Polyunsaturated fatty acids are recognized to be essential for humans and animals for two reasons: synthesis of lipid mediators and production of membranes that have the optimum lipid bilayer structure and functional properties. The main function of ro-3 fatty acids is related to membrane structure, and DHA plays a role. The polyunsaturated ro-6 fatty acids have a role in formation of lipid mediators, which includes the eicosanoids and the inositol phosphoglycerides. Arachidonic acid is the main fatty acid of the ro-6 family involved in this process; however, EPA also can play a role in the formation of eicosanoids.3
The ro-3 fatty acids are major structural components of membrane phospholipids of tissues and influence membrane fluidity and ion transports. These polyunsaturated fatty acids are rich in the brain, myocardium, and retina, are essential for proper functioning and growth, and modulate many physiological processes.26 Along with arachidonic acid, DHA is the major polyunsaturated fatty acid found in the brain and is recognized to be important for brain development and function.29 These two types of fatty acids have an effect on the neuronal membrane fluidity index, and it appears that the relative amounts of ro-3 and ro-6 fatty acids in the cell membranes are responsible for affecting cellular function.30 Reported effects of polyunsaturated fatty acids on brain functions are modifications of membrane fluidity, the activity of membrane-bound enzymes, the number and affinity of receptors, the function of ion channels, and the production and activity of neurotransmitters and signal transduction.31
Omega-3 fatty acids have been recognized to lower the risk of coronary heart disease (CHD) and protect against sudden cardiac death through antiarrhythmic, antiatherogenic, antithrombotic, and vasoprotective mechanisms. Omega-3 fatty effects on inhibition of platelet aggregation and serum lipids (triglycerides and high-density lipoprotein [HDL]) may prevent CHD. However, the antiarrhythmic rather than the antiatherothrombotic effect seems to be the major function of ro-3 fatty acids in preventing CHD.26
The ro-3 fatty acid DHA is also essential for proper development of the retina, particularly at the synapse and the outer segment of photoreceptors,32 by influencing membrane fluidity in the temporal response of the G-protein-coupled signaling system in the retinal rod outer segment (ROS).829
One of the most characterized physiological functions of these polyunsaturated fatty acids is the role of eicosanoids. Eicosanoids work as autocrine/paracrine hormones and mediate a variety of functions, such as immune response, blood pressure regulation, blood coagulation,33 movement of calcium and other substances into and out of cells, relaxation and contraction of muscles, and cell division and growth.34 The eicosanoids include substances such as prostaglandins and leukotrienes. Arachi-donic acid is the precursor for a group of eicosanoids that include series-2 prostaglandins and series-4 leukotrienes, while EPA is the precursor for a group of eicosanoids that include series-3 prostaglandins and series-5 leukotrienes.28 Eicosanoids derived from ro-6 fatty acids are pro-inflammatory and pro-aggregatory agonists, while those derived from ro-3 fatty acids tend to inhibit platelet aggregation and be anti-inflammatory.3536 Adequate production of the series-3 prostaglandins is thought to have protective effects against heart attacks.28
The effects of ro-3 and ro-6 fatty acids in inflammation and autoimmune diseases have been linked to the concentration of the type of prostaglandins and leukotrienes present in the cells. EPA-derived eicosanoids are less potent inducers of inflammation than the arachidonic acid-derived eicosanoids,5 and competition between ro-3 and ro-6 fatty acids occurs in the prostaglandins' formation.36 For instance, an increased level of interleukin (IL) 1, a pro-inflamatory cytokine, is present in CHD, depression, aging, and cancer. Similarly, high levels of IL1 and series-4 leukotrienes produced by ro-6 fatty acids are present in arthritis and other autoimmune diseases.
Other probable effects of ro-3 fatty acids are membrane-mediated processes, such as insulin transduction signals; activity of lipases, which are affected by alteration of the fatty acid composition of the membrane phospholipids; and regulation of genes involved in lipid and glucose metabolism, and adipogenesis.37
A newly discovered metabolite (10,17S-docosatriene) synthesized in the brain has been indicated to have a response to an ischemic insult and to have opposite anti-inflammatory effects.29
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