Linoleic acid (LA) is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

with the formula COOH(CH₂)₇CH=CHCH₂CH=CH(CH₂)₄CH₃. Both alkene groups are ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

or ester of this acid. Linoleic acid is a

polyunsaturated In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food. The term often refers specifically to triglycerides (triple es ...

omega-6 fatty acid Omega-6 fatty acids (also referred to as ω-6 fatty acids or ''n''-6 fatty acids) are a family of polyunsaturated fatty acids that have in common a final carbon-carbon double bond in the ''n''-6 position, that is, the sixth bond, counting from ...

. It is a colorless liquid that is virtually insoluble in water but soluble in many organic solvents. It typically occurs in nature as a triglyceride (ester of

glycerin Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

) rather than as a

free fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

. It is one of two

essential fatty acid Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them. Only two fatty acids are known to be essential for humans: alpha-linolenic ac ...

s for humans, who must obtain it through their diet, and the most essential, because the body uses it as a base to make the others. The word "linoleic" derives from the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

''linum'' "flax" + ''oleum'' "oil", reflecting the fact that it was first isolated from linseed oil.

History

In 1844, F. Sacc, working at the laboratory of Justus von Liebig, isolated linoleic acid from linseed oil. In 1886, K. Peters determined the existence of two double bonds.(1886), ''Monatsch''., volume 7, pages 522- Its essential role in human diet was discovered by

G. O. Burr G is the seventh letter of the Latin alphabet. G may also refer to: Places * Gabon, international license plate code G * Glasgow, UK postal code G * Eastern Quebec, Canadian postal prefix G * Melbourne Cricket Ground in Melbourne, Australia, g ...

and others in 1930. Its chemical structure was determined by T.P. Hilditch and others in 1939, and it was synthesized by R. A. Raphael and F. Sondheimer in 1950.

In physiology

The consumption of linoleic acid is vital to proper health, as it is an essential fatty acid.

Metabolism and eicosanoids

Linoleic acid (LA: C₁₈ H₃₂ O₂; 18: 2, ω-6) is a precursor to arachidonic acid (AA: C₂₀H₃₂O₂; 20: 4, ω-6) with elongation and saturation. AA is the precursor to some prostaglandins,

leukotriene Leukotrienes are a family of eicosanoid inflammatory mediators produced in leukocytes by the oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme arachidonate 5-lipoxygenase. Leukotrienes ...

s (LTA, LTB, LTC),

thromboxane Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring. Thromboxane is named for i ...

(TXA) and the N-acylethanolamine (NAE) arachidonoylethanolamine (AEA: C₂₂ H₃₇ NO₂; 20:4, ω-6), and other

endocannabinoids Cannabinoids () are several structural classes of compounds found in the cannabis plant primarily and most animal organisms (although insects lack such receptors) or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid tet ...

and

eicosanoids Eicosanoids are signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosanoids are a sub- ...

. The metabolism of LA to AA begins with the conversion of LA into gamma-linolenic acid (GLA), effected by Δ⁶desaturase.Evidence suggests that infants must acquire Δ⁶desaturase breast milk. 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 (DGLA), the immediate precursor to AA. LA is also converted by various

lipoxygenase Lipoxygenases () are a family of (non-heme) iron-containing enzymes most of which catalyze the dioxygenation of polyunsaturated fatty acids in lipids containing a cis,cis-1,4- pentadiene into cell signaling agents that serve diverse roles as aut ...

cyclooxygenase Cyclooxygenase (COX), officially known as prostaglandin-endoperoxide synthase (PTGS), is an enzyme (specifically, a family of isozymes, ) that is responsible for formation of prostanoids, including thromboxane and prostaglandins such as pr ...

cytochrome P450 Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...

enzymes (the CYP monooxygenases), 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, catalyze oxidation of LA to epoxide products viz., its 12,13-epoxide, Vernolic acid, and its 9,10-epoxide,

Coronaric acid Coronaric acid (isoleukotoxin) is a mono-unsaturated, epoxide derivative of the di-saturated fatty acid, linoleic acid (i.e. 9(''Z''),12(''Z'') octadecadienoic acid. It is a mixture of the two optically active isomers of 12(''Z'') 9,10-epoxy-octa ...

. These linoleic acid products are implicated in human physiology and pathology. Hydroperoxides derived from the metabolism of

anandamide Anandamide (ANA), also known as ''N''-arachidonoylethanolamine (AEA), is a fatty acid neurotransmitter. Anandamide was the first endocannabinoid to be discovered: it participates in the body's endocannabinoid system by binding to cannabinoid r ...

(AEA: C₂₂H₃₇ NO₂; 20:4,n-6), or its linoleoyl analogues, is by a

action found to be competitive inhibitors of

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...

and

immune cell White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from multi ...

FAAH, the

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

that breaks down AEA and other endocannabinoids, and the compound linoleoyl-ethanol-amide (C₂₀H₃₇NO₂; 18:2,n-6), an N-acylethanolamine, - the ethanolamide of linoleic acid (LA: C₁₈ H₃₂ O₂; 18: 2, n-6) and its metabolized incorporated ethanolamine (MEA: C₂H₇NO), is the first natural inhibitor of FAAH, discovered.

Uses and reactions

Linoleic acid is a component of quick-

drying oil A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air, at room temperature. The oil hardens through a chemical reaction in which the components crosslink (and hence, polymerize) by the action of oxygen (no ...

s, which are useful in oil paints and varnishes. These applications exploit the lability of the doubly allylic C-H groups (-CH=CH-C''H''₂-CH=CH-) toward oxygen in air ( autoxidation). Addition of oxygen leads to crosslinking and formation of a stable film. Reduction of the carboxylic acid group 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. Linoleic acid is also used in some bar of soap products.

Dietary sources

It is abundant in

safflower Safflower (''Carthamus tinctorius'') is a highly branched, herbaceous, thistle-like annual plant in the family Asteraceae. It is commercially cultivated for vegetable oil extracted from the seeds and was used by the early Spanish colonies along ...

, and corn oil, and comprises over half their composition by weight. It is present in medium quantities in soybean oils, sesame, and almonds.

Other occurrences

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.

Health effects

Consumption of linoleic acid has been associated with lowering the risk of cardiovascular disease and premature death.

References

F. Sacc (1844) "Ueber das Leinöl, seine physicalischen und chemischen Eigenscharften und seine Oxydationsproducte". ''Liebig Annalen'', volume 51, pages 213-230. F. Sacc (1845):
Expériences sur les propriétés physiques et chimiques de l'huile de Lin
. ''SChweizer. Gesell. N. Dekschr.'', volume 7 (1930: ''J Biol Chem'', volume 86, pages 587- R. A. Raphael and Franz Sondheimer (1950): "The synthesis of long-chain aliphatic acids from acetylenic compounds. Part III. The synthesis of linoleic acid". ''Journal of the Chemical Society'' (''Resumed''), article 432,

External links

Linoleic acid MS Spectrum
AOCS Lipid Library {{DEFAULTSORT:Linoleic Acid 5α-Reductase inhibitors Alkenoic acids Aromatase inhibitors Essential fatty acids Essential nutrients Fatty acids