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. The ...

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 quantitie ...

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

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 A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''wikt:tri-#Prefix, tri-'' and ''glyceride''). Triglycerides are the main constituents of body fat in humans and other ...

(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 Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colourless to yellowish oil obtained from the dried, ripened seeds of the flax plant (''Linum usitatissimum''). The oil is obtained by pressing, sometimes followed by ...

History

In 1844, F. Sacc, working at the laboratory of

Justus von Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at t ...

, 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

prostaglandin The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derive ...

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 its ...

(TXA) and the

N-acylethanolamine An ''N''-acylethanolamine (NAE) is a type of fatty acid amide where one of several types of acyl groups is linked to the nitrogen atom of ethanolamine, and highly metabolic formed by intake of essential fatty acids through diet by 20:4, n-6 an ...

(NAE)

arachidonoylethanolamine 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 rec ...

(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 Dihomo-γ-linolenic acid (DGLA) is a 20-carbon ω−6 fatty acid. (also called, cis,cis,cis-8,11,14-Eicosatrienoic acid) In physiological literature, it is given the name 20:3 (ω−6). DGLA is a carboxylic acid with a 20-carbon chain and thr ...

(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 Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...

enzymes (the CYP

monooxygenase Monooxygenases are enzymes that incorporate one hydroxyl group (−OH) into substrates in many metabolic pathways. In this reaction, the two atoms of dioxygen are reduced to one hydroxyl group and one H2O molecule by the concomitant oxidation o ...

s), and non-enzymatic

autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...

mechanisms to mono-

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

products viz., 13-Hydroxyoctadecadienoic acid, and

9-Hydroxyoctadecadienoic acid 9-Hydroxyoctadecadienoic acid (or 9-HODE) has been used in the literature to designate either or both of two stereoisomer metabolites of the essential fatty acid, linoleic acid: 9(''S'')-hydroxy-10(''E''),12(''Z'')-octadecadienoic acid (9(''S'')- ...

; 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

epoxygenase Epoxygenases are a set of membrane-bound, heme-containing cytochrome P450 (CYP P450 or just CYP) enzymes that metabolize polyunsaturated fatty acids to epoxide products that have a range of biological activities. The most thoroughly studied substr ...

s, catalyze oxidation of LA to epoxide products viz., its 12,13-epoxide,

Vernolic acid Vernolic acid (leukotoxin) is a long chain fatty acid that is monounsaturated and contains an epoxide. It is the R,R-''cis'' epoxide derived from the C12–C13 alkene of linoleic acid. Vernolic acid was first definitively characterized in 1954. I ...

, 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 Hydroperoxides or peroxols are compounds containing the hydroperoxide functional group (ROOH). If the R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. O ...

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 rec ...

(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 v ...

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. A ...

that breaks down AEA and other endocannabinoids, and the compound linoleoyl-ethanol-amide (C₂₀H₃₇NO₂; 18:2,n-6), an

, - the

ethanolamide Ethanolamides are chemical compounds which are amides formed from carboxylic acids and ethanolamine. Some ethanolamides are naturally occurring, such as anandamide, palmitoylethanolamide and prostamides, which play physiological roles as lipid neur ...

of linoleic acid (LA: C₁₈ H₃₂ O₂; 18: 2, n-6) and its metabolized incorporated

ethanolamine Ethanolamine (2-aminoethanol, monoethanolamine, ETA, or MEA) is an organic chemical compound with the formula or . The molecule is bifunctional, containing both a primary amine and a primary alcohol. Ethanolamine is a colorless, viscous liquid w ...

(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 paint Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. The viscosity of the paint may be modified by the addition of a solvent such as turpentine or white spirit, and varn ...

s 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 (

). Addition of oxygen leads to

crosslinking Cross-linking may refer to *Cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers ca ...

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 Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...

oil, and comprises over half their composition by weight. It is present in medium quantities in

soybean oil Soybean oil (British English: soyabean oil) is a vegetable oil extracted from the seeds of the soybean (''Glycine max''). It is one of the most widely consumed cooking oils and the second most consumed vegetable oil. As a drying oil, processed s ...

sesame Sesame ( or ; ''Sesamum indicum'') is a flowering plant in the genus ''Sesamum'', also called benne. Numerous wild relatives occur in Africa and a smaller number in India. It is widely naturalized in tropical regions around the world and is cu ...

, and

almonds The almond (''Prunus amygdalus'', Synonym (taxonomy)#Botany, syn. ''Prunus dulcis'') is a species of tree native to Iran and surrounding countries, including the Levant. The almond is also the name of the edible and widely cultivated seed of th ...

Other occurrences

Cockroach Cockroaches (or roaches) are a paraphyletic group of insects belonging to Blattodea, containing all members of the group except termites. About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known as ...

es 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 Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, h ...

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