Lipids are described either by systematic names or common/trivial names. The generally accepted guidelines for lipid systematic names have been defined by the International Union of Pure and Applied Chemists and the International Union of Biochemistry and Molecular Biology (IUPAC-IUBMB) Commission on Biochemical Nomenclature. Because the IUPAC convention tends to be lengthy and sometimes confusing, Cayman’s naming system for prostaglandins is taken from the literature, which often uses the trivial names and historically accepted conventions for the description of substituents (e.g., prostaglandins and their analogs). The scientific community widely uses Rokach nomenclature, as those in the lipid field learn directly from their mentors/peers and continue to follow precedent. It is not always obvious which positions are meant by “tetranor”, “dinor”, “iso”, “ent”, epoxides, diols, etc., or the intended stereochemistry when dealing with pairs of enantiomers. Furthermore, not all commercial lipid suppliers use the same rules, so it is best to compare lipid products across companies by CAS number through a universal database such as SciFinder. Cayman’s product descriptions offer appropriate synonyms when multiple names are given and present structures as relative stereochemistry when various isomers are possible. Please contact a technical support scientist if you need clarification. 

Prostanoic acid is the core molecule of any prostaglandin. The five-membered cyclopentane ring can be modified with the addition of a double bond and/or hydroxyl or carbonyl groups at the 9 and 11 positions. These structure variances determine receptor binding, biological activity, and metabolic fate. The different families of prostaglandins are named by letters following the PG abbreviation and this designates the functional substituent added: A, B, C, D, E, F, J, and K. All naturally occurring PGs derive from the G/H-type (PGH₂). Some PGs are more stable following dehydration or decomposition. Dehydration of the D and E structures leads to formation of the A, B, C, and J structures. The I-type and thromboxane (TXA and TXB) structures are unique versions of the conversion of PGH2 by specific enzymes. G/H-, I-, and TX-types are all unstable molecules. Despite high instability, I and TX have potent activity before rapidly decomposing to more stable forms.

Prostaglandin nomenclature.

Another portion of the naming system lies in the number of double bonds in the product and is represented by the subscripted number in the compound name. The number of double bonds in the parent molecule of each compound is dependent upon the starting fatty acid oxidized in the COX reaction. Hence, DGLA, AA, and EPA provide the subscripted numbers of 1, 2, and 3 (e.g., PGE₁, PGE₂, and PGE₃), respectively.

Prostaglandin formation from different C-20 fatty acids.

Isoprostanoids derive from non-enzymatic, free radical peroxidation of fatty acids and differ largely in their physiochemical properties from those of its corresponding prostaglandins. Because these species are formed without the direct action of enzymes, the substituents at the five-membered ring are racemic. According to the Tabor system, the family name is derived from the substitution pattern of the five-membered ring, but in contrast to the PG system, the letter indicating the isoprostanoid family type is listed in front (e.g., A-isoprostane). Four classes of F2-isoprostanes arise from arachidonic acid, six classes of F3-isoprostanes from EPA, two classes of E1- and F1-isoprostanes arise from α-linolenic and γ-linolenic acids, and eight classes of D4-isoprostanes, eight classes of E4-isoprostanes, and eight classes of F4-neuroprostanes arise from docosahexaenoic acid. Each of the classes comprise up to eight racemic isomers. Alternatively, the Rokach system abbreviates isoprostanes with iP followed by the letter indicating family type, a subscripted number indicating number of double bonds in the side chains, and a Roman numeral indicating the type of carbon skeleton (e.g., iPF2-III).

Isoprostane nomenclature. Reilly, M.P., Praticò, D., Delanty, N., et al. Circulation 98(25), 2822-2828, 1998.

Additional elements further define stereochemical differences from the default structure and are listed below.

Oxylipins formed from various polyunsaturated fatty acids are named according to the fatty acid from which they derived. Here is a list of their abbreviated names.

Eicosadienoic Acid
HEDE	hydroxy - eicosadienoic acid
HpEDE	hydroperoxy - eicosadienoic acid
Eicosatrienoic Acid
DiHEDE	dihydroxy - eicosadienoic acid
EpEDE	epoxy - eicosadienoic acid
HETrE	hydroxy - eicosatrienoic acid
HpETrE	hydroperoxy - eicosatrienoic acid
Eicosatetraenoic Acid (Arachidonic Acid)
DiHETE	dihydroxy - eicosatetraenoic acid*
DiHETrE	dihydroxy - eicosatrienoic acid
EpETrE (EET)	epoxy - eicosatrienoic acid
HETE	hydroxy - eicosatetraenoic acid
HHTrE	hydroxy - heptadecatrienoic acid
HpETE	hydroperoxy - eicosatetraenoic acid
OxoETE	oxo - eicosatetraenoic acid
Eicosapentaenoic Acid
DiHETE	dihydroxy - eicosatetraenoic acid*
EpETE	epoxy - eicosatetraenoic acid
HEPE	hydroxy - eicosapentaenoic acid
HpEPE	hydroperoxy - eicosapentaenoic acid
Docosahexaenoic Acid
DiHDPE	dihydroxy - docosapentaenoic acid
EpDPE	epoxy - docosapentaenoic acid
HDoHE	hydroxy - docosahexaenoic acid
HpDoHE	hydroperoxy - docosahexaenoic acid
Octadecadienoic Acid (Linoleic Acid)
DiHOME	dihydroxy - octadecamonoenoic acid
EpOME	epoxy - octadecamonoenoic acid
HODE	hydroxy - octadecadienoic acid
HpODE	hydroperoxy - octadecadienoic acid
Octadecatrienoic Acid (Linolenic Acid)
HOTrE	hydroxy - octadecatrienoic acid
HpOTrE	hydroperoxy - octadecatrienoic acid

* The product formed depends on the metabolic route and positions of the hydroxyl groups. DiHETEs derive from either AA (via the double lipoxygenase route) or from EPA (via the epoxidation/epoxide hydrolase route).

Note on relative configuration of epoxides and diols

Many classes of lipids are comprised of multiple racemic variations. For simplicity of depiction of all possible racemates, Cayman’s product descriptions for fatty acid epoxide and diols show one out of four possible isomers at the chiral centers. A note is added to the structure to acknowledge the limits of depicting relative stereochemistry in two-dimensional space.

(±)14(15)-EET (Item No. 50651)

References

Lipid Maps nomenclature tutorial. In: LIPID MAPS^® Lipidomics Gateway [Internet]. Available from: http://www.lipidmaps.org/resources/tutorials/lipid_cns.html

Nelson, N.A. Prostaglandin nomenclature. J. Med. Chem. 17(9), 911-918 (1974). [PMID: 4415344]

Mueller, M.J. Isoprostane nomenclature: Inherent problems may cause setbacks for the development of the isoprostanoid field. Prostaglandins Leukot. Essent. Fatty Acids 82(2-3), 71-81 (2010). [PMID: 20034775]

Rokach, J., Khanapure, S.P., Hwang, S.-W., et al. Nomenclature of isoprostanes: A proposal. Prostaglandins 54(6), 853–873 (1997). [PMID: 9533181 ]

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