Linoleic acid (LA) is an organic compound with the formula . 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. 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. It typically occurs in nature as a triglyceride (ester of glycerin) rather than as a free fatty acid. It is one of two essential fatty acids 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 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. Its essential role in human diet was discovered by G. O. Burr 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: ; 18:2, ω-6) is a precursor to arachidonic acid (AA: ; 20:4, ω-6) with elongation and saturation. AA is the precursor to some prostaglandins, leukotrienes (LTA, LTB, LTC), thromboxane (TXA) and the N-acylethanolamine (NAE) arachidonoylethanolamine (AEA: ; 20:4, ω-6), and other endocannabinoids and eicosanoids. The metabolism of LA to AA begins with the conversion of LA into gamma-linolenic acid (GLA), effected by Δ6desaturase. 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. Hydroperoxides derived from the metabolism of anandamide (AEA: ; 20:4,n-6), or its linoleoyl analogues,