Human cytochrome P450 3A4: enzymatic properties of a purified recombinant fusion protein containing NADPH-P450 reductase

Allosteric modulation of cytochrome P450 enzymes by the NADPH cytochrome P450 reductase FMN-containing domain

NADPH-cytochrome P450 reductase delivers electrons required by heme oxygenase, squalene monooxygenase, fatty acid desaturase, and 48 human cytochrome P450 enzymes. While conformational changes supporting reductase intramolecular electron transfer are well defined, intermolecular interactions with these targets are poorly understood, in part because of their transient association. Herein the reductase FMN domain responsible for interacting with targets was fused to the N-terminus of three drug-metabolizing and two steroidogenic cytochrome P450 enzymes to increase the probability of interaction. These artificial fusion enzymes were profiled for their ability to bind their respective substrates and inhibitors and to perform catalysis supported by cumene hydroperoxide. Comparisons with the isolated P450 enzymes revealed that even the oxidized FMN domain causes substantial and diverse effects on P450 function. The FMN domain could increase, decrease, or not affect total ligand binding and/or dissociation constants depending on both P450 enzyme and ligand. As examples, FMN domain fusion has no effect on inhibitor ketoconazole binding to CYP17A1 but substantially altered CYP21A2 binding of the same compound. FMN domain fusion to CYP21A2 resulted in differential effects dependent on whether the ligand was 17α-hydroxyprogesterone versus ketoconazole. Similar enzyme-specific effects were observed on steady-state kinetics. These observations are most consistent with FMN domain interacting with the proximal P450 surface to allosterically impact P450 ligand binding and metabolism separate from electron delivery. The variety of effects on different P450 enzymes and on the same P450 with different ligands suggests intricate and differential allosteric communication between the P450 active site and its proximal reductase-binding surface.

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.

Oxidative Stress in Takotsubo Syndrome-Is It Essential for an Acute Attack? Indirect Evidences Support Multisite Impact Including the Calcium Overload-Energy Failure Hypothesis

Indirect evidences in reviews and case reports on Takotsubo syndrome (TTS) support the fact that the existence of oxidative stress (OS) might be its common feature in the pre-acute stage. The sources of OS are exogenous (environmental factors including pharmacological and toxic influences) and endogenous, the combination of both may be present, and they are being discussed in detail. OS is associated with several pathological conditions representing TTS comorbidities and triggers. The dominant source of OS electrones are mitochondria. Our analysis of drug therapy related to acute TTS shows many interactions, e.g., cytostatics and glucocorticoids with mitochondrial cytochrome P450 and other enzymes important for OS. One of the most frequently discussed mechanisms in TTS is the effect of catecholamines on myocardium. Yet, their metabolic influence is neglected. OS is associated with the oxidation of catecholamines leading to the synthesis of their oxidized forms - aminochromes. Under pathological conditions, this pathway may dominate. There are evidences of interference between OS, catecholamine/aminochrome effects, their metabolism and antioxidant protection. The OS offensive may cause fast depletion of antioxidant protection including the homocystein-methionine system, whose activity decreases with age. The alteration of effector subcellular structures (mitochondria, sarco/endoplasmic reticulum) and subsequent changes in cellular energetics and calcium turnover may also occur and lead to the disruption of cellular function, including neurons and cardiomyocytes. On the organ level (nervous system and heart), neurocardiogenic stunning may occur. The effects of OS correspond to the effect of high doses of catecholamines in the experiment. Intensive OS might represent "conditio sine qua non" for this acute clinical condition. TTS might be significantly more complex pathology than currently perceived so far.

Advanced Understanding of the Electron Transfer Pathway of Cytochrome P450s

Cytochrome P450s are heme-thiolate enzymes that participate in carbon source assimilation, natural compound biosynthesis and xenobiotic metabolism in all kingdoms of life. P450s can catalyze various reactions by using a wide range of organic compounds, thus exhibiting great potential in biotechnological applications. The catalytic reactions of P450s are driven by electron equivalents that are sourced from pyridine nucleotides and delivered by cognate or matching redox partners (RPs). The electron transfer (ET) route from RPs to P450s involves one or more redox center-containing domains. As the rate of ET is one of the main determinants of P450 efficacy, an in-depth understanding of the P450 ET pathway should increase our knowledge of these important enzymes and benefit their further applications. Here, the various P450 RP systems along with current understanding of their ET routes will be reviewed. Notably, state-of-the-art structural studies of the two main types of self-sufficient P450 will also be summarized.

Chimeric cytochrome P450 3A4 used for in vitro prediction of food-drug interactions

Inhibition of cytochrome P450 (CYP)-mediated drug metabolism by dietary substances is the main cause of drug-food interactions in humans. The present study reports on the in vitro inhibition assays of human CYP3A4 genetically linked to the reductase domain of bacterial BM3 of Bacillus megaterium (BMR) resulting in the chimeric protein CYP3A4-BMR. The activity of this chimeric enzyme was initially measured colorimetrically with erythromycin as the substrate where K_M values similar to published data were determined. Subsequently, the inhibition assays with three different dietary products, grapefruit juice, curcumin, and resveratrol, were carried out with the chimeric enzyme both in solution and immobilized on electrode surfaces. For the solution studies, nicotinamide adenine dinucleotide phosphate was added as the electron donor, whereas the need for this cofactor was obviated in the immobilized enzyme as it was supplied by the electrode. Inhibition of the N-demethylation of erythromycin by CYP3A4-BMR chimera was measured at increasing concentrations of the different dietary compounds with calculated IC₅₀ values of 0.5%, 31 μM, and 250 μM for grapefruit juice, curcumin, and resveratrol measured in solution compared with 0.7%, 24 μM, and 208 μM measured electrochemically, respectively. These data demonstrate the feasibility of the use of both CYP3A4-BMR chimera as well as bioelectrochemistry for in vitro studies of not only drug-food interactions but also prediction of adverse drug reactions in this important P450 enzyme.

Effects of triazole fungicides on androgenic disruption and CYP3A4 enzyme activity

Triazole fungicides are widely used as broad-spectrum fungicides, non-steroidal antiestrogens and for various industrial applications. Their residues have been frequently detected in multiple environmental and human matrices. The increasingly reported toxicity incidents have led triazole fungicides as emerging contaminants of environmental and public health concern. However, whether triazole fungicides behave as endocrine disruptors by directly mimicking environmental androgens/antiandrogens or exerting potential androgenic disruption indirectly through the inhibition of cytochrome P450 (CYP450) enzyme activity is yet an unresolved question. We herein evaluated five commonly used triazole fungicides including bitertanol, hexaconazole, penconazole, tebuconazole and uniconazole for the androgenic and anti-androgenic activity using two-hybrid recombinant human androgen receptor (AR) yeast bioassay and comparatively evaluated their effects on enzymatic activity of CYP3A4 by P450-Glo™ CYP3A4 bioassay. All five fungicides showed moderate anti-androgenic activity toward human AR with the IC₅₀ ranging from 9.34 μM to 79.85 μM. The anti-androgenic activity remained no significant change after the metabolism mediated by human liver microsomes. These fungicides significantly inhibited the activity of CYP3A4 at the environmental relevant concentrations and the potency ranks as tebuconazole > uniconazole > hexaconazole > penconazole > bitertanol with the corresponding IC₅₀ of 0.81 μM, 0.93 μM, 1.27 μM, 2.22 μM, and 2.74 μM, respectively. We found that their anti-androgenic activity and the inhibition potency toward CYP3A4 inhibition was significantly correlated (R² between 0.83 and 0.97, p < 0.001). Our results indicated that the risk assessment of triazole pesticides and structurally similar chemicals should fully consider potential androgenic disrupting effects and the influences on the activity of CYP450s.

Nanoscale electron transport measurements of immobilized cytochrome P450 proteins

Gold nanopillars, functionalized with an organic self-assembled monolayer, can be used to measure the electrical conductance properties of immobilized proteins without aggregation. Measurements of the conductance of nanopillars with cytochrome P450 2C9 (CYP2C9) proteins using conducting probe atomic force microscopy demonstrate that a correlation exists between the energy barrier height between hopping sites and CYP2C9 metabolic activity. Measurements performed as a function of tip force indicate that, when subjected to a large force, the protein is more stable in the presence of a substrate. This agrees with the hypothesis that substrate entry into the active site helps to stabilize the enzyme. The relative distance between hopping sites also increases with increasing force, possibly because protein functional groups responsible for electron transport (ETp) depend on the structure of the protein. The inhibitor sulfaphenazole, in addition to the previously studied aniline, increased the barrier height for electron transfer and thereby makes CYP2C9 reduction more difficult and inhibits metabolism. This suggests that P450 Type II binders may decrease the ease of ETp processes in the enzyme, in addition to occupying the active site.

P-LinK: A method for generating multicomponent cytochrome P450 fusions with variable linker length

Fusion protein construction is a widely employed biochemical technique, especially when it comes to multi-component enzymes such as cytochrome P450s. Here we describe a novel method for generating fusion proteins with variable linker lengths, protein fusion with variable linker insertion (P-LinK), which was validated by fusing P450cin monooxygenase (CinA) to the flavodoxin shuttle protein (CinC). CinC was fused to the C terminus of CinA through a series of 16 amino acid linkers of different lengths in a single experiment employing 3 PCR amplifications. Screening for 2-β-hydroxy-1,8-cineole production by CinA-CinC fusion proteins revealed that enzymatically active variants possessed linker lengths of more than 5 amino acids, reaching optimum enzyme activity at a linker length of 10 amino acids. Our P-LinK method not only minimizes experimental effort and significantly reduces time demands but also requires only a single cloning and transformation step in order to generate multiple linker variants (1 to 16 amino acids long), making the approach technically simple and robust.

Studying cytochrome P450 kinetics in drug metabolism

BACKGROUND

Determination of cytochrome P450 enzyme-mediated kinetics in vitro can be useful for predicting drug dosing and clearance in humans. Expressed P450s, human liver microsomes, human hepatocytes (both fresh and cryopreserved), and human liver slices are used to estimate K(m) and V(max) values for determination of intrinsic clearance of the drug for scale-up to predict in vivo clearance.

OBJECTIVE

To describe the advantages and disadvantages of the various in vitro systems used to estimate kinetic parameters for disposition of drugs and the various kinetic profiles that can be observed.

METHODS

A review of the literature was conducted to evaluate the utility of the various in vitro preparations, the methods for determining kinetic parameters and the types of kinetic profiles that may be observed.

RESULTS/CONCLUSIONS

The choice of in vitro system for determining kinetic parameters will depend on the objective of the studies, as each system has advantages and disadvantages. Kinetic parameter determinations must be carefully assessed to assure that the correct kinetic model is applied and the most accurate kinetic parameters are determined.

Engineering and assaying of cytochrome P450 biocatalysts

Cytochrome P450s constitute a highly fascinating superfamily of enzymes which catalyze a broad range of reactions. They are essential for drug metabolism and promise industrial applications in biotechnology and biosensing. The constant search for cytochrome P450 enzymes with enhanced catalytic performances has generated a large body of research. This review will concentrate on two key aspects related to the identification and improvement of cytochrome P450 biocatalysts, namely the engineering and assaying of these enzymes. To this end, recent advances in cytochrome P450 development are reported and commonly used screening methods are surveyed.

Analysis of the interactions of cytochrome b5 with flavocytochrome P450 BM3 and its domains

Interactions between a soluble form of microsomal cytochrome b(5) (b(5)) from Musca domestica (housefly) and Bacillus megaterium flavocytochrome P450 BM3 and its component reductase (CPR), heme (P450) and FAD/NADPH-binding (FAD) domains were analyzed by a combination of steady-state and stopped-flow kinetics methods, and optical spectroscopy techniques. The high affinity binding of b(5) to P450 BM3 induced a low-spin to high-spin transition in the P450 heme iron (K(d) for b(5) binding = 0.44 microM and 0.72 microM for the heme domain and intact flavocytochrome, respectively). The b(5) had modest inhibitory effects on steady-state turnover of P450 BM3 with fatty acids, and the ferrous-carbon monoxy P450 complex was substantially stabilized on binding b(5). Single turnover reduction of b(5) by BM3 using stopped-flow absorption spectroscopy (k(lim) = 116 s(-1)) was substantially faster than steady-state reduction of b(5) by P450 BM3 (or its CPR and FAD domains), indicating rate-limiting step(s) other than BM3 flavin-to-b(5) heme electron transfer in the steady-state reaction. Steady-state b(5) reduction by P450 BM3 was considerably accelerated at high ionic strength. Pre-reduction of P450 BM3 by NADPH decreased the k(lim) for b(5) reduction approximately 10-fold, and also resulted in a lag phase in steady-state b(5) reduction that was likely due to BM3 conformational perturbations sensitive to the reduction state of the flavocytochrome. Ferrous b(5) could not reduce the ferric P450 BM3 heme domain under anaerobic conditions, consistent with heme iron reduction potentials of the two proteins. However, rapid oxidation of both hemoproteins occurred on aeration of the ferrous protein mixture (and despite the much slower autoxidation rate of b(5) in isolation), consistent with electron transfer occurring from b(5) to the oxyferrous P450 BM3 in the complex. The results demonstrate that strong interactions occur between a eukaryotic b(5) and a model prokaryotic P450. Binding of b(5) perturbs BM3 heme iron spin-state equilibrium, as is seen in many physiologically relevant b(5) interactions with eukaryotic P450s. These results are consistent with the conservation of structure of P450s (particularly at the heme proximal face) between prokaryotes and eukaryotes, and may point to as yet undiscovered roles for b(5)-like proteins in the control of activities of certain prokaryotic P450s.

Diversification of catalytic function in a synthetic family of chimeric cytochrome p450s

We report initial characterization of a synthetic family of more than 3000 cytochrome P450s made by SCHEMA recombination of 3 bacterial CYP102s. A total of 16 heme domains and their holoenzyme fusions with each of the 3 parental reductase domains were tested for activity on 11 different substrates. The results show that the chimeric enzymes have acquired significant functional diversity, including the ability to accept substrates not accepted by the parent enzymes. K-means clustering analysis of the activity data allowed the enzymes to be classified into five distinct groups based on substrate specificity. The substrates can also be grouped such that one can be a "surrogate" for others in the group. Fusion of a functional chimeric heme domain with a parental reductase domain always reconstituted a functional holoenzyme, indicating that key interdomain interactions are conserved upon reductase swapping.

Cytochrome P450--redox partner fusion enzymes

The cytochromes P450 (P450s) are a broad class of heme b-containing mono-oxygenase enzymes. The vast majority of P450s catalyse reductive scission of molecular oxygen using electrons usually derived from coenzymes (NADH and NADPH) and delivered from redox partner proteins. Evolutionary advantages may be gained by fusion of one or more redox partners to the P450 enzyme in terms of e.g. catalytic efficiency. This route was taken by the well characterized flavocytochrome P450(BM3) system (CYP102A1) from Bacillus megaterium, in which soluble P450 and cytochrome P450 reductase enzymes are covalently linked to produce a highly efficient electron transport system for oxygenation of fatty acids and related molecules. However, genome analysis and ongoing enzyme characterization has revealed that there are a number of other novel classes of P450-redox partner fusion enzymes distributed widely in prokaryotes and eukaryotes. This review examines our current state of knowledge of the diversity of these fusion proteins and explores their structural composition and evolutionary origins.

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.

Functional characterisation of an engineered multidomain human P450 2E1 by molecular Lego

The human cytochrome P450s constitute an important family of monooxygenase enzymes that carry out essential roles in the metabolism of endogenous compounds and foreign chemicals. We present here results of a fusion between a human P450 enzyme and a bacterial reductase that for the first time is shown does not require the addition of lipids or detergents to achieve wild-type-like activities. The fusion enzyme, P450 2E1-BMR, contains the N-terminally modified residues 22-493 of the human P450 2E1 fused at the C-terminus to residues 473-1049 of the P450 BM3 reductase (BMR). The P450 2E1-BMR enzyme is active, self-sufficient and presents the typical marker activities of the native human P450 2E1: the hydroxylation of p-nitrophenol (KM=1.84+/-0.09 mM and kcat of 2.98+/-0.04 nmol of p-nitrocatechol formed per minute per nanomole of P450) and chlorzoxazone (KM=0.65+/-0.08 mM and kcat of 0.95+/-0.10 nmol of 6-hydroxychlorzoxazone formed per minute per nanomole of P450). A 3D model of human P450 2E1 was generated to rationalise the functional data and to allow an analysis of the surface potentials. The distribution of charges on the model of P450 2E1 compared with that of the FMN domain of BMR provides the ground for the understanding of the interaction between the fused domains. The results point the way to successfully engineer a variety of catalytically self-sufficient human P450 enzymes for drug metabolism studies in solution.

A virus-directed enzyme prodrug therapy (VDEPT) strategy for lung cancer using a CYP2B6/NADPH-cytochrome P450 reductase fusion protein

Virus-directed enzyme prodrug therapy (VDEPT) is an emerging strategy against cancer. Our approach is a P450-based VDEPT that consists of using cyclophosphamide (CPA) as a prodrug and a Cytochrome P450 2B6/NADPH cytochrome P450 reductase fusion protein (CYP2B6/RED) as a prodrug-activating enzyme. Due to the heterogenous expression of proteins in tumor cells, basal reductase activity may not be sufficient to supply CYP2B6 with electrons, the fusion protein should enable the expression of both proteins at high levels in tumor cells. CYP/RED fusion proteins have never been previously expressed in mammalian cells, to enable expression the fusion protein was cloned into an adenoviral vector and subsequently several pulmonary tumor cell lines were infected. The CYP2B6/RED fusion protein was detected by Western blot, its mRNA by Northern blot, and its heme incorporation into an active form by spectral analysis. Infection with the fusion gene increased RED activity in microsomes by a factor of 3 compared to the control. After infection and treatment with CPA, in cell lines with low endogenous RED, the fusion protein mediated significantly higher CPA-induced cytotoxicity compared to cells expressing solely CYP2B6. In conclusion, the fusion protein is functional for VDEPT by providing one protein for higher levels of CPA metabolism.

Genetically engineered bacterial cells co-expressing human cytochrome P450 with NADPH-cytochrome P450 reductase: prediction of metabolism and toxicity of drugs in humans

Genetically engineered bacterial cells expressing human cytochrome P450 (CYP) have been developed as new tools to predict the metabolism and toxicity of drugs in humans. There are various host cells for the heterologous expression of a form of CYP. Among them, bacterial cells such as Escherichia coli (E. coli) have advantages with regard to ease of use and high yield of protein. CYP protein could be first expressed by the modification of the N-terminal amino acid sequence in E. coli cells in 1991. Since then, many forms of human CYP have been successfully expressed in E. coli cells. Since the E. coli cells do not possess endogeneous electron transport systems to support the full catalytic activity of CYP, E. coli strains co-expressing both human CYP and NADPH-cytochrome P450 reductase (OR) have been established. Each form of CYP expressed in the E. coli cells efficiently catalyzed the oxidation of a representative substrate at an efficient rate, indicating that the OR was sufficiently expressed to support the catalytic activity of CYP. According to the studies performed so far, the modification of the N-terminal amino acid sequence of CYP did not seem to affect the catalytic properties of CYP. The human CYP expressed in the E. coli cells were applicable for studies to determine a metabolic pathway(s) of drugs and to estimate kinetic parameters of drug metabolism by human CYP. Drug-drug interactions caused by inhibition of the metabolism of drugs by human CYP could also be examined by in vitro inhibition studies with CYP expressed in the E. coli cells. Recently, human CYP was co-expressed with the OR in Salmonella typhimurium (S. typhimurium) cells used for mutation assay (Ames test) by applying the technology for the expression of human CYP and the OR in E. coli cells, to evaluate whether chemicals including drugs are metabolically activated by human CYP and show mutagenicity. These strains of bacteria are considered as useful tools to study the metabolism and the toxicity of drugs in humans.

Optimizing bacterial expression of catalytically active human cytochromes P450: comparison of CYP2C8 and CYP2C9

1. Methods for the co-expression in Escherichia coli of human cytochrome P450 (CYP) 2C8 and CYP2C9 with NADPH-cytochrome P450 reductase (OxR) to produce a catalytically active system were compared. 2. Approaches assessed were expression of a CYP:OxR fusion construct, bicistronic plasmids, simultaneous transformation with CYP and OxR plasmids, and separate expression of CYP and OxR with reconstitution of activity by mixing the bacterial membranes. Two N-terminal modifications (Delta3-20 and 17alpha-leader) of the individual P450s were additionally investigated. 3. Each approach gave efficient expression of CYP2C8 and CYP2C9, but the bicistronic constructs under the expression conditions used gave low OxR expression and low catalytic activity. CYP expression was higher with the Delta3-20 construct for CYP2C9 and with the 17alpha-presequence construct for CYP2C8. 4. Using torsemide as substrate, all methods gave catalytically active systems with K(m) values similar to human liver microsomes. Mixing bacterial membranes containing separately expressed CYP and OxR reconstituted a catalytically active system with the Delta3-20 construct for CYP2C9 but not for CYP2C8, and with neither of the 17alpha- presequence constructs. OxR co-expressed with CYP in the same membrane interacted with CYP to reconstitute activity more effectively than addition of exogenous OxR membranes. 5. Expression construct and OxR co-expression strategy should be individualized for CYP isoforms.

METABOLISM OF ALFENTANIL BY CYTOCHROME P4503A (CYP3A) ENZYMES

The synthetic opioid alfentanil is an analgesic and an in vivo probe for hepatic and first-pass CYP3A activity. Alfentanil is a particularly useful CYP3A probe because pupil diameter change is a surrogate for plasma concentrations, thereby affording noninvasive assessment of CYP3A. Alfentanil undergoes extensive CYP3A4 metabolism via two major pathways, forming noralfentanil and N-phenylpropionamide. This investigation evaluated alfentanil metabolism in vitro to noralfentanil and N-phenylpropionamide, by expressed CYP3A5 and CYP3A7 in addition to CYP3A4, with and without coexpressed or exogenous cytochrome b(5). Effects of the CYP3A inhibitors troleandomycin and ketoconazole were also determined. Rates of noralfentanil and N-phenylpropionamide formation by CYP3A4 and 3A5 in the absence of b(5) were generally equivalent, although the metabolite formation ratio differed, whereas those by CYP3A7 were substantially less. CYP3A4 and 3A5 were equipotently inhibited by troleandomycin, whereas ketoconazole was an order of magnitude more potent toward CYP3A4. Cytochrome b(5) qualitatively and quantitatively altered alfentanil metabolism, with b(5) coexpression having a greater effect than exogenous addition. Addition or coexpression of b(5) markedly stimulated the formation of both metabolites and changed the formation of noralfentanil but not N-phenylpropionamide from apparent single-site to multisite Michaelis-Menten kinetics. These results demonstrate that alfentanil is a substrate for CYP3A5 in addition to CYP3A4, and the effects of the CYP3A inhibitors troleandomycin and ketoconazole are CYP3A enzyme-selective. Alfentanil is one of the few CYP3A substrates that is metabolized in vitro as avidly by both CYP3A4 and 3A5. Polymorphic CYP3A5 expression may contribute to inter-individual variability in alfentanil metabolism.

In vitro metabolism of fipronil by human and rat cytochrome P450 and its interactions with testosterone and diazepam

Fipronil (5-amino-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-[(trifluoromethyl)sulfinyl]-1H-pyrazole-3-carbonitrile) is a highly active, broad spectrum insecticide from the phenyl pyrazole family, which targets the gamma-amino butyric acid (GABA) receptor. Although fipronil is presently widely used as an insecticide and acaricide, little information is available with respect to its metabolic fate and disposition in mammals. This study was designed to investigate the in vitro human metabolism of fipronil and to examine possible metabolic interactions that fipronil may have with other substrates. Fipronil was incubated with human liver microsomes (HLM) and several recombinant cytochrome P450 (CYP) isoforms obtained from BD Biosciences. HPLC was used for metabolite identification and quantification. Fipronil sulfone was the predominant metabolite via CYP oxidation. The K(m) and V(max) values for human liver microsomes are 27.2 microM and 0.11 nmol/mg proteinmin, respectively; for rat liver microsomes (RLM) the K(m) and V(max) are 19.9 microM and 0.39 nmol/mg proteinmin, respectively. CYP3A4 is the major isoform responsible for fipronil oxidation in humans while CYP2C19 is considerably less active. Other human CYP isoforms have minimal or no activity toward fipronil. Co-expression of cytochrome b(5) (b(5)) is essential for CYP3A4 to manifest high activity toward fipronil. Ketoconazole, a specific inhibitor of CYP3A4, inhibits 78% of the HLM activity toward fipronil at a concentration of 2 microM. Oxidative activity toward fipronil in 19 single-donor HLMs correlated well with their ability to oxidize testosterone. The interactions of fipronil and other CYP3A4 substrates, such as testosterone and diazepam, were also investigated. Fipronil metabolism was activated by testosterone in HLM but not in CYP3A4 Supersomes. Testosterone 6beta-hydroxylation in HLM was inhibited by fipronil. Fipronil inhibited diazepam demethylation but had little effect on diazepam hydroxylation. The results suggest that fipronil has the potential to interact with a wide range of xenobiotics or endogenous chemicals that are CYP3A4 substrates and that fipronil may be a useful substrate for the characterization of CYP3A4 in HLM.

Effects of cytochrome b(5) on drug oxidation activities of human cytochrome P450 (CYP) 3As: similarity of CYP3A5 with CYP3A4 but not CYP3A7

Effects of cytochrome b(5) (b(5)) on catalytic activities of human cytochrome P450 (CYP) 3A5, CYP3A4, and CYP3A7 coexpressed with human NADPH-cytochrome P450 reductase in Escherichia coli membranes were investigated using 14 substrates. The activities of CYP3A5 were enhanced by addition of b(5) in approximately one third of the substrates employed in this study. Such enhancement by b(5) was roughly similar to that of CYP3A4, while the activities of CYP3A7 were not enhanced by b(5) with any substrates employed. V(max) values for midazolam 1'-hydroxylation and amitriptyline N-demethylation by CYP3A5 were increased about twice by addition of b(5), which was also seen with CYP3A4, although the extent of the effects of b(5) on S(50) (K(m)) and Hill coefficient differed dependent on substrates used. In contrast, b(5) did not alter any of these kinetic parameters of CYP3A7. The effects of b(5) on kinetic parameters of CYP3A5 were similar to those of CYP3A4 but not CYP3A7. These results suggest that roles of b(5) in drug oxidation activities of CYP3A5 and CYP3A4 are different from those of CYP3A7.

A novel sterol 14alpha-demethylase/ferredoxin fusion protein (MCCYP51FX) from Methylococcus capsulatus represents a new class of the cytochrome P450 superfamily

Sterol 14alpha-demethylase encoded by CYP51 is a member of the cytochrome P450 (CYP) superfamily of enzymes and has been shown to have an essential role in sterol biosynthesis in eukaryotes, with orthologues recently being described in some bacteria. Examination of the genome sequence data for the proteobacterium Methylococcus capsulatus, a bacterial species known to produce sterol, revealed the presence of a single CYP with strong homology to CYP51, particularly to a form in Mycobacterium tuberculosis. This M. capsulatus CYP51 protein represents a new class of CYP consisting of the CYP domain naturally fused to a ferredoxin domain at the C terminus via an alanine-rich linker. Expression of the M. capsulatus MCCYP51FX fusion in Escherichia coli yielded a P450, which, when purified to homogeneity, had the predicted molecular mass approximately 62 kDa on SDS/PAGE and bound lanosterol as a putative substrate. Sterol 14alpha-demethylase activity was shown (0.24 nmol of lanosterol metabolized per minute per nanomole of MCCYP51FX fusion) by gas chromatography/mass spectrometry with the activity dependent upon the presence of ferredoxin reductase and NADPH. Our unique findings describe a new class of naturally existing cytochrome P450, which will provide pivotal information for CYP structure/function in general.

Epoxidation of benzo[a]pyrene-7,8-dihydrodiol by human CYP1A1 in reconstituted membranes. Effects of charge and nonbilayer phase propensity of the membrane

Human cytochrome P4501A1 (CYP1A1) is one of the key enzymes in the bioactivation of environmental pollutants such as benzo[a]pyrene (B[a]P) and other polycyclic aromatic hydrocarbons. To evaluate the effect of membrane properties and distinct phospholipids on the activity of human CYP1A1 purified insect cell-expressed human CYP1A1 and of human NADPH-P450 reductase were reconstituted into phospholipid vesicle membranes. Conversion rates of up to 36 pmol x min(-1) x pmol(-1) CYP1A1 of the enantiomeric promutagens (-)- and (+)-trans-7,8-dihydroxy-7,8-dihydro-B[a]P (7,8-diol) to the genotoxic diolepoxides were achieved. The highest rates were obtained when negatively charged lipids such as phosphatidylserine and phosphatidylinositol and/or nonbilayer phospholipids such as phosphatidylethanolamine were present in the membrane together with neutral lipids. Both Vmax and Km values were changed. This suggests a rather complex mechanism of stimulation which might include altered substrate binding as well as more effective interaction between CYP1A1 and NADPH-P450 reductase. Furthermore, the ratio of r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydro-B[a]P (DE2) to r-7,t-8-dihydroxy-c-9,10-epoxy-7,8,9,10-tetrahydro-B[a]P (DE1) formed from (-)-7,8-diol was significantly increased by the introduction of anionic lipids, but not by that of nonbilayer lipids. Thus, charged lipids affect the stereoselectivity of the epoxidation by leading to the formation of a larger amount of the ultimate mutagen DE2 than of DE1, which is far less carcinogenic. These data suggest that membrane properties such as negative charge and nonbilayer phase propensity are important for the efficiency and selectivity of enzymatic function of human CYP1A1.

Expression, purification, and biochemical characterization of a human cytochrome P450 CYP2D6-NADPH cytochrome P450 reductase fusion protein

Cytochrome P450 CYP2D6 metabolizes a wide range of pharmaceutical compounds. A CYP2D6 fusion enzyme (CYP2D6F), containing an amino-terminal human CYP2D6 sequence and a carboxyterminal human NADPH-cytochrome P450 oxidoreductase (CPR) moiety, was constructed. High levels of expression were achieved in Escherichia coli (60-100 nmol/liter) and the enzyme was catalytically active with optimal activities achieved in the presence of the antioxidant, GSH. Turnover values for bufuralol 1'-hydroxylation, metoprolol alpha-hydroxylation, O-desmethylation, and dextromethorphan O-demethylation, using membranes expressing the fusion enzyme, were 5.6, 0.4, 0.72, and 6.19 min(-1), respectively. These values were similar to E. coli membranes which coexpressed human CYP2D6 and CPR (CYP2D6/R). The K(m) and k(cat) values for bufuralol metabolism were estimated to be 10.2 microM and 4.1 min(-1), respectively. The enzyme was purified using ion-exchange chromatography, affinity chromatography (2'-5' ADP-Sepharose), and gel filtration. Estimated turnover rates for bufuralol 1'-hydroxylation, metoprolol alpha-hydroxylation, O-desmethylation, and dextromethorphan O-demethylation were 1.2, 0.52, 0.79, and 0.76 min(-1), respectively. Bufuralol 1'-hydroxylase activity by purified CYP2D6F was enhanced by phospholipids and added CPR. The CYP2D6F enzyme was able to stimulate CYP3A4 testosterone 6beta-hydroxylase activity in a reconstitution system indicating that electron transfer may be largely intermolecular. The catalytically self-sufficient CYP2D6F enzyme will facilitate investigations of P450-CPR interactions and the development of new biocatalysts.

alpha-Naphthoflavone acts as activator and reversible or irreversible inhibitor of rabbit microsomal CYP3A6

This report describes the effect of alpha-naphthoflavone (alpha-NF), a known substrate, inhibitor and activator of several cytochromes P450 (CYP), on rabbit CYP3A6. Hepatic microsomes of rabbit pretreated with rifampicine (RIF), enriched with CYP3A6, as well as purified CYP3A6 reconstituted with isolated NADPH:CYP reductase were used as enzymatic systems in this study. The data from difference spectroscopy experiments showed that alpha-NF does yield a type I binding spectrum. This compound is oxidized by microsomal CYP3A6 into two metabolites (5,6-epoxide and trans-7,8-dihydrodiol). While alpha-NF is a substrate of CYP3A6, it also acts as an enzyme modulator. Under the conditions used, stimulation of 17beta-estradiol 2-hydroxylation by alpha-NF was observed. In contrast, this compound reversibly inhibited N-demethylation of erythromycin and tamoxifen, competitively with respect to these substrates, having the K(i) values of 51.5 and 18.0 microM, respectively. Moreover, alpha-NF was found to be an effective inactivator of progesterone and testosterone 6beta-hydroxylation catalyzed by CYP3A6 in RIF-microsomes. In addition, time- and concentration-dependent inactivation of human CYP3A4-mediated 6beta-hydroxylation of testosterone by alpha-NF, was determined. The inactivation of CYP3A6 followed pseudo-first-order kinetics and was dependent on both NADPH and alpha-NF. The concentrations required for half-maximal inactivation (K(i)) were 80.1 and 108.5 microM and the times required for half of the enzyme to be inactivated were 10.0 and 11.9 min for 6beta-hydroxylation of progesterone and testosterone, respectively. The loss of the enzyme activity was not recovered following dialysis, while 90% of the ability to form a reduced CO complex remained. This indicates the binding of alpha-NF to a CYP apoprotein molecule rather than to a heme moiety. Protection from inactivation was seen in the presence of all tested CYP3A substrates. Progesterone and testosterone protected CYP3A6 against inactivation competitively with respect to inactivator, erythromycin non-competitively and 17beta-estradiol showed a mixed type of protection. Here, we described for the first time that alpha-NF is capable of irreversible inhibition of microsomal rabbit CYP3A6 and human CYP3A4. The obtained results strongly suggest that the CYP3A active center contains at least two and probably three distinct binding sites for substrates.

Stimulation of cytochrome P450 reactions by apo-cytochrome b5: evidence against transfer of heme from cytochrome P450 3A4 to apo-cytochrome b5 or heme oxygenase

Many cytochrome P450 (P450)-dependent reactions have been shown to be stimulated by another microsomal protein, cytochrome b(5) (b(5)). Two major explanations are (i) direct electron transfer from b(5) and (ii) a conformational effect in the absence of electron transfer. Some P450s (e.g. 3A4, 2C9, 17A, and 4A7) are stimulated by either b(5) or b(5) devoid of heme (apo-b(5)), indicating a lack of electron transfer, whereas other P450s (e.g. 2E1) are stimulated by b(5) but not by apo-b(5). Recently, a proposal has been made by Guryev et al. (Biochemistry 40, 5018-5031, 2001) that the stimulation by apo-b(5) can be explained only by transfer of heme from P450 preparations to apo-b(5), enabling electron transfer. We have repeated earlier findings of stimulation of catalytic activity of testosterone 6beta-hydroxylation activities with four P450 preparations, in which nearly all of the heme was accounted for as P450. Spectral analysis of mixtures indicated that only approximately 5% of the heme can be transferred to apo-b(5), which cannot account for the observed stimulation. The presence of the heme scavenger apomyoglobin did not inhibit the stimulation of P450 3A4-dependent testosterone or nifedipine oxidation activity. Further evidence against the presence of loosely bound P450 3A4 heme was provided in experiments with apo-heme oxygenase, in which only 3% of the P450 heme was converted to biliverdin. Finally, b(5) supported NADH-b(5) reductase/P450 3A4-dependent testosterone 6beta-hydroxylation, but apo-b(5) did not. Thus, apo-b(5) can stimulate P450 3A4 reactions as well as b(5) in the absence of electron transfer, and heme transfer from P450 3A4 to apo-b(5) cannot be used to explain the catalytic stimulation.

Fused p47phox and p67phox truncations efficiently reconstitute NADPH oxidase with higher activity and stability than the individual components

Activation of the neutrophil NADPH oxidase occurs via assembly of the cytosolic regulatory proteins p47(phox), p67(phox), and Rac with the membrane-associated flavocytochrome b(558). Following cell-free activation, enzymatic activity is highly labile (Tamura, M., Takeshita, M., Curnutte, J. T., Uhlinger, D. J., and Lambeth, J. D. (1992) J. Biol. Chem. 267, 7529-7538). To try to stabilize the activity and investigate the nature of the complex, fusion proteins between p47N-(1-286) and p67N-(1-210) were constructed. In a cell-free system, a fusion protein, p67N-p47N, had an 8-fold higher efficiency and produced a higher activity than the individual proteins, and also resulted in an 8-fold improved efficiency for Rac and a lowered K(m) for NADPH. O(2) generating activity was remarkably stabilized by using p67N-p47N. The cytosolic proteins fused in the opposite orientation, p47N-p67N, showed similar activity and stability as individual proteins, but with a 4-fold improved efficiency compared with the individual cytosolic factors. In the system efficiency for Rac and affinity for NADPH were also higher than those with the nonfused components. Interestingly, the p67N-p47N showed nearly full activation in the absence of an anionic amphifile in a cell-free system containing cytochrome b(558) relipidated with phosphatidylinositol- or phosphatidylserine-enriched phospholipid mixtures. From the results we consider multiple roles of anionic amphifiles in a cell-free activation, which could be substituted by our system. The fact that a fusion produces a more stable complex indicates that interactions among components determine the longevity of the complex. Based on the findings we propose a model for the topology among p47N, p67N, and cytochrome b(558) in the active complex.

An enzymatically active chimeric protein containing the hydrophilic form of NADPH-cytochrome P450 reductase fused to the membrane-binding domain of cytochrome b5

The microsomal flavoprotein NADPH-cytochrome P450 reductase (CPR) contains an N-terminal hydrophobic membrane-binding domain required for reconstitution of hydroxylation activities with cytochrome P450s. In contrast, cytochrome b5 (b5) contains a C-terminal hydrophobic membrane-binding domain required for interaction with P450s. We have constructed, expressed and purified a chimeric flavoprotein (hdb5-CPR) where the C-terminal 45 amino acid residues of b5 have replaced the N-terminal 56 amino acid domain of CPR. This hybrid flavoprotein retains the catalytic properties of the native CPR and is able to reconstitute fatty acid and steroid hydroxylation activities with CYP4A1 and CYP17A. However hdb5-CPR is much less effective than CPR for reconstituting activity with CYP3A4. We conclude that differences on the surface of the P450s reflect unique and specific information essential for the recognition needed to establish reactions of intermolecular electron transfer from the flavoprotein CPR.

Expression, purification, and physical properties of recombinant flavocytochrome fusion proteins containing rat cytochrome b(5) linked to NADPH-cytochrome P450 reductase by different membrane-binding segments

Reconstitution of the enzymatic activities using purified microsomal cytochrome P450s (P450) requires the presence of a membrane-binding segment in the mammalian flavoprotein, NADPH--cytochrome P450 reductase (CPR), and the hemeprotein, cytochrome b(5) (b(5)). The mechanism(s) by which the membrane-binding segments of these proteins exert such a critical role in influencing the reconstitution of the NADPH-supported activity of a P450 remains undefined. In the present work we describe the construction, expression, and purification of four different types of recombinant flavocytochromes containing rat b(5) and rat CPR linked by various membrane-binding segments. The physical properties of these artificial fusion proteins have been studied to determine their ability to serve as electron transfer agents. These studies are a prelude to the subsequent study (accompanying paper) evaluating the functional roles of the hydrophobic (membrane-binding) sequences of b(5) and CPR in the reconstitution of P450 activities. The present study shows that the purified recombinant fusion proteins can serve as active electron transport carriers from NADPH to cytochrome c as well as b(5) by intramolecular as well as intermolecular reactions. It is shown here that the electron transport properties of these purified fusion proteins are influenced by high concentrations of KCl, suggesting a role for charged amino acids in protein-protein interactions. The present study illustrates the application of artificial recombinant flavocytochromes as useful proteins for the study of intramolecular electron transport reactions for comparison with intermolecular interactions.

Oral delivery of taxanes

Oral treatment with cytotoxic agents is to be preferred as this administration route is convenient to patients, reduces administration costs and facilitates the use of more chronic treatment regimens. For the taxanes paclitaxel and docetaxel, however, low oral bioavailability has limited development of treatment by the oral route. Preclinical studies with mdr1a P-glycoprotein knock-out mice, which lack functional P-glycoprotein activity in the gut, have shown significant bioavailability of orally administered paclitaxel. Additional studies in wild-type mice revealed good bioavailability after oral administration when paclitaxel was combined with P-glycoprotein blockers such as cyclosporin A or the structurally related compound SDZ PSC 833. Based on the extensive preclinical research, the feasibility of oral administration of paclitaxel and docetaxel in cancer patients was recently demonstrated in our Institute. Co-administration of cyclosporin A strongly enhanced the oral bioavailability of both paclitaxel and docetaxel. For docetaxel in combination with cyclosporin A an oral bioavailability of 90% was achieved with an interpatient variability similar to that after intravenous drug administration; for paclitaxel the oral bioavailability is estimated at approximately 50%. The safety of the oral route for both taxanes is good. A phase II study of weekly oral docetaxel in combination with cyclosporin A is currently ongoing.

Cooperativity of alpha-naphthoflavone in cytochrome P450 3A-dependent drug oxidation activities in hepatic and intestinal microsomes from mouse and human

1. The effects of several CYP3A substrates (alpha-naphthoflavone (alphaNF), terfenadine, midazolam, erythromycin) on nifedipine oxidation and testosterone 6beta-hydroxylation activities were investigated in hepatic and intestinal microsomes from mouse and human. 2. alphaNF (10 microM) and terfenadine (100 microM) inhibited nifedipine oxidation activities (at substrate concentration of 100 microM) in mouse hepatic microsomes to approximately 50%, but not in mouse intestinal microsomes. alphaNF (30 microM) stimulated nifedipine oxidation activities in mouse and human intestinal microsomes and in human hepatic microsomes to approximately 1.3-1.8-fold. Inhibitory potencies (50% inhibition concentration, IC50) of midazolam and erythromycin for nifedipine oxidations were calculated to be approximately 90 microM in human intestinal microsomes. In contrast, testosterone (100 microM) stimulated the nifedipine oxidation activities approximately 1.5-fold in hepatic and intestinal microsomes from mouse and human. 3. alphaNF showed different effects on the kinetic parameters including the Hill coefficients of nifedipine oxidation and testosterone 6beta-hydroxylation catalysed by hepatic and intestinal microsomes from mouse and human. Cooperativity in nifedipine oxidation was increased by the addition of alphaNF to pooled human hepatic microsomes, but little effects of alphaNF could be observed in individual human intestinal microsomes. 4. These results suggest that CYP3A enzymes in liver and intestine might have different characteristics and that observations from hepatic microsomes should not be directly applicable to intestine metabolism in some cases. Studies of drug-drug interactions of CYP3A substrates are recommended to be performed using intestinal samples.

The effect of different doses of cyclosporin A on the systemic exposure of orally administered paclitaxel

The objective of this study was to define the minimally effective dose of cyclosporin A (CsA) that would result in a maximal increase of the systemic exposure to oral paclitaxel. Six evaluable patients participated in this randomized cross-over study in which they received at two occasions two doses of 90 mg/m(2) oral paclitaxel 7 h apart in combination with 10 or 5 mg/kg CsA. Dose reduction of CsA from 10 to 5 mg/kg resulted in a statistically significant decrease in the area under the plasma concentration-time curve (AUC) and time above the threshold concentrations of 0.1 microM (T>0.1 microM) of oral paclitaxel. The mean (+/-SD) AUC and T>0.1 microM values of oral paclitaxel with CsA 10 mg/kg were 4.29+/-0.88 microM x h and 12.0+/-2.1 h, respectively. With CsA 5 mg/kg these values were 2.75+/-0.63 microM x h and 7.0+/-2.1 h, respectively (p=0.028 for both parameters). In conclusion, dose reduction of CsA from 10 to 5 mg/kg resulted in a significant decrease in the AUC and T>0.1 microM values of oral paclitaxel. Because CsA 10 mg/kg resulted in similar paclitaxel AUC and T>0.1 microM values compared to CsA 15 mg/kg (data which we have published previously), the minimally effective dose of CsA is determined at 10 mg/kg.

Co-administration of GF120918 significantly increases the systemic exposure to oral paclitaxel in cancer patients

Oral bioavailability of paclitaxel is very low, which is due to efficient transport of the drug by the intestinal drug efflux pump P-glycoprotein (P-gp). We have recently demonstrated that the oral bioavailability of paclitaxel can be increased at least 7-fold by co-administration of the P-gp blocker cyclosporin A (CsA). Now we tested the potent alternative orally applicable non-immunosuppressive P-gp blocker GF120918. Six patients received one course of oral paclitaxel of 120 mg/m(2)in combination with 1000 mg oral GF120918 (GG918, GW0918). Patients received intravenous (i.v.) paclitaxel 175 mg/m(2)as a 3-hour infusion during subsequent courses. The mean area under the plasma concentration-time curve (AUC) of paclitaxel after oral drug administration in combination with GF120918 was 3.27 +/- 1.67 microM x h. In our previously performed study of 120 mg/m(2)oral paclitaxel in combination with CsA the mean AUC of paclitaxel was 2.55 +/- 2.29 microM x h. After i.v. administration of paclitaxel the mean AUC was 15.92( )+/- 2.46 microM x h. The oral combination of paclitaxel with GF120918 was well tolerated. The increase in systemic exposure to paclitaxel in combination with GF120918 is of the same magnitude as in combination with CsA. GF120918 is a good and safe alternative for CsA and may enable chronic oral therapy with paclitaxel.

Protein engineering of cytochromes P-450

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

Recombinant in vitro tools to predict drug metabolism and safety

Drug metabolism determines several pharmacological and toxicological properties of pharmaceuticals and is catalysed by drug metabolizing enzymes. Prediction of drug metabolism in humans based on animal experiments is complicated by species differences in the catalytic properties of these enzymes. This review describes and evaluates the use of recombinant models that contain human drug metabolizing enzymes to facilitate the prediction of pharmacokinetic properties of candidate drugs in humans.

Metabolism of N,N-dialkylated amphetamines, including deprenyl, by CYP2D6 expressed in a human cell line

1. Five N,N-dialkylated amphetamines, N-methyl-N-propargylamphetamine (deprenyl; DEP), N-benzyl-N-methylamphetamine (benzphetamine; BPA), N-allyl-N-methylamphetamine (AMA), N,N-diallylamphetamine (DAA) and N-methyl-N-propylamphetamine (MPA), were metabolized in vitro with a microsomal preparation from cells expressing human CYP2D6 to determine what influence the N,N-dialkyl substituents had on the extent of N-dealkylation and/or aromatic ring oxidation. 2. The results obtained from experiments with the first two substrates, DEP and BPA, were surprisingly different. Whereas DEP was N-demethylated and N-depropargylated by the CYP2D6 enzyme system, no metabolites were formed from BPA. Subsequently, it was determined that AMA, DAA and MPA also underwent CYP2D6-catalysed N-dealkylation. Both N-methyl- and N-allylamphetamine were identified as products of AMA metabolism; similarly, metabolism of MPA produced both N-methyl- and N-propargylamphetamine, and N-allylamphetamine was the sole metabolite of DAA. 3. No N,N-didealkylated product (i.e. amphetamine) was isolated from incubates of any of the five substrates, and none of the N,N-dialkylated substrates was metabolized to a ring-hydroxylated product. 4. Rates of these CYP2D6-catalysed reactions were dependent on the nature and degree of unsaturation of the N-substituents.