9-Hydroxyoctadecadienoic acid
9-Hydroxyoctadecadienoic acid,12 has been used in the literature to designate either or both of two stereoisomer metabolites of the essential fatty acid, linoleic acid: 9-hydroxy-10,12-octadecadienoic acid and 9-hydroxy-10,12-octadecadienoic acid ; these two metabolites differ in having their hydroxy residues in the S or R configurations, respectively. The accompanying figure gives the structure for 9-HETE. Two other 9-hydroxy linoleic acid derivatives occur in nature, the 10E,12E isomers of 9-HODE and 9-HODE viz., 9-hydroxy-10E,12E-octadecadienoic acid and 9-hydroxy-10E,12E-octadecadienoic acid ; these two derivatives have their double bond at carbon 12 in the E or trans configuration as opposed to the Z or cis configuration. The four 9-HODE isomers, particularly under conditions of oxidative stress, may form together in cells and tissues; they have overlapping but not identical biological activities and significances. Because many studies have not distinguished between the S and R stereoisomers and, particularly in identifying tissue levels, the two EE'' isomers, 9-HODE is used here when the isomer studied is unclear.
A similar set of 13-Hydroxyoctadecadienoic acid metabolites, 13-HODE, 13, and 13 also occurs naturally and, again particularly under conditions of oxidative stress, may form concurrently with 9-HODEs; these 13-HODEs also have overlapping and complementary but not identical activities with the 9-HODEs. Some recent studies measuring HODE levels in tissue have lumped the four 9-HODEs and four 13-HODEs together to report only on total HODEs : tHODEs have been proposed to be markers for certain human disease. Other recent studies have lumped together the 9-, 9, 13 -, and 13-HODE along with the two ketone metabolites of these HODEs, 9-oxoODE,12 and 13-oxoODE, reporting only on total OXLAMs ; the OXLAMs have been implicated in working together to signal for pain perception.
Pathways making 9-HODEs
Cyclooxygenases 1 and 2
The enzymes cyclooxygenase 1 and cyclooxygenase 2, which are best known for metabolizing arachidonic acid to prostaglandins, are also able to metabolize linoleic acid predominantly to 9-hydroperoxy-10,12 and lesser amounts of 9-hydroperoxy-10,12-octadecadienoic acid ; in cells and tissues, the two hydroperoxy metabolites are rapidly reduce to 9-HODE and 9-HODE, respectively. COX-2 exhibits a greater preference for linoleic acid than does Cox-1 and is therefore credited with producing most of these products in cells expressing both COX enzymes. The COXs also metabolize linoleic acid to 13-hydroperoxy-octadecadionoic acid -HpODE and lesser amounts of 13-hydroperoxy-octadecadienoic acid -HpODE, which are then rapidly reduced to 13 and 13-HODE; the two enzymes therefore metabolize linoleic acid predominantly to the R stereoisomer of 9-HODE and stereoisomer of 13-HODE with the 13-HODE products predominating over the 9-HODE products.Cytochrome P450
enzymes metabolize linoleic acid to a mixture of 9-HpODE and 9-HpODE which are subsequently reduced to their corresponding hydroxy products; these reactions produce racemic mixtures in which the R stereoisomer predominates, for instance by a R/S ratio of 80%/20% in human liver microsomes. In cells and tissues, the cytochrome enzymes concurrently metabolize linoleic acid to 13-HpODE and 13-HpODE which are reduced to 13-HODE and 13-HODE in an R/S ratio similar to than of the 9-HODES, i.e. 80%/20%.Free-radical and singlet-oxygen oxidations
in cells and tissues produces Free-radical-induced and singlet-oxygen-induced oxidations of linoleic acid to generate the various racemic mixtures of 9-HpODE and 9-HODE in non-enzymatic reactions that produce, or are suspected but not proven to produce, approximately equal amounts of their S and R stereoisomers. These oxidations are credited with being the major contributors to 9-HODE and 13-HODE isomer production in tissues undergoing oxidative stress such as occurs in any tissue suffering inadequate blood flow, inflammation, or other serious insult, in liver steatohepatitis, in the atheroma plaques of cardiovascular disease, in nerve tissues of neurodegenerative diseases, and in the various tissues compromised by diabetes. Free-radical oxidation of linoleic acid produces racemic mixtures of 9-HODE and 9-EE-HODE; singlet-oxygen attack on linoleic acid produces racemic mixtures of 9-HODE, 10-hydroxy-8E,12Z-octadecadienoic acid, and 12-hydroxy-9Z-13-E-octadecadienoic acid. Since free-radical-induced and singlet-oxygen-induced oxidations of linoleic acid produce a similar set of 13-HODE metabolites, since both free radicals and singlet oxygen attack not only free linoleic acid but also linoleic acid bound to phospholipids, glycerides, cholesterol, and other lipids, and since free-radical and singlet-oxygen reactions may occur together, oxygen-stressed tissues often contain an array of free and lipid-bound 9-HODE and 13-HODE products. For example, laboratory studies find that 9-HODE and 9-EE-HODE are found in the phospholipid and cholesterol components of low-density lipoproteins that have been oxidized by human monocytes; the reaction appears due to the in situ free-radical- and/or superoxide-induced oxidation of the lipoproteins.Mouse 8(''S'')-lipoxygenase
The murine homolog of human 15-lipoxygenase-2, 8-lipoxygenase, while preferring arachidonic acid over linoleic acid, metabolizes linoleic acid predominantly to -HpODE, which in tissues and cells is rapidly reduced to 9-HODE. However, ALOX15B, similar to human 15-lipoxygenase-1, metabolizes linoleic acid to 13-HODE but not to 9-HODEs.Metabolism
Like most unsaturated fatty acids, the 9-HODEs formed in cells are incorporated into cellular phospholipids principally at the sn-2 position of the phospholipid ; since, however, the linoleic acid bound to cellular phospholipids is susceptible to non-enzymatic peroxidation and free-radical attack, the 9-HODEs in cellular phospholipids may also derive more directly from in-situ oxidation. 9-HODE esterified to the sn-2 position of phosphatidylserine is subject to be released as free 9-HODE by the action of cytosol and therefore may serve as a storage pool that is mobilized by cell stimulation.9-HODE may be further metabolized to 9-oxo-10,12-octadecadienoic acid, possibly by the same hydroxy-fatty-acid dehydrogenase which metabolizes other hydroxy fatty acids, such as 13-HODE, to their oxo derivatives.
Direct actions
9-HODE, 9-oxoODE, and 9-EE-HODE directly activate peroxisome proliferator-activated receptor gamma. This activation appears responsible for the ability of 13-HODE to induce the transcription of PPARγ-inducible genes in human monocytes as well as to stimulate the maturation of these cells to macrophages. 13-HODE also stimulate the activation of peroxisome proliferator-activated receptor beta in a model cell system; 13-HODE are also proposed to contribute to the ability of oxidized low-density lipoprotein to activate PPARβl: LDL containing phospholipid-bound 13-HODE is taken up by the cell and then acted on by phospholipases to release the HODEs which in turn directly activate PPARβl.13-HODE, 13-HODE and 13-oxoODE, along with their 9-HODE counterparts, also act on cells through TRPV1. TRPV1 is the transient receptor potential cation channel subfamily V member 1 receptor. These 6 HODEs, dubbed, oxidized linoleic acid metabolites, individually but also and possibly to a greater extent when acting together, stimulate TRPV1-dependent responses in rodent neurons, rodent and human bronchial epithelial cells, and in model cells made to express rodent or human TRPV1. This stimulation appears due to a direct interaction of these agents on TRPV1 although reports disagree on the potencies of the with, for example, the most potent OXLAM, 9-HODE, requiring at least 10 micromoles/liter or a more physiological concentration of 10 nanomoles/liter to activate TRPV1 in rodent neurons. The OXLAM-TRPV1 interaction is credited with mediating pain sensation in rodents.
9-HODE and with progressively lesser potencies 9-HpODE, a racemic mixture of 9-HODE, 13-HpODE, and 13-HODE directly activate human GPR132 in Chinese hamster ovary cells made to express these receptors; 9-HODE was also a more potent stimulator of human G2A than a series of mono-hydroxy arachidonic acid metabolites. GPR132 was initially described as a pH sensing receptor; the role of 9-HODEs as well as other linoleic and arachidonic acid metabolites in activating GPR132 under the physiological and pathological conditions in which it is implicated to be involved have not yet been determined. This determination, as it might apply to humans, is made difficult by the inability of these HODEs to activate rodent GPR132 and therefore to be analyzed in rodent models.
Biological and clinical relevancy
As markers of disease involving oxidative stress
Various measurements of tissue and blood levels of reactive oxygen species have been used as markers of diseases in which these species are generated and may contribute to tissue injury and systemic disturbances; examples of such diseases include a wide range of neurological, cardiovascular, infectious, autoimmune, and genetic diseases. HODEs measurements have been evaluated as markers for many of these oxygen-stress-related diseases. These measurements commonly use saponification methods to release HODEs bound by acylation to other molecules; they therefore measure not only free HODEs but also HODEs acylated to phospholipids, glycerides, cholesterol, and other lipids.Studies find that 1) 9 9-HODE total HODEs 9-HODE and 9-oxoODE levels of the hydroperoxy precursors to 9-HODE and 13-HODE are elevated in the plasma and/or red blood cells of patients with Alzheimer's disease, atherosclerosis, diabetes, diabetic nephritis, non-alcoholic steatohepatitis, and alcoholic steatohepatitis compared to healthy subjects. These studies suggest that high levels of the HODEs may be useful to indicate the presence and progression of the cited diseases. Since, however, the absolute values of HODEs found in different studies vary greatly, since HODE levels vary with dietary linoleic acid intake, since HODEs may form during the processing of tissues, and since abnormal HODE levels are not linked to a specific disease, the use of these metabolites as markers has not attained clinical usefulness. HODE markers may find usefulness as markers of specific disease, type of disease, and/or progression of disease when combined with other disease markers.