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

Cytochrome P450 168A1 from Pseudomonas aeruginosa is involved in the hydroxylation of biologically relevant fatty acids

The cytochrome P450 CYP168A1 from Pseudomonas aeruginosa was cloned and expressed in Escherichia coli followed by purification and characterization of function. CYP168A1 is a fatty acid hydroxylase that hydroxylates saturated fatty acids, including myristic (0.30 min-1), palmitic (1.61 min-1) and stearic acids (1.24 min-1), at both the ω-1- and ω-2-positions. However, CYP168A1 only hydroxylates unsaturated fatty acids, including palmitoleic (0.38 min-1), oleic (1.28 min-1) and linoleic acids (0.35 min-1), at the ω-1-position. CYP168A1 exhibited a catalytic preference for palmitic, oleic and stearic acids as substrates in keeping with the phosphatidylcholine-rich environment deep in the lung that is colonized by P. aeruginosa.

Riboflavin and pyrroloquinoline quinone generate carbon monoxide in the presence of tissue microsomes or recombinant human cytochrome P-450 oxidoreductase: implications for possible roles in gasotransmission

Carbon monoxide (CO), an endogenously produced gasotransmitter, regulates inflammation and vascular tone, suggesting that delivery of CO may be therapeutically useful for pathologies like preeclampsia where CO insufficiency is implicated. Our strategy is to identify chemicals that increase the activity of endogenous CO-producing enzymes, including cytochrome P-450 oxidoreductase (CPR). Realizing that both riboflavin and pyrroloquinoline quinone (PQQ) are relatively nontoxic, even at high doses, and that they share chemical properties with toxic CO activators that we previously identified, our goal was to determine whether riboflavin or PQQ could stimulate CO production. Riboflavin and PQQ were incubated in sealed vessels with rat and human tissue extracts and CO generation was measured with headspace-gas chromatography. Riboflavin and PQQ increased CO production ∼60% in rat spleen microsomes. In rat brain microsomes, riboflavin and PQQ increased respective CO production approximately fourfold and twofold compared to baseline. CO production by human placenta microsomes increased fourfold with riboflavin and fivefold with PQQ. In the presence of recombinant human CPR, CO production was threefold greater with PQQ than with riboflavin. These observations demonstrate for the first time that riboflavin and PQQ facilitate tissue-specific CO production with significant contributions from CPR. We propose a novel biochemical role for these nutrients in gastransmission.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

Drug-enhanced carbon monoxide production from heme by cytochrome P450 reductase

Carbon monoxide (CO) formed endogenously is considered to be cytoprotective, and the vast majority of CO formation is attributed to the degradation of heme by heme oxygenases-1 and -2 (HO-1, HO-2). Previously, we observed that brain microsomes containing HO-2 produced many-fold more CO in the presence of menadione and its congeners; herein we explored these observations further. We determined the effects of various drugs on CO production of rat brain microsomes and recombinant human cytochrome P450 reductase (CPR); CO was measured by gas chromatography with reductive detection. Brain microsomes of Sprague-Dawley rats or recombinant human cytochrome P450 reductase (CPR) were incubated with NADPH and various drugs in closed vials in phosphate buffer at pH 7.4 and 37°C. After 15 minutes, the reaction was stopped by cooling in dry ice, and the headspace gas was analyzed for CO production using gas chromatography with reductive (mercuric oxide) detection. We observed drug-enhanced CO production in the presence of both microsomes and recombinant CPR alone; the presence of HO was not required. A range of structurally diverse drugs were capable of amplifying this CO formation; these molecules had structures consistent with redox cycling capability. The addition of catalase to a reaction mixture, that contained activating drugs, inhibited the production of CO. Drug-enhanced CO formation can be catalyzed by CPR. The mechanism of CPR activation was not through classical drug-receptor mediation. Redox cycling may be involved in the drug-induced amplification of CO production by CPR through the production of reactive oxygen species.

Catalytic Activities of Tumor-Specific Human Cytochrome P450 CYP2W1 Toward Endogenous Substrates

CYP2W1 is a recently discovered human cytochrome P450 enzyme with a distinctive tumor-specific expression pattern. We show here that CYP2W1 exhibits tight binding affinities for retinoids, which have low nanomolar binding constants, and much poorer binding constants in the micromolar range for four other ligands. CYP2W1 converts all-transretinoic acid (atRA) to 4-hydroxyatRA and all-transretinol to 4-OH all-transretinol, and it also oxidizes retinal. The enzyme much less efficiently oxidizes 17β-estradiol to 2-hydroxy-(17β)-estradiol and farnesol to a monohydroxylated product; arachidonic acid is, at best, a negligible substrate. These findings indicate that CYP2W1 probably plays an important role in localized retinoid metabolism that may be intimately linked to its involvement in tumor development.

Altered inflammatory responses to Citrobacter rodentium infection, but not bacterial lipopolysaccharide, in mice lacking the Cyp4a10 or Cyp4a14 genes

Murine hepatic Cyp4a mRNAs are markedly downregulated during inflammation. Here, we investigated the roles of Cyp4a10 and Cyp4a14 in the response to infection with C. rodentium. Absence of either Cyp4a gene attenuated or abrogated the changes in spleen weight, colon crypt length, hepatic cytokine, and acute phase protein mRNAs, and serum acute phase proteins and cytokines caused by infection. Cyp4a10(-/-) mice on a low-salt diet had a similar hepatic acute phase response as those mice on a high-salt diet, suggesting that hypertension associated with this genotype is not the cause of their altered inflammatory response. In contrast, wild-type, Cyp4a10(-/-), and Cyp4a14(-/-) mice showed similar responses to injected LPS. These results implicate Cyp4a10 and Cyp4a14 in the regulation of the host inflammatory response to enteropathogenic bacterial infection but not to acute aseptic inflammation. Understanding the mechanism of this role may lead to novel therapeutic approaches in some inflammatory diseases.

NADPH P450 oxidoreductase: structure, function, and pathology of diseases

Cytochrome P450 oxidoreductase (POR) is an enzyme that is essential for multiple metabolic processes, chiefly among them are reactions catalyzed by cytochrome P450 proteins for metabolism of steroid hormones, drugs and xenobiotics. Mutations in POR cause a complex set of disorders that often resemble defects in steroid metabolizing enzymes 17α-hydroxylase, 21-hydroxylase and aromatase. Since our initial reports of POR mutations in 2004, more than 200 different mutations and polymorphisms in POR gene have been identified. Several missense variations in POR have been tested for their effect on activities of multiple steroid and drug metabolizing P450 proteins. Mutations in POR may have variable effects on different P450 partner proteins depending on the location of the mutation. The POR mutations that disrupt the binding of co-factors have negative impact on all partner proteins, while mutations causing subtle structural changes may lead to altered interaction with specific partner proteins and the overall effect may be different for each partner. This review summarizes the recent discoveries related to mutations and polymorphisms in POR and discusses these mutations in the context of historical developments in the discovery and characterization of POR as an electron transfer protein. The review is focused on the structural, enzymatic and clinical implications of the mutations linked to newly identified disorders in humans, now categorized as POR deficiency.

Role of cysteine residues in heme binding to human heme oxygenase-2 elucidated by two-dimensional NMR spectroscopy

Human heme oxygenases 1 and 2 (HO-1 and HO-2) degrade heme in the presence of oxygen and NADPH-cytochrome P450 reductase, producing ferrous iron, CO, and biliverdin. HO-1 is an inducible enzyme, but HO-2 is constitutively expressed in selected tissues and is involved in signaling and regulatory processes. HO-2 has three cysteine residues that have been proposed to modulate the affinity for heme, whereas HO-1 has none. Here we use site-specific mutagenesis and two-dimensional NMR of l-[3-(13)C]cysteine-labeled proteins to determine the redox state of the individual cysteines in HO-2 and assess their roles in binding of heme. The results indicate that in the apoprotein, Cys(282) and Cys(265) are in the oxidized state, probably in an intramolecular disulfide bond. The addition of a reducing agent converts them to the reduced, free thiol state. Two-dimensional NMR of site-specific mutants reveals that the redox state of Cys(265) and Cys(282) varies with the presence or absence of other Cys residues, indicating that the microenvironments of the Cys residues are mutually interdependent. Cys(265) appears to be in a relatively hydrophilic, oxidizable environment compared with Cys(127) and Cys(282). Chemical shift data indicate that none of the cysteines stably coordinates to the heme iron atom. In the oxidized state of the apoprotein, heme is bound 2.5-fold more tightly than in the reduced state. This small difference in heme affinity between the oxidized and reduced states of the protein is much less than previously reported, suggesting that it is not a significant factor in the physiological regulation of cellular heme levels.

The molecular action of the novel insecticide, Pyridalyl

Pyridalyl is a recently discovered insecticide that exhibits high insecticidal activity against Lepidoptera and Thysanoptera. Pyridalyl action requires cytochrome P450 activity, possibly for production of a bioactive derivative, Pyridalyl metabolism being prevented by general P450 inhibitors. Apoptosis is apparently not involved in the cytotoxicity. Continuous culture of Spodoptera frugiperda Sf21 cells in sub-lethal doses of Pyridalyl, results in a Pyridalyl-resistant cell line. Probing the molecular action of Pyridalyl by comparison of the proteomes of Pyridalyl-resistant and -susceptible cell lines, revealed differential expression of a number of proteins, including the up-regulation of thiol peroxiredoxin (TPx), in the resistant cells. Treatment of Bombyx mori larvae with Pyridalyl, followed by comparison of the midgut microsomal sub-proteome, revealed the up-regulation of three proteasome subunits. Such subunits, together with Hsp70 stress proteins, glyceraldehyde 3-phosphate dehydrogenases (GAPDHs) and thiol peroxiredoxin (TPx) were also up-regulated in the whole proteome of B. mori BM36 cells following treatment with the insecticide. The foregoing results lead to the hypothesis that cytochrome P450 action leads to an active Pyridalyl metabolite, which results in production of reactive oxygen species (ROS), that leads to damage to cellular macromolecules (e.g., proteins) and enhanced proteasome activity leads to increased protein degradation and necrotic cell death.

Substrate-specific modulation of CYP3A4 activity by genetic variants of cytochrome P450 oxidoreductase

OBJECTIVES

CYP3A4 receives electrons from P450 oxidoreductase (POR) to metabolize about 50% of clinically used drugs. There is substantial inter-individual variation in CYP3A4 catalytic activity that is not explained by CYP3A4 genetic variants. CYP3A4 is flexible and distensible, permitting it to accommodate substrates varying in shape and size. To elucidate the mechanisms of variability in CYP3A4 catalysis, we examined the effects of genetic variants of POR, and explored the possibility that substrate-induced conformational changes in CYP3A4 differentially affect the ability of POR variants to support catalysis.

METHODS

We expressed human CYP3A4 and four POR variants (Q153R, A287P, R457H, A503 V) in bacteria, reconstituted them in vitro and measured the Michaelis constant and maximum velocity with testosterone, midazolam, quinidine and erythromycin as substrates.

RESULTS

POR A287P and R457H had low activity with all substrates; Q153R had 76-94% of wild-type (WT) activity with midazolam and erythromycin, but 129-150% activity with testosterone and quinidine. The A503 V polymorphism reduced the CYP3A4 activity to 61-77% of WT with testosterone and midazolam, but had nearly WT activity with quinidine and erythromycin.

CONCLUSION

POR variants affect CYP3A4 activities. The impact of a POR variant on catalysis by CYP3A4 is substrate-specific, probably because of substrate-induced conformational changes in CYP3A4.

Effects of genetic variants of human P450 oxidoreductase on catalysis by CYP2D6 in vitro

OBJECTIVES

Cytochrome P450 (P450) oxidoreductase (POR) donates electrons to all microsomal cytochrome P450s, including drug-metabolizing and steroidogenic enzymes. Severe POR mutations cause skeletal malformations and disordered steroidogenesis. The POR polymorphism A503V is found on approximately 28% of human alleles and decreases activities of CYP3A4 and steroidogenic CYP17, but not the activities of steroidogenic CYP21 or drug-metabolizing CYP1A2 and CYP2C19. CYP2D6 metabolizes about 25% of clinically used drugs; we assessed the capacity of POR variants to support the activities of human CYP2D6.

METHODS

N-27 forms of wildtype (WT), Q153R, A287P, R457H and A503V POR, and WT CYP2D6 were expressed in Escherichia coli. POR proteins in bacterial membranes were reconstituted with purified CYP2D6. Support of CYP2D6 was measured by metabolism of EOMCC (2H-1-benzopyran-3-carbonitrile,7-(ethoxy-methoxy)-2-oxo-(9Cl)), dextromethorphan and bufuralol. Michaelis constant (K(m)) and maximum velocity (V(max)) were determined in three triplicate experiments for each reaction; catalytic efficiency is expressed as V(max)/K(m).

RESULTS

Compared with WT POR, disease-causing POR mutants A287P and R457H supported no detectable CYP2D6 activity with EOMCC, but A287P supported approximately 25% activity with dextromethorphan and bufuralol. Q153R had increased function with CYP2D6 (128% with EOMCC, 198% with dextromethorphan, 153% with bufuralol). A503V supported decreased CYP2D6 activity: 85% with EOMCC, 62% with dextromethorphan and 53% with bufuralol.

CONCLUSION

POR variants have different effects depending on the substrate metabolized. Disease-causing POR mutations R457H and A287P had poor activities, suggesting that diminished drug metabolism should be considered in affected patients. The common A503V polymorphism impaired CYP2D6 activities with two commonly used drugs by 40-50%, potentially explaining some genetic variation in drug metabolism.

Cytochrome P450 7A1 cholesterol 7alpha-hydroxylation: individual reaction steps in the catalytic cycle and rate-limiting ferric iron reduction

Cytochrome P450 (P450) 7A1 is well known as the cholesterol 7α-hydroxylase, the first enzyme involved in bile acid synthesis from cholesterol. The human enzyme has been reported to have the highest catalytic activity of any mammalian P450. Analyses of individual steps of cholesterol 7α-hydroxylation reaction revealed several characteristics of this reaction: (i) two-step binding of cholesterol to ferric P450, with an apparent K(d) of 0.51 μM, (ii) a rapid reduction rate in the presence of cholesterol (∼10 s(-1) for the fast phase), (iii) rapid formation of a ferrous P450-cholesterol-O(2) complex (29 s(-1)), (iv) the lack of a non-competitive kinetic deuterium isotope effect, (v) the lack of a kinetic burst, and (vi) the lack of a deuterium isotope effect when the reaction was initiated with the ferrous P450-cholesterol complex. A minimum kinetic model was developed and is consistent with all of the observed phenomena and the rates of cholesterol 7α-hydroxylation and H(2)O and H(2)O(2) formation. The results indicate that the first electron transfer step, although rapid, becomes rate-limiting in the overall P450 7A1 reaction. This is a different phenomenon compared with other P450s that have much lower rates of catalysis, attributed to the much more efficient substrate oxidation steps in this reaction.

Isocyanides inhibit human heme oxygenases at the verdoheme stage

Heme oxygenases (HO) catalyze the oxidative cleavage of heme to generate biliverdin, CO, and free iron. In humans, heme oxygenase-1 (hHO-1) is overexpressed in tumor tissues, where it helps to protect cancer cells from anticancer agents, while HOs in fungal pathogens, such as Candida albicans, function as the primary means of iron acquisition. Thus, HO can be considered a potential therapeutic target for certain diseases. In this study, we have examined the equilibrium binding of three isocyanides, isopropyl, n-butyl, and benzyl, to the two major human HO isoforms (hHO-1 and hHO-2), Candida albicans HO (CaHmx1), and human cytochrome P450 CYP3A4 using electronic absorption spectroscopy. Isocyanides coordinate to both ferric and ferrous HO-bound heme, with tighter binding by the more hydrophobic isocyanides and 200-300-fold tighter binding to the ferrous form. Benzyl isocyanide was the strongest ligand to ferrous heme in all the enzymes. Because the dissociation constants (KD) of the ligands for ferrous heme-hHO-1 were below the limit of accuracy for equilibrium titrations, stopped-flow kinetic experiments were used to measure the binding parameters of the isocyanides to ferrous hHO-1. Steady-state activity assays showed that benzyl isocyanide was the most potent uncompetitive inhibitor with respect to heme with a KI = 0.15 microM for hHO-1. Importantly, single turnover assays revealed that the reaction was completely stopped by coordination of the isocyanide to the verdoheme intermediate rather than to the ferric heme complex. Much tighter binding of the inhibitor to the verdoheme intermediate differentiates it from inhibition of, for example, CYP3A4 and offers a possible route to more selective inhibitor design.

Isoporphyrin intermediate in heme oxygenase catalysis. Oxidation of alpha-meso-phenylheme

Human heme oxygenase-1 (hHO-1) catalyzes the O2- and NADPH-dependent oxidation of heme to biliverdin, CO, and free iron. The first step involves regiospecific insertion of an oxygen atom at the alpha-meso carbon by a ferric hydroperoxide and is predicted to proceed via an isoporphyrin pi-cation intermediate. Here we report spectroscopic detection of a transient intermediate during oxidation by hHO-1 of alpha-meso-phenylheme-IX, alpha-meso-(p-methylphenyl)-mesoheme-III, and alpha-meso-(p-trifluoromethylphenyl)-mesoheme-III. In agreement with previous experiments (Wang, J., Niemevz, F., Lad, L., Huang, L., Alvarez, D. E., Buldain, G., Poulos, T. L., and Ortiz de Montellano, P. R. (2004) J. Biol. Chem. 279, 42593-42604), only the alpha-biliverdin isomer is produced with concomitant formation of the corresponding benzoic acid. The transient intermediate observed in the NADPH-P450 reductase-catalyzed reaction accumulated when the reaction was supported by H2O2 and exhibited the absorption maxima at 435 and 930 nm characteristic of an isoporphyrin. Product analysis by reversed phase high performance liquid chromatography and liquid chromatography electrospray ionization mass spectrometry of the product generated with H2O2 identified it as an isoporphyrin that, on quenching, decayed to benzoylbiliverdin. In the presence of H218O2, one labeled oxygen atom was incorporated into these products. The hHO-1-isoporphyrin complexes were found to have half-lives of 1.7 and 2.4 h for the p-trifluoromethyl- and p-methyl-substituted phenylhemes, respectively. The addition of NADPH-P450 reductase to the H2O2-generated hHO-1-isoporphyrin complex produced alpha-biliverdin, confirming its role as a reaction intermediate. Identification of an isoporphyrin intermediate in the catalytic sequence of hHO-1, the first such intermediate observed in hemoprotein catalysis, completes our understanding of the critical first step of heme oxidation.

Genetics of P450 oxidoreductase: sequence variation in 842 individuals of four ethnicities and activities of 15 missense mutations

P450 oxidoreductase (POR) is an electron-donating flavoprotein required for the activity of all microsomal cytochrome P450 enzymes. We sequenced 5,655 bp of the POR gene in a representative population of 842 healthy unrelated individuals in four ethnic groups: 218 African Americans, 260 Caucasian Americans, 179 Chinese Americans, and 185 Mexican Americans. One hundred forty SNPs were detected, of which 43 were found in >/=1% of alleles. Twelve SNPs were in the POR promoter region. Fifteen of 32 exonic variations altered the POR amino acid sequence; 13 of these 15 are previously undescribed missense variations. We found eight indels, only one of which was in the coding region. A previously described variant, A503V, was found on 27.9% of all alleles with some ethnic predilection (19.1% in African Americans, 26.4% in Caucasian Americans, 36.7% Chinese Americans, and 31.0% in Mexican Americans). We built cDNA expression vectors for the 13 previously undescribed missense variants, expressed each protein lacking 27 N-terminal residues in Escherichia coli, and assayed the apparent K(m) and V(max) of each in four assays: reduction of cytochrome c, oxidation of NADPH, 17alpha-hydroxylase activity of P450c17, and 17,20 lyase activity of P450c17. The catalytic activities of several missense mutants differed substantially in these assays, indicating that each POR mutant must be assayed separately with each potential target P450 enzyme. The activity of A503V was reduced to a modest but statistically significant degree in all four assays, suggesting that it may play an important role in interindividual variation in drug response.

Clinical, structural and functional implications of mutations and polymorphisms in human NADPH P450 oxidoreductase

Cytochrome P450 proteins are involved in metabolism of drugs and xenobiotics. In the endoplasmic reticulum a single nicotinamide adenine dinucleotide phosphate (NADPH) P450 oxidoreductase (POR) supplies electrons to all microsomal P450s for catalytic activity. POR is a flavoprotein that contains both flavin mononucleotide and flavin adenine dinucleotide as cofactors and uses NADPH as the source of electrons. We have recently reported a number of POR mutations in the patients with disordered steroidogenesis. In the first report we had described missense mutations (A287P, R457H, V492E, C569Y, and V608F) identified in four patients with defects in steroid production. Each POR variant was produced as recombinant N-27 form of the enzyme in bacteria and as full-length form in yeast. Membranes from bacteria or yeast expressing normal or variant POR were purified and their activities were characterized in cytochrome c and CYP17A1 assays. Later we have published a larger study that described a whole range of POR mutations and characterized the mutants/polymorphisms A115V, T142A, M263V, Y459H, A503V, G539R, L565P, R616X, V631I, and F646del from the sequencing of patient DNA. We also studied POR variants Y181D, P228L, R316W, G413S, and G504R that were available in public databases or published literature. Three-dimensional structure of rat POR is known and we have used this structure to deduce the structure-function correlation of POR mutations in human. The missense mutations found in patients with disordered steroidogenesis are generally in the co-factor binding and functionally important domains of POR and the apparent polymorphisms are found in regions with lesser structural importance. A variation in POR can alter the activity of all microsomal P450s, and therefore, can affect the metabolism of drugs and xenobiotics even when the P450s involved are otherwise normal. It is important to study the genetic and biochemical basis of POR variants in human population to gain information about possible differences in P450 mediated reactions among the individuals carrying a variant or polymorphic form of POR that could impact their metabolism.

Biochemical analysis of mutations in P450 oxidoreductase

All microsomal P450s require POR (cytochrome P450 reductase) for catalytic activity. Most of the clinically used drugs are metabolized by a small number of P450s and polymorphisms in the cytochrome P450s are known to cause changes in drug metabolism. We have recently found a number of POR missense mutations in the patients with disordered steroidogenesis. Our initial report described five missense mutations (A284P, R454H, V489E, C566Y and V605F) identified in four patients. We built bacterial expression vectors for each POR variant, purified the membranes expressing normal or variant POR and characterized their activities with cytochrome c and P450c17 assays. We have recently completed an extensive study of the range of POR mutations and characterized the mutants/polymorphisms A112V, T139A, M260V, Y456H, A500V, G536R, L562P, R613X, V628I and F643del from sequencing of patient DNA. We also studied POR variants Y179D, P225L, R313W, G410S and G501R that were available in databases or the published literature. We analysed the mutations with a three-dimensional model of human POR that was based on an essentially similar rat POR with known crystal structure. The missense mutations found in patients with disordered steroidogenesis mapped to functionally important domains of POR and the apparent polymorphisms mapped to less crucial regions. Since a variation in POR can alter the activity of all microsomal P450s, it can also affect the drug metabolism even with a normal P450. Understanding the genetic and biochemical basis of POR-mediated drug metabolism will provide valuable information about possible differences in P450-mediated reactions among the individuals carrying a variant or polymorphic form of POR.

Repeated whiskey binges promote liver injury in rats fed a choline-deficient diet

BACKGROUND/AIMS

Alcoholic liver disease is associated with nutritional deficiency and it may aggravate within the context of fatty liver. We investigated the relationship between alcohol intake (whiskey binge drinking) and a choline-deficient diet (CD) and assessed whether stellate cells could contribute to liver injury in this model.

RESULTS

Rats fed the CD diet plus whiskey showed increased liver damage compared to rats fed the CD diet, as demonstrated by H&E staining, elevated transaminases, steatosis, TNF-alpha levels, enhanced CYP2E1 activity, impaired antioxidant defense, elevated lipid peroxidation, and protein carbonyls. The combined treatment triggered an apoptotic response as determined by elevated Bax, caspase-3 activity, cytochrome-c release, and decreased Bcl-2 and Bcl-XL. Stellate cells were activated as increased expression of alpha-Sma was observed over that by the CD diet alone. The combined treatment shifted extracellular matrix remodeling towards a pro-fibrogenic response due to up-regulation of collagen I, TIMP1, and Hsp47 proteins, along with down-regulation of MMP13, MMP2, and MMP9 expression, proteases which degrade collagen I. These events were accompanied by increased phosphorylation of p38, a kinase that elevates collagen I.

CONCLUSIONS

Repeated alcohol binges in the context of mild steatosis may promote activation of stellate cells and contribute to liver injury.

Motexafin gadolinium-induced cell death correlates with heme oxygenase-1 expression and inhibition of P450 reductase-dependent activities

Heme oxygenase-1 (HO1), which oxidizes heme to biliverdin, CO, and free iron, conveys protection against oxidative stress and is antiapoptotic. Under stress conditions, some porphyrin derivatives can inhibit HO1 and trigger cell death. Motexafin gadolinium (MGd) is an expanded porphyrin that selectively targets cancer cells through a process of futile redox cycling that decreases intracellular reducing metabolites and protein thiols. Here, we report that hematopoietic-derived cell lines that constitutively express HO1 are more susceptible to MGd-induced apoptosis than those that do not. MGd used in combination with tin protoporphyrin IX, an inhibitor of HO1, resulted in synergistic cell killing. Consistent with these cell culture observations, we found that MGd is an inhibitor of heme oxygenase-1 activity in vitro. We demonstrate that inhibition of HO1 reflects an interaction of MGd with NADPH-cytochrome P450 reductase, the electron donor for HO1, that results in diversion of reducing equivalents from heme oxidation to oxygen reduction. In accord with this mechanism, MGd is also an in vitro inhibitor of CYP2C9, CYP3A4, and CYP4A1. Inhibition of HO1 by MGd may contribute to its anticancer activity, whereas its in vitro inhibition of a broad spectrum of P450 enzymes indicates that a potential exists for drug-drug interactions.

P450 oxidoreductase deficiency: a new disorder of steroidogenesis

Microsomal P450 enzymes, which metabolize drugs and catalyze steroid biosynthesis require electron donation from NADPH via P450 oxidoreductase (POR). POR knockout mice are embryonically lethal, but we found recessive human POR missense mutations causing disordered steroidogenesis and Antley-Bixler syndrome (ABS), a skeletal malformation syndrome featuring craniosynostosis. Dominant mutations in exons 8 and 10 of fibroblast growth factor receptor 2 (FGFR2) cause phenotypically related craniosynostosis syndromes and were reported in patients with ABS and normal steroidogenesis. Sequencing POR and FGFR2 exons in 32 patients with ABS and/or hormonal findings suggesting POR deficiency showed complete genetic segregation of POR and FGFR2 mutations. Fifteen patients carried POR mutations on both alleles, four carried POR mutations on 1 allele, nine carried FGFR2/3 mutations on one allele and no mutation was found in three patients. The 34 affected POR alleles included 10 with A287P, 7 with R457H, 9 other missense mutations and 7 frameshifts. These 11 missense mutations and 10 others identified by database mining were expressed in E. coli, purified to apparent homogeneity, and their catalytic capacities were measured in four assays: reduction of cytochrome c, oxidation of NADPH, and support of the 17alpha-hydroxylase and 17,20 lyase activities of human P450c17. As assessed by Vmax/Km, 17,20 lyase activity provided the best correlation with clinical findings. Modeling human POR on the X-ray crystal structure of rat POR shows that these mutant activities correlate well with their locations in the structure. POR deficiency is a new disease, distinct from the craniosynostosis syndromes caused by FGFR mutations.

Functional interaction of nitrogenous organic bases with cytochrome P450: A critical assessment and update of substrate features and predicted key active-site elements steering the access, binding, and orientation of amines

The widespread use of nitrogenous organic bases as environmental chemicals, food additives, and clinically important drugs necessitates precise knowledge about the molecular principles governing biotransformation of this category of substrates. In this regard, analysis of the topological background of complex formation between amines and P450s, acting as major catalysts in C- and N-oxidative attack, is of paramount importance. Thus, progress in collaborative investigations, combining physico-chemical techniques with chemical-modification as well as genetic engineering experiments, enables substantiation of hypothetical work resulting from the design of pharmacophores or homology modelling of P450s. Based on a general, CYP2D6-related construct, the majority of prospective amine-docking residues was found to cluster near the distal heme face in the six known SRSs, made up by the highly variant helices B', F and G as well as the N-terminal portion of helix C and certain beta-structures. Most of the contact sites examined show a frequency of conservation < 20%, hinting at the requirement of some degree of conformational versatility, while a limited number of amino acids exhibiting a higher level of conservation reside close to the heme core. Some key determinants may have a dual role in amine binding and/or maintenance of protein integrity. Importantly, a series of non-SRS elements are likely to be operative via long-range effects. While hydrophobic mechanisms appear to dominate orientation of the nitrogenous compounds toward the iron-oxene species, polar residues seem to foster binding events through H-bonding or salt-bridge formation. Careful uncovering of structure-function relationships in amine-enzyme association together with recently developed unsupervised machine learning approaches will be helpful in both tailoring of novel amine-type drugs and early elimination of potentially toxic or mutagenic candidates. Also, chimeragenesis might serve in the construction of more efficient P450s for activation of amine drugs and/or bioremediation.

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.

Diversity and function of mutations in p450 oxidoreductase in patients with Antley-Bixler syndrome and disordered steroidogenesis

P450 oxidoreductase (POR) is the obligatory flavoprotein intermediate that transfers electrons from reduced nicotinamide adenine dinucleotide phosphate (NADPH) to all microsomal cytochrome P450 enzymes. Although mouse Por gene ablation causes embryonic lethality, POR missense mutations cause disordered steroidogenesis, ambiguous genitalia, and Antley-Bixler syndrome (ABS), which has also been attributed to fibroblast growth factor receptor 2 (FGFR2) mutations. We sequenced the POR gene and FGFR2 exons 8 and 10 in 32 individuals with ABS and/or hormonal findings that suggested POR deficiency. POR and FGFR2 mutations segregated completely. Fifteen patients carried POR mutations on both alleles, 4 carried mutations on only one allele, 10 carried FGFR2 or FGFR3 mutations, and 3 patients carried no mutations. The 34 affected POR alleles included 10 with A287P (all from whites) and 7 with R457H (four Japanese, one African, two whites); 17 of the 34 alleles carried 16 "private" mutations, including 9 missense and 7 frameshift mutations. These 11 missense mutations, plus 10 others found in databases or reported elsewhere, were recreated by site-directed mutagenesis and were assessed by four assays: reduction of cytochrome c, oxidation of NADPH, support of 17alpha-hydroxylase activity, and support of 17,20 lyase using human P450c17. Assays that were based on cytochrome c, which is not a physiologic substrate for POR, correlated poorly with clinical phenotype, but assays that were based on POR's support of catalysis by P450c17--the enzyme most closely associated with the hormonal phenotype--provided an excellent genotype/phenotype correlation. Our large survey of patients with ABS shows that individuals with an ABS-like phenotype and normal steroidogenesis have FGFR mutations, whereas those with ambiguous genitalia and disordered steroidogenesis should be recognized as having a distinct new disease: POR deficiency.

Catalytic Activity and Isoform-Specific Inhibition of Rat Cytochrome P450 4F Enzymes

Arachidonic acid is omega-hydroxylated to 20-hydroxyeicosatetraenoic acid (20-HETE), which has effects on vasoactivity and renal tubular transport and has been implicated in the regulation of blood pressure. Cytochrome p450 (p450) 4A isoforms are generally considered the major arachidonic acid omega-hydroxylases; however, little is known about the role of rat CYP4F isoforms in 20-HETE formation. The rat CYP4F isoforms, CYP4F1, CYP4F4, CYP4F5, and CYP4F6, were heterologously expressed in Escherichia coli, and their substrate specificity in fatty acid metabolism was characterized. Substrate-binding assays indicated that leukotriene B(4) (LTB(4)) and arachidonic acid bound CYP4F1 and CYP4F4 in a type-I manner with a K(s) of 25 to 59 microM, and lauric acid bound CYP4F4 poorly. Reconstituted CYP4F1 and CYP4F4 catalyzed the omega-hydroxylation of LTB(4) with a K(m) of 24 and 31 microM, respectively, and CYP4F5 had minor activity in LTB(4) metabolism. Importantly, CYP4F1 and CYP4F4 catalyzed the omega-hydroxylation of arachidonic acid with an apparent k(cat) of 9 and 11 min(-1), respectively. Lauric acid was a poor substrate for all of the CYP4F isoforms, and CYP4F6 had no detectable fatty acid omega-hydroxylase activity. The p450 omega-hydroxylase inhibitors 17-octadecynoic acid, 10-undecynyl sulfate, and N-methylsulfonyl-12,12-dibromododec-11-enamide showed isoform-specific inhibition of CYP4F1- and CYP4F4-catalyzed omega-hydroxylation of arachidonic acid and potency differences between the CYP4A and CYP4F isoforms. These data support a significant role for CYP4F1 and CYP4F4 in the formation of 20-HETE and identify p450 inhibitors that can be used to understand the relative contribution of the CYP4A and CYP4F isoforms to renal 20-HETE formation.

Revisiting the mechanism of P450 enzymes with the radical clocks norcarane and spiro[2,5]octane

Norcarane (1) and spiro[2.5]octane (2) yield different product distributions depending on whether they are oxidized via concerted, radical, or cationic mechanisms. For this reason, these two probes were used to investigate the mechanisms of hydrocarbon hydroxylation by two mammalian and two bacterial cytochrome P450 enzymes. Products indicative of a radical intermediate with a lifetime ranging from 16 to 52 ps were detected during the oxidation of norcarane by P450(cam) (CYP101), P450(BM3) (CYP102), CYP2B1, and CYP2E1. Trace amounts of the cation rearrangement product were observed with norcarane for all but CYP2E1, while no cation or radical rearrangement products were observed for spiro[2.5]octane. The results for the oxidation of norcarane with a radical rearrangement rate of 2 x 10(8) s(-1) are consistent with the involvement of a two-state radical rebound mechanism, while for the slower (5 x 10(7) s(-1)) spiro[2,5]oct-4-yl radical rearrangement products were beyond detection. Taken together with earlier data for the hydroxylation of bicyclo[2.1.0]pentane, which also suggested a 50 ps radical lifetime, these three structurally similar and functionally simple substrates show a consistent pattern of rearrangement that supports a radical rebound mechanism for this set of cytochrome P450 enzymes.

Autocatalytic mechanism and consequences of covalent heme attachment in the cytochrome P4504A family

The prosthetic heme group in the CYP4A family of cytochrome P450 enzymes is covalently attached to an I-helix glutamic acid residue. This glutamic acid is conserved in the CYP4 family but is absent in other P450 families. As shown here, the glutamic acid is linked, presumably via an ester bond, to a hydroxyl group on the heme 5-methyl group. Mutation of the glutamic acid to an alanine in CYP4A1, CYP4A3, and CYP4A11 suppresses covalent heme binding. In wild-type CYP4A3 68% of the heme is covalently bound to the heterologously expressed protein, but in the CYP4A3/E318D mutant, 47% of the heme is unchanged, 47% is present as noncovalently bound 5-hydroxymethylheme, and only 6% is covalently bound to the protein. In the CYP4A3/E318Q mutant, the majority of the heme is unaltered, and <2% is covalently linked. The proportion of covalently bound heme in the recombinant CYP4A proteins increases with time under turnover conditions. The catalytic activity is sensitive in some, but not all, CYP4A enzymes to the extent of covalent heme binding. Mutations of Glu(318) in CYP4A3 decrease the apparent k(cat) values for lauric acid hydroxylation. The key conclusions are that (a) covalent heme binding occurs via an ester bond to the heme 5-methyl group, (b) covalent binding of the heme is mediated by an autocatalytic process, and (c) fatty acid oxidation is sensitive in some CYP4A enzymes to the presence or absence of the heme covalent link.

Peroxisomal beta-oxidation and peroxisome proliferator-activated receptor alpha: an adaptive metabolic system

beta-Oxidation occurs in both mitochondria and peroxisomes. Mitochondria catalyze the beta-oxidation of the bulk of short-, medium-, and long-chain fatty acids derived from diet, and this pathway constitutes the major process by which fatty acids are oxidized to generate energy. Peroxisomes are involved in the beta-oxidation chain shortening of long-chain and very-long-chain fatty acyl-coenzyme (CoAs), long-chain dicarboxylyl-CoAs, the CoA esters of eicosanoids, 2-methyl-branched fatty acyl-CoAs, and the CoA esters of the bile acid intermediates di- and trihydroxycoprostanoic acids, and in the process they generate H2O2. Long-chain and very-long-chain fatty acids (VLCFAs) are also metabolized by the cytochrome P450 CYP4A omega-oxidation system to dicarboxylic acids that serve as substrates for peroxisomal beta-oxidation. The peroxisomal beta-oxidation system consists of (a) a classical peroxisome proliferator-inducible pathway capable of catalyzing straight-chain acyl-CoAs by fatty acyl-CoA oxidase, L-bifunctional protein, and thiolase, and (b) a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs by branched-chain acyl-CoA oxidase (pristanoyl-CoA oxidase/trihydroxycoprostanoyl-CoA oxidase), D-bifunctional protein, and sterol carrier protein (SCP)x. The genes encoding the classical beta-oxidation pathway in liver are transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPAR alpha). Evidence derived from mice deficient in PPAR alpha, peroxisomal fatty acyl-CoA oxidase, and some of the other enzymes of the two peroxisomal beta-oxidation pathways points to the critical importance of PPAR alpha and of the classical peroxisomal fatty acyl-CoA oxidase in energy metabolism, and in the development of hepatic steatosis, steatohepatitis, and liver cancer.

Nonalcoholic steatosis and steatohepatitis. II. Cytochrome P-450 enzymes and oxidative stress

Oxidative stress is present in the liver of humans with steatosis and nonalcoholic steatohepatitis (NASH) and is a plausible mediator of cellular injury, inflammatory recruitment, and fibrogenesis. CYPs 2E1 and 4A are the microsomal oxidases involved with fatty acid oxidation. Both enzymes can reduce molecular oxygen to produce prooxidant species, which, if not countered efficiently by antioxidants, create oxidative stress. In this theme article, we present the evidence that, in the context of hepatic steatosis, CYPs 2E1 and 4A could generate the "second hit" of cellular injury, particularly when antioxidant reserves are depleted, and propose ways in which this could contribute to the pathogenesis of NASH.

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.

Cloning and expression of a pig liver taurochenodeoxycholic acid 6alpha-hydroxylase (CYP4A21): a novel member of the CYP4A subfamily

A cytochrome P450 expressed in pig liver was cloned by polymerase chain reaction using oligonucleotide primers based on amino acid sequences of the purified taurochenodeoxycholic acid 6alpha-hydroxylase. This enzyme catalyzes a 6alpha-hydroxylation of chenodeoxycholic acid, and the product hyocholic acid is considered to be a primary bile acid specific for the pig. The cDNA encodes a protein of 504 amino acids. The primary structure of the porcine taurochenodeoxycholic acid 6alpha-hydroxylase, designated CYP4A21, shows about 75% identity with known members of the CYP4A subfamily in rabbit and man. Transfection of the cDNA for CYP4A21 into COS cells resulted in the synthesis of an enzyme that was recognized by antibodies raised against the purified pig liver enzyme and catalyzed 6alpha-hydroxylation of taurochenodeoxycholic acid. The hitherto known CYP4A enzymes catalyze hydroxylation of fatty acids and prostaglandins and have frequently been referred to as fatty acid hydroxylases. A change in substrate specificity from fatty acids or prostaglandins to a steroid nucleus among CYP4A enzymes is notable. The results of mutagenesis experiments indicate that three amino acid substitutions in a region around position 315 which is highly conserved in all previously known CYP4A and CYP4B enzymes could be involved in the altered catalytic activity of CYP4A21.

Protein engineering of cytochromes P-450

The cytochromes P-450 are an immensely important superfamily of heme-containing enzymes. They catalyze the monooxygenation of an enormous range of substrates. In bacteria, cytochromes P-450 are known to catalyze the hydroxylation of environmentally significant substrates such as camphor, phenolic compounds and many herbicides. In eukaryotes, these enzymes perform key roles in the synthesis and interconversion of steroids, while in mammals hepatic cytochromes P-450 are vital for the detoxification of many drugs. As such, the cytochromes P-450 are of considerable interest in medicine and biotechnology and are obvious targets for protein engineering. The purpose of this article is to illustrate the ways in which protein engineering has been used to investigate and modify the properties of cytochromes P-450. Illustrative examples include: the manipulation of substrate selectivity and regiospecificity, the alteration of membrane binding properties, and probing the route of electron transfer.

Replacement of the distal glycine 139 transforms human heme oxygenase-1 into a peroxidase

The human heme oxygenase-1 crystal structure suggests that Gly-139 and Gly-143 interact directly with iron-bound ligands. We have mutated Gly-139 to an alanine, leucine, phenylalanine, tryptophan, histidine, or aspartate, and Gly-143 to a leucine, lysine, histidine, or aspartate. All of these mutants bind heme, but absorption and resonance Raman spectroscopy indicate that the water coordinated to the iron atom is lost in several of the Gly-139 mutants, giving rise to mixtures of hexacoordinate and pentacoordinate ligation states. The active site perturbation is greatest when large amino acid side chains are introduced. Of the Gly-139 mutants investigated, only G139A catalyzes the NADPH-cytochrome P450 reductase-dependent oxidation of heme to biliverdin, but most of them exhibit a new H(2)O(2)-dependent guaiacol peroxidation activity. The Gly-143 mutants, all of which have lost the water ligand, have no heme oxygenase or peroxidase activity. The results establish the importance of Gly-139 and Gly-143 in maintaining the appropriate environment for the heme oxygenase reaction and show that Gly-139 mutations disrupt this environment, probably by displacing the distal helix, converting heme oxygenase into a peroxidase. The principal role of the heme oxygenase active site may be to suppress the ferryl species formation responsible for peroxidase activity.

A single amino acid substitution (F363I) converts the regiochemistry of the spearmint (-)-limonene hydroxylase from a C6- to a C3-hydroxylase

The essential oils of peppermint and spearmint are distinguished by the position of oxygenation on the constituent p-menthane monoterpenes. Peppermint produces monoterpenes bearing an oxygen at C3, whereas spearmint produces monoterpenes bearing an oxygen at C6. Branching of the monoterpene biosynthetic pathways in these species is determined by two distinct cytochrome P450s that catalyze the regiospecific hydroxylation of (-)-4S-limonene at C3 or C6 exclusively. cDNAs encoding the limonene-3-hydroxylase from peppermint and the limonene-6-hydroxylase from spearmint have been isolated, shown to be 70% identical at the amino acid level, and functionally expressed. A combination of domain swapping and reciprocal site-directed mutagenesis between these two enzymes demonstrated that the exchange of a single residue (F363I) in the spearmint limonene-6-hydroxylase led to complete conversion to the regiospecificity and catalytic efficiency of the peppermint limonene-3-hydroxylase.

Reaction intermediates and single turnover rate constants for the oxidation of heme by human heme oxygenase-1

Heme oxygenase converts heme to biliverdin, iron, and CO in a reaction with two established intermediates, alpha-meso-hydroxyheme and verdoheme. Transient kinetic studies show that the conversion of Fe(3+)-heme to Fe(3+)-verdoheme is biphasic. Electron transfer to the heme (0.11 s(-1) at 4 degrees C and 0.49 s(-1) at 25 degrees C) followed by rapid O(2) binding yields the ferrous dioxy complex. Transfer of an electron (0.056 s(-1) at 4 degrees C and 0.21 s(-1) at 25 degrees C) to this complex triggers the formation of alpha-meso-hydroxyheme and its subsequent O(2)-dependent fragmentation to Fe(3+)-verdoheme. The conversion of Fe(3+)-verdoheme to Fe(3+)-biliverdin is also biphasic. Thus, reduction of Fe(3+) to Fe(2+)-verdoheme (0.15 s(-1) at 4 degrees C and 0.55 s(-1) at 25 degrees C) followed by O(2) binding and an electron transfer produces Fe(3+)-biliverdin (0.025 s(-1) at 4 degrees C and 0.10 s(-1) at 25 degrees C). The conversion of Fe(3+)-biliverdin to free biliverdin is triphasic. Reduction of Fe(3+)-biliverdin (0.035 s(-1) at 4 degrees C and 0.15 s(-1) at 25 degrees C), followed by rapid release of Fe(2+) (0.19 s(-1) at 4 degrees C and 0.39 s(-1) at 25 degrees C), yields the biliverdin-enzyme complex from which biliverdin slowly dissociates (0.007 s(-1) at 4 degrees C and 0.03 s(-1) at 25 degrees C). The rate of Fe(2+) release agrees with the rate of Fe(3+)-biliverdin reduction. Fe(2+) release clearly precedes biliverdin dissociation. In the absence of biliverdin reductase, biliverdin release is the rate-limiting step, but in its presence biliverdin release is accelerated and the overall rate of heme degradation is limited by the conversion of Fe(2+)-verdoheme to the Fe(3+)-biliverdin.

Omega-hydroxyceramides are required for corneocyte lipid envelope (CLE) formation and normal epidermal permeability barrier function

Omega-hydroxyceramides (omega-OHCer) are the predominant lipid species of the corneocyte lipid envelope in the epidermis. Moreover, their omega-esterified-derivatives (acylCer) are major components of the stratum corneum extracellular lamellae, which regulate cutaneous permeability barrier function. Because epidermal omega-OHCer appear to be generated by a cytochrome P450-dependent process, we determined the effects of a mechanism-based inhibitor of omega-hydroxylation, aminobenzotriazole (ABT), on epidermal omega-OH Cer formation and barrier function. We first ascertained that ABT, but not hydroxybenzotriazole (OHBT), a chemical relative with no P450 inhibitory activity, inhibited the incorporation of [14C]-acetate into the omega-OH-containing Cer species in cultured human keratinocytes (68.1% +/- 6.9% inhibition versus vehicle-treated controls; p < 0.001), without altering the synthesis of other Cer and fatty acid species. In addition, ABT significantly inhibited the omega-hydroxylation of very long-chain fatty acids in cultured human keratinocytes. Topical application of ABT, but not OHBT, when applied to the skin of hairless mice following acute barrier disruption by tape-stripping, resulted in a significant delay in barrier recovery (e.g., 38.3% delay at 6 h versus vehicle-treated animals), assessed as increased transepidermal water loss. The ABT-induced barrier abnormality was associated with: (i) a significant decrease in the quantities of omega-OHCer in both the unbound and the covalently bound Cer pools; (ii) marked alterations of lamellar body structure and contents; and (iii) abnormal stratum corneum extracellular lamellar membrane structures, with no signs of cellular toxicity. Furthermore, pyridine-extraction of ABT- versus vehicle-treated skin, which removes all of the extracellular lamellae, leaving the covalently attached lipids, showed numerous foci with absent corneocyte lipid envelope in ABT- versus vehicle-treated stratum corneum. These results provide the first direct evidence for the importance of omega-OHCer for epidermal permeability function, and suggest further that acylCer and/or corneocyte lipid envelope are required elements in permeability barrier homeostasis.

Structural determination of the substrate specificities and regioselectivities of the rat and human fatty acid omega-hydroxylases

The substrate and regiospecificities of the known CYP4A enzymes from rat (CYP4A1, -4A2, -4A3, and -4A8) and human (CYP4A11) have been determined using lauric (C12), myristic (C14), palmitic (C16), oleic (C18:1), and arachidonic (C20:4) acids. The CYP4A2 and CYP4A8 cDNAs required to complete the enzyme set were cloned from a rat kidney library. All five proteins were expressed in Escherichia coli and were purified with the help of a six-histidine tag at the carboxyl terminus. Two complementary CYP4A2-CYP4A3 chimeras fused at residue 119 (CYP4A2) and 122 (CYP4A3) were constructed to explore the roles of the 18 amino acid differences between the parent proteins in determining their catalytic profiles. The chimera in which the first 119 amino acids are from CYP4A2 indicates that the first 120 amino acids control the substrate specificity. The chimera in which the first 122 amino acids are from CYP4A3 is inactive due to a defect in electron transfer to the heme group. The highest activity for lauric acid was obtained with CYP4A1 and CYP4A8, but for all the proteins the activity decreased with increasing fatty acid chain length. The fact that none of the rat and human CYP4A enzymes exhibits a high activity with arachidonic acid appears to limit their role as catalysts for the physiologically important conversion of arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-HETE).

Production in vitro by the cytochrome P450 CYP94A1 of major C18 cutin monomers and potential messengers in plant-pathogen interactions: enantioselectivity studies

The major C(18) cutin monomers are 18-hydroxy-9,10-epoxystearic and 9,10,18-trihydroxystearic acids. These compounds are also known messengers in plant-pathogen interactions. We have previously shown that their common precursor 9,10-epoxystearic acid was formed by the epoxidation of oleic acid in Vicia sativa microsomes (Pinot, Salaün, Bosch, Lesot, Mioskowski and Durst (1992) Biochem. Biophys. Res. Commun. 184, 183-193). Here we determine the chirality of the epoxide produced as (9R,10S) and (9S,10R) in the ratio 90:10 respectively. We further show that microsomes from yeast expressing the cytochrome P450 CYP94A1 are capable of hydroxylating the methyl terminus of 9,10-epoxystearic and 9,10-dihydroxystearic acids in the presence of NADPH to form the corresponding 18-hydroxy derivatives. The reactions were not catalysed by microsomes from yeast transformed with a void plasmid or in absence of NADPH. After incubation of a synthetic racemic mixture of 9,10-epoxystearic acid with microsomes of yeast expressing CYP94A1, the chirality of the residual epoxide was shifted to 66:34 in favour of the (9S,10R) enantiomer. Both enantiomers were incubated separately and V(max)/K(m) values of 16 and 3.42 ml/min per nmol of P450 for (9R, 10S) and (9S,10R) respectively were determined, demonstrating that CYP94A1 is enantioselective for the (9R,10S) enantiomer, which is preferentially formed in V. sativa microsomes. Compared with the epoxide, the diol 9,10-dihydroxystearic acid was a much poorer substrate for the omega-hydroxylase, with a measured V(max)/K(m) of 0.33 ml/min per nmol of P450. Our results indicate that the activity of CYP94A1 is strongly influenced by the stereochemistry of the 9, 10-epoxide and the nature of substituents on carbons 9 and 10, with V(max)/K(m) values for epoxide>>oleic acid>diol.

Unique properties of purified, Escherichia coli-expressed constitutive cytochrome P4504A5

Cytochromes P450 of the 4A family metabolize a variety of fatty acids, prostaglandins, and eicosanoids mainly at the terminal carbon (omega-hydroxylation) and, to a lesser extent, at the penultimate carbon [(omega-1)-hydroxylation]. In the present study, cytochrome P4504A5 (4A5) has been successfully expressed in Escherichia coli, with an average yield of enzyme of approximately 80 nmol/liter of cells. Spectroscopic characterization of the purified enzyme, using electron paramagnetic resonance and absolute and substrate-perturbed optical difference spectroscopy, showed that the heme of resting 4A5 is primarily low spin, but is converted primarily to high spin by substrate binding. The kcat and Km values for laurate omega-hydroxylation were 41 min-1 and 8.5 microM, respectively, in the absence of cytochrome b5, and 138 min-1 and 38 microM, respectively, in the presence of cytochrome b5. Hydroxylation of palmitate was dependent on the presence of cytochrome b5; kcat and Km values were 48 min-1 and 122 microM, respectively. Hydroxylation of arachidonic acid was barely detectable and was unchanged by the addition of cytochrome b5.

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.

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.