A universal approach to the expression of human and rabbit cytochrome P450s of the 2C subfamily in Escherichia coli

Recombinant Technologies Facilitate Drug Metabolism, Pharmacokinetics, and General Biomedical Research

The development of safe and effective medications requires a profound understanding of their pharmacokinetic (PK) and pharmacodynamic properties. PK studies have been built through investigation of enzymes and transporters that drive drug absorption, distribution, metabolism, and excretion (ADME). Like many other disciplines, the study of ADME gene products and their functions has been revolutionized through the invention and widespread adoption of recombinant DNA technologies. Recombinant DNA technologies use expression vectors such as plasmids to achieve heterologous expression of a desired transgene in a specified host organism. This has enabled the purification of recombinant ADME gene products for functional and structural characterization, allowing investigators to elucidate their roles in drug metabolism and disposition. This strategy has also been used to offer recombinant or bioengineered RNA (BioRNA) agents to investigate the posttranscriptional regulation of ADME genes. Conventional research with small noncoding RNAs such as microRNAs (miRNAs) and small interfering RNAs has been dependent on synthetic RNA analogs that are known to carry a range of chemical modifications expected to improve stability and PK properties. Indeed, a novel transfer RNA fused pre-miRNA carrier-based bioengineering platform technology has been established to offer consistent and high-yield production of unparalleled BioRNA molecules from Escherichia coli fermentation. These BioRNAs are produced and processed inside living cells to better recapitulate the properties of natural RNAs, representing superior research tools to investigate regulatory mechanisms behind ADME. SIGNIFICANCE STATEMENT: This review article summarizes recombinant DNA technologies that have been an incredible boon in the study of drug metabolism and PK, providing investigators with powerful tools to express nearly any ADME gene products for functional and structural studies. It further overviews novel recombinant RNA technologies and discusses the utilities of bioengineered RNA agents for the investigation of ADME gene regulation and general biomedical research.

Opportunities for Accelerating Drug Discovery and Development by Using Engineered Drug-Metabolizing Enzymes

The study of drug metabolism is fundamental to drug discovery and development (DDD) since by mediating the clearance of most drugs, metabolic enzymes influence their bioavailability and duration of action. Biotransformation can also produce pharmacologically active or toxic products, which complicates the evaluation of the therapeutic benefit versus liability of potential drugs but also provides opportunities to explore the chemical space around a lead. The structures and relative abundance of metabolites are determined by the substrate and reaction specificity of biotransformation enzymes and their catalytic efficiency. Preclinical drug biotransformation studies are done to quantify in vitro intrinsic clearance to estimate likely in vivo pharmacokinetic parameters, to predict an appropriate dose, and to anticipate interindividual variability in response, including from drug-drug interactions. Such studies need to be done rapidly and cheaply, but native enzymes, especially in microsomes or hepatocytes, do not always produce the full complement of metabolites seen in extrahepatic tissues or preclinical test species. Furthermore, yields of metabolites are usually limiting. Engineered recombinant enzymes can make DDD more comprehensive and systematic. Additionally, as renewable, sustainable, and scalable resources, they can also be used for elegant chemoenzymatic, synthetic approaches to optimize or synthesize candidates as well as metabolites. Here, we will explore how these new tools can be used to enhance the speed and efficiency of DDD pipelines and provide a perspective on what will be possible in the future. The focus will be on cytochrome P450 enzymes to illustrate paradigms that can be extended in due course to other drug-metabolizing enzymes. SIGNIFICANCE STATEMENT: Protein engineering can generate enhanced versions of drug-metabolizing enzymes that are more stable, better suited to industrial conditions, and have altered catalytic activities, including catalyzing non-natural reactions on structurally complex lead candidates. When applied to drugs in development, libraries of engineered cytochrome P450 enzymes can accelerate the identification of active or toxic metabolites, help elucidate structure activity relationships, and, when combined with other synthetic approaches, provide access to novel structures by regio- and stereoselective functionalization of lead compounds.

Heterologous Expression of Recombinant Human Cytochrome P450 (CYP) in Escherichia coli: N-Terminal Modification, Expression, Isolation, Purification, and Reconstitution

Cytochrome P450 (CYP) enzymes play important roles in metabolising endogenous and xenobiotic substances. Characterisations of human CYP proteins have been advanced with the rapid development of molecular technology that allows heterologous expression of human CYPs. Among several hosts, bacteria systems such as Escherichia coli (E. coli) have been widely used thanks to their ease of use, high level of protein yields, and affordable maintenance costs. However, the levels of expression in E. coli reported in the literature sometimes differ significantly. This paper aims to review several contributing factors, including N-terminal modifications, co-expression with a chaperon, selections of vectors and E. coli strains, bacteria culture and protein expression conditions, bacteria membrane preparations, CYP protein solubilizations, CYP protein purifications, and reconstitution of CYP catalytic systems. The common factors that would most likely lead to high expression of CYPs were identified and summarised. Nevertheless, each factor may still require careful evaluation for individual CYP isoforms to achieve a maximal expression level and catalytic activity. Recombinant E. coli systems have been evidenced as a useful tool in obtaining the ideal level of human CYP proteins, which ultimately allows for subsequent characterisations of structures and functions.

Exploring optimal Taxol® CYP725A4 activity in Saccharomyces cerevisiae

Background

CYP725A4 catalyses the conversion of the first Taxol® precursor, taxadiene, to taxadiene-5α-ol (T5α-ol) and a range of other mono- and di-hydroxylated side products (oxygenated taxanes). Initially known to undergo a radical rebound mechanism, the recent studies have revealed that an intermediate epoxide mediates the formation of the main characterised products of the enzyme, being T5α-ol, 5(12)-oxa-3(11)-cyclotaxane (OCT) and its isomer, 5(11)-oxa-3(11)-cyclotaxane (iso-OCT) as well as taxadienediols. Besides the high side product: main product ratio and the low main product titre, CYP725A4 is also known for its slow enzymatic activity, massively hindering further progress in heterologous production of Taxol® precursors. Therefore, this study aimed to systematically explore the key parameters for improving the regioselectivity and activity of eukaryotic CYP725A4 enzyme in a whole-cell eukaryotic biocatalyst, Saccharomyces cerevisiae.

Results

Investigating the impact of CYP725A4 and reductase gene dosages along with construction of self-sufficient proteins with strong prokaryotic reductases showed that a potential uncoupling event accelerates the formation of oxygenated taxane products of this enzyme, particularly the side products OCT and iso-OCT. Due to the harmful effect of uncoupling products and the reactive metabolites on the enzyme, the impact of flavins and irons, existing as prosthetic groups in CYP725A4 and reductase, were examined in both their precursor and ready forms, and to investigate the changes in product distribution. We observed that the flavin adenine dinucleotide improved the diterpenoids titres and biomass accumulation. Hemin was found to decrease the titre of iso-OCT and T5α-ol, without impacting the side product OCT, suggesting the latter being the major product of CYP725A4. The interaction between this iron and the iron precursor, δ-Aminolevulinic acid, seemed to improve the production of these diterpenoids, further denoting that iso-OCT and T5α-ol were the later products. While no direct correlation between cellular-level oxidative stress and oxygenated taxanes was observed, investigating the impact of salt and antioxidant on CYP725A4 further showed the significant drop in OCT titre, highlighting the possibility of enzymatic-level uncoupling event and reactivity as the major mechanism behind the enzyme activity. To characterise the product spectrum and production capacity of CYP725A4 in the absence of cell growth, resting cell assays with optimal neutral pH revealed an array of novel diterpenoids along with higher quantities of characterised diterpenoids and independence of the oxygenated product spectra from the acidity effect. Besides reporting on the full product ranges of CYP725A4 in yeast for the first time, the highest total taxanes of around 361.4 ± 52.4 mg/L including 38.1 ± 8.4 mg/L of T5α-ol was produced herein at a small, 10-mL scale by resting cell assay, where the formation of some novel diterpenoids relied on the prior existence of other diterpenes/diterpenoids as shown by statistical analyses.

Conclusions

This study shows how rational strain engineering combined with an efficient design of experiment approach systematically uncovered the promoting effect of uncoupling for optimising the formation of the early oxygenated taxane precursors of Taxol®. The provided strategies can effectively accelerate the design of more efficient Taxol®-producing yeast strains.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01922-1.

Spectroscopic observations of β-eudesmol binding to human cytochrome P450 isoforms 3A4 and 1A2, but not to isoforms 2C9, 2C19 and 2D6

β-Eudesmol is a sesquiterpenoid component o Atractylodes lancea with cytotoxic activity against cholangiocarcinoma. Its lipophilic nature makes β-eudesmol a likely substrate of human cytochrome P450 (P450) enzymes.Using ligand-binding difference spectroscopy, the affinities of this compound to recombinant CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 were investigated in Escherichia coli membrane preparations.CYP3A4 showed a type I spectral change, with a binding constant Ks of 77 ± 23 (mean ± SD) μM at 0.5 μM P450 (Ks/[P450] ≈ 155). The reference substrate testosterone and the inhibitor fluconazole bound to the enzyme with apparent affinities of 86 ± 4 μM (type I) and 21 μM (type II), respectively. β-Eudesmol was bound to CYP3A4 in a non-cooperative manner (Hill coefficient n ≈ 0.8). CYP1A2 showed reverse type I difference spectra with either β-eudesmol or caffeine. The CYP1A2 affinity for β-eudesmol was higher (0.23 mM) than for caffeine (0.37 mM) but lower than for phenacetin (0.11 mM, type I). β-Eudesmol did not bind significantly to CYP2C9, CYP2C19, and CYP2D6.Confirmation of metabolic activity and studies on the involvement of other human P450 isoforms studies are required. Double-beam spectrometry is needed to validate Ks measurements made with a plate reader.

Insights into the Genetic Variations of Human Cytochrome P450 2C9: Structural Analysis, Characterization and Comparison

Cytochromes P450 (CYP) are one of the major xenobiotic metabolizing enzymes with increasing importance in pharmacogenetics. The CYP2C9 enzyme is responsible for the metabolism of a wide range of clinical drugs. More than sixty genetic variations have been identified in CYP2C9 with many demonstrating reduced activity compared to the wild-type (WT) enzyme. The CYP2C9*8 allele is predominantly found in persons of African ancestry and results in altered clearance of several drug substrates of CYP2C9. The X-ray crystal structure of CYP2C9*8, which represents an amino acid variation from arginine to histidine at position 150 (R150H), was solved in complex with losartan. The overall conformation of the CYP2C9*8-losartan complex was similar to the previously solved complex with wild type (WT) protein, but it differs in the occupancy of losartan. One molecule of losartan was bound in the active site and another on the surface in an identical orientation to that observed in the WT complex. However, unlike the WT structure, the losartan in the access channel was not observed in the *8 complex. Furthermore, isothermal titration calorimetry studies illustrated weaker binding of losartan to *8 compared to WT. Interestingly, the CYP2C9*8 interaction with losartan was not as weak as the CYP2C9*3 variant, which showed up to three-fold weaker average dissociation constant compared to the WT. Taken together, the structural and solution characterization yields insights into the similarities and differences of losartan binding to CYP2C9 variants and provides a useful framework for probing the role of amino acid substitution and substrate dependent activity.

A double-Flp-in method for stable overexpression of two genes

Overexpression of single genes in mammalian cells is widely used to investigate protein function in basic and applied biosciences and in drug research. A better understanding of interactions of two proteins is an important next step in the advancement of our understanding of complex biological systems. However, simultaneous and robust overexpression of two or more genes is challenging. The Flp-In system integrates a vector into cell lines at a specific genomic locus, but has not been used for integration of more than one gene. Here we present a modification of the Flp-In system that enables the simultaneous targeted integration of two genes. We describe the modification and generation of the vectors required and give the complete protocol for transfection and validation of correct genomic integration and expression. We also provide results on the stability and reproducibility, and we functionally validated this approach with a pharmacologically relevant combination of a membrane transporter facilitating drug uptake and an enzyme mediating drug metabolism.

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

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

Biophysical characterization of Aptenodytes forsteri cytochrome P450 aromatase

Cytochrome P450 19 (CYP19, aromatase) catalyzes the conversion of androgens to estrogens in a sequence of three reactions that each depend on NADPH and O2. Aromatase is a phylogenetically-ancient enzyme and its breadth of expression in other species has highlighted distinct physiological functions. In songbirds, estrogen production is required for programming the neural circuits controlling song and in the determination of sex in fish and reptiles. This work describes the expression, purification, and biophysical characterization of Aptenodytes forsteri (Emperor penguin, af) aromatase. Using human cytochrome P450 reductase as a redox partner, afCYP19 displayed similar substrate turnover and LC/MS/MS confirmed that afCYP19 catalyzes the transformations through the intermediates 19-hydroxy- and 19-oxo-androstenedione. Androstenedione and anastrozole had the highest affinity for the enzyme and were followed closely by 19-hydroxyandrostenedione and testosterone. The affinity of 19-oxo-androstenedione for afCYP19 was ten-fold lower. The time-dependent changes in the Soret bands observed in stopped-flow mixing experiments of the steroidal ligands and the inhibitor anastrozole with afCYP19 were best described by a two-step binding mechanism. In summary, these studies describe the first biophysical characterization of an avian aromatase that displays strikingly similar enzyme kinetics and ligand binding properties to the human enzyme and could serve as a convenient model system for studies of the enigmatic transformation of androgens to estrogens.

Heterologous expression and characterization of plant Taxadiene-5α-Hydroxylase (CYP725A4) in Escherichia coli

Taxadiene-5α-Hydroxylase (CYP725A4) is a membrane-bound plant cytochrome P450 that catalyzes the oxidation of taxadiene to taxadiene-5α-ol. This oxidation is a key step in the production of the valuable cancer therapeutic and natural plant product, taxol. In this work, we report the bacterial expression and purification of six different constructs of CYP725A4. All six of these constructs are N-terminally modified and three of them are fused to cytochrome P450 reductase to form a chimera construct. The construct with the highest yield of CYP725A4 protein was then selected for substrate binding and kinetic analysis. Taxadiene binding followed type-1 substrate patterns with an observed K_D of 2.1 ± 0.4 μM. CYP725A4 was further incorporated into nanoscale lipid bilayers (nanodiscs) and taxadiene metabolism was measured. Taxadiene metabolism followed Michaelis-Menten kinetics with an observed V_max of 30 ± 8 pmol/min/nmol_CYP725A4 and a K_M of 123 ± 52 μM. Additionally, molecular operating environment (MOE) modeling was performed in order to gain insight into the interactions of taxadiene with CYP725A4 active site. Taken together, we demonstrate the successful expression and purification of the functional membrane-bound plant CYP, CYP725A4, in E. coli.

Identification of a Novel Laser Dye Substrate of Mammalian Cytochromes P450: Application in Rapid Kinetic Analysis, Inhibitor Screening, and Directed Evolution

The author sought to develop a high-throughput activity screening assay to carry out rapid kinetic analysis, inhibitor screening, and directed evolution of cytochrome P450 2C enzymes. Initially, of the 9 fluorescent substrates and 10 P450 2C enzymes tested, several P450 2C enzymes showed > 1 nmol/min/nmol P450 activity in cumene hydroperoxide (CuOOH)—supported reaction with a laser dye, 7-dimethylamino-4-trifluoromethylcoumarin (C152). A high-throughput steady-state kinetic analysis of the human P450 2C8, 2C9, and 2C19 showed 1) kcat = 3 to 6 min—1, 2) Km, CuOOH = 100 to 200 µM, and 3) S50, C152 = 10 to 20 µM in the CuOOH system. In addition, P450 2C9 and 2C19 showed a very high kcat (27 and 38 min—1, respectively) in the nicotinamide adenine dinucleotide phosphate (NADPH)—supported reaction. Subsequently, when mammalian P450s from the other subfamilies were tested, P450 2B1dH, 2B4dH, 2B5dH, 3A4, and 3A5 exhibited a significant activity in both CuOOH and NADPH systems. Furthermore, a high-throughput activity screening assay using whole-cell suspensions of the human P450 2C8, 2C9, and 2C19 was optimized. Overall, the data suggested that C152 can be used as a model substrate for mammalian P450s in CuOOH-supported reaction to perform rapid kinetic analysis, inhibitor screening, and directed evolution. ( Journal of Biomolecular Screening 2007:677-682)

In Situ Proteolysis for Crystallization of Membrane Bound Cytochrome P450 17A1 and 17A2 Proteins from Zebrafish

Fish and human cytochrome P450 (P450) 17A1 catalyze both steroid 17α-hydroxylation and 17α,20-lyase reactions. Fish P450 17A2 catalyzes only 17α-hydroxylation. Both enzymes are microsomal-type P450s, integral membrane proteins that bind to the membrane through their N-terminal hydrophobic segment, the signal anchor sequence. The presence of this N-terminal region renders expression of full-length proteins challenging or impossible. For some proteins, variable truncation of the signal anchor sequence precludes expression or results in poor expression levels. To crystallize P450 17A1 and 17A2 in order to gain insight into their different activities, we used an alternative N-terminal sequence to boost expression together with in situ proteolysis. Key features of our approach to identify crystallizable P450 fragments were the use of an N-terminal leader sequence, a screen composed of 12 proteases to establish optimal cleavage, variations of protease concentration in combination with an SDS-PAGE assay, and analysis of the resulting fragments using Edman sequencing. Described in this unit are protocols for vector preparation, expression, purification, and in situ proteolytic crystallization of two membrane-bound P450 proteins.

Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase: STRUCTURE OF THE ENZYME·PROGESTERONE SUBSTRATE COMPLEX AND RATE-LIMITING C-H BOND CLEAVAGE

Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase, and deficiency of this enzyme is involved in ∼95% of cases of human congenital adrenal hyperplasia, a disorder of adrenal steroidogenesis. A structure of the bovine enzyme that we published previously (Zhao, B., Lei, L., Kagawa, N., Sundaramoorthy, M., Banerjee, S., Nagy, L. D., Guengerich, F. P., and Waterman, M. R. (2012) Three-dimensional structure of steroid 21-hydroxylase (cytochrome P450 21A2) with two substrates reveals locations of disease-associated variants. J. Biol. Chem. 287, 10613-10622), containing two molecules of the substrate 17α-hydroxyprogesterone, has been used as a template for understanding genetic deficiencies. We have now obtained a crystal structure of human P450 21A2 in complex with progesterone, a substrate in adrenal 21-hydroxylation. Substrate binding and release were fast for human P450 21A2 with both substrates, and pre-steady-state kinetics showed a partial burst but only with progesterone as substrate and not 17α-hydroxyprogesterone. High intermolecular non-competitive kinetic deuterium isotope effects on both kcat and kcat/Km, from 5 to 11, were observed with both substrates, indicative of rate-limiting C-H bond cleavage and suggesting that the juxtaposition of the C21 carbon in the active site is critical for efficient oxidation. The estimated rate of binding of the substrate progesterone (kon 2.4 × 10(7) M(-1) s(-1)) is only ∼2-fold greater than the catalytic efficiency (kcat/Km = 1.3 × 10(7) M(-1) s(-1)) with this substrate, suggesting that the rate of substrate binding may also be partially rate-limiting. The structure of the human P450 21A2-substrate complex provides direct insight into mechanistic effects of genetic variants.

Guidelines for development and implementation of biocatalytic P450 processes

Biocatalytic reactions performed by cytochrome P450 monooxygenases are interesting in pharmaceutical research since they are involved in human drug metabolism. Furthermore, they are potentially interesting as biocatalysts for synthetic chemistry because of the exquisite selectivity of the chemistry they undertake. For example, selective hydroxylation can be undertaken on a highly functionalized molecule without the need for functional group protection. Recent progress in the discovery of novel P450s as well as protein engineering of these enzymes strongly encourages further development of their application, including use in synthetic processes. The biological characteristics of P450s (e.g., cofactor dependence) motivate the use of whole-cell systems for synthetic processes, and those processes implemented in industry are so far dominated by growing cells and native host systems. However, for an economically feasible process, the expression of P450 systems in a heterologous host with sufficient biocatalyst yield (g/g cdw) for non-growing systems or space-time yield (g/L/h) for growing systems remains a major challenge. This review summarizes the opportunities to improve P450 whole-cell processes and strategies in order to apply and implement them in industrial processes, both from a biological and process perspective. Indeed, a combined approach of host selection and cell engineering, integrated with process engineering, is suggested as the most effective route to implementation.

Structural and kinetic basis of steroid 17α,20-lyase activity in teleost fish cytochrome P450 17A1 and its absence in cytochrome P450 17A2

Cytochrome P450 (P450) 17A enzymes play a critical role in the oxidation of the steroids progesterone (Prog) and pregnenolone (Preg) to glucocorticoids and androgens. In mammals, a single enzyme, P450 17A1, catalyzes both 17α-hydroxylation and a subsequent 17α,20-lyase reaction with both Prog and Preg. Teleost fish contain two 17A P450s; zebrafish P450 17A1 catalyzes both 17α-hydroxylation and lyase reactions with Prog and Preg, and P450 17A2 is more efficient in pregnenolone 17α-hydroxylation but does not catalyze the lyase reaction, even in the presence of cytochrome b5. P450 17A2 binds all substrates and products, although more loosely than P450 17A1. Pulse-chase and kinetic spectral experiments and modeling established that the two-step P450 17A1 Prog oxidation is more distributive than the Preg reaction, i.e. 17α-OH product dissociates more prior to the lyase step. The drug orteronel selectively blocked the lyase reaction of P450 17A1 but only in the case of Prog. X-ray crystal structures of zebrafish P450 17A1 and 17A2 were obtained with the ligand abiraterone and with Prog for P450 17A2. Comparison of the two fish P450 17A-abiraterone structures with human P450 17A1 (DeVore, N. M., and Scott, E. E. (2013) Nature 482, 116-119) showed only a few differences near the active site, despite only ∼50% identity among the three proteins. The P450 17A2 structure differed in four residues near the heme periphery. These residues may allow the proposed alternative ferric peroxide mechanism for the lyase reaction, or residues removed from the active site may allow conformations that lead to the lyase activity.

Functional studies of N-terminally modified CYP2J2 epoxygenase in model lipid bilayers

CYP2J2 epoxygenase is a membrane bound cytochrome P450 that converts omega-3 and omega-6 fatty acids into physiologically active epoxides. In this work, we present a comprehensive comparison of the effects of N-terminal modifications on the properties of CYP2J2 with respect to the activity of the protein in model lipid bilayers using Nanodiscs. We demonstrate that the complete truncation of the N-terminus changes the association of this protein with the E.coli membrane but does not disrupt incorporation in the lipid bilayers of Nanodiscs. Notably, the introduction of silent mutations at the N-terminus was used to express full length CYP2J2 in E. coli while maintaining wild-type functionality. We further show that lipid bilayers are essential for the productive use of NADPH for ebastine hydroxylation by CYP2J2. Taken together, it was determined that the presence of the N-terminus is not as critical as the presence of a membrane environment for efficient electron transfer from cytochrome P450 reductase to CYP2J2 for ebastine hydroxylation in Nanodiscs. This suggests that adopting the native-like conformation of CYP2J2 and cytochrome P450 reductase in lipid bilayers is essential for effective use of reducing equivalents from NADPH for ebastine hydroxylation.

Structural characterization of human cytochrome P450 2C19: active site differences between P450s 2C8, 2C9, and 2C19

To identify the structural features underlying the distinct substrate and inhibitor profiles of P450 2C19 relative to the closely related human enzymes, P450s 2C8 and 2C9, the atomic structure (Protein Data Bank code 4GQS) of cytochrome P450 2C19 complexed with the inhibitor (2-methyl-1-benzofuran-3-yl)-(4-hydroxy-3,5-dimethylphenyl)methanone (Protein Data Bank chemical component 0XV) was determined to 2.87 Å resolution by x-ray crystallography. The conformation of the peptide backbone of P450 2C19 is most similar to that of P450 2C8, but the substrate-binding cavity of P450 2C8 is much larger than that of P450 2C19 due to differences in the amino acid residues that form the substrate-binding cavities of the two enzymes. In contrast, the substrate-binding cavity of P450 2C19 is much more similar in size to that of the structure of the P450 2C9 flurbiprofen complex than to that of a modified P450 2C9 or that of P450 2C8. The cavities of the P450 2C19 0XV complex and the P450 2C9 flurbiprofen complex differ, however, because the helix B-C loops of the two enzymes are dissimilar. These conformational differences reflect the effects of adjacent structural elements that interact with the B-C loops and that differ between the two enzymes. The availability of a structure for 2C19 will facilitate computational approaches for predictions of substrate and inhibitor binding to this enzyme.

Simultaneous and sensitive LC-MS/MS determination of tetrahydrocannabinol and metabolites in human plasma

Cannabis is not only a widely used illicit drug but also a substance which can be used in pharmacological therapy because of its analgesic, antiemetic, and antispasmodic properties. A very rapid and sensitive method for determination of ∆(9)-tetrahydrocannabinol (THC), the principal active component of cannabis, and two of its phase I metabolites in plasma has been developed and validated. After solid-phase extraction of plasma (0.2 mL), the clean extracts were analyzed by tandem mass spectrometry after a 5-min liquid chromatographic separation. The linear calibration ranges were from 0.05 to 30 ng mL(-1) for THC and 11-nor-∆(9)-carboxy-tetrahydrocannabinol (THC-COOH) and from 0.2 to 30 ng mL(-1) for ∆(9)-(11-OH)-tetrahydrocannabinol (11-OH-THC). Imprecision and inaccuracy were always below 7 and 12 % (expressed as relative standard deviation and relative error), respectively. The method has been successfully applied to determination of the three analytes in plasma obtained from healthy volunteers after oral administration of 20 mg dronabinol.

In vitro assessment of the allelic variants of cytochrome P450

The cytochrome P450 (CYP) superfamily is one of the most important groups of enzymes involved in drug metabolism. It is responsible for the metabolism of a large number of drugs. Many CYP isoforms are expressed polymorphically, and catalytic alterations of allelic variant proteins can affect the metabolic activities of many drugs. The CYP2D6, CYP2C9, CYP2C19, and CYP2B6 genes are particularly polymorphic, whereas CYP1A1, CYP1A2, CYP2E1, and CYP3A4 are relatively well conserved without common functional polymorphisms. In vitro studies using cDNA expression systems are useful tools for evaluating functional alterations of the allelic variants of CYP, particularly for low-frequency alleles. Recombinant CYPs have been successfully expressed in bacteria, yeast, baculoviruses, and several mammalian cells. Determination of CYP variant-mediated kinetic parameters (Km and Vmax) in vitro can be useful for predicting drug dosing and clearance in humans. This review focuses on the advantages and disadvantages of the various cDNA-expression systems used to determine the kinetic parameters for CYP allelic variants, the methods for determining the kinetic parameters, and the findings of in vitro studies on highly polymorphic CYPs, including CYP2D6, CYP2C9, CYP2C19, and CYP2B6.

CYP2C8 exists as a dimer in natural membranes

CYP2C8 with a modified N-terminal sequence (2C8H) crystallizes as a dimer, but it is not known whether native CYP2C8 exists as a dimer in natural membranes. We have examined the organization of 2C8H and CYP2C8 expressed in bacterial membranes and mammalian endoplasmic reticulum membranes, respectively, by cysteine scanning and cross-linking or oxidation of sulfhydryl groups. In both forms of CYP2C8, cross-linked dimers were observed that were eliminated by mutation of Cys-24 in the linker region. Introduction of individual cysteines in the N-terminal 21-amino acid membrane-spanning signal anchor resulted in a pattern of cross-linking consistent with an α-helical structure for the signal anchor. In the linker region, cross-linking was observed for cysteine substituted at residues 22, 23, or 24, just before three Arg residues, indicating close apposition of the two linker sequences despite the neighboring positive charges. Introduction into the F-G loop region of cysteine pairs optimally located for cross-linking based on the crystal structure resulted in cross-linked dimers in the Cys-24 mutant. Deletion of the signal anchor sequence eliminated cross-linking mediated by Cys-24 or by cysteines introduced in the F-G loop regions, indicating that the signal anchor interaction is required for stable dimer formation. These results indicate that the signal anchor sequence and the F-G loop region form interfaces for CYP2C8 intermolecular interactions in natural membranes.

Pharmacogenetics of cytochrome P450 (CYP) in the elderly

The genetics of cytochrome P450 (CYP) is a very active area of multidisciplinary research, overlapping the interest of medicine, biology and pharmacology, being the CYP enzyme system responsible for the metabolism of more than 80% of the commercially available drugs. Variations in CYP encoding genes are responsible for inter-individual differences in CYP production or function, with severe clinical consequences as therapeutic failures (TFs) and adverse drug reactions (ADRs), being ADRs worldwide primary causes of morbidity and mortality in elderly people. In fact, the prevalence of both TFs and ADRs strongly increased in the presence of multiple pharmacological treatments, a common status in subjects aging 65 years and over. The present article explored some basic concepts of human genetics that have important implications in the genetics of CYP. An attempted to transfer these basic concepts to the genetic data reported by the Home Page of The Human Cytochrome P450 (CYP) Allele Nomenclature Committee was also made, focusing on the current knowledge of CYP genetics. The status of what we know and what we need to know is the base for the clinical applications of pharmacogenetics, in which personalized drug treatments constituted the main aim, in particular in patients attending a geriatric ward.

Microbial expression of alkaloid biosynthetic enzymes for characterization of their properties

A wide variety of secondary metabolites are produced in higher plants. These metabolites are synthesized in specific organs/cells at certain developmental stages and/or under specific environmental conditions. Since these biosynthetic activities are rather restricted and difficult to detect, the biochemical characterization of biosynthetic enzymes involved in secondary metabolism has been limited compared to those involved in primary metabolism. Recently, however, progress in tissue culture and molecular biology has made it easier to study biosynthetic enzymes. Here we describe protocols for expressing some biosynthetic enzymes in Escherichia coli expression systems, since this system is both efficient and cost-effective. First, we describe a standard system for expressing biosynthetic enzymes as a soluble protein under the T7 promoter of the pET expression system in E. coli. In addition, the successful expression of cytochrome P450 in E. coli in an active soluble form with N-terminal modification is discussed, since P450 is the critical enzyme in secondary metabolite biosynthesis.

Kinetic analysis of the three-step steroid aromatase reaction of human cytochrome P450 19A1

Cytochrome P450 19A1 (P450 19A1), the aromatase, catalyzes the conversion of androgens to estrogens through a sequential three-step reaction, generating 19-hydroxy and 19-aldehyde intermediates en route to the product estrogen. A procedure for the heterologous expression and purification of P450 19A1 in Escherichia coli was developed (k(cat) of 0.06 s(-1) for the conversion of androstenedione to estrone). Binding of the substrate and intermediates show low micromolar dissociation constants and are at least two-step processes. Rates of reduction of the iron were fast in the presence of substrate, either intermediate, or product. P450 19A1 is a distributive rather than a processive enzyme, with the sequential reaction allowing free dissociation of the intermediates as revealed by pulse-chase experiments. Conversion of androstenedione to estrone (under single turnover conditions) generated a progress curve showing changes in the concentrations of the substrate, intermediates, and product. A minimal kinetic model containing the individual rate constants for the steps in P450 19A1 catalysis was developed to globally fit the time course of the overall reaction, the dissociation constants, the two-step ligand binding, the distributive character, the iron-reduction rates, and the steady-state conversion of the 19-hydroxy androstenedione and 19-aldehyde androstenedione intermediates to estrone.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

mRNA distribution and heterologous expression of orphan cytochrome P450 20A1

Cytochrome P450 (P450) 20A1 is one of the so-called "orphan" P450s without assigned biological function. mRNA expression was detected in human liver, and extrahepatic expression was noted in several human brain regions, including substantia nigra, hippocampus, and amygdala, using conventional polymerase chain reaction and RNA dot blot analysis. Adult human liver contained 3-fold higher overall mRNA levels than whole brain, although specific regions (i.e., hippocampus and substantia nigra) exhibited higher mRNA expression levels than liver. Orthologous full-length and truncated transcripts of P450 20A1 were transcribed and sequenced from rat liver, heart, and brain. In rat, the concentrations of full-length transcripts were 3- to 4-fold higher in brain and heart than in liver. In situ hybridization of rat whole brain sections showed an mRNA expression pattern similar to that observed for human P450 20A1, indicating expression in substantia nigra, hippocampus, and amygdala. A number of N-terminal modifications of the codon-optimized human P450 20A1 sequence were prepared and expressed in Escherichia coli, and two of the truncated derivatives showed characteristic P450 spectra (200-280 nmol of P450/l). Although the recombinant enzyme system oxidized NADPH, no catalytic activity was observed with the heterologously expressed protein when a number of potential steroids and biogenic amines were surveyed as potential substrates. The function of P450 20A1 remains unknown; however, the sites of mRNA expression in human brain and the conservation among species may suggest possible neurophysiological function.

Determinants of cytochrome P450 2C8 substrate binding: structures of complexes with montelukast, troglitazone, felodipine, and 9-cis-retinoic acid

Although a crystal structure and a pharmacophore model are available for cytochrome P450 2C8, the role of protein flexibility and specific ligand-protein interactions that govern substrate binding are poorly understood. X-ray crystal structures of P450 2C8 complexed with montelukast (2.8 A), troglitazone (2.7 A), felodipine (2.3 A), and 9-cis-retinoic acid (2.6 A) were determined to examine ligand-protein interactions for these chemically diverse compounds. Montelukast is a relatively large anionic inhibitor that exhibits a tripartite structure and complements the size and shape of the active-site cavity. The inhibitor troglitazone occupies the upper portion of the active-site cavity, leaving a substantial part of the cavity unoccupied. The smaller neutral felodipine molecule is sequestered with its dichlorophenyl group positioned close to the heme iron, and water molecules fill the distal portion of the cavity. The structure of the 9-cis-retinoic acid complex reveals that two substrate molecules bind simultaneously in the active site of P450 2C8. A second molecule of 9-cis-retinoic acid is located above the proximal molecule and can restrain the position of the latter for more efficient oxygenation. Solution binding studies do not discriminate between cooperative and noncooperative models for multiple substrate binding. The complexes with structurally distinct ligands further demonstrate the conformational adaptability of active site-constituting residues, especially Arg-241, that can reorient in the active-site cavity to stabilize a negatively charged functional group and define two spatially distinct binding sites for anionic moieties of substrates.

Genetic polymorphism analysis of CYP2C19 in Chinese Han populations from different geographic areas of mainland China

Aims: Although many studies have been performed on CYP2C19, most of them have mainly examined the effects of the defective alleles CYP2C19*2 and CYP2C19*3. A comprehensive search for genetic polymorphisms of the CYP2C19 gene in the Chinese Han population has rarely been conducted. The present study was designed to determine the genetic basis of CYP2C19 polymorphisms. Materials & Methods: We investigated the 5´-regulatory region, all the exons and their surrounding introns of the CYP2C19 gene in 400 unrelated healthy Chinese Han volunteers from four different geographical locations, namely Shanghai, Shantou, Shenyang and Xi’an, with a sample of 100 subjects in each population, using direct sequencing. Results: A total of 14 different CYP2C19 polymorphisms, including one novel variant (-2306G>A) in the enhancer region and a novel nonsynonymous one (905C>G, T302R) were identified. In addition, CYP2C19*1, *2, *3, *15 and *17 alleles showed frequencies of 69.7%, 24.7%, 3.3%, 1.2% and 1.2%, respectively, and CYP2C19*15 was the first detected in an Asian population. The frequencies of the prevalent defective alleles CYP2C19*2 and CYP2C19*3 in Chinese Han populations are similar to those in other Asians, and much higher than those reported in American European and other Caucasian populations. Haplotype analysis demonstrated CATCGG was the dominating haplotype with a frequency of 38.6% in the Chinese Han population. Furthermore, homology modeling analysis for CYP2C19 indicates that Thr302Arg could cause the displacement of heme. Conclusion: This is the first study that systematically screened the polymorphisms of the whole CYP2C19 gene in a large Chinese Han population. The results suggest that a few low frequent variants show significant differences among the four populations, while the prevalent polymorphisms show no differences. Therefore, our database provides important information on CYP2C19 polymorphisms in the Chinese population, and could be helpful for future personalized medicine studies in Asian populations generally.

Important amino acid residues that confer CYP2C19 selective activity to CYP2C9

Although CYP2C9 and CYP2C19 display 91% sequence identity at the amino acid level, the two enzymes have distinct substrate specificities for compounds such as diclofenac, progesterone and (S)-mephenytoin. Amino acid substitutions in CYP2C9 were made based on an alignment of CYP2C9, CYP2C19 and monkey CYP2C43 sequences. Mutants of CYP2C9 were expressed in Escherichia coli. Sixteen amino acids, which are common to both CYP2C19 and CYP2C43 but different between CYP2C9 and CYP2C19, were substituted in CYP2C9 (CYP2C9-16aa). Next, the mutated amino acids in CYP2C9-16aa were individually reverted to those of CYP2C9 to examine the effect of each substitution on the enzymatic activity for CYP2C marker substrates. In addition, the role of the F-G loop in CYP2C9 and CYP2C19 was examined for substrate specificity and enzymatic activity. Our results showed: (i) CYP2C9-16aa displays 11% (S)-mephenytoin 4'-hydroxylase and full omeprazole 5-hydroxylase activity compared with that of CYP2C19; (ii) residue 286 is important for conferring CYP2C9-like enzyme activity on CYP2C9-16aa and residue 442 in CYP2C19 may be involved in the interaction with NADPH-P450 reductase; (iii) substitution of the F-G loop in CYP2C9 to that of CYP2C19 enhances tolbutamide p-methyhydroxylase and diclofenac 4'-hydroxylase activities and confers partial (S)-mephenytoin 4'-hydroxylase and omeprazole 5-hydroxylase activities, which are attributed to CYP2C19.

A shuffled CYP2C library with a high degree of structural integrity and functional versatility

Cytochrome P450 (CYP) enzymes involved in mammalian xenobiotic metabolism are attractive targets for the engineering of biocatalysts since they have broad and overlapping substrate and reaction substrate specificities. In this report, a library of chimeric mutants was prepared from CYP2C8, CYP2C9, CYP2C18 and CYP2C19 by DNA family shuffling. Twelve randomly selected clones were fully sequenced and showed 9+/-2 crossovers and 1.5+/-0.5 spontaneous mutations per approximately 1.5kbp open reading frame. CYP hemoprotein expression was observed in 50% (microaerobic culture) to 54% (aerobic culture) of clones. The functional diversity of the library was assessed using three luminogenic substrates, diclofenac and indole as probe substrates. A random sample of 26 clones revealed two clones with activity towards luciferin ME, one towards luciferin H and five towards diclofenac 4'-hydroxylation. One mutant showed activity towards all three substrates. Of 96 clones screened on solid media, one showed elevated indigo production compared to the parental forms. Turnover rates for luciferin ME and H metabolism by CYP2C9 and mutants were at least one order of magnitude higher in experiments with membranes compared to whole cells, consistent with impaired product egress from cells. Apparent K(m) values were increased in whole cell incubations with luciferin H suggesting impaired access of the substrate to the active site of the enzymes in whole cells. Finally screening with a panel of CYP2C ligands using CYP2C9 or active mutants revealed different patterns of inhibition and heteroactivation of metabolism of luciferin analogs.

Genotyping and haplotyping of CYP2C19 functional alleles on thin-film biosensor chips

OBJECTIVES

Numerous functional polymorphisms in the CYP2C19 gene have been identified; some alleles (e.g. CYP2C19*2 and CYP2C19*3) are associated with poor metabolism of CYP2C19 substrate drugs. Studies have found that the proportion of poor metabolizers, explained by CYP2C19*2 and CYP2C19*3, varies from less than 50% to more than 90% of poor metabolizers. Therefore, phenotype-genotype correlation studies should cover more than CYP2C19*2 and CYP2C19*3. A broader coverage, however, requires an easy-to-use and high-throughput genotyping platform. This broader coverage should also include the recently identified functional allele, CYP2C19*10, which involves a nucleotide change adjacent to the altered nucleotide change in CYP2C19*2. The currently used restriction fragment length polymorphism-based method for genotyping CYP2C19*2 cannot distinguish between CYP2C19*2 and CYP2C19*10. We aim to develop a simple platform that can genotype all CYP2C19 functional alleles.

METHODS

We have developed a thin-film biosensor chip platform to genotype 16 exonic CYP2C19 variants, including two sets of two adjacent single nucleotide polymorphisms and 12 single single nucleotide polymorphisms, using a ligation strategy.

RESULTS

We demonstrate that this is a rapid, accurate, and inexpensive method for genotyping CYP2C19 variants using individual's genomic DNA samples. We further demonstrate that this genotyping platform can be used to construct a haplotype structure of the CYP2C19 variants in a population, and to assign a haplotype combination to each individual on the basis of his/her genotype results.

CONCLUSION

This assay can be applied in pharmacogenomic studies in both basic research and clinical laboratories. It is also an ideal technology for pharmacogenomic tests in both developed and developing countries.

Molecular cloning and functional characterization of psoralen synthase, the first committed monooxygenase of furanocoumarin biosynthesis

Ammi majus L. accumulates linear furanocoumarins by cytochrome P450 (CYP)-dependent conversion of 6-prenylumbelliferone via (+)-marmesin to psoralen. Relevant activities, i.e. psoralen synthase, are induced rapidly from negligible background levels upon elicitation of A. majus cultures with transient maxima at 9-10 h and were recovered in labile microsomes. Expressed sequence tags were cloned from elicited Ammi cells by a nested DD-RT-PCR strategy with CYP-specific primers, and full-size cDNAs were generated from those fragments correlated in abundance with the induction profile of furanocoumarin-specific activities. One of these cDNAs representing a transcript of maximal abundance at 4 h of elicitation was assigned CYP71AJ1. Functional expression in Escherichia coli or yeast cells initially failed but was accomplished eventually in yeast cells after swapping the N-terminal membrane anchor domain with that of CYP73A1. The recombinant enzyme was identified as psoralen synthase with narrow substrate specificity for (+)-marmesin. Psoralen synthase catalyzes a unique carbon-chain cleavage reaction concomitantly releasing acetone by syn-elimination. Related plants, i.e. Heracleum mantegazzianum, are known to produce both linear and angular furanocoumarins by analogous conversion of 8-prenylumbelliferone via (+)-columbianetin to angelicin, and it was suggested that angelicin synthase has evolved from psoralen synthase. However, (+)-columbianetin failed as substrate but competitively inhibited psoralen synthase activity. Analogy modeling and docked solutions defined the conditions for high affinity substrate binding and predicted the minimal requirements to accommodate (+)-columbianetin in the active site cavity. The studies suggested that several point mutations are necessary to pave the road toward angelicin synthase evolution.

Engineering human cytochrome P450 enzymes into catalytically self-sufficient chimeras using molecular Lego

The membrane-bound human cytochrome P450s have essential roles in the metabolism of endogenous compounds and drugs. Presented here are the results on the construction and characterization of three fusion proteins containing the N-terminally modified human cytochrome P450s CYP2C9, CY2C19 and CYP3A4 fused to the soluble NADPH-dependent oxidoreductase domain of CYP102A1 from Bacillus megaterium. The constructs, CYP2C9/BMR, CYP2C19/BMR and CYP3A4/BMR are well expressed in Escherichia coli as holo proteins. The chimeras can be purified in the absence of detergent and the purified enzymes are both active and correctly folded in the absence of detergent, as demonstrated by circular dichroism and functional studies. Additionally, in comparison with the parent P450 enzyme, these chimeras have greatly improved solubility properties. The chimeras are catalytically self-sufficient and present turnover rates similar to those reported for the native enzymes in reconstituted systems, unlike previously reported mammalian cytochrome P450 fusion proteins. Furthermore the specific activities of these chimeras are not dependent on the enzyme concentration present in the reaction buffer and they do not require the addition of accessory proteins, detergents or phospholipids to be fully active. The solubility, catalytic self-sufficiency and wild-type like activities of these chimeras would greatly simplify the studies of cytochrome P450 mediated drug metabolism in solution.

Genetic variations and haplotypes of CYP2C19 in a Japanese population

Forty-eight single nucleotide variations, including 27 novel ones, were found in the 5'- regulatory region, all of the exons and their surrounding introns of CYP2C19 in 253 Japanese subjects (134 diabetic patients and 119 healthy volunteers). Identified novel variations were as follows: -2772G>A, 2767_-2760delGGTGAACA, -2720T>C, -2547delG, -2545G>T, -2545_-2544 delGC, and -2040C>T in the enhancer region; -778C>T, -777G>A, -529G>C, -189C>A, and -185A>G in the promoter region; 151A>G (S51G), 481G>C (A161P), 986G>A (R329H), 1078G>A (D360N), and 1119C>T (D373D) in the exons, and IVS1+128T>A, IVS3+163G>A, IVS4+271A>G, IVS5-49A>G, IVS6-210C>T, IVS6-196T>A, IVS6-32T>A, IVS7+84G>A, IVS7-174C>T, and IVS8+64C>T in the introns. Since we found no significant differences in the variation frequencies between healthy volunteers and diabetic patients, the data for all subjects were treated as one group in further analysis. The allele frequencies were 0.265 for IVS6-196T>A, 0.045 for -2772G>A and -2720T>C, 0.024 for -2040C>T, 0.014 for IVS7-174C>T, 0.010 for -529G>C, 0.006 for IVS1+128T>A and 481G>C (A161P), 0.004 for -2767_-2760delGGTGAACA and IVS6-210C>T, and 0.002 for the other 17 variations. In addition, the two known nonsynonymous single nucleotide polymorphisms, 681G>A (splicing defect, (*)2 allele) and 636G>A (W212X; (*)3 allele) were detected at 0.267 and 0.128 frequencies, respectively. No variation was detected in the known binding sites for constitutive androstane receptor and glucocorticoid receptor. Linkage disequilibrium analysis showed several close linkages of variations throughout the gene. By using the variations, thirty-one haplotypes of CYP2C19 and their frequencies were estimated. Our results would provide fundamental and useful information for genotyping CYP2C19 in the Japanese and probably other Asian populations.

Recombinant enzymes overexpressed in bacteria show broad catalytic specificity of human cytochrome P450 2W1 and limited activity of human cytochrome P450 2S1

Human cytochromes P450 2S1 and 2W1 have received only limited attention with regard to characterization of function. Both cytochromes P450 have been reported to be overexpressed in human tumors, and cytochrome P450 2S1 is induced by carcinogenic polycyclic hydrocarbons. We report methods for high-level expression and purification of both cytochromes P450 from Escherichia coli, with the goal of establishing function. The level of expression of human cytochrome P450 2W1 achieved using codon optimization for E. coli was 1800 nmol of cytochrome P450 per liter of culture, the highest level achieved in this laboratory to date. Assays with a number of the typical cytochrome P450 substrates showed no detectable activity, including some for which qualitative reports have appeared in the literature. Cytochrome P450 2W1 catalyzed benzphetamine N-demethylation (k(cat), 3.8/min) and arachidonic acid oxidation, albeit at a very low rate (approximately 0.05/min). In a umu genotoxicity screen, cytochrome P450 2W1 catalyzed the activation of several procarcinogens, particularly polycyclic hydrocarbon diols, but cytochrome P450 2S1 did not. The bioactivation of procarcinogens by cytochrome P450 2W1 may be of significance in the context of reports of preferential expression of the enzyme in tumors, in that activation of procarcinogens could lead to the accumulation of mutations and enhance the carcinogenic process.

Cynomolgus Monkey Cytochrome P450 2C43: cDNA Cloning, Heterologous Expression, Purification and Characterization

The cDNA of cytochrome P450 (CYP) 2C43 was cloned from cynomolgus monkey liver by RT-PCR. The deduced amino acid sequence showed 93% and 91% identity to human CYP2C9 and CYP2C19, respectively. The cDNA was expressed in Escherichia coli and purified by a series of chromatography steps, yielding a specific content of 11.5 nmol P450/mg protein. The substrate specificity of the purified CYP2C43 was examined in a reconstitution system comprising NADPH-P450 reductase, lipid, cytochrome b(5) and CYP2C marker substrates. The purified CYP2C43 showed high activity for testosterone 17-oxidation and progesterone 21-hydroxylation, which were also observed for CYP2C19 but not CYP2C9. In addition, CYP2C43 showed activity for (S)-mephenytoin 4'-hydroxylation, a marker reaction for CYP2C19. With CYP2C9 marker substrates, CYP2C43 exhibited low activity for diclofenac 4'-hydroxylation and no activity for tolbutamide p-methylhydroxylation. Therefore, in terms of substrate specificity, our results indicate that CYP2C43 is similar to CYP2C19, rather than CYP2C9.

P450 2C18 catalyzes the metabolic bioactivation of phenytoin

The safe clinical use of phenytoin (PHT) is compromised by a drug hypersensitivity reaction, hypothesized to be due to bioactivation of the drug to a protein-reactive metabolite. Previous studies have shown PHT is metabolized to the primary phenol metabolite, HPPH, then converted to a catechol which then autoxidizes to produce reactive quinone. PHT is known to be metabolized to HPPH by cytochromes P450 (P450s) 2C9 and 2C19 and then to the catechol by P450s 2C9, 2C19, 3A4, 3A5, and 3A7. However, the role of many poorly expressed or extrahepatic P450s in the metabolism and/or bioactivation of PHT is not known. The aim of this study was to assess the ability of other human P450s to catalyze PHT metabolism. P450 2C18 catalyzed the primary hydroxylation of PHT with a kcat (2.46 +/- 0.09 min-1) more than an order of magnitude higher than that of P450 2C9 (0.051 +/- 0.004 min-1) and P450 2C19 (0.054 +/- 0.002 min-1) and Km (45 +/- 5 microM) slightly greater than those of P450 2C9 (12 +/- 4 microM) and P450 2C19 (29 +/- 4 microM). P450 2C18 also efficiently catalyzed the secondary hydroxylation of PHT as well as covalent drug-protein adduct formation from both PHT and HPPH in vitro. While P450 2C18 is expressed poorly in the liver, significant expression has been reported in the skin. Thus, P450 2C18 may be important for the extrahepatic tissue-specific bioactivation of PHT in vivo.