Eicosanoid Series Nomenclature
- For details on the metabolic pathways for eicosanoids in each series, see the main articles for prostaglandins (PG), thromboxanes (TX), prostacyclins (PGI) and leukotrienes (LK).
Eicosanoids are signalling molecules derived from the essential fatty acids (EFA); they are a major pathway by which the EFAs act in the body. There are four classes of eicosanoid and two or three series within each class. Before discussing eicosanoid action, we will explain the series nomenclature.
Cell's outer membranes contain phospholipid fat. Each phospholipid molecule contains two fatty acids. Some of these fatty acids are 20-carbon polyunsaturated essential fatty acids – AA, EPA or DGLA. In response to a variety of inflammatory signals, these EFAs are cleaved out of the phospholipid and released as free fatty acids. Next, the EFA is oxygenated (by either of two pathways), then further modified, yielding the eicosanoids. Cyclooxygenase (COX) oxidation removes two C=C double bonds, leading to the TX, PG and PGI series. Lipoxygenase oxidation removes no C=C double bonds, and leads to the LK.
After oxidation, the eicosanoids are further modified, making a series. Members of a series are differentiated by an ABC... letter, and are numbered by the number of double bonds, which does not change within a series. For example, cyclooxygenase action upon AA (with 4 double bonds) leads to the series-2 thromboxanes (TXA2, TXB2... ) each with two double bonds. Cyclooxygenase action on EPA (with 5 double bonds) leads to the series-3 thromboxanes (TXA3, TXB3... ) each with three double bonds. There are exceptions to this pattern, some of which indicate stereochemistry (PGF2α).
Table (1) shows these sequences for AA (20:4 ω-6). The sequences for EPA (20:5 ω-3) and DGLA (20:3 ω-6) are analogous.
| Dietary Essential Fatty Acid |
Abbr | Formula carbons:double bonds ω |
Eicosanoid product series | ||
|---|---|---|---|---|---|
| TX PG PGI |
LK | Effects | |||
| Gamma-linolenic acid via Dihomo gamma linolenic acid |
GLA DGLA |
18:3ω6 20:3ω6 |
series-1 | series-3 | less inflammatory |
| Arachidonic acid | AA | 20:4ω6 | series-2 | series-4 | more inflammatory |
| Eicosapentaenoic acid | EPA | 20:5ω3 | series-3 | series-5 | less inflammatory |
All the prostenoids are substituted prostanoic acids. Cyberlipid Center's Prostenoid page illustrates the parent compound and the rings associated with each series–letter.
The IUPAC and the IUBMB use the equivalent term Icosanoid.
Read more about this topic: Essential Fatty Acid Interactions
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“There is in every either-or a certain naivete which may well befit the evaluator, but ill- becomes the thinker, for whom opposites dissolve in series of transitions.”
—Robert Musil (1880–1942)