Nomenclature
- See related detail at Essential Fatty Acid Interactions—Nomenclature
"Eicosanoid" (eicosa-, Greek for "twenty"; see icosahedron) is the collective term for oxygenated derivatives of three different 20-carbon essential fatty acids:
- Eicosapentaenoic acid (EPA), an ω-3 fatty acid with 5 double bonds;
- Arachidonic acid (AA), an ω-6 fatty acid, with 4 double bonds;
- Dihomo-gamma-linolenic acid (DGLA), an ω-6, with 3 double bonds.
Current usage limits the term to the leukotrienes (LT) and three types of prostanoids—prostaglandins (PG) prostacyclins (PGI), and thromboxanes (TX). This is the definition used in this article. However, several other classes can technically be termed eicosanoid, including the hepoxilins, resolvins, isofurans, isoprostanes, lipoxins, epi-lipoxins, epoxyeicosatrienoic acids (EETs) and endocannabinoids. LTs and prostanoids are sometimes termed 'classic eicosanoids' in contrast to the 'novel', 'eicosanoid-like' or 'nonclassic eicosanoids'.
A particular eicosanoid is denoted by a four-character abbreviation, composed of:
- Its two letter abbreviation (above),
- One A-B-C sequence-letter; and
- A subscript, indicating the number of double bonds.
Examples are:
- The EPA-derived prostanoids have three double bonds, (e.g. PGG3, PGH3, PGI3, TXA3) while its leukotrienes have five, (LTB5).
- The AA-derived prostanoids have two double bonds, (e.g. PGG2, PGH2, PGI2, TXA2) while its leukotrienes have four, (LTB4).
Furthermore, stereochemistry may differ among the pathways, indicated by Greek letters, e.g. for (PGF2α).
Read more about this topic: Eicosanoid