15-Hydroxyeicosatetraenoic acid (also termed 15-HETE, 15(S)-HETE, and 15S-HETE) is an eicosanoid, i.e. a metabolite of arachidonic acid. Various cell types metabolize arachidonic acid to 15(S)-hydroperoxyeicosatetraenoic acid (15(S)-HpETE). This initial hydroperoxide product is extremely short-lived in cells: if not otherwise metabolized, it is rapidly reduced to 15(S)-HETE. Both of these metabolites, depending on the cell type which forms them, can be further metabolized to 15-oxo-eicosatetraenoic acid (15-oxo-ETE), 5(S),15(S)-dihydroxy-eicosatetraenoic acid (5(S),15(S)-diHETE), 5-oxo-15(S)-hydroxyeicosatetraenoic acid (5-oxo-15(S)-HETE), a subset of specialized pro-resolving mediators viz., the lipoxins, a class of pro-inflammatory mediators, the eoxins, and other products that have less well-defined activities and functions. Thus, 15(S)-HETE and 15(S)-HpETE, in addition to having intrinsic biological activities, are key precursors to numerous biologically active derivatives.
Some cell types (e.g. platelets) metabolize arachidonic acid to the stereoisomer of 15(S)-HpETE, 15(R)-HpETE. Both stereoisomers may also be formed as result of the metabolism of arachidonic acid by cellular microsomes or as a result of arachidonic acid auto-oxidation. Similar to 15(S)-HpETEs, 15(R)-HpETE may be rapidly reduced to 15(R)-HETE. These R,S stereoisomers differ only in having their hydroxy residue in opposite orientations. While the two R stereoisomers are sometimes referred to as 15-HpETE and 15-HETE, proper usage should identify them as R stereoisomers. 15(R)-HpETE and 15(R)-HETE lack some of the activity attributed to their S stereoisomers but can be further metabolized to bioactive products viz., the 15(R) class of lipoxins (also termed epi-lipoxins).
15(S)-HETE, 15(S)-HpETE, and many of their derivative metabolites are thought to have physiologically important functions. They appear to act as hormone-like autocrine and paracrine signaling agents that are involved in regulating inflammatory and perhaps other responses. Clinically, drugs that are stable analogs, and therefore mimic the anti-inflammatory actions of the lipoxins and drugs that block the production or actions of the pro-inflammatory eoxins may prove useful for treating acute and chronic inflammatory disorders.
Nomenclature and stereoisomers 15(S)-HETE is unambiguously designated by a shortened version of its IUPAC name viz., 15(S)-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid. In this terminology S refers to the absolute configuration of the chirality of the hydroxy functional group at carbon position 15. Its 15(R) enantiomer is designated 15(R)-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid. Z and E give the cis–trans isomerism about each double bond at carbon positions 5, 8, 11, and 13 with Z indicating cis and E indicating trans isomerism. Both stereoisomers are produced from their corresponding S and R 15-HpETE stereoisomers, i.e. 15(S)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15(S)-HpETE) and 15(R)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15(R)-HpETE).
Production Human cells release arachidonic acid (i.e. 5Z,8Z,11Z,14Z-eicosatetraenoic acid) from its storage site in phospholipids by reactions that involve phospholipase C and/or lipase enzymes. This release is stimulated or enhanced by cell stimulation. The freed arachidonic acid is then converted to 15-hydroperoxy/hydroxy products by one or more of the following five pathways.
15-Lipoxygenase-1: Cells metabolize arachidonic acid with 15-lipoxygenase-1 (i.e., 15-LO-1, ALOX15) to form 15(S)-HpETE as a major product and 12(S)-hydroperoxy-5Z,8Z,10E,15Z-eicosatetraenoic acid (12(S)-HpETE) and 14(S),15(S)-trans-oxido-5Z,8Z,11Z-14,15-leukotriene A4 as minor products; 15(S)-HpETE and 12(S)-HpETE are rapidly converted to 15(S)-HETE and 12(S)-hydroxy-5Z,8Z,10E,15Z-eicosatetraenoic acid (12(S)-hydroxyeicosatetraenoic acid), (i.e. 12(S)-HETE), respectively, or further metabolized through other enzyme pathways; 14(S),15(S)-trans-oxido-5Z,8Z,11Z-14,15-leukotriene A4 is metabolized by 15-LO-1 to various isomers of 8,15(S)-dihydroxy-5S,8S,11Z,13S-eicosatetraenoic acids, e.g. 8,15(S)-LTB4's.
15-Lipoxygenase-2: Cells also used 15-lipoxygenase 2 (i.e. 15-LOX-2 or ALOX15B) to make 15(S)-HpETE and 15(S)-HETE. However this enzyme has a preference for metabolizing linoleic acid rather than arachidonic acid. It therefore forms linoleic acid metabolites (e.g. 13-hydoxyperoxy/hydroxy-octadecadienoic and 9-hydroperoxy/hydroxyl-octadecadienoic acids) in greater amounts than 15(S)-HpETE and 15(S)-HETE. 15-LOX-2 also differs from 15-LOX-1 in that it does not make 12(S)-HpETE or the leukotriene A4 isomer cited above.
Cyclooxygenase: Cells can use prostaglandin-endoperoxide synthase 1 (i.e. cyclooxygenenase-1 or COX-1) and prostaglandin-endoperoxide synthase 2 (COX-2) to metabolize arachidonic acid primarily to prostaglandins but also to small amounts of 11(R)-HETE and a racemic mixture of 15-HETEs composed of ~22% 15(R)-HETE and ~78% 15(S)-HETE. When pretreated with aspirin, however, COX-1 is inactive while COX-2 attacks arachidonic acid to produce almost exclusively 15(R)-HETE along with its presumed precursor 15(R)-HpETE.
Microsome metabolism: Human and rat microsomal cytochrome P450s, e.g. CYP2C19, metabolize arachidonic acid to a racemic mixture of 15-HETEs, i.e., 15(R,S)-HETEs, >90% of which is the 15(R) stereoisomer.
Autoxidation: The spontaneous and non-enzymatically induced autoxidation of arachidonic acid yields 15(R,S)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acids. This non-enzymatic reaction is promoted in cells undergoing oxidative stress. Cells forming this racemic mixture of 15-hydroperoxy products may convert then to 15(R,S)-HETEs and other products. However, the uncontrolled overproduction of the 15-hydroperoxy products may react with other elements to produce cell injury.
Further metabolism The newly formed products formed by the pathways cited in the previous section are bioactive but may also flow into down-stream pathways to form other metabolites with a different sets of bioactivity. The initially formed 15(S)-HpETE may be further metabolized by its parent cell or pass it to nearby cell by a process termed transcellular metabolism.
15(S)-HpETE may be: Rapidly reduced to 15(S)-HETE by ubiquitous cellular peroxidase reactions including those possessed by prostaglandin-endoperoxide synthase-1 and -2, prostacyclin synthase, thromboxane synthase, and various glutathione peroxidases. Acylated into membrane phospholipids, particularly phosphatidylinositols and phosphatidylethanolamine. The 15(S)-HpETE is bound primarily at the sn-2 position of these phospholipids (see Phospholipase) and may be reduced to 15(S)-HETE thereby forming their 15(S)-HETE-bound phospholipoid analogs. Phosphatidylinositol phospholipids with 15(S)-HETE in the sn-2 position can be attacked by phospholipase C to form corresponding diglycerides with 15(S)-HETE at their sn-2 positions. Metabolized by 15-LO-1 to its 14,15-trans-epoxide, 14,15-trans-epoxide-oxido-5Z,8Z,10E,13E-eicosatetraenoic acid (i.e., eoxin A4 or EXA4), and thereafter to 14(R)-glutothionyl-15(S)-hydroxy-5Z,8Z,10E,13E-eicosatetraenoic acid (i.e. eoxin C4 or EXC4) by leukotriene C4 synthase. EXC4 contains glutathione (i.e. γ-L-glutamyl-L-cysteinylglycine) bound in the R configuration to carbon 14. EXC4 is further metabolized by removal of the γ-L-glutamyl residue to form EXD4 which is in turn further metabolized by removal of the glycine residue to form EXE4. These metabolic transformations are similar to those in the pathway that metabolizes arachidonic acid to LTA4, LTC4, LTD4, and LTE4 and presumed to be conducted by the same enzymes (Eoxins are also termed 14,15-leukotrienes or 14,15-LTs). Metabolized alternatively by 15-LO-1 to various 8,15-diHETEs including the two 8(R) and 8(S) diastereomers of 8,15(S)-dihydroxy-5,9,11,13-eicosatetraenoic acid (8,15-leukotrienes B4) and to two isomeric erythro-14,15-dihydroxy-5-cis-8,10,12-eicosatetraenoic acids (14,15-leukotrienes B4). Metabolized by 15-LOX-2 to 11(S)-hydroxy-14(S),15(S)-epoxy-5(Z),8(Z),12(E)-eicosatrienoic acid and 13(R)-hydroxy-14(S),15(S)-epoxy-5(Z),8(Z),11(Z)-eicosatrienoic acid; these two products are novel hepoxilins produced by ALOX15 rather than ALOX12, the enzyme responsible for making the various other hepoxilins in humans. The two novel hepoxilins are termed respectively 14,15-HXA3 and 14,15-HXB3. 14,15-HXA3 can be further metabolized by glutathione transferases to 11(S),15(S)-dihydroxy-14(R)-glutathionyl-(5Z),8(Z),12(E)-eicosatrienoic acid (14,15-HXA3C) which is then further metabolized to 11(S),15(S)-dihydroxy-14(R)-cysteinyl-glycyl-(5Z),8(Z),12(E)-eicosatrienoic acid (14,15-HXA3D). Isomerized to 15(S)-hydroxy-11,12-cis-epoxy-5Z,8Z,13E-eicosatrienoic acid (i.e., 15-H-11,12-EETA) by a hydroperoxide isomerase activity and then to 11,12,15-trihydroxy-5Z,8Z,12E-eicosatrienoic acid (i.e. 11,12,15-THETA) and 11,14,15-trihydroxy-5Z,8Z,12E-eicosatrienoic acid (i.e., 11,14,15-THETA) by a soluble epoxide hydrolase activity or, by acid in a non-enzymatic reaction (the R, S configuration of the hydroxy residues in the latter two metabolites has not been defined). Isomerized to threo and erythro diastereoisomers of 13-hydroxy-14,15-cis-epoxy-5Z,8Z,11Z-eicosatrienoic acid (i.e., 15-H-11,12-EETA) by a hydroperoxide isomerase activity, possibly a cytochrome P450, i.e. CYP2J2. Metabolized by cytochrome P450 (CYP) enzymes such as CYP1A1, CYP1A2, CYP1B1, and CYP2S1 to 15-oxo-ETE. Metabolized in skin epidermis by epidermis-type lipoxygenase 3 (eLOX3, encoded by the ALOXE3 gene) to make two products, hepoxilin A3 (HxA3, i.e., 13R-hydroxy-14(S),15(S)-epoxy-5Z,8Z,11Z-eicosatetraenoic acid) and 15-oxo-ETE). Converted to its 14,15-epoxide derivative, eoxin A4, and further metabolized to eoxin C4, eoxin D4, and eoxin E4 (there is no eoxin B4). Degraded non-enzymatically to various cell-injuring electrophiles such as 4-hydroxy-2(E)-nonenal and 4-oxo-2(E)-nonenal.
15(S)-HETE may be: Oxidized to its keto analog, 15-oxo-ETE, by the same enzyme that converts prostaglandins of the A, E, and F series to their 15-keto analogs viz., NAD+-dependent 15-hydroxyprostaglandin dehydrogenase; 15-oxo-ETE, similar to 15(S)-HETE, may be acylated into membrane phosphatidylethanolamine or, similar to 15(S)-HpETE, conjugated with glutathione to form a 13-cysteinyl-glycyl-glutamine adduct viz., 13-glutatione,15-oxo-5(S),8(Z),11(E)-eicosatrienoic acid; the latter metabolite is attacked by γ-glutamyl-transferase to form 13-cysteinyl-glycine,15-oxo-5(S),8(Z),11(E)-eicosatrienoic acid. Acylated into membrane phospholipids, particularly phosphatidylinositol and phosphatidylethanolamine. Phospholipid products contain this 15(S)-HETE most likely at the sn-2 position. 15(S)-HETE-containing-phospholipids may also be made directly by the action of 15-LO-1 on membrane phosphatidylinositols or phosphatidylethanolamines containing arachidonic acid at the sn-2 positions. The phosphatidylethanolamine-bound 15-HETE may be converted to phosphatidylethanolamine-bound 15-oxo-ETE. Oxygenated by 5-lipoxygenase (ALOX5) to form its 5,6-trans epoxide derivative which may then rearrange to the lipoxins (LX), LXA4 (i.e. 5(S),6(R),15(S)-trihydroxy-7E,9E,11Z,13E-eicosatetraenoic acid) and LXB4 (i.e., 5(S),14(R),15(S)-trihydroxy-6E,8Z,10E,12E-eicosatetraenoic acid) or to 5(S),15(S)-dihydroperoxy-6E,8Z,11Z,13E-eicosatetraenoate (i.e., 5(S),15(S)-diHETE). 5(S),15(S)-diHETE may then be oxidized to 5-oxo-15(S)-hydroxy-6E,8Z,11Z,13E-eicosatetraenoate (i.e., 5-oxo-15(S)-hydroxy-ETE). The latter two metabolites may also be made by 15-LO's metabolism of 5-hydroxyeicosatetraenoic acid (i.e. 5-HETE) and 5-oxo-eicosatetraenoic acid (i.e. 5-oxo-ETE), respectively.
15(R)-HpETE may be: Reduced to 15(R)-HETE by the same pathway that reduces 5(S)-HpETE to 15(S)-HETE.