Linoleic acid: structure, properties, metabolism, food sources

Chemical structure of linoleic acid

Linoleic acid (LA) (18 carbon atoms), from the Latin linon, meaning flax, plus oleic, meaning oil or olive oil, was isolated by Sacc F. in 1844 from linseed oil.
The exact structure was clarified by Hilditch T.P. et al. in 1939, and was synthesized by Raphael R.A. and Sondheimer F. in 1950.
It is a polyunsaturated fatty acid (PUFA) with two cis (Z) double bounds (the first one from the methyl end is in omega-6 (ω-6) or n-6, so in shorthand 18:2n-6) member of the sub-group called long chain fatty acids (LCFA), from 14 to 18 carbon atoms.

Properties of linoleic acid

Linoleic Acid
Fig. 1 – LA

Molecular weight: 280.44548 g/mol
Molecular formula: C18H32O2
IUPAC name: (9Z,12Z)-octadeca-9,12-dienoic acid
CAS registry number: 60-33-3
PubChem: 5280450

In purified form it is a colorless oil easily oxidized by air, with melting point at -12 °C (10.4 °F; 261.15 K) and boiling point at 202 °C (395.6 °F; 475.15 K) at 1.4 mm Hg or 230 °C (446 °F; 503.15 K) at 16 mm Hg.

Other names of linoleic acid

  • LA
  • (Z,Z)-9,12-octadecadienoic acid
  • a-linoleic acid
  • linolic acid
  • 9-cis,12-cis-linoleic acid
  • 9Z,12Z-linoleic acid
  • cis-delta9,12-octadecadienoic acid
  • 9,12-linoleic acid
  • 18:2n-6

 Metabolism of linoleic acid

Metabolism of Linoleic Acid
Fig. 2 – Metabolism of LA

It is produced de novo, from oleic acid, only by plant.
It is a primary product of plant polyunsaturated fatty acid synthesis, and the commonest one in plants and animal tissue.
Animals can’t synthesize it because they lack of Δ12-desaturase, the enzyme that catalyzes its synthesis, and are obliged to obtain it from plant foodstuff: so it is an essential fatty acid (EFA).

Linoleic acid, omega-6, and arachidonic acid

Linoleic Acid
Fig. 3 – Omega-6 Fatty Acid Synthesis

It is the precursor of all the omega-6 fatty acids.
It is converted in gamma-linolenic acid in the reaction catalyzed by Δ6-desaturase; then it is elongated to form dihomo-gamma-linolenic acid, which in turn is desaturated in the reaction catalyzed by Δ5-desaturase to form arachidonic acid, which is elongated to form adrenic acid (so far all omega-6).
LA is important in that it is required for the synthesis of arachidonic acid, the precursor of many eicosanoids, such as:

  • prostaglandins;
  • thromboxanes;
  • leukotrienes.

For this role in eicosanoids synthesis, its deficiency causes growth retardation, wound healing retardation and dermatitis, reproductive failure, fatty liver and polydipsia.

Sources of linoleic acid

Although nature produces LA at concentrations fairly equitable with those of alpha-linolenic acid, modern agriculture has greatly enriched its plant content and consequently some plant foodstuff and, in minor extent, livestock fed diets particularly rich in this fatty acid, can be good source for humans.
It occurs in large amount as glycerol ester in fats of walnut, peanut, seeds of cotton, poppy, sunflower, grape, corn, sesame and soy and in ther oils.
With oleic and palmitic acids, it is one of the three most abundant fatty acids in triacylglycerols of adipose tissue and plasma lipoproteins.

References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

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