Eicosapentaenoic acid (EPA) is a long-chain (≥ 12 carbon (C) atoms), polyunsaturated (> 1 double bond) fatty acid (English : PUFAs, polyunsaturated fatty acids) belonging to the group of omega-3 fatty acids (n-3 FS, first double bond is present – as seen from the methyl (CH3) end of the fatty acid chain – at the third C-C bond) – C20:5; n-3. EPA can be supplied both through the diet, mainly through oils of fatty marine fish, such as mackerel, herring, eel, and salmon, and synthesized (formed) in the human organism from the essential (vital) n-3 FS alpha-linolenic acid (C18:3).
Synthesis
Alpha-linolenic acid is the precursor (precursor) for the endogenous (endogenous) synthesis of EPA and enters the body exclusively through the diet, mainly through vegetable oils, such as flax, walnut, canola, and soybean oils. Through desaturation (insertion of double bonds, turning a saturated compound into an unsaturated one) and elongation (lengthening of the fatty acid chain by 2 C atoms), alpha-linolenic acid is metabolized (metabolized) to EPA in the smooth endoplasmic reticulum (structurally rich cell organelle with a channel system of cavities surrounded by membranes) of leukocytes (white blood cells) and liver cells. The conversion of alpha-linolenic acid to EPA proceeds as follows.
- Alpha-linolenic acid (C18:3) → C18:4 by delta-6 desaturase (enzyme that inserts a double bond at the sixth C-C bond – as seen from the carboxyl (COOH) end of the fatty acid chain – by transferring electrons).
- C18:4 → C20:4 by fatty acid elongase (enzyme that elongates fatty acids by a C2 body).
- C20:4 → eicosapentaenoic acid (C20:5) by delta-5 desaturase (enzyme that inserts a double bond at the fifth C-C bond – as seen from the carboxyl (COOH) end of the fatty acid chain – by transferring electrons).
Women exhibit more effective EPA synthesis from alpha-linolenic acid compared to men, which may be attributed to the effects of estrogen. While healthy young women convert about 21% of alpha-linolenic acid supplied alimentarily (through food) to EPA, only about 8% of alpha-linolenic acid from food is converted to EPA in healthy young men. To ensure endogenous synthesis of EPA, sufficient activity of both delta-6 and delta-5 desaturases is required. Both desaturases require certain micronutrients, particularly pyridoxine (vitamin B6), biotin, calcium, magnesium and zinc, to maintain their function. Deficiency of these micronutrients leads to a decrease in desaturase activity and subsequently to impaired EPA synthesis. In addition to micronutrient deficiency, delta-6 desaturase activity is also inhibited by the following factors:
- Increased intake of saturated and unsaturated fatty acids, such as oleic acid (C18:1; n-9-FS) and linoleic acid (C18:2; n-6-FS).
- Alcohol consumption in high doses and over a long period of time, chronic alcohol consumption.
- Increased cholesterol
- Insulin-dependent diabetes mellitus
- Viral infections
- Stress – release of lipolytic hormones, such as epinephrine, which leads to cleavage of triglycerides (TG, triple esters of the trivalent alcohol glycerol with three fatty acids) and release of saturated and unsaturated fatty acids through stimulation of triglyceride lipase.
- Aging
In addition to EPA synthesis from alpha-linolenic acid, delta-6 and delta-5 desaturase and fatty acid elongase are also responsible for the conversion of linoleic acid (C18:2; n-6-FS) to arachidonic acid (C20:4; n-6-FS) and oleic acid (C18:1; n-9-FS) to eicosatrienoic acid (C20:3; n-9-FS), respectively. Thus, alpha-linolenic acid and linoleic acid compete for the same enzyme systems in the synthesis of other biologically important polyunsaturated fatty acids, with alpha-linolenic acid having a higher affinity (binding strength) for delta-6 desaturase compared to linoleic acid.If, for example, more linoleic acid than alpha-linolenic acid is supplied in the diet, there is an increased endogenous synthesis of the proinflammatory (inflammation-promoting) omega-6 fatty acid arachidonic acid and a reduced endogenous synthesis of the anti-inflammatory (anti-inflammatory) omega-3 fatty acid EPA. This illustrates the relevance of a quantitatively balanced ratio of linoleic acid to alpha-linolenic acid in the diet. According to the German Nutrition Society (DGE), the ratio of omega-6 to omega-3 fatty acids in the diet should be 5:1 in terms of a preventively effective composition. The excessive intake of linoleic acid – in accordance with today’s diet (through cereal germ oils, sunflower oil, vegetable and diet margarine, etc.) and the suboptimal enzyme activity, especially of delta-6 desaturase due to frequent micronutrient deficiencies, nutrient interactions, hormonal influences, etc., are the reason why EPA synthesis from alpha-linolenic acid in humans is very slow and at a low level (maximum 10% on average), which is why EPA is considered an essential (vital) compound from today’s perspective. To reach the required amount of 1 g EPA, the intake of about 20 g pure alpha-linolenic acid – corresponding to about 40 g linseed oil – is necessary. However, this amount is not practical, which makes the consumption of EPA-rich cold–water fish, such as herring and mackerel, (2 fish meals/week, corresponding to 30-40 g fish/day) or the direct administration of EPA through fish oil capsules so significant. Only a diet rich in EPA ensures optimal concentrations of this highly unsaturated fatty acid in the human body.
Absorption
EPA can be present in the diet both in free form and bound in triglycerides (TG, triple esters of the trivalent alcohol glycerol with three fatty acids) and phospholipids (PL, phosphorus-containing amphiphilic lipids as essential components of cell membranes), which are subject to mechanical and enzymatic degradation in the gastrointestinal tract (mouth, stomach, small intestine). Through mechanical dispersion – chewing, gastric and intestinal peristalsis – and under the action of bile, dietary lipids are emulsified and thus broken down into small oil droplets (0.1-0.2 µm) that can be attacked by lipases (enzymes that cleave free fatty acids (FFS) from lipids → lipolysis). Pregastric (base of tongue, primarily in early infancy) and gastric (stomach) lipases initiate cleavage of triglycerides and phospholipids (10-30% of dietary lipids). However, the main lipolysis (70-90% of lipids) occurs in the duodenum (duodenal) and jejunum (jejunum) under the action of pancreatic (pancreatic) esterases, such as pancreatic lipase, carboxylester lipase, and phospholipase, whose secretion (secretion) is stimulated by cholecystokinin (CCK, peptide hormone of the gastrointestinal tract). The monoglycerides (MG, glycerol esterified with a fatty acid, such as EPA), lyso-phospholipids (glycerol esterified with a phosphoric acid), and free fatty acids, including EPA, resulting from TG and PL cleavage combine in the small intestinal lumen together with other hydrolyzed lipids, such as cholesterol, and bile acids to form mixed micelles (spherical structures 3-10 nm in diameter, in which the lipid molecules are arranged so that the water-soluble molecule portions are turned outward and the water-insoluble molecule portions are turned inward) – micellar phase for solubilization (increase in solubility) – that enable the uptake of lipophilic (fat-soluble) substances into the enterocytes (cells of the small intestinal epithelium) of the duodenum and jejunum. Diseases of the gastrointestinal tract associated with increased acid production, such as Zollinger-Ellison syndrome (increased synthesis of the hormone gastrin by tumors in the pancreas or upper small intestine), can lead to impaired absorption of lipid molecules and thus to steatorrhea (pathologically increased fat content in the stool), because the tendency to form micelles decreases with a decrease in pH in the intestinal lumen. Fat absorption under physiological conditions is between 85-95% and can occur by two mechanisms.On the one hand, MG, lyso-PL, cholesterol and EPA can pass through the phospholipid double membrane of enterocytes by means of passive diffusion due to their lipophilic nature, and on the other hand, by the involvement of membrane proteins, such as FABPpm (fatty acid-binding protein of the plasma membrane) and FAT (fatty acid translocase), which are present in other tissues besides the small intestine, such as liver, kidney, adipose tissue – adipocytes (fat cells), heart and placenta, to allow lipid uptake into the cells. A high-fat diet stimulates intracellular (inside the cell) expression of FAT. In enterocytes, EPA, which has been incorporated (taken up) as a free fatty acid or in the form of monoglycerides and released under the influence of intracellular lipases, is bound to FABPc (fatty acid-binding protein in the cytosol), which has a higher affinity for unsaturated than for saturated long-chain fatty acids and is expressed (formed) particularly in the brush border of the jejunum. Subsequent activation of protein-bound EPA by adenosine triphosphate (ATP)-dependent acyl-coenzyme A (CoA) synthetase (→ EPA-CoA) and transfer of EPA-CoA to ACBP (acyl-CoA-binding protein), which serves as an intracellular pool and transporter of activated long-chain fatty acids (acyl-CoA), enables the resynthesis of triglycerides and phospholipids in the smooth endoplasmic reticulum (richly branched channel system of planar cavities enclosed by membranes) on the one hand, and – by removing fatty acids from diffusion equilibrium – the incorporation of further fatty acids into enterocytes on the other. This is followed by incorporation of EPA-containing TG and PL, respectively, into chylomicrons (CM, lipoproteins) composed of lipids – triglycerides, phospholipids, cholesterol and cholesterol esters – and apolipoproteins (protein portion of lipoproteins, function as structural scaffolds and/or recognition and docking molecules, for example, for membrane receptors), such as apo B48, AI, and AIV, and are responsible for the transport of dietary lipids absorbed in the intestine to peripheral tissues and the liver. Instead of being stored in chylomicrons, EPA-containing TGs and PLs, respectively, can also be transported to tissues in VLDLs (very low density lipoproteins). The removal of absorbed dietary lipids by VLDL occurs particularly in the starvation state. Reesterification of lipids in enterocytes and their incorporation into chylomicrons may be impaired in certain diseases, such as Addison’s disease (adrenocortical insufficiency) and gluten-induced enteropathy (chronic disease of the mucosa of the small intestine due to gluten intolerance), resulting in decreased fat absorption and ultimately steatorrhea (pathologically increased fat content in the stool).
Transport and distribution
Lipid-rich chylomicrons (consisting of 80-90% triglycerides) are secreted (secreted) into the interstitial spaces of enterocytes by exocytosis (transport of substances out of the cell) and transported away via the lymph. Via the truncus intestinalis (unpaired lymphatic collecting trunk of the abdominal cavity) and ductus thoracicus (lymphatic collecting trunk of the thoracic cavity), the chylomicrons enter the subclavian vein (subclavian vein) and jugular vein (jugular vein), respectively, which converge to form the brachiocephalic vein (left side) – angulus venosus (venous angle). The venae brachiocephalicae of both sides unite to form the unpaired superior vena cava (superior vena cava), which opens into the right atrium of the heart. By the pumping force of the heart, chylomicrons are introduced into the peripheral circulation, where they have a half-life (time in which a value that decreases exponentially with time is halved) of approximately 30 minutes. During transport to the liver, most of the triglycerides from chylomicrons are cleaved into glycerol and free fatty acids, including EPA, under the action of lipoprotein lipase (LPL) located on the surface of endothelial cells of blood capillaries, which are taken up by peripheral tissues, such as muscle and adipose tissue, partly by passive diffusion and partly carrier-mediated – FABPpm; FAT. Through this process, chylomicrons are degraded to chylomicron remnants (CM-R, low-fat chylomicron remnant particles), which, mediated by apolipoprotein E (ApoE), bind to specific receptors in the liver.Uptake of CM-R into the liver occurs via receptor-mediated endocytosis (invagination of the cell membrane → strangulation of CM-R-containing vesicles (endosomes, cell organelles) into the cell interior). The CM-R-rich endosomes fuse with lysosomes (cell organelles with hydrolyzing enzymes) in the cytosol of liver cells, resulting in the cleavage of free fatty acids, including EPA, from the lipids in CM-Rs. After binding of the released EPA to FABPc, its activation by ATP-dependent acyl-CoA synthetase and transfer of EPA-CoA to ACBP, reesterification of triglycerides and phospholipids occurs. The resynthesized lipids may be further metabolized (metabolized) in the liver and/or incorporated into VLDL (very low density lipoproteins) to pass through them via the bloodstream to extrahepatic (“outside the liver”) tissues. As VLDL circulating in the blood binds to peripheral cells, the triglycerides are cleaved by action of LPL and the fatty acids released, including EPA, are internalized by passive diffusion and transmembrane transport proteins, such as FABPpm and FAT, respectively. This results in the catabolism of VLDL to IDL (intermediate density lipoproteins) and subsequently to LDL (low density lipoproteins; cholesterol-rich low density lipoproteins), which supplies peripheral tissues with cholesterol. In the cells of target tissues, such as blood, liver, brain, heart, and skin, EPA can be incorporated – depending on the function and needs of the cell – into the phospholipids of cell membranes as well as the membranes of cell organelles, such as mitochondria (“energy power plants” of cells) and lysosomes (cell organelles with acidic pH and digestive enzymes), used as a starting substance for the synthesis of anti-inflammatory (anti-inflammatory) eicosanoids (hormone-like substances that act as immune modulators and neurotransmitters), such as series 3 prostaglandins and series 5 leukotrienes, or stored in the form of triglycerides. Numerous studies have shown that the fatty acid pattern of phospholipids in cell membranes is strongly dependent on the fatty acid composition of the diet. Thus, high EPA intake causes an increase in the proportion of EPA in plasma membrane phospholipids by displacing arachidonic acid, thereby increasing membrane fluidity, which in turn has effects on membrane-ligand interactions, permeability (permeability), intercellular interactions, and enzyme activities.
Degradation
Catabolism (degradation) of fatty acids occurs in all body cells and is localized in mitochondria (“energy powerhouses” of cells). Exceptions are erythrocytes (red blood cells), which lack mitochondria, and nerve cells, which lack the enzymes that break down fatty acids. The reaction process of fatty acid catabolism is also called ß-oxidation, since oxidation occurs at the ß-C atom of the fatty acids. In ß-oxidation, the previously activated fatty acids (acyl-CoA) are oxidatively degraded to several acetyl-CoA (activated acetic acid consisting of 2 C atoms) in a cycle that is run repeatedly. In this process, acyl-CoA is shortened by 2 C atoms – corresponding to one acetyl-CoA – per “run”. In contrast to saturated fatty acids, whose catabolism occurs according to the ß-oxidation spiral, unsaturated fatty acids, such as EPA, undergo several conversion reactions during their degradation – depending on the number of double bonds – because they are cis-configured in nature (both substituents are on the same side of the reference plane), but for ß-oxidation they must be in trans-configuration (both substituents are on opposite sides of the reference plane). In order to be made available for ß-oxidation, EPA bound in triglycerides and phospholipids, respectively, must first be released by hormone-sensitive lipases. In starvation and stress situations, this process (→ lipolysis) is intensified due to increased release of lipolytic hormones such as adrenaline. EPA released during lipolysis can be fed directly to ß-oxidation in the same cell or also in other tissues to which it reaches via the bloodstream bound to albumin. In the cytosol of cells, EPA is activated by ATP-dependent acyl-CoA synthetase (→ EPA-CoA) and transported across the inner mitochondrial membrane into the mitochondrial matrix with the help of carnitine, a receptor molecule for activated long-chain fatty acids.In the mitochondrial matrix, EPA-CoA is introduced into ß-oxidation, the cycle of which is run once – as follows.
- Acyl-CoA → alpha-beta-trans-enoyl-CoA (unsaturated compound) → L-beta-hydroxyacyl-CoA → beta-ketoacyl-CoA → acyl-CoA (Cn-2).
The result is an EPA shortened by 2 C atoms, which must be enzymatically trans-configured at its cis double bond before entering the next reaction cycle. Since the first double bond of EPA – as seen from the COOH end of the fatty acid chain – is localized on an odd-numbered C atom (→ beta-gamma-cis-enoyl-CoA), isomerization to alpha-beta-trans-enoyl-CoA, which is an intermediate of ß-oxidation, occurs directly under the action of an isomerase. After two ß-oxidation cycles have been run again and the fatty acid chain has been shortened by a further 2 x 2 C atoms, the trans configuration of the next cis double bond of EPA takes place, which – viewed from the COOH end of the fatty acid chain – is located on an even-numbered C atom (→ alpha-beta-cis-enoyl-CoA). For this purpose, alpha-beta-cis-enoyl-CoA is hydrated to D-beta-hydroxyacyl-CoA by a hydratase (enzyme that incorporates H2O into a molecule) and subsequently isomerized to L-beta-hydroxyacyl-CoA by an epimerase (enzyme that changes the asymmetric arrangement of a C atom in a molecule). The latter can be directly introduced into its reaction cycle as an intermediate of ß-oxidation. Until the activated EPA is completely degraded to acetyl-CoA, 3 further conversion reactions (2 isomerase reactions, 1 hydratase-epimerase reaction) and 5 further ß-oxidation cycles are necessary, so that ß-oxidation is run through 9 times in total, 5 conversion reactions (3 isomerase, 2 hydratase-epimerase reactions) – corresponding to 5 existing cis-double bonds – take place and 10 acetyl-CoA as well as reduced coenzymes (9 NADH2 and 4 FADH2) are formed. The acetyl-CoA resulting from EPA catabolism are introduced into the citrate cycle, in which oxidative degradation of organic matter occurs for the purpose of obtaining reduced coenzymes, such as NADH2 and FADH2, which together with the reduced coenzymes from ß-oxidation in the respiratory chain are used to synthesize ATP (adenosine triphosphate, universal form of immediately available energy). Although unsaturated fatty acids require conversion reactions (cis → trans) during ß-oxidation, whole-body analyses in fat-free fed rats revealed that labeled unsaturated fatty acids exhibit similar rapid degradation as saturated fatty acids.
Excretion
Under physiological conditions, fat excretion in feces should not exceed 7% at a fat intake of 100 g/day because of the high absorption rate (85-95%). Malassimilation syndrome (impaired nutrient utilization due to reduced breakdown and/or absorption), for example, due to inadequate bile acid and pancreatic juice secretion and small intestinal disease, respectively, may lead to reduction of intestinal fat absorption and thus to steatorrhea (pathologically increased fat content (>7%) in the stool).
