a carboxylic acid with a 20-carbon chain and five cis double bonds; the first double bond is located at the third carbon from the omega end.
?)EPA is a polyunsaturated fatty acid (PUFA) that acts as a precursor for prostaglandin-3 (which inhibits platelet aggregation), thromboxane-3, and leukotriene-5 eicosanoids. EPA is both a precursor and the hydrolytic breakdown product of eicosapentaenoyl ethanolamide (EPEA: C22H35NO2; 20:5,n-3).[1] Although studies of fish oil supplements, which contain both docosahexaenoic acid (DHA) and EPA, have failed to support claims of preventing heart attacks or strokes,[2][3][4] a recent multi-year study of Vascepa (ethyl eicosapentaenoate, the ethyl ester of the free fatty acid), a prescription drug containing only EPA, was shown to reduce heart attack, stroke, and cardiovascular death by 25% relative to a placebo in those with statin-resistant hypertriglyceridemia.[5][6]
Sources
EPA is obtained in the human diet by eating oily fish or fish oil, e.g., cod liver, herring, mackerel, salmon, menhaden and sardine and various types of edible algae. It is also found in human breast milk.
Fish can synthesize EPA from fatty acids found in their alimentation[7] or obtain it from the algae they consume.[8] It is available to humans from some non-animal sources (e.g., commercially, from Yarrowia lipolytica,[9] and from microalgae such as Monodus subterraneus, Chlorella minutissima and Phaeodactylum tricornutum,[10][11] which are being developed as a commercial source).[12] EPA is not usually found in higher plants, but it has been reported in trace amounts in purslane.[13] In 2013, it was reported that a genetically modified form of the plant camelina produced significant amounts of EPA.[14][15]
The human body converts a portion of absorbed alpha-linolenic acid (ALA) to EPA. ALA is itself an essential fatty acid, and humans need an appropriate supply of it. The efficiency of the conversion of ALA to EPA, however, is much lower than the absorption of EPA from food containing it. Because EPA is also a precursor to docosahexaenoic acid (DHA), ensuring a sufficient level of EPA on a diet containing neither EPA nor DHA is harder both because of the extra metabolic work required to synthesize EPA and because of the use of EPA to metabolize into DHA. Medical conditions like diabetes or certain allergies may significantly limit the human body's capacity for metabolization of EPA from ALA.
Biosynthesis of Eicosapentaenoic Acid
The biosynthesis of Eicosapentaenoic acid (EPA) in prokaryotes and eukaryotes involves polyketide synthase (PKS). The polyketide pathway includes six enzymes namely, 3-ketoacyl synthase (KS), 2 ketoacyl-ACP-reductase(KR), dehydrase (DH), enoyl reductase (ER), dehydratase/2-trans 3-cos isomerase (DH/2,3I), dehydratase/2-trans, and 2-cis isomerase(DH/2,2I). The biosynthesis of EPA varies in marine species, but most of the marine species’ ability to convert C18 PUFA to LC-PUFA is dependent on the fatty acyl desaturase and elongase enzymes. The molecule basis of the enzymes will dictate where the double bond is formed on the resulting molecule.[16]
Here is an overview of the possible biosynthesis pathways of EPA from fatty acid synthesis (FAS). The reactions are mediated by desaturases enzymes with Δx specificity and elongated by elongases of fatty acid chains.
The proposed polyketide synthesis pathway of EPA in Shewanella is a repetitive reaction of reduction, dehydration, and condensation that uses acetyl coA and malonyl coA as building blocks. The mechanism of α-linolenic acid to EPA involves the condensation of malonyl-CoA to the pre-existing α-linolenic acid by KS. The resulting structure is converted by NADPH dependent reductase, KR, to form an intermediate that is dehydrated by the DH enzyme. The final step is the NADPH-dependent reduction of a double bond in trans-2-enoly-ACP via ER enzyme activity. The process is repeated to form EPA.[17]
Clinical significance
The US National Institute of Health's MedlinePlus lists medical conditions for which EPA (alone or in concert with other ω-3 sources) is known or thought to be an effective treatment.[18] Most of these involve its ability to lower inflammation.
Intake of large doses (2.0 to 4.0 g/day) of long-chain omega-3 fatty acids as prescription drugs or dietary supplements are generally required to achieve significant (> 15%) lowering of triglycerides, and at those doses the effects can be significant (from 20% to 35% and even up to 45% in individuals with levels greater that 500 mg/dL).
It appears that both EPA and DHA lower triglycerides; however, DHA appears to raise low-density lipoprotein (the variant which drives atherosclerosis, sometimes inaccurately called "bad cholesterol") and LDL-C values (always only a calculated estimate; not measured by labs from person's blood sample for technical and cost reasons), while EPA does not.
EPA and DHA ethyl esters (all forms) may be absorbed less well, thus work less well, when taken on an empty stomach or with a low-fat meal.[19]
Omega-3 fatty acids, particularly EPA, have been studied for their effect on autistic spectrum disorder (ASD). Some have theorized that, since omega-3 fatty acid levels may be low in children with autism, supplementation might lead to an improvement in symptoms. While some uncontrolled studies have reported improvements, well-controlled studies have shown no statistically significant improvement in symptoms as a result of high-dose omega-3 supplementation.[20][21]
In addition, studies have shown that omega-3 fatty acids may be useful for treating depression.
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