Linoleic acid is a polyunsaturatedomega-6 fatty acid and is one of two essential fatty acids for humans, who must obtain it through their diet. It is a colorless or white oil that is virtually insoluble in water. The word "linoleic" derives from the Greek word linon. Oleic means "of, relating to, or derived from oil of olive" or "of or relating to oleic acid" because saturating the omega-6 double bond produces oleic acid.
Chemistry
Linoleic acid is a fatty acid. It is an 18-carbon chain with two double bondsin cis configuration. A shorthand notation like "18:2 " or "18:2 cis-9,12" may be used in literature. It typically occurs in nature as a triglyceride ester; free fatty acids, the form not combined with glycerol to form triglyceride, are typically low in foods. It is very soluble in acetone, benzene, diethyl ether and ethanol.
In physiology
Linoleic acid is a polyunsaturated fatty acid used in the biosynthesis of arachidonic acid with elongation and saturation, and thus some prostaglandins, leukotrienes, and thromboxane. It is found in the lipids of cell membranes. It is abundant in safflower, sunflower, corn, and comprises over half their composition by weight. It is present in medium quantities in soybean oils, sesame, and almonds. The consumption of linoleic acid is vital to proper health, as it is an essential fatty acid. In rats, a diet deficient in linoleate has been shown to cause mild skin scaling, hair loss, and poor wound healing. However, chronic consumption of high levels of LA may be associated with the development of ulcerative colitis. Cockroaches release oleic and linoleic acid upon death, which discourages other roaches from entering the area. This is similar to the mechanism found in ants and bees, which release oleic acid upon death.
Metabolism and eicosanoids
The first step in the metabolism of linoleic acid is performed by Δ6desaturase, which converts LA into gamma-Linolenic acid. There is evidence suggesting that infants lack Δ6desaturase of their own, and must acquire it through breast milk. Studies show that breast-milk fed babies have higher concentrations of GLA than formula-fed babies, while formula-fed babies have elevated concentrations of LA. GLA is converted to dihomo-γ-linolenic acid, which in turn is converted to arachidonic acid. One of the possible fates of AA is to be transformed into a group of metabolites called eicosanoids during the inflammatory response and during physical activity; eicosanoids are a class of paracrine hormones. The three types of eicosanoids are prostaglandins, thromboxanes, and leukotrienes. Eicosanoids produced from AA tend to promote inflammation and promote growth during and after physical activity in healthy humans. For example, both AA-derived thrombaxane and leukotrieneB4 are proaggregatory and vasoconstrictive eicosanoids during inflammation. The oxidized metabolic products of linoleic acid, such as 9-hydroxyoctadecanoic acid and 13-hydroxyoctadecanoic acid, have also been shown to activate TRPV1, the capsaicin receptor, and through this might play a major role in hyperalgesia and allodynia. There are some suggested negative health effects related to this inflammation promoting function of linoleic acid as an omega-6 fatty acid. LA is also converted by various lipoxygenases, cyclooxygenases, certain cytochrome P450 enzymes, and non-enzymatic autoxidation mechanisms to mono-hydroxyl products viz., 13-Hydroxyoctadecadienoic acid, and 9-Hydroxyoctadecadienoic acid; these two hydroxy metabolites are enzymatically oxidized to their keto metabolites, 13-oxo-octadecadienoic acid and 9-oxo-octadecdienoic acid. Certain cytochrome P450 enzymes, the CYP epoxygenases, metabolize LA to epoxide products viz., its 12,13-epoxide, Vernolic acid, and its 9,10-epoxide, Coronaric acid. All of these linoleic acid products have bioactivity and are implicated in human physiology and pathology.
Uses
Industrial uses
Linoleic acid is used in making quick-drying oils, which are useful in oil paints and varnishes. These applications exploit the easy reaction of the linoleic acid with oxygen in air, which leads to crosslinking and formation of a stable film called linoxyn. Reduction of linoleic acid yields linoleyl alcohol. Linoleic acid is a surfactant with a critical micelle concentration of 1.5 x 10−4 M @ pH 7.5. Linoleic acid has become increasingly popular in the beauty products industry because of its beneficial properties on the skin. Research points to linoleic acid's anti-inflammatory, acne reductive, skin-lightening and moisture retentive properties when applied topically on the skin.
Use in research
Linoleic acid lipid radicals can be used to show the antioxidant effect of polyphenols and natural phenols. Experiments on linoleic acid subjected to 2,2'-Azobis dihydrochloride induced oxidation of linoleic acid, thus producing lipid radicals; the use of different combinations of phenolics then show that binary mixtures can lead to either a synergetic antioxidant effect, or to an antagonistic effect towards the lipid radicals. Research like this is useful in discovering which phenols prevent the autoxidation of lipids in vegetable oils.
