Transformation of an arachidonic acid hydroperoxide into epoxyhydroxy and trihydroxy fatty acids by liver microsomal cytochrome P-450

Oxygen Surrogate Systems for Supporting Human Drug-Metabolizing Cytochrome P450 Enzymes

Oxygen surrogates (OSs) have been used to support cytochrome P450 (P450) enzymes for diverse purposes in drug metabolism research, including reaction phenotyping, mechanistic and inhibition studies, studies of redox partner interactions, and to avoid the need for NADPH or a redox partner. They also have been used in engineering P450s for more cost-effective, NADPH-independent biocatalysis. However, despite their broad application, little is known of the preference of individual P450s for different OSs or the substrate dependence of OS-supported activity. Furthermore, the biocatalytic potential of OSs other than cumene hydroperoxide (CuOOH) and hydrogen peroxide (H₂O₂) is yet to be explored. Here, we investigated the ability of the major human drug-metabolizing P450s, namely CYP3A4, CYP2C9, CYP2C19, CYP2D6, and CYP1A2, to use the following OSs: H₂O₂, tert-butyl hydroperoxide (tert-BuOOH), CuOOH, (diacetoxyiodo)benzene, and bis(trifluoroacetoxy)iodobenzene. Overall, CuOOH and tert-BuOOH were found to be the most effective at supporting these P450s. However, the ability of P450s to be supported by OSs effectively was also found to be highly dependent on the substrate used. This suggests that the choice of OS should be tailored to both the P450 and the substrate under investigation, underscoring the need to employ screening methods that reflect the activity toward the substrate of interest to the end application. SIGNIFICANCE STATEMENT: Cytochrome P450 (P450) enzymes can be supported by different oxygen surrogates (OSs), avoiding the need for a redox partner and costly NADPH. However, few data exist comparing relative activity with different OSs and substrates. This study shows that the choice of OS used to support the major drug-metabolizing P450s influences their relative activity and regioselectivity in a substrate-specific fashion and provides a model for the more efficient use of P450s for metabolite biosynthesis.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Hypoxia activates 15-PGDH and its metabolite 15-KETE to promote pulmonary artery endothelial cells proliferation via ERK1/2 signalling

BACKGROUND AND PURPOSE

Dysfunction and injury of endothelial cells in the pulmonary artery play critical roles in the hypertension induced by chronic hypoxia. One consequence of hypoxia is increased activity of 15-hydroxyprostaglandin dehydrogenase (PGDH). Here, we have explored, in detail, the effects of hypoxia on the proliferation of pulmonary artery endothelial cells.

EXPERIMENTAL APPROACH

We used bromodeoxyuridine incorporation, cell-cycle analysis, immunohistochemistry and Western blot analysis to study the effects of hypoxia, induced 15-PGDH) activity and its product, 15-keto-6Z, 8Z, 11Z, 13E-eicosatetraenoic acid (15-KETE), on endothelial cell proliferation. Scratch-wound and tube formation assays were also used to study migration of endothelial cells.

KEY RESULTS

15-KETE increased DNA synthesis and enhanced the transition from the G0 /G1 phase to the S phase in hypoxia. Inhibition of 15-PGDH or siRNA for 15-PGDH reversed these effects. 15-KETE also activated the ERK1/2 signalling pathway. 15-KETE-induced cell migration and tube formation were reversed by blocking ERK1/2, but not the p38 MAPK pathway.

CONCLUSIONS AND IMPLICATIONS

Hypoxia-induced endothelial proliferation and migration, an important underlying mechanism contributing to hypoxic pulmonary vascular remodelling, appears to be mediated by 15-PGDH and 15-KETE, via the ERK1/2 signalling pathway.

Regio- and enantioselective catalytic monoepoxidation of conjugated dienes: synthesis of chiral allylic cis-epoxides

Ti(IV)-salan 4 catalyzes the diastereo- and enantioselective monoepoxidation of conjugated dienes using 30% H2O2 at rt or below even in the presence of other olefins and adjacent stereocenters. Its enantiomer, ent-4, provides access to the opposite diastereomer or enantiomer. The resultant chiral allylic epoxides, and the triols derived from them, are versatile synthetic intermediates as well as substructures present in many bioactive natural products. The epoxidation is highly specific for Z-olefins. For 1-acyl(silyl)oxypenta-2,4-dienes, epoxidation of the distal olefin is generally favored in contrast to the adjacent regioselectivity characteristic of Sharpless, peracid, and other directed epoxidations of hydroxylated dienes.

Monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 enzymes

This review examines the monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 (CYP) enzymes in bacterial, archaeal and mammalian systems. CYP enzymes catalyze monooxygenation reactions by inserting one oxygen atom from O2 into an enormous number and variety of substrates. The catalytic versatility of CYP stems from its ability to functionalize unactivated carbon-hydrogen (C-H) bonds of substrates through monooxygenation. The oxidative prowess of CYP in catalyzing monooxygenation reactions is attributed primarily to a porphyrin π radical ferryl intermediate known as Compound I (CpdI) (Por•+FeIV=O), or its ferryl radical resonance form (FeIV-O•). CYP-mediated hydroxylations occur via a consensus H atom abstraction/oxygen rebound mechanism involving an initial abstraction by CpdI of a H atom from the substrate, generating a highly-reactive protonated Compound II (CpdII) intermediate (FeIV-OH) and a carbon-centered alkyl radical that rebounds onto the ferryl hydroxyl moiety to yield the hydroxylated substrate. CYP enzymes utilize hydroperoxides, peracids, perborate, percarbonate, periodate, chlorite, iodosobenzene and N-oxides as surrogate oxygen atom donors to oxygenate substrates via the shunt pathway in the absence of NAD(P)H/O2 and reduction-oxidation (redox) auxiliary proteins. It has been difficult to isolate the historically elusive CpdI intermediate in the native NAD(P)H/O2-supported monooxygenase pathway and to determine its precise electronic structure and kinetic and physicochemical properties because of its high reactivity, unstable nature (t½~2 ms) and short life cycle, prompting suggestions for participation in monooxygenation reactions of alternative CYP iron-oxygen intermediates such as the ferric-peroxo anion species (FeIII-OO-), ferric-hydroperoxo species (FeIII-OOH) and FeIII-(H2O2) complex.

Cytochrome P450 2S1 is reduced by NADPH-cytochrome P450 reductase

Cytochrome P450 (P450) 2S1 is one of the orphan P450s without a clear physiological function. Controversy has arisen as to whether it can interact with NADPH-P450 reductase and accept electrons. The reduction of 1,4-bis{[2-(dimethylamino-N-oxide)ethyl]amino}-5,8-dihydroxyanthracene-9,10-dione (AQ4N) by P450 2S1 was confirmed, and the NADPH consumption rates were measured aerobically and anaerobically in the absence and presence of the drug. The reduction kinetics of P450 2S1 were rapid, as measured by stopped-flow kinetics. These results confirm that P450 2S1 can be reduced by NADPH-P450 reductase and suggest normal mixed-function oxidase roles of P450 2S1 to be revealed.

Role of arachidonic acid lipoxygenase metabolites in acetylcholine-induced relaxations of mouse arteries

Arachidonic acid (AA) metabolites function as EDHFs in arteries of many species. They mediate cyclooxygenase (COX)- and nitric oxide (NO)-independent relaxations to acetylcholine (ACh). However, the role of AA metabolites as relaxing factors in mouse arteries remains incompletely defined. ACh caused concentration-dependent relaxations of the mouse thoracic and abdominal aorta and carotid, femoral, and mesentery arteries (maximal relaxation: 57 ± 4%, 72 ± 4%, 82 ± 3%, 80 ± 3%, and 85 ± 3%, respectively). The NO synthase inhibitor nitro-L-arginine (L-NA; 30 μM) blocked relaxations in the thoracic aorta, and L-NA plus the COX inhibitor indomethacin (10 μM) inhibited relaxations in the abdominal aorta and carotid, femoral, and mesenteric arteries (maximal relaxation: 31 ± 10%, 33 ± 5%, 41 ± 8%, and 73 ± 3%, respectively). In mesenteric arteries, NO- and COX-independent relaxations to ACh were inhibited by the lipoxygenase (LO) inhibitors nordihydroguaiaretic acid (NDGA; 10 μM) and BW-755C (200 μM), the K(+) channel inhibitor apamin (1 μM), and 60 mM KCl and eliminated by endothelium removal. They were not altered by the cytochrome P-450 inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (20 μM) or the epoxyeicosatrienoic acid antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (10 μM). AA relaxations were attenuated by NDGA or apamin and eliminated by 60 mM KCl. Reverse-phase HPLC analysis revealed arterial [(14)C]AA metabolites that comigrated with prostaglandins, trihydroxyeicosatrienoic acids (THETAs), hydroxyepoxyeicosatrienoic acids (HEETAs), and hydroxyeicosatetraenoic acids (HETEs). Epoxyeicosatrienoic acids were not observed. Mass spectrometry confirmed the identity of 6-keto-PGF(1α), PGE(2), 12-HETE, 15-HETE, HEETAs, 11,12,15-THETA, and 11,14,15-THETA. AA metabolism was blocked by NDGA and endothelium removal. 11(R),12(S),15(S)-THETA relaxations (maximal relaxation: 73 ± 3%) were endothelium independent and blocked by 60 mM KCl. Western immunoblot analysis and RT-PCR of the aorta and mesenteric arteries demonstrated protein and mRNA expression of leukocyte-type 12/15-LO. Thus, in mouse resistance arteries, 12/15-LO AA metabolites mediate endothelium-dependent relaxations to ACh and AA.

Human CYP2S1 metabolizes cyclooxygenase- and lipoxygenase-derived eicosanoids

CYP2S1 is a recently described dioxin-inducible cytochrome P450. We previously demonstrated that human CYP2S1 oxidizes a number of carcinogens but only via the peroxide shunt. In this article, we investigated whether human CYP2S1 can metabolize cyclooxygenase- and lipoxygenase-derived lipid peroxides in a NADPH-independent fashion. Human CYP2S1 metabolizes prostaglandin G(2) (PGG(2)) (K(m) = 0.267 ± 0.072 μM) into several products including 12S-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT). It also metabolizes prostaglandin H(2) (PGH(2)) (K(m) = 11.7 ± 2.8 μM) into malondialdehyde, 12-HHT, and thromboxane A(2) (TXA(2)). The turnover to 12-HHT by human CYP2S1 (1.59 ± 0.04 min(-1)) is 40-fold higher than that of TXA(2) (0.04 min(-1)). In addition to PGG(2) and PGH(2) metabolism, human CYP2S1 efficiently metabolizes the hydroperoxyeicosatetraenoic acids (5S-, 12S-, and 15S-) and 13S-hydroperoxyoctadecadienoic acid into 5-oxo-eicosatetraenoic acid (turnover = 16.7 ± 0.3 min(-1)), 12-oxo-eicosatetraenoic acid 1 (11.5 ± 0.9 min(-1)), 15-oxo-eicosatetraenoic acid (16.9 ± 0.8 min(-1)), and 13-octadecadienoic acid (20.2 ± 0.9 min(-1)), respectively. Other cytochromes P450 such as CYP1A1, 1A2, 1B1, and 3A4 underwent similar conversions but at slower rates. The fatty acid hydroperoxides were also converted by human CYP2S1 to several epoxyalcohols. Our data indicate that fatty acid endoperoxides and hydroperoxides represent endogenous substrates of CYP2S1 and suggest that the enzyme CYP2S1 may play an important role in the inflammatory process because some of the products that CYP2S1 produces play important roles in inflammation.

Identification of 13-hydroxy-14,15-epoxyeicosatrienoic acid as an acid-stable endothelium-derived hyperpolarizing factor in rabbit arteries

Arachidonic acid (AA) is metabolized by endothelial 15-lipoxygenase (15-LO) to several vasodilatory eicosanoids such as 11,12,15-trihydroxyeicosatrienoic acid (11,12,15-THETA) and its proposed unstable precursor 15-hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-EETA). In the present study, the acid-stable 13-hydroxy-trans-14,15-epoxy-eicosatrienoic acid (13-H-14,15-EETA) was identified and its vascular activities characterized. Rabbit aorta, mesenteric arteries, and the combination of 15-LO and cytochrome P450 2J2 converted AA to two distinct HEETA metabolites. The HEETA metabolites were resistant to acidic hydrolysis but were hydrolyzed by recombinant sEH to a more polar metabolite identified by mass spectrometry as 13,14,15-THETA. Mass spectrometric analyses and HPLC comigration identified the HEETAs as threo- and erythro-diastereomers of 13-H-trans-14,15-EETA. Erythro- and threo-diastereomers of 13-H-trans-14,15-EETA relaxed endothelium-denuded rabbit small mesenteric arteries with maximum relaxations of 22.6 +/- 6.0% and 8.6 +/- 4.3%, respectively. Apamin (10(-7) m) inhibited the relaxations to the erythro-isomer (maximum relaxation = 1.2 +/- 5.6%) and increasing [K(+)](o) from 4.6 to 30 mm blocked relaxations to both isomers. In cell-attached patches of mesenteric arterial smooth muscle cells (SMCs), erythro-13-H-trans-14,15-EETA (1-3 x 10(-6) m) increased mean open time of small conductance K(+) channels (13-14 pS) from 0.0007 +/- 0.0007 to 0.0053 +/- 0.0042. This activation was inhibited by apamin. The erythro, but not the threo, isomer blocked angiotensin II-stimulated aortic SMC migration. These studies demonstrate that 13-H-14,15-EETAs induces vascular relaxation via K(+) channel activation to cause SMC hyperpolarization. Thus, 13-H-14,15-EETA represents a new endothelial factor.

Reaction mechanisms of 15-hydroperoxyeicosatetraenoic acid catalyzed by human prostacyclin and thromboxane synthases

Prostacyclin synthase (PGIS) and thromboxane synthase (TXAS) are atypical cytochrome P450s. They do not require NADPH or dioxygen for isomerization of prostaglandin H(2) (PGH(2)) to produce prostacyclin (PGI(2)) and thromboxane A(2) (TXA(2)). PGI(2) and TXA(2) have opposing actions on platelet aggregation and blood vessel tone. In this report, we use a lipid hydroperoxide, 15-hydroperoxyeicosatetraenoic acid (15-HPETE), to explore the active site characteristics of PGIS and TXAS. The two enzymes transformed 15-HPETE not only into 13-hydroxy-14,15-epoxy-5,8,11-eicosatrienoic acid (13-OH-14,15-EET), like many microsomal P450s, but also to 15-ketoeicosatetraenoic acid (15-KETE) and 15-hydroxyeicosatetraenoic acid (15-HETE). 13-OH-14,15-EET and 15-KETE result from homolytic cleavage of the O-O bond, whereas 15-HETE results from heterolytic cleavage, a common peroxidase pathway. About 80% of 15-HPETE was homolytically cleaved by PGIS and 60% was homolytically cleaved by TXAS. The V(max) of homolytic cleavage is 3.5-fold faster than heterolytic cleavage for PGIS-catalyzed reactions (1100 min(-1)vs. 320 min(-1)) and 1.4-fold faster for TXAS (170 min(-1)vs. 120 min(-1)). Similar K(M) values for homolytic and heterolytic cleavages were found for PGIS ( approximately 60 microM 15-HPETE) and TXAS ( approximately 80 microM 15-HPETE), making PGIS a more efficient catalyst for the 15-HPETE reaction.

Rabbit aorta converts 15-HPETE to trihydroxyeicosatrienoic acids: potential role of cytochrome P450

Previous work showed that rabbit aorta metabolizes arachidonic acid via 15-lipoxygenase to 15-hydroperoxyeicosatetraenoic acid (15-HPETE), which undergoes an enzymatic rearrangement to 11-hydroxy-14,15-epoxyeicosatrienoic acid (11-H-14,15-EETA) and 15-hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-EETA). Hydrolysis of the epoxy group results in the formation of 11,14,15- and 11,12,15-trihydroxyeicosatrienoic acids (THETAs). Endothelial cells have several heme-containing enzymes including cytochromes P450 (CYP), nitric oxide synthase (eNOS), and prostacyclin (PGI(2)) synthase that catalyze the rearrangement of 15-HPETE to HEETAs. Incubation of arachidonic acid and 15-lipoxygenase, or 15-HPETE with rabbit aortic microsomes or rat liver microsomes, a rich source of CYP, resulted in the formation of a product that comigrated with THETAs and HEETAs on HPLC. Immunoblot analysis showed the presence of CYP2C8 and CYP2J2 in aortic tissue and when CYP2J2 or CYP2C8 was incubated with arachidonic acid and 15-lipoxygenase, the major products were 11,12,15- and 11,14,15-THETAs. Incubation of purified hematin, CYP2C11, eNOS or PGI(2) synthase enzymes with arachidonic acid and 15-lipoxygenase produced a different pattern of metabolites from rabbit aortic microsomes. Clotrimazole, a non-specific CYP inhibitor, and ebastine and terfenadone, specific CYP2J2 inhibitors, blocked the ability of aortic microsomes to produce THETAs while specific inhibitors of PGI(2) synthase, eNOS or CYP2C8/2C9 had no effect on THETA production. We suggest that a CYP, possibly CYP2J2, may function as the hydroperoxide isomerase converting 15-HPETE to HEETAs in rabbit vascular tissue. Further hydrolysis of the epoxy group of the HEETAs results in the formation of 11,12,15- and 11,14,15-THETAs. The HEETAs and THETAs are both vasodilators and may function as important regulators of vascular tone.

Metabolism of 12-hydroperoxyeicosatetraenoic acid to vasodilatory trioxilin C3 by rabbit aorta

Arachidonic acid is metabolized by both the cyclooxygenase and lipoxygenase pathways by rabbit aorta. We investigated the metabolism of 12-hydroperoxyeicosatetraenoic acid by aortic homogenates and microsomes. Rabbit aortic homogenates were incubated in the presence of (14)C-arachidonic acid plus 12-lipoxygenase and analyzed by reversed-phase high-pressure liquid chromatography (HPLC). Under these experimental conditions, there was a (14)C-metabolite that migrated at 17.6 min. This (14)C-metabolite was not observed when aortic homogenates were incubated in the absence of 12-lipoxygenase. Similar results were obtained with aortic microsomes. Further analysis using a different HPLC solvent system resolved the (14)C-metabolite into a number of products. Gas chromatography/mass spectrometric (GC-MS) analysis of the major product (labeled peak 3) after conversion to the methyl ester-trimethylsilyl derivative showed two major compounds (compounds A and B) eluting at 13.99 and 14.14 min. The two compounds differed in the intensities of the 213 and 243 m/z ions with 243 being greater than 213 in compound A and the opposite in compound B (relative abundance 213 vs. 243; 100% vs. 43% for compound A and 5% vs. 100% for compound B). Based on the mass spectra, peak 3 contained two metabolites identified as the methyl ester-trimethylsilyl ether derivatives of 8,11,12-trihydroxyeicosatrienoic acid (trioxilin A(3)) and 8,9,12-trihydroxyeicosatrienoic acid (trioxilin C(3)). Biological activity of the mixture of two trioxilins isolated from aortic homogenates was tested in phenylephrine-precontracted aortas and found to produce concentration-dependent relaxations (maximal relaxation: 20.1+/-7.6%). Further testing with authentic trioxilin A(3) and C(3) revealed that trioxilin C(3) was the active metabolite (maximal relaxation: 16.6+/-1.3%). In conclusion, trioxilin C(3) acid was isolated and identified as a novel biologically active arachidonic acid metabolite formed by rabbit aorta when 12-lipoxygenase is supplied exogenously.

Identification of the 11,14,15- and 11,12, 15-trihydroxyeicosatrienoic acids as endothelium-derived relaxing factors of rabbit aorta

A number of endothelium-derived relaxing factors have been identified including nitric oxide, prostacyclin, and the epoxyeicosatrienoic acids. Previous work showed that in rabbit aortic endothelial cells, arachidonic acid was metabolized by a lipoxygenase to vasodilatory eicosanoids. The identity was determined by the present study. Aortic homogenates were incubated in the presence of [U-14C]arachidonic acid, [U-14C]arachidonic acid plus 15-lipoxygenase (soybean lipoxidase), or [U-14C]15-hydroxyeicosatetraenoic acid (15-HPETE) and analyzed by reverse phase high pressure liquid chromatography (RP-HPLC). Under both experimental conditions, there was a radioactive metabolite that migrated at 17.5-18.5 min on RP-HPLC. When the metabolite was isolated from aortic homogenates, it relaxed precontracted aortas in a concentration-dependent manner. Gas chromatography/mass spectrometry (GC/MS) of the derivatized metabolite indicated the presence of two products; 11,12,15-trihydroxyeicosatrienoic acid (THETA) and 11,14,15-THETA. A variety of chemical modifications of the metabolite supported these structures and confirmed the presence of a carboxyl group, double bonds, and hydroxyl groups. With the combination of 15-lipoxygenase, arachidonic acid, and aortic homogenate, an additional major radioactive peak was observed. This fraction was analyzed by GC/MS. The mass spectrum was consistent with this peak, containing both the 11-hydroxy-14, 15-epoxyeicosatrienoic acid (11-H-14,15-EETA) and 15-H-11,12-EETA. The hydroxyepoxyeicosatrienoic acid (HEETA) fraction also relaxed precontracted rabbit aorta. Microsomes derived from rabbit aortas also synthesized 11,12,15- and 11,14,15-THETAs from 15-HPETE, and pretreatment with the cyctochrome P450 inhibitor, miconazole, blocked the formation of these products. The present studies suggest that arachidonic acid is metabolized by 15-lipoxygenase to 15-HPETE, which undergoes an enzymatic rearrangement to 11-H-14,15-EETA and 15-H-11,12-EETA. Hydrolysis of the epoxy group results in the formation of 11,14,15- and 11,12,15-THETA, which relaxed rabbit aorta. Thus, the 15-series THETAs join prostacyclin, nitric oxide, and epoxyeicosatrienoic acids as new members of the family of endothelium-derived relaxing factors.

Glucuronic acid-conjugated dihydroxy fatty acids in the urine of patients with generalized peroxisomal disorders

Urine extracts from children diagnosed with generalized peroxisomal disorders were screened by continuous flow-negative ion fast atom bombardment-mass spectrometry. In 45 of 60 children with generalized peroxisomal disorders, we observed one or more intense ions (m/z 489, 505, 461, and others) that are infrequently found in children with cholestatic liver disease or normal children. Compounds giving rise to these ions were isolated using reverse phase and anion exchange chromatography. After appropriate derivatization and/or methanolysis the compounds were analyzed using capillary gas chromatography-mass spectrometry. The major compounds were found to be 12,13-dihydroxy-9-octadecenoic acid and 9,10-dihydroxy-12-octadecenoic acid, with one of the hydroxyl groups in glycosidic linkage with glucuronic acid. Minor compounds were glucuronic acid conjugates of 9,10-dihydroxy-octadecanoic acid, and 12,13-dihydroxy-6,9-, 15,16-dihydroxy-9,12-, and 9, 10-dihydroxy-12,15-octadecadienoic acids. A series of hexadecanoic, hexadecenoic, and hexadecadienoic acid glucuronides which appear to be beta-oxidation products of the C18 fatty acids were also observed, with the major species being 10, 11-dihydroxy-7-hexadecenoic acid glucuronide. In all, 16 C16 and C18 dihydroxy fatty acids were identified by gas chromatography-mass spectrometry. A series of at least 11 trihydroxy fatty acids was also observed but not fully characterized. Measurement of these compounds may prove to be useful in the diagnosis of some peroxisomal disorders.

Bacterial metabolism of human polymorphonuclear leukocyte-derived arachidonic acid

Evidence for transcellular bacterial metabolism of phagocyte-derived arachidonic acid was sought by exposing human blood polymorphonuclear leukocytes, prelabelled with [3H]arachidonic acid, to opsonized, stationary-phase Pseudomonas aeruginosa (bacteria-to-phagocyte ratio of 50:1) for 90 min at 37 degrees C. Control leukocytes were stimulated with the calcium ionophore A23187 (5 microM) for 5 min. Radiochromatograms of arachidonic acid metabolites, extracted from A23187-stimulated cultures and then separated by reverse-phase high-performance liquid chromatography, revealed leukotriene B4, its omega-oxidation products, and 5-hydroxy-eicosatetraenoic acid. In contrast, two major metabolite peaks, distinct from known polymorphonuclear leukocyte arachidonic acid products by high-performance liquid chromatography or by thin-layer chromatography, were identified in cultures of P. aeruginosa with [3H]arachidonic acid-labelled polymorphonuclear leukocytes. Respective chromatographic characteristics of these novel products were identical to those of two major metabolite peaks produced by incubation of stationary-phase P. aeruginosa with [3H]arachidonic acid. Production of the metabolites was dependent upon pseudomonal viability. UV spectral data were consistent with a conjugated diene structure. Metabolism of arachidonic acid by P. aeruginosa was not influenced by the presence of catalase, superoxide dismutase, nordihydroguaiaretic acid, ethanol, dimethyl sulfoxide, or ferrous ions but was inhibited by carbon monoxide, ketoconazole, and 1,2-epoxy-3,3,3-trichloropropane. Our data suggest that pseudomonal metabolism of polymorphonuclear leukocyte-derived arachidonic acid occurs during phagocytosis, probably by enzymatic epoxidation and hydroxylation via an oxygenase. By this means, potential proinflammatory effects of arachidonic acid or its metabolites may be modulated by P. aeruginosa at sites of infection in vivo.

Lipoxygenase metabolites of arachidonic acid in neuronal transmembrane signalling

Studies of invertebrate and vertebrate nervous tissue have demonstrated that free arachidonic acid and its lipoxygenase metabolites are produced in a receptor-dependent fashion. The intracellular actions of these compounds include the regulation of activity of membrane ion channels and protein kinases. In this article Daniele Piomelli and Paul Greengard review the evidence that these lipophilic molecules constitute a novel class of intracellular second messenger, possibly involved in the modulation of neurotransmitter release.

Possible involvement of arachidonic acid metabolites of cytochrome P450 monooxygenase pathway in vasopressin-stimulated glycogenolysis in isolated rat hepatocytes

Arachidonic acid (AA) is reported to be metabolized by three major pathways, i.e., cyclooxygenase (CO), lipoxygenase (LO), and NADPH-dependent cytochrome P450 monooxygenase (MO) pathways. Monooxygenase metabolites of AA have been proposed to play an important role in hormone action in various cells. Recently it was reported that the MO pathway may exist in rat liver. The present study was carried out to investigate the role of MO metabolites in vasopressin-induced glycogenolysis in isolated rat hepatocytes. The pretreatment of isolated rat hepatocytes with eicosatetraynoic acid (ETYA), an inhibitor of CO, LO, and MO pathways, and ketoconazole and SKF 525A, inhibitors of the MO pathway, dose-dependently reduced vasopressin-induced phosphorylase activation, while the pretreatment with indomethacin, an inhibitor of the CO pathway, had no effect. The increment of cytosolic calcium concentration in vasopressin-stimulated hepatocytes was also dose-dependently decreased by ETYA, ketoconazole, and SKF 525A. In vitro addition of epoxyeicosatrienoic acid (EET) dose-dependently increased both phosphorylase a activity and cytosolic calcium concentration. 14,15-EET was the most potent among four regioisomeric EETs. These results suggest that MO metabolites of AA, most likely EETs, may be involved in vasopressin-induced glycogenolysis probably via the activation of phosphorylase by increasing the cytosolic calcium concentration.

Transformation of arachidonic acid by 5- and 15-lipoxygenase pathways in bovine adrenal fasciculata cells

[1-14C]Arachidonic acid was incubated with isolated bovine adrenal fasciculata cells for 15 min at 37gC. The metabolites were separated and purified by reverse- and straight-phase high performance liquid chromatography, and identified by gas chromatography-mass spectrometry or radioimmunoassay. Identified metabolites were 5-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE), 15-hydroxy-5,8,11,13-eicosatetraenoic acid (15-HETE), leukotriene B4 and 11,14,15-trihydroxy-5,8,12-eicosatrienoic acid (11,14,15-THET). Addition of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HPETE), an intermediate metabolite of 15-lipoxygenase pathway to microsomes of bovine adrenal fasciculata cells resulted in the formation of 11,14,15-THET. The formation of 11,14,15-THET by microsomes was not dependent on the presence of NADPH, while it was dose-dependently suppressed by ketoconazole, a potent inhibitor of cytochrome P-450 dependent enzymes. These results indicate that 5- and 15-lipoxygenase pathways of arachidonic acid may exist in bovine adrenal fasciculata cells and that 15-HPETE is further metabolized to 11,14,15-THET by adrenal microsomal cytochrome P-450.

Purification and characterization of an anticonvulsant-induced human cytochrome P-450 catalysing cyclosporin metabolism

A form of human hepatic microsomal cytochrome P-450 (P450hA7) with subunit Mr 50,400 has been purified from an epileptic who had been receiving long-term treatment with anticonvulsant drugs. P450hA7 metabolized the immunosuppressant drug cyclosporin A and the dihydropyridine calcium channel antagonist nifedipine, but did not metabolize a similar dihydropyridine drug, nicardipine, nor a series of alkoxyresorufin model substrates. The hepatic microsomal concentration of P450hA7 was higher in five individuals who had been receiving long-term anticonvulsant treatment than in any of 21 individuals who had not been similarly treated. The mean P450hA7 concentration in the treated individuals was 5-fold higher than the mean concentration in the untreated individuals. It is concluded that P450hA7 is a member of the cytochrome P450III family which is induced by anticonvulsant drugs in man.

Products of heme-catalyzed transformation of the arachidonate derivative 12-HPETE open S-type K+ channels in Aplysia

In Aplysia mechanosensory neurons, the neuropeptide FMRFamide increases the opening of the background S-K+ channel. This action is mediated by activation of arachidonic acid metabolism. Arachidonic acid in Aplysia nervous tissue is transformed through the 12-lipoxygenase pathway to 12-HPETE, which undergoes further metabolism. In intact sensory cells, 12-HPETE simulates the FMRFamide response, raising the question of whether 12-HPETE is the messenger molecule ultimately acting on the S-K+ channel. Here we show that in cell-free (inside-out) patches from sensory cells, 12-HPETE fails to modulate the S-K+ channel, but in the presence of hematin (which catalyzes 12-HPETE metabolism), it triggers sharp increases in the channel opening probability. We also found that SKF-525A, an inhibitor of the cytochrome P450, reduces the response to FMRFamide, arachidonic acid, and 12-HPETE in intact cells. We conclude that a heme-catalyzed transformation of 12-HPETE is necessary and sufficient to promote the opening of the S-K+ channel and a heme-containing enzyme such as cytochrome P450 might play this key role.

Inhibitors of cytochrome P-450-dependent arachidonic acid metabolism

A new generation of heteroatom analogs of arachidonic acid are documented as powerful and selective inhibitors of the cytochrome P-450-dependent arachidonic acid oxygenase reaction (IC50, 5-10 microM) with little effect on either cyclooxygenase or soybean lipoxidase at 100 microM. The imidazole derivatives, ketoconazole and clotrimazole, are potent and selective inhibitors of the arachidonic acid epoxygenase and lipoxidase-like activities of phenobarbital-induced rat liver microsomal fractions (IC50, 2.0 and 0.3 microM, respectively). In contrast, the w/w-1 oxygenase activity of ciprofibrate-induced microsomal fractions was relatively resistant to inhibition by these compounds (IC50, 50 and 25 microM for ketoconazole and clotrimazole, respectively). Nordihydroguaiaretic acid (NDGA), eicosatetraynoic acid (ETYA), and indomethacin, extensively utilized inhibitors of the cyclooxygenase and lipoxygenase branches of the arachidonate cascade, also inhibit cytochrome P-450-dependent arachidonic acid metabolism. In decreasing order of potency, they were NDGA, ETYA, and indomethacin (IC50, 15, 40, and 70 microM, respectively).

Conditions for the formation of the oxo derivatives of arachidonic acid from platelet 12-lipoxygenase and soybean 15-lipoxygenase

Three carbonyl compounds derived from arachidonic acid have recently been characterized in human platelets, namely, 12-ketoeicosatetraenoic acid and two isomeric 12-oxododecatrienoic acids. The conditions for the synthesis of these compounds and for the synthesis of analogous products from soybean lipoxygenase, i.e., 15-ketoeicosatetraenoic acid and 15-oxopentadecatetraenoic acids, were compared with regard to the role of oxygen and fatty acid availability, and heme catalysis. Using platelet homogenates or soybean lipoxygenase and arachidonic acid as a substrate, it was found that the establishment of anaerobic conditions during the incubation was mandatory only for the synthesis of 15-oxopentadecatetraenoic acids. Anaerobic conditions, however, greatly increased the formation of 15-ketoeicosatetraenoic acid and, to a lesser extent, of 12-oxododecatrienoic acids. On the other hand, 12-hydroperoxyeicosatetraenoic acid (12-HPETE) was transformed into 12-ketoeicosatetraenoic acid and 12-oxododecatrienoic acids by platelet homogenates or soybean lipoxygenase. This transformation was increased when the incubation was performed in anaerobic conditions and in the presence of a fatty acid substrate of the enzyme. These data suggest that oxygen deprivation and excess fatty acid could play a stimulatory role in the synthesis of 12-oxo compounds by platelets. Finally, we have compared the heme-catalyzed generation of the 12-oxo and 15-oxo derivatives from their hydroperoxide precursors: whereas 12-oxododecatrienoic acids and 12-ketoeicosatetraenoic acid were formed in the proportion of 8.5: 1.5 from 12-HPETE incubated with hematin (150 nM), 15-ketoeicosatetraenoic acid was the only carbonyl compound generated from 15-HPETE in the same conditions, emphasizing the unique reactivity of the 12-HPETE.