Hepoxilin

Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol (i.e. hydroxyl) residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid derived from acid the (PUFA), arachidonic acid. A second group of less well studied hepoxilins, HxA4, HxB4, and their non-enzymatically formed isomers are nonclassical eicosanoids derived from the PUFA, eicosapentaenoic acid. Recently, 14,15-HxA3 and 14,15-HxB3 have been defined as arachidonic acid derivatives that are produced by a different metabolic pathway than HxA3, HxB3, HxA4, or HxB4 and differ from the aforementioned hepoxilins in the positions of their hydroxyl and epoxide residues. Finally, hepoxilin-like products of two other PUFAs, docosahexaenoic acid and linoleic acid, have been described. All of these epoxyalcohol metabolites are at least somewhat unstable and are readily enzymatically or non-enzymatically to their corresponding trihydroxy counterparts, the trioxilins (TrX). HxA3 and HxB3, in particular, are being rapidly metabolized to TrXA3, TrXB3, and TrXC3. Hepoxilins have various biological activities in animal models and/or cultured mammalian (including human) tissues and cells. The TrX metabolites of HxA3 and HxB3 have less or no activity in most of the systems studied but in some systems retain the activity of their precursor hepoxilins. Based on these studies, it has been proposed that the hepoxilins and trioxilins function in human physiology and pathology by, for example, promoting inflammation responses and dilating arteries to regulate regional blood flow and blood pressure.5Z,8Z,11Z,14Z-eicosatetraenoic acid + O2 → 15(S)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid → 11R-hydroxy-14S,15 S-epoxy-5Z,8Z,12E-eicosatrienoic acid and 13R-hydroxy-14S,15S-epoxy-5Z,8Z,11Z-eicosatrienoic acid Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol (i.e. hydroxyl) residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid derived from acid the (PUFA), arachidonic acid. A second group of less well studied hepoxilins, HxA4, HxB4, and their non-enzymatically formed isomers are nonclassical eicosanoids derived from the PUFA, eicosapentaenoic acid. Recently, 14,15-HxA3 and 14,15-HxB3 have been defined as arachidonic acid derivatives that are produced by a different metabolic pathway than HxA3, HxB3, HxA4, or HxB4 and differ from the aforementioned hepoxilins in the positions of their hydroxyl and epoxide residues. Finally, hepoxilin-like products of two other PUFAs, docosahexaenoic acid and linoleic acid, have been described. All of these epoxyalcohol metabolites are at least somewhat unstable and are readily enzymatically or non-enzymatically to their corresponding trihydroxy counterparts, the trioxilins (TrX). HxA3 and HxB3, in particular, are being rapidly metabolized to TrXA3, TrXB3, and TrXC3. Hepoxilins have various biological activities in animal models and/or cultured mammalian (including human) tissues and cells. The TrX metabolites of HxA3 and HxB3 have less or no activity in most of the systems studied but in some systems retain the activity of their precursor hepoxilins. Based on these studies, it has been proposed that the hepoxilins and trioxilins function in human physiology and pathology by, for example, promoting inflammation responses and dilating arteries to regulate regional blood flow and blood pressure. HxA3 and HxB3 were first identified, named, shown to have biological activity in stimulating insulin secretion in cultured rat pancreatic islets of Langerhans in Canada in 1984 by CR Pace-Asciak and JM Martin. Shortly thereafter, Pace-Asciak identified, named, and showed to have insulin secretagogue activity HxA4 and HxB4. HxA3, HxB3, and their isomers are distinguished from most other eicosanoids (i.e. signaling molecules made by oxidation of 20-carbon fatty acids) in that they contain both epoxide and hydroxyl residues; they are structurally differentiated in particular from two other classes of arachidonic acid-derived eicosanoids, the leukotrienes and lipoxins, in that they lack conjugated double bonds. HxA4 and HxB4 are distinguished from HxA3 and HxB3 by possessing four rather than three double bonds. The 14,15-HxA3 and 14,15-HxB3 non-classical eicosanoids are distinguished from the aforementioned hepoxilins in that they are formed by a different metabolic pathway and differ in the positioning of their epoxide and hydroxyl residues. Two other classes of epoxyalcohol fatty acids, those derived from the 22-carbon polyunsaturated fatty acid, docosahexaenoic acid, and the 18-carbon fatty acid, linoleic acid, are distinguished from the aforementioned hepoxilins by their carbon chain length; they are termed hepoxilin-like rather than hepoxilins. A hepoxilin-like derivative of linoleic acid is formed on linoleic acid that is esterified to a sphingosine in a complex lipid termed esterified omega-hydroxylacyl-sphingosin (EOS). The full structural identities of the hepoxilins and hepoxilin-like compounds in most studies are unclear in two important respects. First, the R versus S chirality of their hydroxy residue in the initial and most studies thereafter is undefined and therefore given with, for example, HxB3 as 10R/S-hydroxy or just 10-hydroxy. Second, the R,S versus S,R chirality of the epoxide residue in these earlier studies likewise goes undefined and given with, for example, HxB3 as 11,12-epoxide. While some later studies have defined the chirality of these residues for the products they isolated, it is often not clear that the earlier studies dealt with products that had exactly the same or a different chirality at these residues. Human HxA3 and HxB3 are formed in a two-step reaction. First, molecular oxygen (O2) is added to carbon 12 of arachidonic acid (i.e. 5Z,8Z,11Z,14Z-eicosatetraenoic acid) and concurrently the 8Z double bond in this arachidonate moves to the 9E position to form the intermediate product, 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (i.e. 12S-hydroperoxyeicosatetraenoic acid or 12S-HpETE). Second, 12S-HpETE is converted to the hepoxilin products, HxA3 (i.e. 8R/S-hydroxy-11,12-oxido-5Z,9E,14Z-eicosatrienoic acid) and HxB3 (i.e. 10R/S-hydroxy-11,12-oxido-5Z,8Z,14Z-eicosatrienoic acid). This two-step metabolic reaction is illustrated below: The second step in this reaction, the conversion of 12(S)-HpETE to HxA3 and HxB3, may be catalyzed by ALOX12 as an intrinsic property of the enzyme. Based on gene knockout studies, however, the epidermal lipoxygenase, ALOXE3, or more correctly, its mouse ortholog Aloxe3, appears responsible for converting 12(S)-HpETE to HxB3 in mouse skin and spinal tissue. It is suggested that ALOXE3 contributes in part or whole to the production of HxB3 and perhaps other hepoxilins by tissues where it is expressed such as the skin. Furthermore, hydroperoxide-containing unsaturated fatty acids can rearrange non-enzymatically to form a variety of epoxyalcohol isomers. The 12(S)-HpETE formed in tissues, it is suggested, may similar rearrange non-enzymatically to form HxA3 and HXB3. Unlike the products made by ALOX12 and ALOXE3, which are stereospecific in forming only HxA3 and HxB3, however, this non-enzymatic production of hepoxilins may form a variety of hepoxilin isomers and occur as an artifact of tissue processing. Finally, cellular peroxidases readily and rapidly reduce 12(S)-HpETE to its hydroxyl analog, 12S-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12S-HETE; see 12-hydroxyeicosatetraenoic acid; this reaction competes with the hepoxilin-forming reaction and in cells expressing very high peroxidase activity may be responsible for blocking the formation of the hepoxilins. ALOX15 is responsible for metabolizing arachidonic acid to 14,15-HxA3 and 14,15-HxB3 as indicated in the following two-step reaction which first forms 15(S)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15S-HpETE) and then two specific isomers of 11S/R-hydroxy-14S,15S-epoxy-5Z,8Z,12E-eicosatrienoic acid (i.e. 14,15-HxA3) and 13S/R)-hydroxy-14S,15S-epoxy-5Z,8Z,11Z-eicosatrienoic acid (i.e. 14,15-HxB3): ALOX15 appears capable of conducting both steps in this reaction although further studies may show that ALOXE3, non-enzymatic rearrangements, and the reduction of 15S-HpETE to 15(S)-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (i.e. 15S-HETE; see 15-hydroxyicosatetraenoic acid) may be involved in the production of 14,15-HxA3 and 14,15-HxB3 as they are in that of HxA3 and HxB3. Production of the hepoxilin-like metabolites of docosahexaenoic acid, 7R/S-hydroxy-10,11-epoxy-4Z,7E,13Z,16Z,19Z-docosapentaenoic acid (i.e. 7-hydroxy-bis-α-dihomo-HxA5) and 10-hydroxy-13,14-epoxy-4Z,7EZ,11E,16Z,19Z-docosapentaenoic acid (i.e. 10-hydroxy-bis-α-dihomo-HxA5) was formed (or inferred to be formed based on the formation of their tihydroxy metabolites (see trioxilins, below) as a result of adding docosahexaenoic acid to the pineal gland or hippocampus isolated from rats; the pathway(s) making these products has not been described.