High-performance liquid chromatography with spectrophotometric and electrochemical detection of a series of manganese(III) cationic porphyrins

Recent studies have revealed potent pharmacological activities of manganese-containing cationic porphyrins. An analytical method employing high-performance liquid chromatography with spectrophotometric and electrochemical detection (HPLC-UV/EC) suitable for in vivo applications is described for a series of manganese(III) cationic porphyrins with good separation and resolution. In particular, this method resolved the four atropisomers of manganese(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP5+ or AEOL-10113), verified by mass spectrometry. Electrochemical and spectrophotometric methods of detection were compared using manganese(III) meso-tetrakis(1,3-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP5+ or AEOL-10150), the lead catalytic antioxidant of this series. Both methods of detection were quantitative, but electrochemical detection, although less specific for in vivo applications, appears to be considerably more sensitive than spectrophotometric detection.     

Construction and engineering of a thermostable self-sufficient cytochrome P450

CYP175A1 is a thermophilic cytochrome P450 and hydroxylates β-carotene. We previously identified a native electron transport system for CYP175A1. In this report, we constructed two fusion proteins consisting of CYP175A1, ferredoxin (Fdx), and ferredoxin-NADP⁺ reductase (FNR): H₂N-CYP175A1-Fdx-FNR-COOH (175FR) and H₂N-CYP175A1-FNR-Fdx-COOH (175RF). Both 175FR and 175RF were expressed in Escherichia coli and purified. The V_max value for β-carotene hydroxylation was 25 times higher with 175RF than 175FR and 9 times higher with 175RF than CYP175A1 (non-fused protein), although the k_m values of these enzymes were similar. 175RF retained 50% residual activity even at 80 °C. Furthermore, several mutants of the CYP175A1 domain of 175RF were prepared and one mutant (Q67G/Y68I) catalyzed the hydroxylation of an unnatural substrate, testosterone. Thus, this is the first report of a thermostable self-sufficient cytochrome P450 and the engineering of a thermophilic cytochrome P450 for the oxidation of an unnatural substrate.     

The effect of iron to manganese substitution on microperoxidase 8 catalysed peroxidase and cytochrome P450 type of catalysis

 This study describes the catalytic properties of manganese microperoxidase 8 [Mn(III)MP8] compared to iron microperoxidase 8 [Fe(III)MP8]. The mini-enzymes were tested for pH-dependent activity and operational stability in peroxidase-type conversions, using 2-methoxyphenol and 3,3′-dimethoxybenzidine, and in a cytochrome P450-like oxygen transfer reaction converting aniline to para-aminophenol. For the peroxidase type of conversions the Fe to Mn replacement resulted in a less than 10-fold decrease in the activity at optimal pH, whereas the aniline para-hydroxylation is reduced at least 30-fold. In addition it was observed that the peroxidase type of conversions are all fully blocked by ascorbate and that aniline para-hydroxylation by Fe(III)MP8 is increased by ascorbate whereas aniline para-hydroxylation by Mn(III)MP8 is inhibited by ascorbate. Altogether these results indicate that different types of reactive metal oxygen intermediates are involved in the various conversions. Compound I/II, scavenged by ascorbate, may be the reactive species responsible for the peroxidase reactions, the polymerization of aniline and (part of) the oxygen transfer to aniline in the absence of ascorbate. The para-hydroxylation of aniline by Fe(III)MP8, in the presence of ascorbate, must be mediated by another reactive iron-oxo species which could be the electrophilic metal(III) hydroperoxide anion of microperoxidase 8 [M(III)OOH MP8]. The lower oxidative potential of Mn, compared to Fe, may affect the reactivity of both compound I/II and the metal(III) hydroperoxide anion intermediate, explaining the differential effect of the Fe to Mn substitution on the pH-dependent behavior, the rate of catalysis and the operational stability of MP8. Received: 29 September 1998 / Accepted: 16 February 1999     

Partially and fully beta-brominated Mn-porphyrins in P450 biomimetic systems: effects of the degree of bromination on electrochemical and catalytic properties

Beta-hexabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br6TCMPP)Cl) was prepared by selective Br2-hexabromation of its parent non-brominated manganese complex (Mn(III)(TCMPP)Cl), whereas the octabrominated analogue beta-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br8TCMPP)Cl) was synthesized via metallation of the corresponding free-base. Beta-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin was obtained by demetallation of its brominated Cu(II) derivative, which, in its turn, was prepared by either a Br2 or an N-bromosuccinimide protocol. Relative to Mn(III)(TCMPP)Cl (E(1/2) = -0.16 V vs. normal hydrogen electrode, CH2Cl2), the Mn(III)/Mn(II) reduction potential of Mn(III)(Br8TCMPP)Cl and Mn(III)(Br6TCMPP)Cl showed anodic shifts of 0.43 and 0.33 V, respectively, which corresponded to a linear shift of 0.05 V per bromine added. These manganese complexes were evaluated as cytochrome P450 mimics in catalytic iodosylbenzene (PhIO)-oxidations of cyclohexane and cyclohexene. In aerobic PhIO-oxidation of cyclohexene, epoxidation and allylic autoxidation reactions were inversely related, competitive processes; the most efficient P450-mimics were the least effective autoxidation catalysts. Mn(III)(Br6TCMPP)Cl was more efficient as epoxidation or hydroxylation catalyst than both its fully and non-beta-brominated counterparts were. There was no linear relationship between the catalytic efficiency and both the number of bromine substituents and the Mn(III)/Mn(II) potential; these observations were compared to Lyons system literature data and discussed. Analogously to enzymatic optimum pH effects, an optimum redox potential effect is suggested as relevant in designing and understanding cytochrome P450 biomimetic catalysts.     

Stereoselectivity in the oxidation of bufuralol,a chiral substrate,by human cytochrome P450s

Bufuralol (BF), a nonselective beta-adrenoceptor blocking agent, has a chiral center in its molecule, yielding the enantiomers 1'R-BF and 1'S-BF. beta-Adrenoceptor blocking potency is much higher in 1'S-BF than in 1'R-BF. One of the metabolic pathways of BF is 1"-hydroxylation of an ethyl group attached at the aromatic 7-position forming a carbinol metabolite (1"-hydroxybufuralol, 1"-OH-BF), and further oxidation (or dehydrogenation) produces a ketone metabolite (1-oxobufuralol, 1"-Oxo-BF). Both 1"-OH-BF and 1"-Oxo-BF are known to have beta-adrenoceptor blocking activities comparable to or higher than those of the parent drug. The 1"-hydroxylation introduces another chiral center into the BF molecule and four 1"-OH-BF diastereomers are formed from BF racemate in mammals, including humans, making elucidation of the metabolic profiles complicated. HPLC methods employing derivatization, reversed phase, or chiral columns have been developed to efficiently separate the four 1"-OH-BF diastereomers formed from BF enantiomers or racemate. Accumulated in vitro experimental results revealed that 1'R-BF is a much more preferential substrate than 1'S-BR for BF 1"-hydroxylation in human liver microsomes. Kinetic studies using recombinant human cytochrome P450 (CYP) enzymes indicate that CYP2D6 serves as a major BF 1"-hydroxylase and that CYP1A2 and CYP2C19 also contribute to BF 1"-hydroxylation in human livers. This mini-review summarizes the knowledge reported so far on the pharmacology of BF and its metabolites and the profiles of BF metabolism, especially focusing on the stereoselectivity in the oxidation of BF mainly in human livers and recombinant CYP enzymes.     

Suppression in the expression of a male-specific cytochrome P450, P450-male: difference in the effect of chemical inducers on P450-male mRNA and protein in rat livers

Hypophysectomy of male adult rats caused a 70% decrease in the hepatic level of mRNA hybridized to two specific oligonucleotide probes for the sequence of coding and 3'-noncoding regions of P450(M-1) (H. Yoshioka et al., (1987) J. Biol. Chem. 262, 1706-1711), which corresponds to P450-male. Treatment of hypophysectomized male and female rats with subcutaneous injection of human growth hormone twice a day for 7 days increased the mRNA to a level similar to that of normal male rats. In contrast, the mRNA was decreased by treatment with continuous infusion. These results correlated well with those on the amounts of P450-male protein, indicating that growth hormone regulates the hepatic level of P450-male protein mainly by acting at the pretranslational step. Treatment of adult male rats with phenobarbital (PB), dexamethasone (Dex), or 3-methylcholanthrene (MC) decreased the content of P450-male protein by 68, 36, and 46%, respectively. The content of P450-male protein was also decreased to 65% in Dex-treated hypophysectomized male rats, but was not changed by treatment of hypophysectomized male rats with PB or MC, suggesting that PB and MC decrease P450-male protein through a pituitary growth hormone-mediated process. However, the level of mRNA hybridizable to the P450-male oligonucleotide probe was not decreased, but rather it increased in PB- or Dex-treated hypophysectomized male rats. A similar inconsistent change in protein and mRNA was also observed in PB-treated normal rats. These results indicate that PB and Dex have an additional effect of increasing the hepatic level of the specific mRNA of P450-male/(M-1) or a closely related form. Noncoordinate changes in the level of P450-male protein and mRNA also suggest that the hepatic level of P450-male protein is regulated by plural mechanisms: pretranslational and translational regulation in which pituitary growth hormone and/or other endocrine factors are involved.     

Enantioselective 2-hydroxylation of RS-8359, a selective and reversible MAO-A inhibitor, by cytochrome P450 in mouse and rat liver microsomes

RS-8359, (+/-)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]-pyrimidine is a racemic compound with a selective and reversible monoamine oxidase A (MAO-A) inhibition activity. The substrate and product enantioselectivity with respect to 2-hydroxylation of RS-8359 enantiomers was studied using mouse and rat liver microsomes. In mice, the (S)-enantiomer was transformed to the cis-diol metabolite, whereas the (R)-enantiomer to the trans-diol metabolite. The Vmax/Km value for the formation of the cis-diol metabolite from the (S)-enantiomer was sevenfold greater than that for the formation of the trans-diol metabolite from the (R)-enantiomer. The greater Vmax/Km value for the (S)-enantiomer was due to the tenfold smaller Km value compared to that for the (R)-enantiomer. The results were in fair agreement with the previously reported low plasma concentrations of the (S)-enantiomer and the high recovery of the cis-diol metabolite derived from the (S)-enantiomer in urine after oral administration of RS-8359 to mice. Similarly to mice, in rats the (R)-enantiomer was transformed to the trans-diol metabolite, whereas the (S)-enantiomer yielded the cis-diol and trans-diol metabolites. The Vmax/Km value for the (R)-enantiomer was larger than that for the (S)-enantiomer in rats, indicating that the low plasma concentration of the (S)-enantiomer in rats might be caused by a metabolic reaction other than P450-dependent hydroxylation. CYP3A was shown to be responsible for the trans-diol formation from the (R)-enantiomer.     

Human cytochrome P450 4F11: Heterologous expression in bacteria, purification, and characterization of catalytic function

Human cytochrome P450 (P450) 4F11 is still considered an “orphan” because its function is not well characterized. A bacterial expression system was developed for human P450 4F11, producing ∼230 nmol P450 from a 3-l culture of Escherichia coli. P450 4F11 was purified and utilized for untargeted substrate searches in human liver extract using a liquid chromatography/mass spectrometry-based metabolomic and isotopic labeling approach (Tang et al., 2009 [19]). Four fatty acids—palmitic, oleic, arachidonic, and docosahexaenoic—were identified in human liver and verified as substrates of P450 4F11. The products were characterized as ω-hydroxylated fatty acids by gas chromatography-mass spectrometry analysis of their trimethylsilyl derivatives. Kinetic analysis of the oxidation products confirmed that the fatty acids are substrates oxidized by P450 4F11. P450 4F11 also exhibited low activity for some drug N-demethylation reactions but none for activation of several pro-carcinogens.     

Modification of the fatty acid specificity of cytochrome P450 BM-3 from Bacillus megaterium by directed evolution: a validated assay

Cytochrome P450 BM-3 (CYP102) catalyzes the subterminal hydroxylation of fatty acids with a chain length of 12–22 carbons. The paper focuses on the regioselectivity and substrate specificity of the purified wild-type enzyme and five mutated variants towards caprylic, capric, and lauric acid. The enzymes were obtained by random mutagenic fine-tuning of the mutant F87A(LARV). F87A(LARV) was selected as the best enzyme variant in a previous study in which the single mutant F87A was subjected to rational evolution to achieve hydroxylation activity for short chain length substrates using a p-nitrophenolate-based spectrophotometric assay.

The best mutants, F87V(LAR) and F87V(LARV), show a higher catalytic activity towards ω-(p-nitrophenoxy)decanoic acid (10-p-NCA) than F87A(LARV). In addition, they proved capable of hydroxylating ω-(p-nitrophenoxy)octanoic acid (8-p-NCA) which the wild-type enzyme is unable to do. Both variants catalyzed hydroxylation of capric acid, which is not a substrate for the wild-type, with a conversion rate of up to 57%. The chain length specificity of the mutants in fatty acid hydroxylation processes shows a good correlation with their activity towards p-NCA pseudosubstrates. The p-NCA assay therefore, allows high-throughput screening of large mutant libraries for the identification of enzyme variants with the desired catalytic activity towards fatty acids as the natural substrates.     

The influence of B-group vitamins on monooxygenase activity of cytochrome P450 3A4: Pharmacokinetics and electro analysis of the catalytic properties

Simultaneous administration of loading doses of B-group vitamins and diclofenac allow to decrease the daily dose of this drug without reduction of its analgesic effect. In all three schemes of the diclofenac intake (diclofenac alone, diclofenac plus 2 tablets of Gitagamp (a mixture of B-group vitamins), and diclofenac plus 4 tablets of Gitagamp—maximal concentration of blood diclofenal (C_max) was observed 1 h after the treatment. In the case of diclofenac treatment alone, with 2 tablets of Gitagamp, and with 4 tablets of Gitagamp C_max values were 1137.2 ± 82.4, 1326.7 ± 122.5 and 2200.4 ± 111.3 ng/mL, respectively. Thus, loading doses of B-group vitamins caused a statistically significant effect on the C_max value of blood diclofenac concentration; they also reduced manifestations of the pain syndrome. Pharmacodynamics and pharmacokinetics data were confirmed in electrochemical studies of cytochrome P450 3A4 (CYP3A4) activity. This enzyme was immobilized onto screen printed graphite electrodes modified with gold nanoparticles and synthetic membrane-like compound didodecyldimethylammonium bromide (DDAB/Au). Electrochemical analysis revealed the influence of B-group vitamins on metabolism of the non-steroidal anti-inflammatory drug diclofenac catalyzed by cytochrome P450 3A4. Comparative analysis of the effect of 300 μM vitamins of the B-group (B₁, B₂, and B₆) demonstrated that riboflavin was the most effective inhibitor of diclofenac hydroxylation catalyzed by CYP3A4. These data support possibility of regulation of pharmacokinetic parameters and manifestation of pharmacodynamic effects by loading doses of B-group vitamins, which regulate the catalytic activity of drug metabolizing enzymes such as cytochrome P450 3A4.     

X-ray crystal structure and catalytic properties of Thr252Ile mutant of cytochrome P450cam: roles of Thr252 and water in the active center

The structure-function relationship in cytochrome P450cam monooxygenase was studied by employing its active site mutant Thr252Ile. X-ray crystallographic analyses of the ferric d-camphor-bound form of the mutant revealed that the mutation caused a structural change in the active site giving an enlarged oxygen-binding pocket that did not contain any hydrophilic group such as the OH group of Thr and H(2)O. The enzyme showed a low monooxygenase activity of ca. 1/10 of the activity of the wild-type enzyme. Kinetic analyses of each catalytic step revealed that the rate of proton-coupled reduction of the oxygenated intermediate of the enzyme, a ternary complex of dioxygen and d-camphor with the ferrous enzyme, decreased to about 1/30 of that of the wild-type enzyme, while the rates of other catalytic steps including the reduction of the ferric d-camphor-bound form by reduced putidaredoxin did not change significantly. These results indicated that a hydrophilic group(s) such as water and/or hydroxyl group in the active site is prerequisite to a proton supply for the reduction of the oxygenated intermediate, thereby giving support for the operation of a proton transfer network composed of Thr252, Asp251, and two other amino acids and water proposed by previous investigators.     

Degradation of morpholine, piperidine, and pyrrolidine by mycobacteria: evidences for the involvement of a cytochrome P450.

Nine bacterial strains that grew on morpholine and pyrrolidine as sole carbon, nitrogen, and energy sources were isolated from three different environments with no known morpholine contamination. One of these strains could also degrade piperidine. These bacteria were identified as Mycobacterium strains. A phylogenetic analysis based on the partial 16S rDNA sequences indicated that the isolated strains clustered within the fast growing group of mycobacteria. When the above-mentioned cyclic amines were used as growth substrates, the synthesis of a soluble cytochrome P450 was induced in all these bacteria. Other laboratory strains, Mycobacterium fortuitum and Mycobacterium smegmatis mc(2)155, were tested for their abilities to degrade morpholine. Neither of them degraded morpholine but could use pyrrolidine and piperidine. The growth of M. fortuitum and M. smegmatis mc(2)155 on these compounds involved a soluble cytochrome P450, suggesting that mycobacterial strains are naturally able to use pyrrolidine and have developed a similar enzymatic pathway to metabolize this amine.     

A medium-throughput screening assay to determine catalytic activities of oxygen-consuming enzymes: a new tool for functional characterization of cytochrome P450 and other oxygenases

A challenge of the post-genomic era is to determine the functions of a plethora of orphan genes. This is a more acute problem when dealing with large gene families, such as the superfamily encoding cytochrome P450 enzymes in higher plants. We propose here a new, simple, medium-throughput methodology to screen for potential substrates of orphan P450 mono-oxygenases. The same technique can also be applied to screening for inhibitors of the oxygenases involved in the biosynthesis of compounds essential for plant development, such as growth regulators. The method is based on a commercially available microplate system, which detects the oxygen consumed by the catalytic reaction via an oxygen-sensing fluorophore. It is optimized using as a model CYP73A1, the cinnamic acid hydroxylase from Helianthus tuberosus, expressed in yeast. We show that the procedure is suitable not only for the detection and real-time monitoring, but also for the quantitative evaluation of enzyme activity. This new method has broad application for the identification of candidate substrates and inhibitors in chemical libraries, to support determination of physiological substrates, development of plant growth regulators, investigations on herbicide and pollutant metabolism, synthesis of valuable compounds and drug design. It also provides a fast-assay platform for determination of catalytic and inhibition parameters. The method applies to plant P450 enzymes, but also to cytochromes P450 from other organisms, and all types of oxygenases. The critical steps, calculation of oxygen consumption from fluorescence signal, and limits of the methods are discussed.     

Expression and characterization of the two flavodoxin proteins of Bacillus subtilis, YkuN and YkuP: biophysical properties and interactions with cytochrome P450 BioI

The two flavodoxins (YkuN and YkuP) from Bacillus subtilis have been cloned, overexpressed in Escherichia coli and purified. DNA sequencing, mass spectrometry, and flavin-binding properties showed that both YkuN and YkuP were typical short-chain flavodoxins (158 and 151 amino acids, respectively) and that an error in the published B. subtilis genome sequence had resulted in an altered reading frame and misassignment of YkuP as a long-chain flavodoxin. YkuN and YkuP were expressed in their blue (neutral semiquinone) forms and reoxidized to the quinone form during purification. Potentiometry confirmed the strong stabilization of the semiquinone form by both YkuN and YkuP (midpoint reduction potential for oxidized/semiquinone couple = -105 mV/-105 mV) with respect to the hydroquinone (midpoint reduction potential for semiquinone/hydroquinone couple = -382 mV/-377 mV). Apoflavodoxin forms were generated by trichloroacetic acid treatment. Circular dichroism studies indicated that flavin mononucleotide (FMN) binding led to considerable structural rearrangement for YkuP but not for YkuN. Both apoflavodoxins bound FMN but not riboflavin avidly, as expected for short-chain flavodoxins. Structural stability studies with the chaotrope guanidinium chloride revealed that there is moderate destabilization of secondary and tertiary structure on FMN removal from YkuN, but that YkuP apoflavodoxin has similar (or slightly higher) stability compared to the holoprotein. Differential scanning calorimetry reveals further differences in structural stability. YkuP has a lower melting temperature than YkuN, and its endotherm is composed of a single transition, while that for YkuN is biphasic. Optical and fluorimetric titrations with oxidized flavodoxins revealed strong affinity (K(d) values consistently <5 microM) for their potential redox partner P450 BioI, YkuN showing tighter binding. Stopped-flow reduction studies indicated that the maximal electron-transfer rate (k(red)) to fatty acid-bound P450 BioI occurs from YkuN and YkuP at approximately 2.5 s(-1), considerably faster than from E. coli flavodoxin. Steady-state turnover with YkuN or YkuP, fatty acid-bound P450 BioI, and E. coli NADPH-flavodoxin reductase indicated that both flavodoxins supported lipid hydroxylation by P450 BioI with turnover rates of up to approximately 100 min(-1) with lauric acid as substrate. Interprotein electron transfer is a likely rate-limiting step. YkuN and YkuP supported monohydroxylation of lauric acid and myristic acid, but secondary oxygenation of the primary product was observed with both palmitic acid and palmitoleic acid as substrates.     

Iron-porphyrin catalysis of the oxidative dealkylation of para-nitro-anisole and 7-ethoxycoumarin by cumylhydroperoxide: a possible model for the corresponding cytochrome P 450-dependent reactions

A biphasic system containing an iron porphyrin, Fe (TPP) (C1)¹ or [Fe(TPP)]₂O, efficiently catalyzes the cumyl-or tertiobutyl-hydroperoxide-supported dealkylation of p-nitroanisole and 7-ethoxycoumarin to the corresponding phenol and formaldehyde. Stoichiometric amounts of iron porphyrin and hydroperoxide give a quantitative reaction. Catalytic amounts of iron porphyrin give reaction rates and yields which are proportional to substrate concentration. With increasing hydroperoxide concentrations, the rates level offto limit values and the yield rapidly decreases. The maximum rates obtained approach those of the reactions mediated by cytochrome P 450-dependent monooxygenases.     

Crystal structures of cytochrome P450nor and its mutants (Ser286-->Val, Thr) in the ferric resting state at cryogenic temperature: a comparative analysis with monooxygenase cytochrome P450s

Cytochrome P450nor (P450nor) is a heme enzyme isolated from the denitrifying fungus Fusarium oxysporum and catalyzes the NO reduction to N2O. Crystal structures of the wild type and two Ser286 mutants (Ser286-->Val, Ser286-->Thr) of P450nor have been determined for the ferric resting forms at a 1.7 A resolution at cryogenic temperature (100 K). We carried out three comparative analyses: (1) between the structures of P450nor at room temperature and cryogenic temperature, (2) between the structures of P450nor and four monooxygenase P450s, and (3) between the structures of the WT and the Ser286 mutant enzymes of P450nor. Comparison of the charge distribution on the protein surface suggests that proton and electron flow to the heme site is quite different in P450nor than in monooxygenase P450s. On the basis of the mutant structures, it was found that a special hydrogen-bonding network, Wat99-Ser286-Wat39-Asp393-solvent, acts as a proton delivery pathway in NO reduction by P450nor. In addition, the positively charged cluster located beneath the B'-helix is suggested as possible NADH binding site in P450nor, from which the direct two-electron transfer to the heme site allows to generate the characteristic intermediate in the NO reduction. These structural characteristics were not observed in structures of monooxygenase P450s, implying that these are factors determining the unique NO reduction activity of P450nor.     

Escherichia coli MTC, a human NADPH P450 reductase competent mutagenicity tester strain for the expression of human cytochrome P450 isoforms 1A1, 1A2, 2A6, 3A4, or 3A5: catalytic activities and mutagenicity studies

We report here on the genetic engineering of four new Escherichia coli tester bacteria, coexpressing human CYP1A1, CYP2A6, CYP3A4 or CYP3A5 with human NADPH cytochrome P450 reductase (RED) by a biplasmid coexpression system, recently developed to express human CYP1A2 in the tester strain MTC. The four new strains were compared for CYP- and RED-expression levels and CYP activities with the formerly developed CYP1A2 expressing strain. CYP1A2 and CYP2A6 were expressed at the highest, CYP1A1 at the lowest and CYP3A4 and CYP3A5 at intermediate expression levels. Membranes of all five tester bacteria demonstrated similar RED-expression levels, except for the two CYP3A-containing bacteria which demonstrated slightly increased RED-levels. CYP-activities were determined as ethoxyresorufin deethylase (CYP1A1 and CYP1A2), coumarin 7-hydroxylase (CYP2A6) and erythromycin N-demethylase (CYP3A4 and CYP3A5) activities. Reaction rates were comparable with those obtained previously for these CYP-enzymes, except for CYP3A5 which demonstrated a lower activity. Benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene demonstrated mutagenicity in the CYP1A1 expressing strain with mutagenic activities, respectively, approximately 10-fold and 100-fold higher as compared with those obtained with the use of rat liver S9 fraction. Aflatoxin B1 demonstrated a significant mutagenicity with all CYP expressing strains, albeit lower as compared to those obtained with the use of rat liver S9. CYP1A2 was approximately 3-fold more effective in generating a mutagenic response of AFB1 as compared to CYP3A4. CYP3A5 and CYP3A4 demonstrated comparable capacities in AFB1 bioactivation which was equal as found for CYP1A1. It is concluded that these four new strains contain stable CYP- and RED-expression, significant CYP-activities and demonstrated significant bioactivation activities with several diagnostic carcinogens.     

Identification of a cytochrome P450 hydroxylase, CYP81A6, as the candidate for the bentazon and sulfonylurea herbicide resistance gene, Bel, in rice

Bentazon and sulfonylureas have been used for selective control of broadleaf weeds and sedges in rice fields for more than 20 years. A bentazon and sulfonylurea susceptible mutant, bel, was previously identified for the purpose of allowing these herbicides to be used for removing false hybrids from hybrid rice. While this mutation has been used successfully in rice breeding, the genetic nature of bel is not known. Using 1,776 susceptible plants from a population of 10,000 F₂ individuals, we constructed a fine map for the Bel locus and delimited it to a 36-kb DNA fragment between two restriction fragment length polymorphism markers, L5 and P17. Bioinformatic analysis indicated that there are five genes within this interval, an ethylene-responsive OsER33 gene and four tandem repeats of cytochrome P450 genes designated as CYP81A5, CYP81A6, CYP81A7, and CYP81A8. Comparative sequencing could not find any differences in the coding regions of the OsER33, CYP81A5, CYP81A7, and CYP81A8 genes between the mutant bel and its wild-type progenitor W6154S, but did identify a single base guanine deletion at position +1,332 bp downstream from the translation start codon of CYP81A6. This deletion introduces a premature stop codon and leads to the loss of the heme-binding motif, which is essential for cytochrome P450 function because it contains an absolutely conserved cysteine that serves as the fifth ligand to the heme iron. CYP81A6 presumably functions as a hydroxylase for the detoxification of bentazon and sulfonylurea herbicides in rice. A gene-specific cleaved amplified polymorphic sequence marker and tightly linked flanking markers were developed that will be very useful for selection of the bel allele when transferred to photoperiod-/thermo-sensitive genic male sterility and CMS lines in hybrid rice breeding programs.     

Identification and characterisation of a cytochrome P450 gene and processed pseudogene from an arachnid: the cattle tick, Boophilus microplus.

We isolated and sequenced the first known cytochrome P450 gene and pseudogene from an arachnid, the cattle tick, Boophilus microplus. Both the gene and pseudogene belong to the family CYP4, but a new subfamily, CYP4W, had to be created for these genes because they are substantially different to other CYP4 genes. The gene, CYP4W1, has greatest homology with CYP4C1 from a cockroach, Blaberus discoidalis. The predicted molecular weight of the protein encoded by CYP4W1 (63 KDa) is greater than that of the other CYP4 genes. The pseudogene, CYP4W1P, is probably a processed pseudogene derived from the functional gene CYP4W1. This is only the third CYP processed pseudogene to be identified. The pseudogene is 98% identical to the functional gene, CYP4W1, therefore we hypothesise that this pseudogene evolved recently from the functional gene. The CYP4 genes from arthropods have diverged from each other more than those of mammals; consequently the phylogeny of the arthropod genes could not be resolved.     

Differential effects of phenobarbitone and 3-methylcholanthrene induction on the hepatic microsomal metabolism and cytochrome P-450-binding of phenoxazone and a homologous series of its n-alkyl ethers (alkoxyresorufins)

The metabolism and cytochrome P-450-binding of phenoxazone and a homologous series of its n-alkyl ethers (1-8C) was studied in hepatic microsomes of control, phenobarbitone-pretreated (PB) and 3-methylcholanthrene-pretreated (3MC) C57/BL10 mice. Phenoxazone and its ethers were hydroxylated and O-dealkylated respectively to a common metabolite, resorufin. The three categories of microsomes differed greatly in activity for the metabolism and binding of the various substrate homologues. The most rapidly metabolised substrates for control microsomes were phenoxazone and its shortest-chain ethers, for PB microsomes phenoxazone and the pentyl ether, and for 3MC microsomes the ethyl and propyl ethers. The variations in activity occurred in Vmax rather than in the apparent K_m-value. All the ethers gave Type I cytochrome P-450-binding spectra. The substrates giving the largest Type I spectra were the same for all microsomes—the ethyl, propyl and butyl ethers—but the magnitudes of the spectra differed in the order 3MC- > PB- > control microsomes. Phenoxazone and resorufin gave Modified Type II cytochrome P-450-binding spectra. PB-induction was most marked for the depentylation reaction (increased 101-fold), whereas 3MC-induction was most marked for depropylation and debutylation (88- and 96-fold).The intermicrosomal differences were interpreted as reflecting the different metabolic specificities of variant forms of cytochrome P-450. Substrate lipophilicity increased with increasing ether chain length and was not a major influence on specificity. The main substrate influence on specificity was steric, due to the presence and length of the ether side chain. The preeminent effect of ether chain length was considered to be on the rate of substrate transformation rather than on substrate interaction with cytochrome P-450.