The catalytic site of rat hepatic lauric acid omega-hydroxylase. Protein versus prosthetic heme alkylation in the omega-hydroxylation of acetylenic fatty acids

Exploring human CYP4 enzymes: physiological roles, function in diseases and focus on inhibitors

The cytochrome P450 (CYP)4 family of enzymes are monooxygenases responsible for the ω-oxidation of endogenous fatty acids and eicosanoids and play a crucial part in regulating numerous eicosanoid signaling pathways. Recently, CYP4 gained attention as a potential therapeutic target for several human diseases, including cancer, cardiovascular diseases and inflammation. Small-molecule inhibitors of CYP4 could provide promising treatments for these diseases. The aim of the present review is to highlight the advances in the field of CYP4, discussing the physiology and pathology of the CYP4 family and compiling CYP4 inhibitors into groups based on their chemical classes to provide clues for the future discovery of drug candidates targeting CYP4. Teaser: This review provides an updated view of the physiology and pathology of CYP4 enzymes. CYP4 inhibitors are compiled based on their skeletons to provide clues for the future discovery of drug candidates targeting CYP4.

Functional Characterization of Drosophila melanogaster CYP6A8 Fatty Acid Hydroxylase

Genomic analysis indicated that the genome of Drosophila melanogaster contains more than 80 cytochrome P450 genes. To date, the enzymatic activity of these P450s has not been extensively studied. Here, the biochemical properties of CYP6A8 were characterized. CYP6A8 was cloned into the pCW vector, and its recombinant enzyme was expressed in Escherichia coli and purified using Ni²⁺-nitrilotriacetate affinity chromatography. Its expression level was approximately 130 nmol per liter of culture. Purified CYP6A8 exhibited a low-spin state in the absolute spectra of the ferric forms. Binding titration analysis indicated that lauric acid and capric acid produced type І spectral changes, with K_d values 28 ± 4 and 144 ± 20 μM, respectively. Ultra-performance liquid chromatography-mass spectrometry analysis showed that the oxidation reaction of lauric acid produced (ω-1)-hydroxylated lauric acid as a major product and ω-hydroxy-lauric acid as a minor product. Steady-state kinetic analysis of lauric acid hydroxylation yielded a k_cat value of 0.038 ± 0.002 min^-1 and a K_m value of 10 ± 2 μM. In addition, capric acid hydroxylation of CYP6A8 yielded kinetic parameters with a k_cat value of 0.135 ± 0.007 min^-1 and a K_m value of 21 ± 4 μM. Because of the importance of various lipids as carbon sources, the metabolic analysis of fatty acids using CYP6A8 in this study can provide an understanding of the biochemical roles of P450 enzymes in many insects, including Drosophila melanogaster.

Acetylenes: cytochrome P450 oxidation and mechanism-based enzyme inactivation

The oxidation of carbon-carbon triple bonds by cytochrome P450 produces ketene metabolites that are hydrolyzed to acetic acid derivatives or are trapped by nucleophiles. In the special case of 17α-ethynyl sterols, D-ring expansion and de-ethynylation have been observed as competing pathways. The oxidation of acetylenic groups is also associated with mechanism-based inactivation of cytochrome P450 enzymes. One mechanism for this inactivation is reaction of the ketene metabolite with cytochrome P450 residues essential for substrate binding or catalysis. However, in the case of monosubstituted acetylenes, inactivation can also occur by addition of the oxidized acetylenic function to a nitrogen of the heme prosthetic group. This addition reaction is not mediated by the ketene metabolite, but rather occurs during oxygen transfer to the triple bond. In some instances, a detectable intermediate is formed that is most consistent with a ketocarbene-iron heme complex. This complex can progress to the N-alkylated heme or revert back to the unmodified enzyme. The ketocarbene complex may intervene in the formation of all the N-alkyl heme adducts, but is normally too unstable to be detected.

Heme Modification Contributes to the Mechanism-Based Inactivation of Human Cytochrome P450 2J2 by Two Terminal Acetylenic Compounds

The mechanism-based inactivation of human CYP2J2 by three terminal acetylenic compounds: N-(methylsulfonyl)-6-(2-propargyloxyphenyl)hexanamide (MS), 17-octadecynoic acid (OD), and danazol (DZ) was investigated. The loss of hydroxyebastine (OHEB) carboxylation activity in a reconstituted system was time- and concentration-dependent and required NADPH for MS and OD, but not DZ. The kinetic constants for the mechanism-based inactivation of OHEB carboxylation activity were: K_I of 6.1 μM and k_inact of 0.22 min^-1 for MS and K_I of 2.5 μM and k_inact of 0.05 min^-1 for OD. The partition ratios for MS and OD were ∼10 and ∼20, respectively. Inactivation of CYP2J2 by MS or OD resulted in a loss of the native heme spectrum and a similar decrease in the reduced CO difference spectrum. A heme adduct was observed in the MS-inactivated CYP2J2. The possible reactive metabolite which covalently modified the prosthetic heme was characterized by analysis of the glutathione conjugates formed by MS or OD following oxygenation of the ethynyl moiety. Liquid chromatography-mass spectrometry showed that inactivation by MS or OD did not lead to modification of apoprotein. Interaction of CYP2J2 with DZ produced a type II binding spectrum with a K_s of 2.8 μM and the IC₅₀ for loss of OHEB carboxylation activity was 0.18 μM. In conclusion, heme modification by MS and OD was responsible for the mechanism-based inactivation of CYP2J2. The results suggest that the ethynyl moiety of MS and OD faces the heme iron, whereas the isoxazole ring of DZ is preferentially oriented toward the heme iron of CYP2J2.

Effects of phenobarbital and valproate on the expression of cytochromes P-450 in co-cultured rat hepatocytes

We have investigated, in hepatocytes co-cultured with epithelial cells, the effects of phenobarbital and sodium valproate treatment on members of the cytochrome P-450 superfamily. The expression of the P450IIB and P450IV families was examined, using Western blotting, at 4, 7 and 14 days of co-culture either in the presence or absence of drug treatment. The amount of P450IIB was increased after exposure of the co-cultured cells to either phenobarbital or sodium valproate. In contrast, P450IV amounts were increased only by sodium valproate treatment. The maximal induction of P450IIB (17-fold) was observed at 7 and 14 days of co-culture, whereas P450IV was induced to the same extent in cells co-cultured for 4, 7 and 14 days. We also examined the expression of these two cytochrome P-450 subfamilies in hepatocytes cultured for 4 days in the absence of epithelial cells. The amounts of both P450IIB and P450IV were low or undetectable in these cells, and the induction of these proteins by either phenobarbital or valproate was less than that observed in co-cultured hepatocytes. Our results demonstrate that the co-culture system is a suitable in vitro system for examining the effects of various foreign compounds on the expression of phase I drug metabolizing enzymes.

Structural control of cytochrome P450-catalyzed ω-hydroxylation

The regiospecific or preferential ω-hydroxylation of hydrocarbon chains is thermodynamically disfavored because the ease of C-H bond hydroxylation depends on the bond strength, and the primary C-H bond of a terminal methyl group is stronger than the secondary or tertiary C-H bond adjacent to it. The hydroxylation reaction will therefore occur primarily at the adjacent secondary or tertiary C-H bond unless the protein structure specifically enforces primary C-H bond oxidation. Here we review the classes of enzymes that catalyze ω-hydroxylation and our current understanding of the structural features that promote the ω-hydroxylation of unbranched and methyl-branched hydrocarbon chains. The evidence indicates that steric constraints are used to favor reaction at the ω-site rather than at the more reactive (ω-1)-site.

Ethynyl and Propynylpyrene Inhibitors of Cytochrome P450

The single-crystal X-ray structures and in vivo activities of three aryl acetylenic inhibitors of cytochromes P450 1A1, 1A2, 2A6, and 2B1 have been determined and are reported herein. These are 1-ethynylpyrene, 1-propy-nylpyrene, and 4-propynylpyrene. To investigate electronic influences on the mechanism of enzyme inhibition, the experimental electron density distribution of 1-ethynylpy-rene has been determined using low-temperature X-ray diffraction measurements, and the resulting net atomic charges compared with various theoretical calculations. A total of 82,390 reflections were measured with Mo Kα radiation to a (sinθ/λ)(max) = 0.985 Å(-1). Averaging symmetry equivalent reflections yielded 8,889 unique reflections. A least squares refinement procedure was used in which multipole parameters were added to describe the distortions of the atomic electron distributions from spherical symmetry. A map of the model electron density distribution of 1-ethynylpyrene was obtained. Net atomic charges calculated from refined monopole population parameters yielded charges that showed that the terminal acetylenic carbon atom (C18) is more negative than the internal carbon (C17). Net atomic charges calculated by ab initio, density functional theory, and semi-empirical methods are consistent with this trend suggesting that the terminal acetylenic carbon atom is more likely to be the site of oxidation. This is consistent with the inhibition mechanism pathway that results in the formation of a reactive ketene intermediate. This is also consistent with assay results that determined that 1-ethynylpyrene acts as a mechanism-based inhibitor of P450s 1A1 and 1A2 and as a reversible inhibitor of P450 2B1. Crystallographic data: 1-ethynylpyrene, C(18)H(10), P2(1)/c, a = 14.571(2) Å, b = 3.9094(5) Å, c = 20.242(3) Å, β = 105.042(2)°, V = 1,113.5(2) Å(3); 1-propynylpyrene, C(19)H(12), P2(1)/n, a = 8.970(2) Å, b = 10.136(1) Å, c = 14.080(3) Å, β = 99.77(2)°, V = 1,261.5(4) Å(3); 4-propynylpyrene, C(19)H(12), Pbca, a = 9.904(1) Å, b = 13.174(2) Å, c = 19.401(1) Å, V = 2,531.4(5) Å(3).

Biochemical and structural characterization of CYP124: a methyl-branched lipid omega-hydroxylase from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) produces a variety of methyl-branched lipids that serve important functions, including modulating the immune response during pathogenesis and contributing to a robust cell wall that is impermeable to many chemical agents. Here, we report characterization of Mtb CYP124 (Rv2266) that includes demonstration of preferential oxidation of methyl-branched lipids. Spectrophotometric titrations and analysis of reaction products indicate that CYP124 tightly binds and hydroxylates these substrates at the chemically disfavored omega-position. We also report X-ray crystal structures of the ligand-free and phytanic acid-bound protein at a resolution of 1.5 A and 2.1 A, respectively, which provide structural insights into a cytochrome P450 with predominant omega-hydroxylase activity. The structures of ligand-free and substrate-bound CYP124 reveal several differences induced by substrate binding, including reorganization of the I helix and closure of the active site by elements of the F, G, and D helices that bind the substrate and exclude solvent from the hydrophobic active site cavity. The observed regiospecific catalytic activity suggests roles of CYP124 in the physiological oxidation of relevant Mtb methyl-branched lipids. The enzymatic specificity and structures reported here provide a scaffold for the design and testing of specific inhibitors of CYP124.

Mechanism-based inactivation of CYP2B1 and its F-helix mutant by two tert-butyl acetylenic compounds: covalent modification of prosthetic heme versus apoprotein

The mechanism-based inactivation of cytochrome CYP2B1 [wild type (WT)] and its Thr205 to Ala mutant (T205A) by tert-butylphenylacetylene (BPA) and tert-butyl 1-methyl-2-propynyl ether (BMP) in the reconstituted system was investigated. The inactivation of WT by BPA exhibited a k(inact)/K(I) value of 1343 min(-1)mM(-1) and a partition ratio of 1. The inactivation of WT by BMP exhibited a k(inact)/K(I) value of 33 min(-1)mM(-1) and a partition ratio of 10. Liquid chromatography/tandem mass spectrometry analysis (LC/MS/MS) of the WT revealed 1) inactivation by BPA resulted in the formation of a protein adduct with a mass increase equivalent to the mass of BPA plus one oxygen atom, and 2) inactivation by BMP resulted in the formation of multiple heme adducts that all exhibited a mass increase equivalent to BMP plus one oxygen atom. LC/MS/MS analysis indicated the formation of glutathione (GSH) conjugates by the reaction of GSH with the ethynyl moiety of BMP or BPA with the oxygen being added to the internal or terminal carbon. For the inactivation of T205A by BPA and BMP, the k(inact)/K(I) values were suppressed by 100- and 4-fold, respectively, and the partition ratios were increased 9- and 3.5-fold, respectively. Only one major heme adduct was detected following the inactivation of the T205A by BMP. These results show that the Thr205 in the F-helix plays an important role in the efficiency of the mechanism-based inactivation of CYP2B1 by BPA and BMP. Homology modeling and substrate docking studies were presented to facilitate the interpretation of the experimental results.

Metabolic activation of mifepristone [RU486; 17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-propynyl)-estra-4,9-dien-3-one] by mammalian cytochromes P450 and the mechanism-based inactivation of human CYP2B6

Mifepristone [RU486; 17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-propynyl)-estra-4,9-dien-3-one] inactivates CYP2B6 in the reconstituted system in a mechanism-based manner. The loss of 7-ethoxy-4-(trifluoromethyl)-coumarin deethylation activity of CYP2B6 is concentration- and time-dependent. The inactivation requires NADPH and is irreversible. The concentration of inactivator required to give the half-maximal rate of inactivation is 2.8 microM, and the maximal rate constant for inactivation at a saturating concentration of the inactivator is 0.07 min(-1). Incubation of CYP2B6 with 20 microM RU486 for 15 min resulted in 61% loss of catalytic activity, 60% loss of the reduced cytochrome P450 (P450)-CO complex, and a 40% loss of native heme. The partition ratio is approximately 5, and the stoichiometry of binding is approximately 0.6 mol RU486/mol P450 inactivated. SDS-polyacrylamide gel electrophoresis and high-pressure liquid chromatography analysis showed that [(3)H]RU486 was irreversibly bound to CYP2B6 apoprotein. RU486 is metabolized to form three major metabolites and bioactivated to give reactive intermediates by purified P450s in the reconstituted system. After incubation of RU486 with the purified P450s and liver microsomes from rats and humans in the presence of glutathione (GSH) and NADPH, GSH conjugates with MH(+) ions at m/z 769, 753, and 751 were detected by liquid chromatography-tandem mass spectrometry. Two GSH conjugates with MH(+) ions at m/z 753 are formed from the reaction of GSH with RU486. The adducts are formed after addition of an activated oxygen to the carbon-carbon triple bond of the propynyl moiety. This suggests that oxirene intermediates may be involved in the mechanism of inactivation. It seems that the potential for drug-drug interactions of RU486 may not be limited only to CYP3A4 and should also be evaluated for drugs metabolized primarily by CYP2B6, such as bupropion and efavirenz.

Functional expression and characterization of cytochrome P450 52A21 from Candida albicans

Candida albicans contains 10 putative cytochrome P450 (CYP) genes coding for enzymes that appear to play important roles in fungal survival and virulence. Here, we report the characterization of CYP52A21, a putative alkane/fatty acid hydroxylase. The recombinant CYP52A21 protein containing a 6x(His)-tag was expressed in Escherichia coli and was purified. The purified protein, reconstituted with rat NADPH-cytochrome P450 reductase, omega-hydroxylated dodecanoic acid to give 12-hydroxydodecanoic acid, but to a lesser extent also catalyzed (omega-1)-hydroxylation to give 11-hydroxydodecanoic acid. When 12,12,12-d(3)-dodecanoic acid was used as the substrate, there was a major shift in the oxidation from the omega- to the (omega-1)-hydroxylated product. The regioselectivity of fatty acid hydroxylation was examined with the 12-iodo-, 12-bromo-, and 12-chlorododecanoic acids. Although all three 12-halododecanoic acids bound to CYP52A21 with similar affinities, the production of 12-oxododecanoic acid decreased as the size of the terminal halide increased. The regioselectivity of CYP52A21 fatty acid oxidation is thus consistent with presentation of the terminal end of the fatty acid chain for oxidation via a narrow channel that limits access to other atoms of the fatty acid chain. This constricted access, in contrast to that proposed for the CYP4A family of enzymes, does not involve covalent binding of the heme to the protein.

The inactivation of cytochrome P450 3A5 by 17alpha-ethynylestradiol is cytochrome b5-dependent: metabolic activation of the ethynyl moiety leads to the formation of glutathione conjugates, a heme adduct, and covalent binding to the apoprotein

17Alpha-ethynylestradiol (EE) inactivates cytochrome P450 3A5 (3A5) in the reconstituted system in a mechanism-based manner. The inactivation is dependent on NADPH, and it is irreversible. The inactivation of 3A5 by EE is also dependent on cytochrome b5 (b5). The values for the K(I) and k(inact) of the 7-benzyloxy-4-(trifluoromethyl)coumarin O-debenzylation activity of 3A5 are 26 microM and 0.06 min(-1), respectively. Incubation of 3A5 with EE resulted in a 62% loss of catalytic activity, 60% loss in the reduced CO difference spectrum, and 40% decrease in native heme with the formation of a heme adduct. The partition ratio was approximately 25, and the stoichiometry of binding was approximately 0.3 mol of EE metabolite bound/mol of P450 inactivated. Four major metabolites were formed during the metabolism of EE by 3A5. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [3H]EE was irreversibly bound to 3A5 apoprotein. Liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) revealed that two glutathione (GSH) conjugates with m/z values of 620 were formed only in the presence of b5. These two conjugates are formed from the reaction of GSH with the ethynyl group with the oxygen being inserted into either the internal or terminal carbon. A heme adduct with the ion at m/z 927 and two dipyrrole adducts with ions at m/z 579 were detected by LC-MS/MS analysis. In conclusion, 3A5 can activate EE to a 17alpha-oxirene-related reactive species that can then partition the oxygen between the internal and terminal carbons of the ethynyl group to form heme and apoprotein adducts, resulting in the inactivation of P450 3A5.

Cloning, Functional Expression, and Characterization of CYP709C1, the First Sub-terminal Hydroxylase of Long Chain Fatty Acid in Plants

We cloned and characterized CYP709C1, a new plant cytochrome P450 belonging to the P450 family, that so far has no identified function except for clustering with a fatty acid metabolizing clade of P450 enzymes. We showed here that CYP709C1 is capable of hydroxylating fatty acids at the omega-1 and omega-2 positions. This work was performed after recoding and heterologous expression of a full-length cDNA isolated from a wheat cDNA library in an engineered yeast strain. Investigation on substrate specificity indicates that CYP709C1 metabolizes different fatty acids varying in their chain length (C12 to C18) and unsaturation. CYP709C1 is the first identified plant cytochrome P450 that can catalyze sub-terminal hydroxylation of C18 fatty acids. cis-9,10-Epoxystearic acid is metabolized with the highest efficiency, i.e. K((m)(app)) of 8 microM and V(max(app)) of 328 nmol/min/nmol P450. This, together with the fact that wheat possesses a microsomal peroxygenase able to synthesize this compound from oleic acid, strongly suggests that it is a physiological substrate. Hydroxylated fatty acids are implicated in plant defense events. We postulated that CYP709C1 could be involved in plant defense by producing such compounds. This receives support from the observation that (i) sub-terminal hydroxylation of 9,10-epoxystearic acid is induced (15-fold after 3 h) in microsomes of wheat seedlings treated with the stress hormone methyl jasmonate and (ii) CYP709C1 is enhanced at the transcriptional level by this treatment. CYP709C1 transcript also accumulated after treatment with a combination of the safener naphthalic acid anhydride and phenobarbital. This indicates a possible detoxifying function for CYP709C1 that we discussed.

A gas chromatography-mass spectrometry method for the measurement of fatty acid omega and omega(-1) hydroxylation kinetics by CYP4A1 using an artificial membrane system

A gas chromatography-mass spectrometry assay method for the analysis of lauric, myristic, and palmitic acids and their omega and omega(-1) hydroxylated metabolites from in vitro incubations of cytochrome P450 CYP4A1, involving solid-phase extraction and trimethysilyl derivatization, was developed. The assay was linear, precise, and accurate over the range 0.5 to 50microM for all the analytes. It has the advantages of a more rapid analysis time, an improved sensitivity, and a wider range of analytes compared with other methods. An artificial membrane system was optimized for application to purified CYP4A1 enzyme by investigating the molar ratios of cytochrome b(5) and cytochrome P450 reductase present in the incubation mixture. Using this method, the kinetics of omega and omega(-1) oxidation of lauric, myristic, and palmitic acids by CYP4A enzymes were measured and compared in rat liver microsomes and an artificial membrane system.

Calibration of the channel that determines the omega-hydroxylation regiospecificity of cytochrome P4504A1: catalytic oxidation of 12-HALODOdecanoic acids

The fatty acid omega-hydroxylation regiospecificity of CYP4 enzymes may result from presentation of the terminal carbon to the oxidizing species via a narrow channel that restricts access to the other carbon atoms. To test this hypothesis, the oxidation of 12-iodo-, 12-bromo-, and 12-chlorododecanoic acids by recombinant CYP4A1 has been examined. Although all three 12-halododecanoic acids bind to CYP4A1 with similar dissociation constants, the 12-chloro and 12-bromo fatty acids are oxidized to 12-hydroxydodecanoic acid and 12-oxododecanoic acid, whereas the 12-iodo analogue is very poorly oxidized. Incubations in H(2)(18)(2)O show that the 12-hydroxydodecanoic acid oxygen derives from water, whereas that in the aldehyde derives from O(2). The alcohol thus arises from oxidation of the halide to an oxohalonium species that is hydrolyzed by water, whereas the aldehyde arises by a conventional carbon hydroxylation-elimination mechanism. No irreversible inactivation of CYP4A1 is observed during 12-halododecanoic acid oxidation. Control experiments show that CYP2E1, which has an omega-1 regiospecificity, primarily oxidizes 12-halododecanoic acids to the omega-aldehyde rather than alcohol product. Incubation of CYP4A1 with 12,12-[(2)H](2)-12-chlorododecanoic acid causes a 2-3-fold increase in halogen versus carbon oxidation. The fact that the order of substrate oxidation (Br > Cl >> I) approximates the inverse of the intrinsic oxidizability of the halogen atoms is consistent with presentation of the halide terminus via a channel that accommodates the chloride and bromide but not iodide atoms, which implies an effective channel diameter greater than 3.90 Angstroms but smaller than 4.30 Angstroms.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Regulation and inhibition of arachidonic acid omega-hydroxylases and 20-HETE formation

Cytochrome P450-catalyzed metabolism of arachidonic acid is an important pathway for the formation of paracrine and autocrine mediators of numerous biological effects. The omega-hydroxylation of arachidonic acid generates significant levels of 20-hydroxyeicosatetraenoic acid (20-HETE) in numerous tissues, particularly the vasculature and kidney tubules. Members of the cytochrome P450 4A and 4F families are the major omega-hydroxylases, and the substrate selectivity and regulation of these enzymes has been the subject of numerous studies. Altered expression and function of arachidonic acid omega-hydroxylases in models of hypertension, diabetes, inflammation, and pregnancy suggest that 20-HETE may be involved in the pathogenesis of these diseases. Our understanding of the biological significance of 20-HETE has been greatly aided by the development and characterization of selective and potent inhibitors of the arachidonic acid omega-hydroxylases. This review discusses the substrate selectivity and expression of arachidonic acid omega-hydroxylases, regulation of these enzymes during disease, and the application of enzyme inhibitors to study 20-HETE function.

A method for determining two substrates binding in the same active site of cytochrome P450BM3: an explanation of high energy omega product formation

A number of enzymes from the cytochrome P450 family show atypical (non-Michaelis-Menten) kinetic behavior resulting from substrate activation, inhibition, partial inhibition, biphasic saturation, or autoactivation. Herein, we provide a technique that can identify multiple substrate occupancy in the same active site of a P450 as a result of an altered kinetic profile. Using an isotope effect on product ratios confirms that the enzyme-substrate (ES) complex responsible for omega hydroxylation of palmitic acid (palmitate) is in rapid equilibrium with the ES complex that leads to omega-1 hydroxylation of palmitate. Co-incubation of a second substrate, lauric acid (laurate), results in a change in the ratio of omega to omega-1 hydroxylated palmitate. Furthermore, an isotope effect on palmitate is observed when deuterated laurate is co-incubated with non-deuterated palmitate. These results are only consistent with both substrates being in the same active site simultaneously. This mode of binding explains how the F87A mutant of P450BM3 is able to produce the omega alcohol, a product that arises from the high-energy primary radical.

Mechanism-based inactivation of cytochrome P450 2B6 by a novel terminal acetylene inhibitor

N-(3,5-Dichloro-4-pyridyl)-3-(cyclopentyloxy)-4-methoxybenzamide (DCMB) is a known marker substrate for cytochrome p450 2B6. Based on the chemical template of DCMB, a novel terminal acetylene compound, N-(3,5-dichloro-4-pyridyl)-4-methoxy-3-(prop-2-ynyloxy)benzamide (TA) was synthesized and evaluated as a mechanism-based inactivator of p450 2B6. The pseudo first-order inactivation of expressed p450 2B6 by TA was both substrate and time-dependent. The kinetics of inhibition resulted in a maximal rate constant (k(inactivation)) of 0.09 min(-1) and an apparent K(I) of 5.1 microM. Incubation of expressed p450 2B6 with TA and NADPH resulted in a 68% loss in enzyme activity and a concurrent 62% loss in the formation of a reduced carbon monoxide complex, suggesting that heme destruction is the primary mode of enzyme inactivation. Enzyme inactivation of p450 2B6 was not reduced by the presence of 10 mM glutathione and was protected by incubation of excess DCMB with TA. The production of the carboxylic acid metabolite, N-(3,5-Dichloro-4-pyridyl)-3-(2-carboxyethoxy)-4-methoxybenzamide (TA-COOH), during the incubation of TA with 2B6 suggests that inactivation proceeds through a ketene intermediate. For 2B6 inactivation, the partition ratio was approximately 1.5 nmol TA-COOH formed/nmol P450 inactivated. Finally, TA was evaluated for mechanism-based inactivation of p450 3A4, 2C9, 2C19, 2D6, and 2E1 using human liver microsomes. In addition to 2B6, p450 2C forms were also found to be sensitive to TA-mediated inactivation, suggesting that subtle changes in the O-alkyl chain of the parent may be critical for the selectivity of enzyme inactivation.

Antihypertensive effect of mechanism-based inhibition of renal arachidonic acid omega-hydroxylase activity

The cytochrome P-450 eicosanoid 20-hydroxyeicosatetraenoic acid (20-HETE) is a potent vasoconstrictor that is implicated in the regulation of blood pressure. The identification of selective inhibitors of renal 20-HETE formation for use in vivo would facilitate studies to determine the systemic effects of this eicosanoid. We characterized the acetylenic fatty acid sodium 10-undecynyl sulfate (10-SUYS) as a potent and selective mechanism-based inhibitor of renal 20-HETE formation. A single dose of 10-SUYS caused an acute reduction in mean arterial blood pressure in 8-wk-old spontaneously hypertensive rats. The decrease in mean arterial pressure was maximal 6 h after 10-SUYS treatment (17.9 +/- 3.2 mmHg; P < 0.05), and blood pressure returned to baseline levels within 24 h after treatment. Treatment with 10-SUYS was associated with a decrease in urinary 20-HETE formation in vivo and attenuation of the vasoconstrictor response of renal interlobar arteries to ANG II in vitro. These results provide further evidence that 20-HETE plays an important role in the regulation of blood pressure in the spontaneously hypertensive rat.

Enhanced electron transfer and lauric acid hydroxylation by site-directed mutagenesis of CYP119

CYP119, a cytochrome P450 from a thermophilic organism for which a crystal structure is available, is shown here to hydroxylate lauric acid in a reaction supported by putidaredoxin and putidaredoxin reductase. This fatty acid hydroxylation activity is increased 15-fold by T214V and D77R mutations. The T214V mutation increases the rate by facilitating substrate binding and enhancing the associated spin state change, whereas the D77R mutation improves binding of the heterologous redox partner putidaredoxin to CYP119 and the rate of electron transfer from it to the heme group. A sequence alignment with P450(cam) can, therefore, be used to identify a part of the binding site for putidaredoxin on an unrelated P450 enzyme. This information can be used to engineer by mutagenesis an improved complementarity of the protein-protein interface that results in improved electron transfer from putidaredoxin to the P450 enzyme. As a result, the catalytic activity of the thermo- and barostable CYP119 has been incorporated into a catalytic system that hydroxylates fatty acids.

Use of kinetic isotope effects to delineate the role of phenylalanine 87 in P450(BM-3)

The substrate oxidation rates of P450(BM-3) are unparalleled in the cytochrome P450 (CYP) superfamily of enzymes. Furthermore, the bacterial enzyme, originating from Bacillus megaterium, has been used repeatedly as a model to study the metabolism of mammalian P450s. A specific example is presented where studying P450(BM-3) substrate dynamics can define important enzyme-substrate characteristics, which may be useful in modeling omega-hydroxylation seen in mammalian P450s. In addition, if the reactive species responsible for metabolism can be controlled to produce specific products this enzyme could be a useful biocatalyst. Based on crystal structures and the fact that the P450(BM-3) F87A mutant produces a large isotope in contrast to the native enzyme, we propose that phenylalanine 87 is responsible for hindering substrate access to the active oxygen species for nonnative substrates. Using kinetic isotopes and two aromatic substrates, p-xylene and 4,4'-dimethylbiphenyl, the role phenylalanine 87 plays in active-site dynamics is characterized. The intrinsic KIE is 7.3 +/- 2 for wtP450(BM-3) metabolism of p-xylene. In addition, stoichiometry differences were measured with the native and mutant enzyme and 4,4'-dimethylbiphenyl. The results show a more highly coupled substrate/NADPH ratio in the mutant than in the wtP450(BM-3).

Hydroxylation of fatty acids by microsomal and reconstituted cytochrome P450 2B1

Understanding the mechanisms by which cytochrome(s) P450 (CYP) discriminate good from poor substrates, and orient them for highly regio- and stereoselective oxidation, has considerable intrinsic and practical importance. Here we present results of a study of fatty acid hydroxylation by CYP2B1 in a reconstituted system and in microsomes from phenobarbital-pretreated rats. The results indicate that 2B1 prefers decanoic acid as the optimum fatty acid substrate (among C(8)-C(16)) and that it hydroxylates all positions five or more methylene groups distant from the carboxylate carbon. That hydroxylation does not occur at carbon atoms closer to the carboxyl group than the C(6) position suggests that these carbons may not reach the ferryl oxygen because the carboxyl group is anchored to a specific site at a fixed distance from the heme iron.

Expressed CYP4A4 metabolism of prostaglandin E(1) and arachidonic acid

Cytochrome P4504A4 (CYP4A4) is a hormonally induced pulmonary cytochrome P450 which metabolizes prostaglandins and arachidonic acid (AA) to their omega-hydroxylated products. Although the physiological function of this enzyme is unknown, prostaglandins play an important role in the regulation of reproductive, vascular, intestinal, and inflammatory systems and 20-hydroxyeicosatetraenoic acid, the omega-hydroxylated product of arachidonate, is a potent vasoconstrictor. Therefore, it is important to obtain sufficient quantities of the protein for kinetic and biophysical characterization. A CYP4A4 construct was prepared and expressed in Escherichia coli. The enzyme was purified, and its activity with substrates prostaglandin E(1) (PGE(1)) and AA was examined in the presence and absence of cytochrome b(5) (cyt b(5)) and with a heme-depleted form of cyt b(5) (apo b(5)). The stimulatory role played by cyt b(5) in this system is not dependent on electron transfer from cyt b(5) to the CYP4A4 as similar stimulation was observed with apo b(5). Rapid kinetic measurement of CYP4A4 electron transfer rates confirmed this result. Both flavin and heme reduction rates were constant in the absence and presence of cyt b(5) or apo b(5). CD spectroscopy demonstrated that a conformational change occurred in CYP4A4 protein upon binding of cyt b(5) or apo b(5). Finally, acetylenic fatty acid inhibitors 17-octadecynoic acid, 12-hydroxy-16-heptadecynoic acid, 15-hexadecynoic acid, and 10-undecynoic acid (10-UDYA) were used to probe the substrate-binding pocket of CYP4A4. The short-chain fatty acid inhibitor 10-UDYA was unable to inhibit either PGE(1) or AA metabolism. All but 10-UDYA were effective inhibitors of CYP4A4.

Roles of cytochrome p450 in development

Cytochrome P450 (CYP) forms are ubiquitous in nature, appearing in almost all phyla, with many forms appearing in any organism. About 50 different forms have been identified in man, and some of these are found in the embryo, some showing temporal dependence. Many of the forms of cytochrome P450 present in one species have homologues in other species. For example, CYP1A2 is present in many species, including man, rabbits, rodents, fish and fowl. The amino acid sequence identity of these homologues is often in excess of 70%. CYP26, too, has more than 61% identity in amino acid sequence between fish, fowl and mammals. In view of the high degree of conservation of sequence as well as of enzymatic activities, it is only reasonable to assume that such strong conservation of sequence also reflects a conservation of function. Since the 'xenobiotic metabolizing' enzymes predate the production of the many xenobiotics they are known to metabolize, perhaps it is reasonable to consider endobiotics as natural substrates for their metabolism. Of the identified forms of cytochrome P450 that are present in embryonic tissue, we consider the possibility that they serve the organism in support of morphogenesis of the embryonic tissue. These forms may either function to generate morphogenic molecules or to keep regions free of them, thereby creating temporal and spatial regions of morphogen action and supporting region-specific changes in cells. One known morphogen, retinoic acid, has the enzymes retinal dehydrogenase (RALDH) and CYP26 maintaining its actions, the former responsible for its generation and the latter for its elimination. Another form of cytochrome P450, CYP1B1 appears also to be involved in differentiation of tissue, with its absence resulting in primary congenital glaucoma. However, the nature of the morphogen it may maintain still remains to be elucidated.

A conservative amino acid substitution alters the regiospecificity of CYP94A2, a fatty acid hydroxylase from the plant Vicia sativa

Fatty acid omega-hydroxylation is involved in the biosynthesis of the plant cuticle, formation of plant defense signaling molecules, and possibly in the rapid catabolism of free fatty acids liberated under stress conditions. CYP94A2 is a cytochrome P450-dependent medium-chain fatty acid hydroxylase that was recently isolated from Vicia sativa. Contrary to CYP94A1 and CYP86A1, two other fatty acid hydroxylases previously characterized in V. sativa and Arabidopsis thaliana, CYP94A2 is not a strict omega-hydroxylase, but exhibits chain-length-dependent regioselectivity of oxidative attack. Sequence alignments of CYP94A2 with CYP94A1 and molecular modeling studies suggested that F494, located in SRS-6 (substrate recognition site) was involved in substrate recognition and positioning. Indeed, a conservative amino acid substitution at that position markedly altered the regiospecificity of CYP94A2. The observed shift from omega toward omega-1 hydroxylation was prominent with lauric acid as substrate and declined with increasing fatty acid chain length.

Covalently linked heme in cytochrome p4504a fatty acid hydroxylases

Three independent experimental methods, liquid chromatography, denaturing gel electrophoresis with heme staining, and mass spectrometry, establish that the CYP4A class of enzymes has a covalently bound heme group even though the heme is not cross-linked to the protein in other P450 enzymes. Covalent binding has been demonstrated for CYP4A1, -4A2, -4A3, -4A8, and -4A11 heterologously expressed in Escherichia coli. However, the covalent link is also present in CYP4A1 isolated from rat liver and is not an artifact of heterologous expression. The extent of heme covalent binding in the proteins as isolated varies and is substoichiometric. In CYP4A3, the heme is attached to the protein via an ester link to glutamic acid residue 318, which is on the I-helix, and is predicted to be within the active site. This is the first demonstration that a class of cytochrome P450 enzymes covalently binds their prosthetic heme group.

New Selective Inhibitors of Cytochrome P450 2B4 and an Activator of Cytochrome P450 3A6 in Rabbit Liver Microsomes

We investigated interactions of adamantane, diamantane and their two substituted derivatives, 2-isopropenyl-2-methyladamantane (2-PMADA,1) and 3-isopropenyl-3-methyldiamantane (3-PMDIA,2), with various isoforms of rabbit cytochrome P450 (CYP). The data of spectroscopic experiments showed that all the substances are bound to the substrate binding site of rabbit CYP2B4 and CYP3A6.1and2are compounds having higher affinities to these CYP isoforms than adamantane and diamantane. All compounds inhibit CYP2B4 specific enzyme activity (the 7-pentoxyresorufinO-depentylase activity). The 50% inhibition of CYP2B4 was due to 3.82, 0.61, 0.66 and 0.37 μM adamantane, diamantane,1and2, respectively. The products formed during the CYP2B4-mediated metabolism of studied substances are less effective inhibitors than parent compounds. An opposite effect of1on CYP3A6 was determined. The specific enzyme activity of CYP3A6 increased to 138% of control when1was used in the presence of 40 μM erythromycin as a substrate. Here, we report the finding of a new activator of CYP3A6 having the structure quite different from that of CYP3A6 activators known to date.

The catalytic site of cytochrome P4504A11 (CYP4A11) and its L131F mutant

CYP4A11, the principal known human fatty acid omega-hydroxylase, has been expressed as a polyhistidine-tagged protein and purified to homogeneity. Based on an alignment with P450BM-3, the CYP4A11 L131F mutant has been constructed and similarly expressed. The two proteins are spectroscopically indistinguishable, but wild-type CYP4A11 primarily catalyzes omega-hydroxylation, and the L131F mutant only omega-1 hydroxylation, of lauric acid. The L131F mutant is highly uncoupled in that it slowly (omega-1)-hydroxylates lauric acid yet consumes NADPH at approximately the same rate as the wild-type enzyme. Wild-type CYP4A11 is inactivated by 1-aminobenzotriazole under turnover conditions but the L131F mutant is not. This observation, in conjunction with the binding affinities of substituted imidazoles for the two proteins, indicates that the L131F mutation decreases access of exogenous substrates to the heme site. Leu-131 thus plays a key role in controlling the regioselectivity of substrate hydroxylation and the extent of coupled versus uncoupled enzyme turnover. A further important finding is that the substituted imidazoles bind more weakly to CYP4A11 and its L131F mutant when these proteins are reduced by NADPH-cytochrome P450 reductase than by dithionite. This finding suggests that the ferric enzyme undergoes a conformational change that depends on both reduction of the iron and the presence of cytochrome P450 reductase and NADPH.

Selectivity of polycyclic inhibitors for human cytochrome P450s 1A1, 1A2, and 1B1

Human cytochrome P450s 1A1, 1A2, and 1B1 are known to have overlapping substrate specificities. All are regulated in part by the Ah locus; P450 1A2 is expressed essentially only in liver, but P450s 1A1 and 1B1 are both expressed in many extrahepatic tissues. Twenty-five polycyclic hydrocarbons, many containing acetylenic side chains, were examined as inhibitors of the three enzymes using 7-ethoxyresorufin O-deethylation as the enzyme assay in all cases. Several compounds were inhibitory at low nanomolar concentrations. 1-(1-Propynyl)pyrene and 2-(1-propynyl)phenanthrene nearly completely inhibited P450 1A1 at concentrations at which no P450 1B1 inhibition was observed. 2-Ethynylpyrene and alpha-naphthoflavone (7, 8-benzoflavone) nearly completely inhibited P450 1B1 at concentrations at which no P450 1A1 inhibition was noted. All four of the above compounds also inhibited P450 1A2. Several polycyclic hydrocarbons devoid of acetylenic groups were also inhibitory with respect to all three P450s. Some of the acetylenic compounds examined showed enhanced inhibition following preincubation with the P450s in the presence of cofactors NADPH and O2. However, of seven compounds (five acetylenes) tested with P450 1B1, only two [2-ethynylpyrene and 4-(1-propynyl)biphenyl] showed such evidence for mechanism-based inactivation. We conclude that (i) several polycyclic hydrocarbons and their oxidation products are very inhibitory with respect to human P450s 1A1, 1A2, and 1B1; (ii) of these inhibitors only some are mechanism-based inactivators; and (iii) some of the inhibitors are potentially useful for distinguishing between human P450s 1A1 and 1B1.

Positive feedback between ethanolamine-specific phospholipid base exchange and cytochrome P450 activities in rat liver microsomes. The effect of clofibric acid

The results of the present investigation relate the effects of the nutritional state and administration of clofibric acid (CLA), a hypolipidaemic drug and peroxisomal proliferator, on phosphatidylethanolamine (PE) synthesis in rat liver and fatty acid metabolism. Fasting and CLA treatment of animals causes an increase in the amount of PE in endoplasmic reticulum (ER) membranes and mitochondria, as well as in the PE/phosphatidylcholine (PC) ratio. Moreover, the activity of the ethanolamine-specific phospholipid base exchange (PLBE) enzyme in liver ER membranes of fasted animals was enhanced by 75% in comparison to that of animals fed ad libitum. The effect of CLA treatment was additive to that of starvation; PE synthesis tested in vitro via the Ca2+-sensitive PLBE reaction increased 3-fold in comparison to rats fed ad libitum. This is confirmed by an increased Vmax for the reaction, but the affinity of the enzyme for ethanolamine was not significantly changed. These effects were accompanied by an enhanced expression of cytochrome P450 CYP4A1 isoform and elevated activity of the enzyme upon CLA administration. The stimulatory effect of CLA administration on the efficiency of the ethanolamine-specific PLBE reaction can be explained by elimination of lauric acid, a known inhibitor of de novo PE synthesis, during the course of omega-hydroxylation catalysed by CYP4A1, and by increased expression of the PLBE enzyme. The products of omega-hydroxylation of lauric acid, which are then converted by dehydrogenase to 1,12-dodecanedioic acid, did not significantly affect the in vitro synthesis of PE.

Inhibition of renal arachidonic acid omega-hydroxylase activity with ABT reduces blood pressure in the SHR

The mechanism-based cytochrome P-450 (CYP) inhibitor 1-aminobenzotriazole (ABT) was characterized as an inhibitor of renal arachidonic acid metabolism and administered to spontaneously hypertensive rats (SHRs) to determine the effect of reduced eicosanoid production on mean arterial pressure (MAP). A single intraperitoneal dose of ABT to Sprague-Dawley rats caused a dose-dependent loss of renal CYP content, arachidonic acid metabolism, and CYP4A protein. In the cortex and outer medulla, ABT showed a high degree of selectivity for the CYP4A enzymes, reflected by the potent inhibition of 19- and 20-hydroxyeicosatetraenoic acid (19- and 20-HETE) formation. A 50 mg/kg dose of ABT reduced cortical 20-HETE formation to 16.1 +/- 0.82% of control and outer medullary 20-HETE formation to 23.8 +/- 0.45% of control. In contrast, there was no inhibition of renal epoxygenase activity at this dose. Renal CYP content, arachidonic acid omega- and (omega-1)-hydroxylase activity, and CYP4A protein levels gradually return to control levels by 72 h after a single dose of ABT. Cortical 20-HETE formation recovered from 17.9 +/- 3.15% of control at 6 h to 84.8 +/- 4.67% of control at 72 h after ABT administration. A single injection of ABT to 7-wk-old SHRs caused an acute reduction in MAP, which remained suppressed for at least 12 h. The effect was maximal within 4 h and averaged 17-23 mmHg during the 4- to 12-h period after administration. 20-HETE formation was inhibited 85% in the cortex and 70-80% in the outer medulla during the period when MAP was reduced. A structurally related ABT analog 1-hydroxybenzotriazole had no effect on blood pressure or renal arachidonic acid metabolism. These results identify ABT as a selective inhibitor of renal CYP4A activity and provide further support for a role for 20-HETE in the regulation of blood pressure.

Glu-320 and Asp-323 are determinants of the CYP4A1 hydroxylation regiospecificity and resistance to inactivation by 1-aminobenzotriazole

Little information is available on the active site structure of the CYP4A family of enzymes or the mechanism by which their omega-hydroxylation regiospecificity is enforced. We report here that the E320A, D323E, and E320/D323E mutations decrease the catalytic rate of CYP4A1 approximately 5-fold and cause up to a 10-fold shift from omega- to (omega-1)-hydroxylation. The decreased catalytic rate is due to an increase in the uncoupled reduction of molecular oxygen. Tighter binding of 1- and 4-substituted imidazoles to the double mutant than to the other proteins suggests that its active site is less constrained. The reaction of these proteins with phenyldiazene causes heme degradation without the detectable formation of a phenyl-iron complex. CYP4A1 and its E320A mutant are not inactivated by 1-aminobenzotriazole (1-ABT), but the D323E and E320A/D323E mutants are inactivated. The resistance of purified CYP4A1 to inactivation by 1-ABT is surprising in view of the fact that 1-ABT causes the loss of the omega-hydroxylase activity both in microsomal preparations and in vivo. Collectively, the results establish that Glu-320, and particularly Asp-323, help to define the active site dimensions, the degree of coupled versus uncoupled versus uncoupled turnover, the omega-versus (omega-1)-hydroxylation regiospecificity, and the susceptibility to inactivation by mechanism-based inhibitors. Furthermore, they provide experimental evidence for a structural analogy between the CYP4A1 and P450BM-3 active sites.

Evaluation of chemicals as inhibitors of trout cytochrome P450s

This study examined the ability of several inhibitors of mammalian cytochrome P450s to affect hepatic P450-mediated monooxygenase activities in microsomes from beta-naphthoflavone (BNF)-treated rainbow trout. Three monooxygenase activities, namely, lauric acid (omega-1)-hydroxylase (LA-OH), 7,12-dimethylbenz[a]anthracene hydroxylase (DMBA-OH), and progesterone 6beta-hydroxylase (PROG-OH) activities were used as functional markers for trout hepatic CYP2K1, CYP1A1, and CYP3A27, respectively. The chemicals that were examined for their inhibitory effects were reversible, quasi-irreversible, or irreversible inhibitors of mammalian P450. At 100 microM concentration, the reversible inhibitors (ketoconazole, miconazole, and clotrimazole) were most potent in inhibiting LA-OH activity. These imidazole compounds, as well as ellipticine, parathion, and alpha-naphthoflavone, were the strongest inhibitors of DMBA-OH and PROG-OH activities. In addition, isosafrole, piperonyl butoxide, gestodene, 17alpha-ethynylestradiol, 1-aminobenzotriazole, and 5,8,11,14-eicosatetraynoic acid strongly inhibited PROG-OH activity. The global inhibitors, metyrapone, chloramphenicol, and allylisopropylacetamide, had very little or no inhibitory effect on trout LA-OH and DMBA-OH activities. Triacetyloleandomycin, a CYP3A inhibitor, did not affect PROG-OH activity catalyzed by trout CYP3A27. Diethyldithiocarbamate was an activator of LA-OH activity. None of the above enzyme activities was selectively inhibited by any of the chemicals when used at a concentration of 100 microM. There was no difference in the inhibition of LA-OH activities by representative P450 inhibitors between liver microsomes from untreated trout and BNF-treated trout. The results of this study suggest that inhibition data from mammalian studies could not be directly extrapolated to trout and likely other fish species and that care must be observed when mammalian P450 inhibitors are used to determine the participation of P450s in the metabolism and toxicity of xenobiotics in nonmammalian species.

The acetaminophen regioisomer 3'-hydroxyacetanilide inhibits and covalently binds to cytochrome P450 2E1

3'-Hydroxyacetanilide has been previously studied as a nontoxic regioisomer of the analgesic acetaminophen (4'-hydroxyacetanilide). The radiolabeled derivative has been shown to covalently bind to liver proteins at levels similar to that observed with hepatotoxic doses of radiolabeled acetaminophen with no evidence of hepatic damage. Using an anti-arylacetamide antiserum the primary protein adduct detected following administration of 3'-hydroxyacetanilide (300 and 600 mg/kg) to mice was a 50 kDa microsomal protein that co-migrated with cytochrome P450 2E1. Cytochrome P450 2E1 enzyme activity (p-nitrophenol hydroxylase) was decreased by 79% in the mice treated with 3'-hydroxyacetanilide (600 mg/kg). Incubation of 3'-hydroxyacetanilide with hepatic microsomes resulted in a time dependent 47% decrease in cytochrome P450 2E1 activity. Pre-incubation of acetaminophen with microsomes did not result in covalent binding to the cytochrome P450 nor was there a decrease in p-nitrophenol hydroxylase activity. These data suggest that 3'-hydroxyacetanilide covalently binds to cytochrome P450 2E1 with preferential loss of activity.

Progesterone and testosterone hydroxylation by cytochromes P450 2C19, 2C9, and 3A4 in human liver microsomes

Roles of human cytochrome P450 (P450 or CYP) 2C9, 2C19, and 3A4 in the oxidation of progesterone and testosterone were studied in recombinant P450 enzymes and in human liver microsomes. In vitro inhibition experiments showed that progesterone and its 17alpha- and 21-hydroxylated metabolites and 11-deoxycortisol suppressed the CYP2C19-dependent R-warfarin 7-hydroxylation activities, with progesterone being the most active. These steroid chemicals also inhibited CYP2C9-dependent S-warfarin 7-hydroxylation activities though lesser extents seen with those in CYP2C19 enzyme. Progesterone was found to be a competitive inhibitor of CYP2C19 and CYP2C9 in human liver microsomes. Recombinant CYP2C19 catalyzed progesterone to form 21-hydroxyprogesterone as a major product and 16alpha- and 17alpha-hydroxyprogesterone as minor products. CYP2C9 also had progesterone 21-hydroxylation activities, although the activities were lower than those catalyzed by CYP2C19. Vmax/Km ratios for the progesterone 21-hydroxylation activity of CYP2C19 were determined to be 13- and 32-fold higher than those of CYP2C9 and 3A4, respectively. CYP3A4 oxidized progesterone to form 16alpha-, 6beta-, and 2beta-hydroxyprogesterone as major products and 21-hydroxyprogesterone as a minor product, but did not produce detectable levels of 17alpha-hydroxyprogesterone. Immunoinhibition experiments suggested that anti-CYP2C9 (which inhibits both CYP2C9 and CYP2C19 catalytic activities) suppressed the progesterone 21-hydroxylation activities catalyzed by liver microsomes of humans and monkeys and that anti-CYP2C11 inhibited the progesterone 21-hydroxylation activities catalyzed by liver microsomes of male rats. CYP2C19 was also found to oxidize testosterone at 17-position to form androstenedione. Androstenedione formation catalyzed by liver microsomes of humans and monkeys and of male rats was suppressed by anti-CYP2C9 and anti-CYP2C11, respectively. These results suggest that CYP2C19 plays important roles in the oxidation of progesterone and testosterone in human liver microsomes, although the physiological significance of these metabolic pathways remains unclear. CYP2C9 may have some, but lesser extent than those by CYP2C19, of the catalytic roles for the metabolism of progesterone and testosterone by human liver microsomes.

Mechanism-based inactivation of bovine cytochrome P-450(11beta) by 18-unsaturated progesterone derivatives

Two 18-unsaturated progesterone derivatives, 18-vinylprogesterone (18-VP) and 18-ethynylprogesterone (18-EP) have proved to be potent inhibitors of the bovine cytochrome P-450(11beta), the enzyme involved in the last steps of aldosterone biosynthesis [Delorme, C., Piffeteau, A., Viger, A. & Marquet, A. (1995) Eur. J. Biochem. 232, 247-256]. In the present study, we demonstrate that these two compounds exhibit the characteristics of mechanism-based inactivators of this enzyme. Inactivation followed pseudo-first-order and saturation kinetics. The kinetic parameters of inactivation were k(i) = 0.11 min(-1) and Ki = 4 microM for 18-VP, and k(i) = 0.12 min(-1) and 22 microM for 18-EP. Inactivation of P-450(11beta) activity was strictly dependent on the presence of NADPH. Protection by the substrate deoxycorticosterone was observed, demonstrating a selective modification at the substrate-binding site. With radiolabeled 18-VP, inactivation was shown to be irreversible with a stoichiometry of 1.4 mol bound [3H]18-VP/mol inactivated cytochrome P-450(11beta). SDS/PAGE analysis of the [3H]18-VP-inactivated enzyme showed that, under conditions preventing heme dissociation, the P-450(11beta) band was labeled, while no labeling of the apoprotein was observed under denaturating conditions. Furthermore, the loss of catalytic activity could be correlated with the destruction of the P-450 chromophore evaluated by the FeII-CO versus FeII difference spectra. These arguments led us to propose that 18-vinylprogesterone inactivates cytochrome P-450(11beta) by heme destruction rather than by protein modification.

Metabolism of the cytochrome P450 mechanism-based inhibitor N-benzyl-1-aminobenzotriazole to products that covalently bind with protein in guinea pig liver and lung microsomes: comparative study with 1-aminobenzotriazole

The metabolism and covalent binding of radioactivity to microsomal protein of the cytochrome P450 (P450) mechanism-based inhibitors 1-amino-[14C]-2,3-benzotriazole ([14C]ABT) and two radiolabeled forms of N-benzyl-1-aminobenzotriazole (BBT), N-benzyl-1-amino-[14C]-2,3-benzotriazole ([14C]-2,3-BBT) and [14C]-N-7-benzyl-1-aminobenzotriazole ([14C]-7-BBT), were examined in hepatic or pulmonary microsomes from untreated and phenobarbital (PB)- or beta-naphthoflavone (betaNF)-induced guinea pigs. [14C]-2,3-BBT and [14C]-7-BBT were converted to multiple metabolites including ABT, benzotriazole, benzaldehyde, 2- or 3-hydroxy-BBT, and 4-hydroxy-BBT by hepatic microsomes, while [14C]ABT, whose primary metabolite was benzotriazole, underwent little biotransformation. Neither ABT nor BBT was extensively metabolized by pulmonary microsomes. Hepatic microsomes from betaNF (vs PB)-treated guinea pigs metabolized [14C]ABT, [14C]-2,3-BBT, and [14C]-7-BBT more extensively. The degree of NADPH-dependent covalent binding of [14C]-2,3-BBT- or [14C]-7-BBT-derived radioactivity (1.0 nmol/mg of protein) was higher than that of [14C]ABT (0.3-0.8 nmol/mg of protein) in hepatic microsomes, especially those from PB-induced animals. Covalent binding per nmol of P450 in pulmonary microsomes was 3-4-fold higher with [14C]-2,3-BBT (2.9 nmol/nmol of P450) than with [14C]-7-BBT (1.0 nmol/nmol of P450), whereas in hepatic microsomes from PB- or betaNF-treated animals the ratio of binding with the two forms of BBT was approximately 1:1. [14C]-2,3-BBT- and [14C]-7-BBT-derived radioactivity was covalently bound to proteins that migrated in the molecular weight region corresponding to P450 on SDS-PAGE following incubation with NADPH. These data indicate that BBT is metabolized to at least two reactive compounds capable of covalent modification of protein and/or a single reactive product is formed which contains both the benzo ring (of benzotriazole) and the benzyl carbon atom (of the N-benzyl group); that P450 apoprotein modification may be an important mechanism of inactivation of pulmonary and hepatic P450 by BBT; and that hepatic microsomes from betaNF-induced guinea pigs generate more metabolites that do not act as mechanism-based P450 inhibitors from BBT than do those from PB-induced animals.

Mechanism-based inactivation of cytochrome P450 2B1 by N-benzyl-1-aminobenzotriazole

The kinetics of inactivation of cytochrome P450 2B1, the major phenobarbital inducible rat hepatic P450, by N-benzyl-1-aminobenzotriazole (BBT) were characterized. Purified, reconstituted P450 2B1 7-ethoxy-4-(trifluoromethyl)coumarin (7-EFC) O-deethylase activity was inhibited by BBT in a mechanism-based manner. The loss of O-deethylase activity followed pseudo-first-order kinetics and was NADPH and BBT dependent. After a 5 min incubation, greater than 90% of the 2B1 activity was lost, whereas more than 70% of the ability of the reduced enzyme to bind CO was maintained. Inclusion of 10 mM glutathione in the inactivation reaction lowered the rate of inactivation (k(inactivation)) and increased the partition ratio without significantly affecting the inactivator concentration required for half-maximal inactivation (K(I)). The maximal rate constant for inactivation at 23 degrees C was 0.24 min(-1) without and 0.15 min(-1) with glutathione. The apparent K(I) was 2 microM in both cases. The extrapolated partition ratios were 4 and 9 without and with 10 mM glutathione, respectively. Consistent with mechanism-based inactivation, the loss of 7-EFC O-deethylase activity was irreversible, was not due to product inhibition, was saturable, and could be slowed by including increasing concentrations of competing substrate. However, the inactivated P450 2B1 was still able to metabolize substrate if iodosobenzene was used as an alternate oxidant. Inactivation of 2B1 with either N-[14C]-7-benzyl-1-aminobenzotriazole (BBT) or N-benzyl-1-amino-[14C]-2,3-benzotriazole resulted in the incorporation of covalent radiolabel into the apoprotein. The stoichiometry of labeled metabolite adduct to protein was approximately 0.4:1 in both cases. Identification of metabolites revealed the formation of 1-aminobenzotriazole, benzotriazole, benzaldehyde, and a new metabolite (27) during catalysis of BBT by P450 2B1. Together, these data suggest that P450 2B1 could be inactivated and labeled by more than one metabolite.

A single mutation in cytochrome P450 BM3 changes substrate orientation in a catalytic intermediate and the regiospecificity of hydroxylation

Phenylalanine 87 of Bacillus megaterium cytochrome P450 BM3, a residue close to the heme in the substrate binding pocket, has been replaced by alanine by site-directed mutagenesis. The substitution had no effect on the rate of hydroxylation of laurate and increased the affinity for laurate of both the intact enzyme and its heme domain by 2.6-6-fold in the ferric state. NMR paramagnetic relaxation measurements showed that in the initial ferric enzyme-substrate complex, where the substrate binds relatively far from the heme, the substitution had no effect on the position or orientation of the bound substrate. However, in the next intermediate in the catalytic cycle, the reduced enzyme, the position of the bound substrate was altered so that the terminal methyl group was 3.1 A from the iron in the mutant, compared to 5.1 A in the wild-type enzyme. Analysis of the products of the action of the enzyme on laurate and myristate showed that the mutant catalyzed hydroxylation almost exclusively at the omega position, in marked contrast to the wild-type enzyme, with which no hydroxylation at this position was observed.

Suicide inactivation of cytochrome P450 by midchain and terminal acetylenes. A mechanistic study of inactivation of a plant lauric acid omega-hydroxylase

Incubation of Vicia sativa microsomes, containing cytochrome P450-dependent lauric acid omega-hydroxylase (omega-LAH), with [1-(14)C]11-dodecynoic acid (11-DDYA) generates a major metabolite characterized as 1,12-dodecandioic acid. In addition to time- and concentration-dependent inactivation of lauric acid and 11-DDYA oxidation, irreversible binding of 11-DDYA (200 pmol of 11-DDYA bound/mg of microsomal protein) at a saturating concentration of 11-DDYA was observed. SDS-polyacrylamide gel electrophoresis analysis showed that 30% of the label was associated with several protein bands of about 53 kDa. The presence of beta-mercaptoethanol in the incubate reduces 1,12-dodecandioic acid formation and leads to a polar metabolite resulting from the interaction of oxidized 11-DDYA with the nucleophile. Although the alkylation of proteins was reduced, the lauric acid omega-hydroxylase activity was not restored, suggesting an active site-directed inactivation mechanism. Similar results were obtained when reconstituted mixtures of cytochrome P450 from family CYP4A from rabbit liver were incubated with 11-DDYA. In contrast, both 11- and 10-DDYA resulted in covalent labeling of the cytochrome P450 2B4 protein and irreversible inhibition of activity. These results demonstrate that acetylenic analogues of substrate are efficient mechanism-based inhibitors and that a correlation between the position of the acetylenic bond in the inhibitor and the regiochemistry of cytochromes P450 oxygenation is essential for enzyme inactivation.

Aryl acetylenes as mechanism-based inhibitors of cytochrome P450-dependent monooxygenase enzymes

Aryl acetylenes have been investigated as inhibitors of cytochrome P450 (P450)-dependent alkoxyresorufin dealkylation dealkylation activities in liver microsomes prepared from rats exposed to beta-naphthoflavone, isosafrole, or phenobarbital. Many of the acetylenes investigated produce pseudo-first-order time-dependent and NADPH-dependent losses of the dealkylation activities characteristic of mechanism-based irreversible inactivation (suicide inhibition). Replacing the terminal hydrogen of aryl acetylenes with a methyl group to convert ethynes into propynes enhances the inhibition of P450 1A enzymes; in some instances, this modification converts a reversible inhibitor of P450s into a suicide inhibitor. In contrast, ethynes are more effective suicide inhibitors of P450 2B-dependent dealkylations than the corresponding propynes. Aryl acetylenes with an ethynyl group on the 2 position of naphthalene or on the 9 position of phenanthrene and arylalkyl acetylenes with alkyl chains containing 2, 3, or 4 methylene groups are selective inhibitors of P450 2B1/2B2 in liver microsomes from rats. Aryl acetylenes also act as suicide inhibitors of P450 1A2 in human liver microsomes, of purified P450 1A2 from rabbit or rat liver in reconstituted systems, and of purified recombinant human P450 1A2 and 1A1 in reconstituted systems. 4-(1-Propynyl)biphenyl (4PBi) inactivated P450 1A2-dependent ethoxyresourfin deethylation (EROD) activity in human liver microsomes in an NADPH-dependent process (k(inactivation), 0.23 min-1; KI, 2.3 microM). 4PBi also inactivated purified recombinant human P450 1A2 (k(inactivation), 0.24 min-1; KI, 4.3 microM). In agreement with previous reports [Yun, C.-H., Hammons, G. J., Jones, G., Martin, M. V., Hopkins, N. E., Alworth, W. L., and Guengerich, F. P. (1992) Biochemistry 31, 10556-10563], 2-ethynylnaphthalene (2EN) was not a suicide inhibitor of the P450 1A2 activity in human liver microsomes but did inactivate purified human P450 1A2. Neither 4PBi nor 2EN affected diagnostic activities of human microsomal P450 2E1, 2C9/10, 3A4, or 2C19. In the systems examined, the losses of P450-dependent activity produced by these aryl acetylenes were not accompanied by corresponding decreases in the measured P450 absorption spectra. Thus P450 inactivation by these aryl acetylenes does not involve labeling and destruction of the heme. Incubation of 4PBi with microsomal P450 1A1 or 1A2 from rat liver under conditions that lead to P450-dependent, enzyme inactivations generates a 2-biphenylylpropionic acid product. This suggests that the suicide inhibition of P450s by propynylaryl acetylenes proceeds via a methylaryl ketene formed by a 1,2-methyl rearrangement, analogous to the mechanism of suicide inhibition by ethynyl acetylenes that proceed via ketene intermediates formed by 1,2-hydrogen shifts [Ortiz de Montellano, P. R., and Kunze, K. L. (1981) Arch. Biochem. Biophys. 209, 710-712].

Mechanism-based inhibition of mouse P4502b-10 by selected arylalkynes

Suicide inhibitors of cytochrome P450 families are excellent tools to predict which isoforms mediate the metabolism/activation of a variety of chemical agents. We compared the inhibitory effects of several arylalkynes on mouse cytochromes P450 with published data for the rat model. The inhibition of P4502b specific dealkylation of benzyloxyresorufin by 2-ethynylnaphthalene (2-EN), 5-phenyl-1-pentyne (PPY), 4-phenyl-1-butyne (PBY), and 9-ethynylphenanthrene (9-EPh) was measured in hepatic microsomes from male mice treated with 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP) to induce cytochrome P4502b. Pulmonary microsomes were prepared from untreated mice. 9-EPh, 2-EN, and PPY caused a time-, concentration-, and NADPH-dependent loss in P4502b activity in both tissues. PBY, however, demonstrated this type of inhibition only in liver microsomes. The IC50 was calculated for both liver and lung microsomes and compared with published Ki (concentration required for half-maximal inhibition) or KI (concentration required for half-maximal inactivation) values for the rat. PPY, PBY, and 9-EPh were equally effective inhibitors of mouse P4502b and rat P4502B1. 2-EN was a 5- to 10-fold less potent inhibitor of mouse P4502b, as compared with the rat, even though it was shown to bind to the active site of the mouse isoform as demonstrated by its metabolism to 2-naphthylacetic acid. These data suggest that the active site of the mouse P4502b enzyme is functionally similar to the rat P4502B isoform, with the exception of the disparity in its susceptibility to inactivation by 2-EN as measured by the Ki values.

Fatty acid discrimination and omega-hydroxylation by cytochrome P450 4A1 and a cytochrome P4504A1/NADPH-P450 reductase fusion protein

The omega-hydroxylation of fatty acids by certain cytochrome P450 enzymes shows a degree of chain-length and regionspecificity which is remarkable in view of the conformational flexibility of these substrates, the strong similarity in properties among homologs, and the lack of polar groups (other than the carboxy terminus) with which to guide and strength enzyme-substrate interactions. To investigate the chemical basis for these features of omega-hydroxylation we designed and synthesized a series of lauric acid analogs and evaluated them as substrates and inhibitors of omega-hydroxylation catalyzed by cytochrome P4504A1 and a cytochrome P450 4A1/NADPH-P450 reductase fusion protein. Among n-alkanoic acids, lauric acid was found to have the optimum chain length for the fusion protein, as it does for native cytochrome P450 4A1. With both enzymes, chain shortening caused a precipitous drop in turnover while chain lengthening caused a gradual drop in turnover. The fusion protein omega-hydroxylated methyl laurate and lauryl alcohol about 1/10th as efficiently as lauric acid, but it did not hydroxylate lauramide. 10-Methoxydecanoic acid underwent O-demethylation (via omega-hydroxylation). The branched substrate 11-methyllauric acid was hydroxylated efficiently and selectively at the omega-position. In contrast, the cyclopropyl analog 11,12-methanolauric acid was not detectably hydroxylated, although it induced Type I binding spectrum and inhibited lauric acid omega-hydroxylation by 43% at equimolar concentrations. omega-(Imidazolyl)-decanoic acid induced a Type II heme-binding spectrum and was an especially potent inhibitor of lauric acid hydroxylation. Collectively these data suggest that the active site of cytochrome P450 4A1 has an elongated tubular shape of definite length (ca. 14 A) with a recognition site for polar groups (including but not limited to carboxyl) at its entrance and the (oxo)heme group at its terminus.

Stereoselective hydroxylation of norcamphor by cytochrome P450cam. Experimental verification of molecular dynamics simulations

The stereoselectivity of cytochrome P450cam hydroxylation has been investigated with the enantiomerically pure substrate analog norcamphor. (1R)- and (1S)-norcamphor (> 92 enantiomeric excess) were characterized in the hydroxylation reaction with cytochrome P450cam with respect to the product profile, steady state kinetics, coupling efficiency, and free energy of substrate dissociation. The experimental results demonstrate regiospecificity that is enantiomer-specific and confirm our previously reported prediction that (1R)-norcamphor is hydroxylated preferentially at the 5-carbon and (1S)-norcamphor at the 6-carbon (Bass, M. B., and Ornstein, R. L. (1993) J. Comput. Chem. 14, 541-548); these simulation results are now compared with simulations involving a ferryl oxygen intermediate. Hydroxylation of (1R)-norcamphor was found at the 5-, 6-, and 3-carbons in a ratio of 65:30:5 (respectively), whereas (1S)-norcamphor was oxidized to produce a 28:62:10 ratio of the same products. With the exception of the regiospecificity, all of the reaction and physical parameters are similar for each enantiomer of norcamphor. These results show that the position of the carbonyl group on the hydrocarbon skeleton of norcamphor plays a role in determining the average orientation of this substrate in the active site and suggests that hydrogen bonding interactions can aid in directing the regiospecificity and stereospecificity of the hydroxylation reaction catalyzed by cytochrome P450cam.