Linoleic acid

Linoleic acid
Names
Preferred IUPAC name
(9Z,12Z)-Octadeca-9,12-dienoic acid
Other names
cis,cis-9,12-Octadecadienoic acid
C18:2 (Lipid numbers)
Identifiers
3D model (JSmol)
3DMet
1727101
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.428
EC Number
  • 200-470-9
57557
KEGG
UNII
  • InChI=1S/C18H32O2/c1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18(19)20/h6-7,9-10H,2-5,8,11-17H2,1H3,(H,19,20)/b7-6-,10-9- checkY
    Key: OYHQOLUKZRVURQ-HZJYTTRNSA-N checkY
  • InChI=1/C18H32O2/c1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18(19)20/h6-7,9-10H,2-5,8,11-17H2,1H3,(H,19,20)/b7-6-,10-9-
    Key: OYHQOLUKZRVURQ-HZJYTTRNBX
  • CCCCC/C=C\C/C=C\CCCCCCCC(=O)O
Properties
C18H32O2
Molar mass 280.452 g·mol−1
Appearance Colorless oil
Density 0.9 g/cm3[1]
Melting point −12 °C (10 °F)[1]
−6.9 °C (19.6 °F)[2]
−5 °C (23 °F)[3]
Boiling point 229 °C (444 °F) at 16 mmHg[2]
230 °C (446 °F) at 21 mbar[3]
230 °C (446 °F) at 16 mmHg[1]
0.139 mg/L[3]
Vapor pressure 16 Torr at 229 °C
Acidity (pKa) 9.24[4]
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
2
1
0
Flash point 112 °C (234 °F)[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references
Not to be confused with linolenic acid, alpha-linolenic acid, lipoic acid, or linolelaidic acid.

Linoleic acid (LA) is an organic compound with the formula HOOC(CH
2)
7CH=CHCH
2CH=CH(CH
2)
4CH
3. 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 or ester of this acid.[5]

Linoleic acid is a polyunsaturated omega-6 fatty acid. It is a colorless liquid that is virtually insoluble in water but soluble in many organic solvents.[2] It typically occurs in nature as a triglyceride (ester of glycerin) rather than as a free fatty acid.[6] It is one of two essential fatty acids for humans, who must obtain it through their diet,[7] and the most essential, because the body uses it as a base to make the others.

The word "linoleic" derives from the Latin 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.[8][9] In 1886, K. Peters determined the existence of two double bonds.[10] Its essential role in human diet was discovered by G. O. Burr and others in 1930.[11] 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.[12]

In physiology
Main articles: Fatty acid desaturase and Fatty acid synthesis

The consumption of linoleic acid is vital to proper health, as it is an essential fatty acid.[13]

Metabolism and eicosanoids

Linoleic acid (LA: C
18H
32O
2; 18:2, ω-6) is a precursor to arachidonic acid (AA: C
20H
32O
2; 20:4, ω-6) with elongation and unsaturation.[13] AA is the precursor to some prostaglandins,[14] leukotrienes (LTA, LTB, LTC), thromboxane (TXA)[15] and the N-acylethanolamine (NAE) arachidonoylethanolamine (AEA: C
22H
37NO
2; 20:4, ω-6),[16] and other endocannabinoids and eicosanoids.[17]

The metabolism of LA to AA begins with the conversion of LA into gamma-linolenic acid (GLA), effected by Δ6-desaturase.[18] GLA is converted to dihomo-γ-linolenic acid (DGLA), the immediate precursor to AA.

LA is also converted by various lipoxygenases, cyclooxygenases, cytochrome P450 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. These linoleic acid products are implicated in human physiology and pathology.[19]

Hydroperoxides derived from the metabolism of anandamide (AEA: C
22H
37NO
2; 20:4,n-6), or its linoleoyl analogues, are by a lipoxygenase action found to be competitive inhibitors of brain and immune cell FAAH, the enzyme that breaks down AEA and other endocannabinoids, and the compound linoleoyl-ethanol-amide (C
20H
37NO
2; 18:2,n-6), an N-acylethanolamine, - the ethanolamide of linoleic acid (LA: C
18H
32O
2; 18:2, n-6) and its metabolized incorporated ethanolamine (MEA: C
2H
7NO),[20] is the first natural inhibitor of FAAH, discovered.[21][22]

Uses and reactions

Linoleic acid is a component of quick-drying oils, which are useful in oil paints and varnishes. These applications exploit the lability of the doubly allylic C−H groups (−CH=CH−CH2−CH=CH−) toward oxygen in air (autoxidation). Addition of oxygen leads to crosslinking and formation of a stable film.[23]

Reduction of the carboxylic acid group of linoleic acid yields linoleyl alcohol.[24]

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.[25][26][27][28]

Linoleic acid is also used in some bar of soap products.

Dietary sources
See also: Vegetable oil § Composition of fats

It is abundant in safflower, and corn oil, and comprises over half their composition by weight. It is present in medium quantities in soybean oils, sesame, and almonds.[29][30]

Name% LA†ref.
Salicornia oil75%
Safflower oil72-78%[31]
Evening Primrose oil65-80%[32]
Melon seed oil70%
Poppyseed oil70%
Grape seed oil69.6%
Prickly Pear seed oil63%
Artichoke oil60%
Hemp oil54.3%[33]
Wheat germ oil55%
Cottonseed oil54%
Corn oil51.9%[34]
Walnut oil51%
Soybean oil50.9%[35]
Sesame oil45%
Rice bran oil39%
Argan oil37%
Pistachio oil32.7%
Peach oil29%[36]
Almonds24%
Canola oil17.8%[37]
Sunflower oil20.5%[38]
Chicken fat18-23%[39]
Peanut oil19.6%[40]
Egg yolk16%
Linseed oil (flax), cold pressed14.2%[41]
Lard10%
Palm oil10%
Olive oil8.4%[42]
Cocoa butter3%
Macadamia oil2%
Butter2%
Coconut oil2%
  †average val

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.[43]

Health effects

Consumption of linoleic acid has been associated with lowering the risk of cardiovascular disease, diabetes and premature death.[44][45][46] There is high-quality evidence that increased intake of linoleic acid decreases total blood cholesterol and low-density lipoprotein.[47]

The American Heart Association advises people to replace saturated fat with linoleic acid to reduce CVD risk.[48]

See also

References
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