14,15-Dihydroxy-5,8,10,12-eicosatetraenoic acid. Enzymatic formation from 14,15-leukotriene A4

Analysis of 12/15-lipoxygenase metabolism of EPA and DHA with special attention to authentication of docosatrienes

A proposed beneficial impact of highly unsaturated “fish oil” fatty acids is their conversion by lipoxygenase (LOX) enzymes to specialized proresolving lipid mediators, including 12/15-LOX products from EPA and DHA. The transformations of DHA include formation of docosatrienes, named for the distinctive conjugated triene of the double bonds. To further the understanding of biosynthetic pathways and mechanisms, herein we meld together biosynthesis and NMR characterization of the unstable leukotriene A (LTA)-related epoxide intermediates formed by recombinant human 15-LOX-1, along with identification of the stable enzymatic products, and extend the findings into the 12/15-LOX metabolism in resident murine peritoneal macrophages. Oxygenation of EPA by 15-LOX-1 converts the initial 15S-hydroperoxide to 14S,15S-trans-epoxy-5Z,8Z,10E,12E,17Z-EPA (appearing as its 8,15-diol hydrolysis products) and mixtures of dihydroperoxy fatty acids, while mainly the epoxide hydrolysis products are evident in the murine cells. DHA also undergoes transformations to epoxides and dihydroperoxides by 15-LOX-1, resulting in a mixture of 10,17-dihydro(pero)xy derivatives (docosatrienes) and minor 7S,17S- and 14,17S-dihydroperoxides. The 10,17S-dihydroxy hydrolysis products of the LTA-related epoxide intermediate dominate the product profile in mouse macrophages, whereas (neuro)protectin D1, the leukotriene B₄-related derivative with trans,trans,cis conjugated triene, was undetectable. In this study, we emphasize the utility of UV spectral characteristics for product identification, being diagnostic of the different double bond configurations and hydroxy fatty acid functionality versus hydroperoxide. LC-MS is not definitive for configurational isomers. Secure identification is based on chromatographic retention times, comparison with authentic standards, and the highly distinctive UV spectra.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

Endogenous anti-inflammatory mediators from arachidonate in human neutrophils

Eicosanoids have been historically involved in the pathogenesis of various inflammatory diseases. Lipoxins (LXs) and epi-LXs show physiological effects relevant to inflammation regulation. In this study, we focused on LX precursors based on the hypothesis that their entrance and metabolism into the cell may facilitate their targeting at the inflammation site. Because compound chirality is of considerable importance in the efficacy of therapeutic agents, our aim was to study the anti-inflammatory effects of various epimers of LXA(4) precursors compared to LXA(4). Blood polymorphonuclear cells (PMNs) were incubated with 15(S)- or 15(R)-hydroxyeicosatetraenoic acid (HETE), 14(R)-,15(S)-, or 14(S),15(S)-diHETE, and LXA(4) and then stimulated with the calcium ionophore A23187. We found that 15(R)-HETE rather than 15(S)-HETE was preferentially metabolized and that 15-epi-LXs were produced in larger amounts than LXs. In contrast, when PMNs were incubated with the diastereoisomers of 14,15(S)-diHETE, 14-epi-LXB(4) was produced in lower amounts than LXB(4). Enantiomers of 15-HETE and diastereoisomers of 14,15-diHETE and LXA(4) were able to significantly decrease LTB(4) release by PMNs. These results suggest a potential resolution of the inflammatory process through endogenous anti-inflammatory mediators released by the way of trans-cellular metabolism.

Mutation of tyrosine 383 in leukotriene A4 hydrolase allows conversion of leukotriene A4 into 5S,6S-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acid. Implications for the epoxide hydrolase mechanism

Leukotriene A4 hydrolase is a bifunctional zinc metalloenzyme that catalyzes the final step in the biosynthesis of the proinflammatory mediator leukotriene B4. In previous studies with site-directed mutagenesis on mouse leukotriene A4 hydrolase, we have identified Tyr-383 as a catalytic amino acid involved in the peptidase reaction. Further characterization of the mutants in position 383 revealed that [Y383H], [Y383F], and [Y383Q] leukotriene A4 hydrolases catalyzed hydrolysis of leukotriene A4 into a novel enzymatic metabolite. From analysis by high performance liquid chromatography, gas chromatography/mass spectrometry of material generated in the presence of H216O or H218O, steric analysis of the hydroxyl groups, treatment with soybean lipoxygenase, and comparison with a synthetic standard, the novel metabolite was assigned the structure 5S, 6S-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acid (5S,6S-DHETE). The kinetic parameters for the formation of 5S,6S-DHETE and leukotriene B4 were found to be similar. Also, both activities were susceptible to suicide inactivation and were equally sensitive to inhibition by bestatin. Moreover, from the stereochemical configuration of the vicinal diol, it could be inferred that 5S, 6S-DHETE is formed via an SN1 mechanism involving a carbocation intermediate, which in turn indicates that enzymatic hydrolysis of leukotriene A4 into leukotriene B4 follows the same mechanism. Inasmuch as soluble epoxide hydrolase utilizes leukotriene A4 as substrate to produce 5S,6R-DHETE, our results also suggest a functional relationship between leukotriene A4 hydrolase and xenobiotic epoxide hydrolases.

The interaction of cytosolic epoxide hydrolase with chiral epoxides

1. The kinetic parameters of the cytosolic epoxide hydrolase were examined with two sets of spectrophotometric substrates. The (2S,3S)- and (2R,3R)-enantiomers of 4-nitrophenyl trans-2,3-epoxy-3-phenylpropyl carbonate had a KM of 33 and 68 microns and a Vmax of 16 and 27 mumol/min/mg, respectively. With the (2S,3S)- and (2R,3R)-enantiomers of 4-nitrophenyl trans-2,3-epoxy-3-(4-nitrophenyl)propyl carbonate, cytosolic epoxide hydrolase had a KM of 8.0 and 15 microM and a Vmax of 7.8 and 5.0 mumol/min/mg, respectively. 2. Glycidyl 4-nitrobenzoate had the lowest I50 of the compounds tested in the glycidyl 4-nitrobenzoate series (I50 = 140 microM). The I50 of the (2R)-enantiomer was 3.7-fold higher. The inhibitor with the lowest I50 in the glycidol series, and the lowest I50 of any compound tested, was (2S,3S)-3-(4-nitrophenyl)glycidol (I50 = 13.0 microM). It also showed the greatest difference in I50 between the enantiomers (330-fold). 3. All enantiomers of glycidyl 4-nitrobenzoates and trans-3-phenylglycidols gave differential inhibition of cytosolic epoxide hydrolase. However, neither the (S,S)-/(S)- or (R,R)-/(R)-enantiomer always had the lower I50. 4. Addition of one or more methyl groups to either enantiomer of glycidyl 4-nitrobenzoate resulted in increased I50. However, addition of a methyl group to C2 of either enantiomer of 3-phenylglycidol resulted in a decreased I50. Finally, when the hydroxyl group of trans-3-(4-nitrophenyl)glycidol was esterified the I50 of the (2S,3S)- but not the (2R,3R)-enantiomer increased.