7-Ethoxycoumarin O-deethylation catalyzed by cytochromes P450 1A2 and 2E1 in human liver microsomes

Metabolomics reveals the role of isopentenyl group in coumarins metabolism

Coumarins are a group of natural compounds commonly found in the families of Rutaceae and Umbelliferae. 7-Isopentenyloxycoumarin (ISC), auraptene (AUR), and umbelliprenin (UM) belong to prenyloxycoumarins (PYCs), which link isopentenyl, geranyl, and farnesyl group at C7 position, respectively. The substituent of 7-ethoxycoumarin (ETC) is the ethyl group. In this study, UPLC-ESI-QTOF-MS (ultra-performance liquid chromatography-electrospray ionization-quadrupole time of flight-MS)-based metabolomics was used to evaluate the in vivo and in vitro metabolism of PYCs. Results showed that ETC produced 10 known metabolites, and ISC was transformed into 17 metabolites in vivo and in vitro, which were undescribed compounds. A total of 35 AUR metabolites, including 34 undescribed metabolites were identified, and 21 metabolites were reported for the first time in UM. The results indicated that hydroxylation and N-acetylcysteine conjugation were the common metabolic reactions for PYCs. The metabolic rates of ETC, ISC, AUR and UM were 26%, 36%, 81%, and 38%, respectively, in human liver microsome, while they were 24%, 40%, 80%, and 37%, respectively, in mouse liver microsomes. In addition, recombinant cytochrome P450s (CYPs) screening showed that CYP1A1, 2C19, 3A4, and 3A5 were the major metabolic enzymes involved in the formation of hydroxylation metabolites. Together, these results suggest that the isopentenyl group plays an important role in the metabolism of PYCs.

Phe-125 and Phe-226 of pig cytochrome P450 1A2 stabilize the binding of aflatoxin B1 and 7-ethoxyresorufin through the key CH/π interactions

Cytochrome P450 1A2 (CYP1A2) plays important roles in the metabolism of many planar and aromatic drugs and also contributes to the bioactivation of aflatoxin B₁ (AFB₁) in vivo. To date, the structural basis for CYP1A2's preference to the planar substrates remains unclear. Herein, we investigated the structure-activity relationships for pig CYP1A2 catalyzing AFB₁ and 7-ethoxyresorufin (7-ER). A molecular docking study was performed based on a constructed model of pig CYP1A2, which predicted the contributions of Thr-118, Thr-124, Phe-125, Phe-226, Leu-260, and Asp-313 to the substrate catalysis. Site-directed mutagenesis and kinetic analyses exhibited the common grounds: Phe-125, Phe-226 and Asp-313 were vital to AFB₁ oxidation (including exo-epoxidation and 9A-hydroxylation) and ethoxyresorufin O-deethylation. Meanwhile, Phe-125 and Phe-226 formed CH/π interactions with AFB₁/7-ER, and Asp-313 formed hydrogen bonds with them. Based on other published reports, this study further emphasizes the critical roles of Phe-125 and Phe-226 in recognizing the planar substrates. Our findings highlight the structural basis of pig CYP1A2 specifically catalyzing AFB₁ and 7-ER, and may help to elucidate the underlying mechanism of CYP1A2's metabolic preference to the planar and aromatic substrates.

Coumarins and P450s, Studies Reported to-Date

Cytochrome P450 enzymes (CYPs) are important phase I enzymes involved in the metabolism of endogenous and xenobiotic compounds mainly through mono-oxygenation reactions into more polar and easier to excrete species. In addition to their role in detoxification, they play important roles in the biosynthesis of endogenous compounds and the bioactivation of xenobiotics. Coumarins, phytochemicals abundant in food and commonly used in fragrances and cosmetics, have been shown to interact with P450 enzymes as substrates and/or inhibitors. In this review, these interactions and their significance in pharmacology and toxicology are discussed in detail.

In vitro Drug Metabolism Investigation of 7-Ethoxycoumarin in Human, Monkey, Dog and Rat Hepatocytes by High Resolution LC-MS/MS

Background:

Recently, it has been an increasing concern on the bioactivation and adverse re-actions associated with consumption of herbal and nature products. 7-Ethoxycoumarin is one of coumarin family compounds, but little information is available regarding its potential reactive metabolites.

Method:

7-ethoxylcoumarin was incubated individually with human, monkey, dog and rat hepatocytes for 2 hr, metabolites were detected, identified and characterized using high resolution liquid chromagraphy – tandem mass spectrometry.

Results:

Twenty-eight metabolites (M1 - M28) were detected and identified. O-deethylation, glucuronida-tion, sulfation, oxygenation, oxidative ring-opening, hydrogenation, glutathionation, dehydrogenation, cysteination, glucosidation, methylation, and hydrolysis were observed. At least sixteen metabolites not reported previously, were newly identified. M1 (O-deethylation, mono-oxygenation and glucuronidation), M3 (O-deethylation and glucuronidation), M5 (hydrolysis and mono-oxygenation), M14 (O-deethylation), M16 (hydrolysis), M22 (oxidative ring-opening and oxygenation) and M27 (mono-oxygenation) exhibited high mass spectrometric responses in human hepatocytes. M3, M5, M8, M13 (mono-oxygenation), M14, M16, M18 (O-deethylation and sulfation), M22 and M27 exhibited high mass spectrometric responses in monkey hepatocytes. M14, M16, M18, M20 (glutathionation and dehy-drogenation) and M27 exhibited high mass spectrometric responses in dog hepatocytes. M1 (O-deethylation, mono-oxygenation and glucuronidation), M3, M5, M13, M14, M16, M17 (cysteination), M18, M20, and M22 exhibited high mass spectrometric responses in rat hepatocytes.

Conclusion:

Most of new metabolites via oxidative ring-opening and glutathionation were identified. Species differences in metabolism of 7-ethoxylcoumarin in hepatocytes were observed. The analysis of metabolites suggests that 7-ethoxylcoumarin may undergo 3,4-epoxidation responsible for formation of glutathione and its derived cysteine conjugates, carboxylic acid and its glucuronides, glucosides and sul-fate.

Mechanistic Understanding of the Synergistic Potential of Azole Fungicides in the Aquatic Invertebrate Gammarus pulex

Azole fungicides are known inhibitors of the important enzyme class cytochrome P450 monooxygenases (CYPs), thereby influencing the detoxification of co-occurring substances via biotransformation. This synergism in mixtures containing an azole has mostly been studied by effect measurements, while the underlying mechanism has been less well investigated. In this study, six azole fungicides (cyproconazole, epoxiconazole, ketoconazole, prochloraz, propiconazole, and tebuconazole) were selected to investigate their synergistic potential and their CYP inhibition strength in the aquatic invertebrate Gammarus pulex. The strobilurin fungicide azoxystrobin was chosen as co-occurring substrate, and the synergistic potential was measured in terms of internal concentrations of azoxystrobin and associated biotransformation products (BTPs). Azoxystrobin is biotransformed by various reactions, and 18 BTPs were identified. By measuring internal concentrations of azoxystrobin and its BTPs with high-resolution tandem mass spectrometry in the presence and absence of azole fungicides followed by toxicokinetic modeling, we showed that the inhibition of CYP-catalyzed biotransformation reactions indeed played a role for the observed synergism. However, synergism was only observed for prochloraz at environmentally realistic concentrations. Increased uptake rate constants, an increase in the total internal concentration of azoxystrobin and its BTPs, in vivo assays for measuring CYP activities, and G. pulex video-tracking suggested that the 2-fold increase in bioaccumulation, and, thereby, the raised toxicity of azoxystrobin in the presence of prochloraz is not only caused by inhibited biotransformation but even more by increased azoxystrobin uptake induced by hyperactivity.

Maintenance of drug metabolism and transport functions in human precision-cut liver slices during prolonged incubation for 5 days

Human precision-cut liver slices (hPCLS) are a valuable ex vivo model that can be used in acute toxicity studies. However, a rapid decline in metabolic enzyme activity limits their use in studies that require a prolonged xenobiotic exposure. The aim of the study was to extend the viability and function of hPCLS to 5 days of incubation. hPCLS were incubated in two media developed for long-term culture of hepatocytes, RegeneMed^®, and Cellartis^®, and in the standard medium WME. Maintenance of phase I and II metabolism was studied both on gene expression as well as functional level using a mixture of CYP isoform-specific substrates. Albumin synthesis, morphological integrity, and glycogen storage was assessed, and gene expression was studied by transcriptomic analysis using microarrays with a focus on genes involved in drug metabolism, transport and toxicity. The data show that hPCLS retain their viability and functionality during 5 days of incubation in Cellartis^® medium. Albumin synthesis as well as the activity and gene expression of phase I and II metabolic enzymes did not decline during 120-h incubation in Cellartis^® medium, with CYP2C9 activity as the only exception. Glycogen storage and morphological integrity were maintained. Moreover, gene expression changes in hPCLS during incubation were limited and mostly related to cytoskeleton remodeling, fibrosis, and moderate oxidative stress. The expression of genes involved in drug transport, which is an important factor in determining the intracellular xenobiotic exposure, was also unchanged. Therefore, we conclude that hPCLS cultured in Cellartis^® medium are a valuable human ex vivo model for toxicological and pharmacological studies that require prolonged xenobiotic exposure.

Mechanism-based inactivation of human cytochrome P450 1A2 and 3A4 isoenzymes by anti-tumor triazoloacridinone C-1305

1. 5-Dimethylaminopropylamino-8-hydroxytriazoloacridinone, C-1305, is a promising anti-tumor therapeutic agent with high activity against several experimental tumors. 2. It was determined to be a potent and selective inhibitor of liver microsomal and human recombinant cytochrome P450 (CYP) 1A2 and 3A4 isoenzymes. Therefore, C-1305 might modulate the effectiveness of other drugs used in multidrug therapy. 3. The objective of this study was to investigate the mechanism of the observed C-1305-mediated inactivation of CYP1A2 and CYP3A4. 4. Our findings indicated that C-1305 produced a time- and concentration-dependent decrease in 7-ethoxycoumarin O-deethylation (CYP1A2, K_I = 10.8 ± 2.14 μM) and testosterone 6β-hydroxylation (CYP3A4, K_I = 9.1 ± 2.82 μM). The inactivation required the presence of NADPH, was unaffected by a nucleophilic trapping agent (glutathione) and a reactive oxygen species scavenger (catalase), attenuated by a CYP-specific substrate (7-ethoxycoumarin or testosterone), and was not reversed by potassium ferricyanide. The estimated partition ratios of 1086 and 197 were calculated for the inactivation of CYP1A2 and CYP3A4, respectively. 5. In conclusion, C-1305 inhibited human recombinant CYP1A2 and CYP3A4 isoenzymes by mechanism-based inactivation. The obtained knowledge about specific interactions between C-1305 and/or its metabolites, and CYP isoforms would be useful for predicting the possible drug-drug interactions in potent multidrug therapy.

[Molecular organization of the microsomal oxidative system: a new connotation for an old term]

The central role that cytochromes P450 play in the metabolism of drugs and other xenobiotics makes these enzymes a major subject for studies of drug disposition, adverse drug effects and drug-drug interactions. Although there has been tremendous success in delineating P450 mechanisms, the concept of the drug-metabolizing ensemble as a functionally integrated system remains undeveloped. However, eukaryotic cells typically possess a multitude of different P450 enzymes that are co-localized in the membrane of endoplasmic reticulum (ER) and interact with each other with the formation of dynamic heteromeric complexes (mixed oligomers). Appreciation of the importance of developing an integral, systems approach to the ensemble of cytochromes P450 as an integral system inspired growing interest of researchers to the molecular organization of microsomal monooxygenase, which remained in the focus of research of academician Archakov for over 40 years. Fundamental studies carried out under his guidance have an important impact on our current concepts in this area. Further exploration of the molecular organization of the system of microsomal monooxygenase as an integral multienzyme and multifunctional system will have an essential impact on our understanding of the key factors that determine the changes in human drug metabolism and other P450-related functions in development, aging, and disease, as well as under influence of drugs, food ingredients, and environmental contaminants.

Functional characterization of CYP1A9 and CYP1C1 from Anguillus japonica

We evaluated the metabolism of several herbicides and progesterone by two P450 proteins (CYP1A9 and CYP1C1) from Japanese eel (Anguilla japonica). Expression vectors harboring CYP1A9 and CYP1C1 sequences were introduced into Escherichia coli. E. coli membrane fractions were incubated with each substrate, and the metabolites were analyzed. CYP1A9 and CYP1C1 deethylated 7-ethoxycoumarin and phenacetin, and demethylated chlorotoluron, diuron, and linuron. CYP1C1 specifically hydroxlyated progesterone at the 6β and 16α positions. Five amino acids of CYP1A9 related to substrate binding were selected for mutation analyses [CYP1A9(F128A), CYP1A9(F229A), CYP1A9(F263A), CYP1A9(V387A), and CYP1A9(I391A)]. Two variants, CYP1A9(F229A) and CYP1A9(F128A), changed the ratio of 16α hydroxyprogesterone to 6β hydroxyprogesterone. Among all the variants, CYP1A9(F263A) showed the highest activity towards substrates used. CYP1A9(V387A) and CYP1A9(I391A) showed higher activities than that of CYP1A9 toward progesterone. The substrate specificity of CYP1A9 may be altered by replacing an amino acid related to substrate binding.

Interactions among cytochromes P450 in microsomal membranes: oligomerization of cytochromes P450 3A4, 3A5, and 2E1 and its functional consequences

The body of evidence of physiologically relevant P450-P450 interactions in microsomal membranes continues to grow. Here we probe oligomerization of human CYP3A4, CYP3A5, and CYP2E1 in microsomal membranes. Using a technique based on luminescence resonance energy transfer, we demonstrate that all three proteins are subject to a concentration-dependent equilibrium between the monomeric and oligomeric states. We also observed the formation of mixed oligomers in CYP3A4/CYP3A5, CYP3A4/CYP2E1, and CYP3A5/CYP2E1 pairs and demonstrated that the association of either CYP3A4 or CYP3A5 with CYP2E1 causes activation of the latter enzyme. Earlier we hypothesized that the intersubunit interface in CYP3A4 oligomers is similar to that observed in the crystallographic dimers of some microsomal drug-metabolizing cytochromes P450 (Davydov, D. R., Davydova, N. Y., Sineva, E. V., Kufareva, I., and Halpert, J. R. (2013) Pivotal role of P450-P450 interactions in CYP3A4 allostery: the case of α-naphthoflavone. Biochem. J. 453, 219-230). Here we report the results of intermolecular cross-linking of CYP3A4 oligomers with thiol-reactive bifunctional reagents as well as the luminescence resonance energy transfer measurements of interprobe distances in the oligomers of labeled CYP3A4 single-cysteine mutants. The results provide compelling support for the physiological relevance of the dimer-specific peripheral ligand-binding site observed in certain CYP3A4 structures. According to our interpretation, these results reveal an important general mechanism that regulates the activity and substrate specificity of the cytochrome P450 ensemble through interactions between multiple P450 species. As a result of P450-P450 cross-talk, the catalytic properties of the cytochrome P450 ensemble cannot be predicted by simple summation of the properties of the individual P450 species.

The chemistry of small-molecule fluorogenic probes

Chemical fluorophores find wide use in biology to detect and visualize different phenomena. A key advantage of small-molecule dyes is the ability to construct compounds where fluorescence is activated by chemical or biochemical processes. Fluorogenic molecules, in which fluorescence is activated by enzymatic activity, light, or environmental changes, enable advanced bioassays and sophisticated imaging experiments. Here, we detail the collection of fluorophores and highlight both general strategies and unique approaches that are employed to control fluorescence using chemistry.

Construction of a CYP2E1-template system for prediction of the metabolism on both site and preference order

We have constructed an in silico system for the prediction of CYP2E1-mediated reaction using a two-dimensional template derived from substrate structures. Although CYP2E1 prefers small-size molecules for the substrates, the enzyme mediates oxidations of large-size molecules, such as benzo[a]pyrene. Overlays of these substrates, to assemble their sites of oxidation into a specific area, suggested a range of regions frequently occupied. The region, having a benzo[a]pyrene-like shape, was thus used as a CYP2E1 template. In this system, atoms in substrates, except for hydrogen atoms, were placed on corners of honeycomb structures of the template after having expanded the structures. Using published data for the metabolism on more than 80 substrates of CYP2E1, the core template was further refined to verify the adjacent area and to define the relative contribution of template positions for the catalysis. The positions on the template were classified into four different point (0-3) groups, depending on relative usage. In addition, we set independent points (-5 to 3) for specific positions to incorporate three-dimensional or functional information. Total scores from both position-occupancy and -function points were calculated for all the orientations of possible conformers of test substrates, and the scores were found to predict the relative abundance (i.e., order) as well as the regioselectivity of human CYP2E1 reactions with high fidelities.

Generation of proliferating human hepatocytes using Upcyte® technology: characterisation and applications in induction and cytotoxicity assays

1. We have developed a novel technique which causes primary human hepatocytes to proliferate by transducing them with genes that upregulate their proliferation. 2. Upcyte(®) hepatocytes did not form colonies in soft agar and are not immortalised anchorage-independent cells. Confluent cultures expressed liver-specific proteins, produced urea and stored glycogen. 3. CYP activities were low but similar to that in 5-day cultures of primary human hepatocytes. CYP1A2 and CYP3A4 were inducible; moreover, upcyte(®) hepatocytes predicted the in vivo induction potencies of known CYP3A4 inducers using the "relative induction score" prediction model. Placing cells into 3D culture increased their basal CYP2B6 and CYP3A4 basal activities and induction responses. 4. Phase 2 activities (UGTs, SULTs and GSTs) were comparable to activities in freshly isolated hepatocytes. 5. Upcyte(®) hepatocytes were markedly more sensitive to the hepatotoxin, α-amanitin, than HepG2 cells, indicating functional OATP1B3 uptake. The cytotoxicity of aflatoxin B(1), was decreased in upcyte(®) hepatocytes by co-incubation with the CYP3A4 inhibitor, ketoconazole. Upcyte(®) hepatocytes also differentiated between ten hepatotoxic and eight non-hepatotoxic compounds. 6. In conclusion, upcyte(®) hepatocyte cultures have a differentiated phenotype and exhibit functional phase 1 and 2 activities. These data support the use of upcyte(®) hepatocytes for CYP induction and cytotoxicity screening.

Unimolecular and bimolecular binding system for the prediction of CYP2D6-mediated metabolism

We have developed a template system for the prediction of CYP2D6-mediated metabolism of compounds. The system is composed of two types of two-dimensional templates (templates A and B), which were generated from mutually occupied areas of typical CYP2D6 substrates. The areas of templates are expressed as hexagonal blocks for application to the two-dimensional structures of chemicals. Experiments with 93 reactions with 69 typical substrates indicated the necessity for two similar but distinct shapes for template A (A1 and A2) for optimal placement. A frequently occupied area for substrates in template A1 was defined as a trigger area in which to capture a substrate for initiation of metabolism. Another frequently occupied area was found near the site of metabolism in template B. Both frequently occupied areas are linked to a pinching area. Occupancy of substrates on two template areas is suggested to be essential for the metabolism of CYPD6 substrates. In cases of CYP2D6 substrates without simultaneous occupancy of both areas, bimolecular placement, in which two molecules are placed coordinately, is proposed. Metabolism of small molecules, including naphthalene and quinoline, became explainable with the use of this idea. Validation of this template system with the use of both good and poor CYP2D6 substrates indicated clear advantages of the present system as a tool for drug modification, in addition to enabling highly accurate estimation of the site of metabolism.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

In vitro evaluation of hepatic and extra-hepatic metabolism of coumarins using rat subcellular fractions: correlation of in vitro clearance with in vivo data

7-Ethoxycoumarin (7-EC) and 7-hydroxycoumarin (7-HC) were chosen as model compounds to study hepatic and extra-hepatic metabolism in rat tissue subcellular (microsomal and S9) fractions and to scale the observed in vitro clearance to in vivo plasma clearance. 7-EC and 7-HC showed significant metabolic degradation in liver subcellular fractions as compared to subcellular fractions obtained from intestine, kidney, lung and brain. The total in vitro metabolic clearance for 7-EC and 7-HC was determined by adding the individual in vitro organ clearance values obtained in hepatic and extra-hepatic microsomes or S9 fractions. The predicted in vivo clearance for 7-HC was 63.6 and 81.6 ml/min/kg by in vitro scaling from microsomes and S9 fractions, respectively. For 7-EC, the values were 78.5 and 76.8 ml/min/kg, respectively. The predicted clearance was found to be reasonably accurate with slight over- and underprediction. Interestingly, the relative contribution of hepatic and extra-hepatic metabolism to the total clearance of 7-EC and 7-HC was remarkably high, ranging from 62-77% and 22-38%, respectively, of the total metabolic clearance. It is concluded that the model of multi-organ subcellular fractions is a useful in vitro tool for the prediction of in vivo metabolic clearance, as it can provide information about the relative contribution of extra-hepatic and hepatic metabolism to total metabolic clearance.

Applications of nanomaterials in electrochemical enzyme biosensors

A biosensor is defined as a kind of analytical device incorporating a biological material, a biologically derived material or a biomimic intimately associated with or integrated within a physicochemical transducer or transducing microsystem. Electrochemical biosensors incorporating enzymes with nanomaterials, which combine the recognition and catalytic properties of enzymes with the electronic properties of various nanomaterials, are new materials with synergistic properties originating from the components of the hybrid composites. Therefore, these systems have excellent prospects for interfacing biological recognition events through electronic signal transduction so as to design a new generation of bioelectronic devices with high sensitivity and stability. In this review, we describe approaches that involve nanomaterials in direct electrochemistry of redox proteins, especially our work on biosensor design immobilizing glucose oxidase (GOD), horseradish peroxidase (HRP), cytochrome P450 (CYP2B6), hemoglobin (Hb), glutamate dehydrogenase (GDH) and lactate dehydrogenase (LDH). The topics of the present review are the different functions of nanomaterials based on modification of electrode materials, as well as applications of electrochemical enzyme biosensors.

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.

In vitro analysis of a hepatic device with intrinsic microvascular-based channels

A novel microfluidics-based bilayer device with a discrete parenchymal chamber modeled upon hepatic organ architecture is described. The microfluidics network was designed using computational models to provide appropriate flow behavior based on physiological data from human microvasculature. Patterned silicon wafer molds were used to generate films with the vascular-based microfluidics network design and parenchymal chamber by soft lithography. The assembled device harbors hepatocytes behind a nanoporous membrane that permits transport of metabolites and small proteins while protecting them from the effects of shear stress. The device can sustain both human hepatoma cells and primary rat hepatocytes by continuous in vitro perfusion of medium, allowing proliferation and maintaining hepatic functions such as serum protein synthesis and metabolism. The design and fabrication processes are scalable, enabling the device concept to serve as both a platform technology for drug discovery and toxicity, and for the continuing development of an improved liver-assist device.

Kinetics of hepatic phase I and II biotransformation reactions in eight finfish species

Hepatic microsomes and cytosols of channel catfish (Ictalurus punctatus), rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), red tilapia (Oreochromis sp.), largemouth bass (Micropterussalmoides), striped bass (Morone saxatilis), hybrid striped bass (M. saxatilis x M. crysops), and bluegill (Lepomis macrochuris) (n=8) were used to study the kinetics of phase I (ECOD, EROD, PROD, BROD) and phase II (UDP-glucuronosyltransferase (UDPGT)-, sulfotransferase (ST)- and glutathione-s-transferase (GST)-mediated) reactions. The best catalytic efficiency for ECOD and GST activities was performed by channel catfish, Atlantic salmon, rainbow trout and tilapia. The highest EROD catalytic efficiency was for Atlantic salmon. None of the species had either PROD or BROD activities. Rainbow trout had very similar UDPGT catalytic efficiency to tilapia, channel catfish, Atlantic salmon, largemouth bass and bluegill. Sulfotransferase conjugation had no significant differences among the species. In summary, tilapia, channel catfish, Atlantic salmon and rainbow trout had the best biotransforming capabilities; striped bass, hybrid striped bass and bluegill were low metabolizers and largemouth bass shared some capabilities with both groups.

Genetic polymorphism in CYP2E1: Population distribution of CYP2E1 activity

Cytochrome P-450 2E1 (CYP2E1) is a key enzyme in the metabolic activation of a variety of toxicants including nitrosamines, benzene, vinyl chloride, and halogenated solvents such as trichloroethylene. CYP2E1 is also one of the enzymes that metabolizes ethanol to acetaldehyde, and is induced by recent ethanol ingestion. There is evidence that interindividual variability in the expression and functional activity of this cytochrome (CYP) may be considerable. Genetic polymorphisms in CYP2E1 were identified and linked to altered susceptibility to hepatic cirrhosis induced by ethanol and esophageal and other cancers in some epidemiological studies. Therefore, it is important to evaluate how such polymorphisms affect CYP2E1 function and whether it is possible to construct a population distribution of CYP2E1 activity based upon the known effects of these polymorphisms and their frequency in the population. This analysis is part of the genetic polymorphism database project described in the lead article in this series and followed the approach described in that article (Ginsberg et al., 2009, this issue). Review of the literature found that there are a variety of CYP2E1 variant alleles but the functional significance of these variants is still unclear. Some, but not all, studies suggest that several upstream 5' flanking mutations affect gene expression and response to inducers such as ethanol or obesity. None of the coding-region variants consistently affects enzyme function. Part of the reason for conflicting evidence regarding genotype effect on phenotype may be due to the wide variety of exposures such as ethanol or dietary factors and physiological factors including body weight or diabetes that modulate CYP2E1 expression. In conclusion, evidence is too limited to support the development of a population distribution of CYP2E1 enzyme activity based upon genotypes. Health risk assessments may best rely upon data reporting interindividual variability in CYP2E1 function for input into physiologically based pharmacokinetic (PBPK) models involving CYP2E1 substrates.

The metabolism of the 5HT3antagonists ondansetron, alosetron and GR87442 I: A comparison ofin vitroandin vivometabolism andin vitroenzyme kinetics in rat, dog and human hepatocytes, microsomes and recombinant human enzymes

The metabolism of the structurally related 5HT3 antagonists ondansetron, alosetron and GR87442 in the rat, dog and human was determined in hepatocytes, liver microsomes and human recombinant microsomes. The profiles of phase I metabolites were similar in human hepatocytes and microsomes. The metabolites of all three compounds produced in rat, dog and human microsomes and hepatocytes were similar to those seen in vivo, with the major routes of metabolism being N-dealkylation and/or hydroxylation. There was more extensive metabolic processing in hepatocytes than in microsomes; however, sequential metabolism was less extensive in vitro compared with in vivo. The pharmacokinetics of the three 5HT3 antagonists investigated were dominated by CYP3A4 (and/or 2C9) compared with CYP1A2 in man, possibly determined by enzyme capacity rather than relative enzyme affinity. These data support the use of rat, dog and human hepatocytes for the prediction of in vivo metabolites of ondansetron, alosetron and GR87442.

Evaluation of HepaRG cells as an in vitro model for human drug metabolism studies

HepaRG cells, a newly developed human hepatoma cell line, differentiate into hepatocyte-like morphology by treatment with dimethyl sulfoxide (DMSO). The expression of cytochrome P450 (P450) enzymes, transporter proteins, and transcription factors was stable in differentiated HepaRG cells over a period of 6 weeks when cultured with DMSO. Compared with human hepatocytes, expression of P450 in HepaRG cells was in general lower with the exception for a considerably higher expression of CYP3A4 and CYP7A1. The expression of P450s generally decreased when DMSO was removed from the medium, whereas transporters and liver-specific factors were unaffected. The relative mRNA content of drug-metabolizing P450s displayed the highest resemblance between human hepatocytes and differentiated HepaRG cells 1 day after removal of DMSO from the medium. The metabolism of midazolam, naloxone, and clozapine in HepaRG cells was similar to human hepatocytes, indicating the function of CYP3A4, CYP1A2, and UDP-glucuronosyltransferase enzymes. However, the metabolism of 7-ethoxycoumarin and dextromethorphan was low, confirming low levels of CYP2E1 and CYP2D6 in HepaRG cells. The P450 probe substrates indicate a decrease in CYP1A2, CYP2B6, CYP2C9, and CYP3A4 activities in HepaRG cells 1 day after removal of DMSO from the medium. The activities were then relatively stable in DMSO-free medium for up to 14 days. Based on the stable expression of liver-specific functions over a long period in culture, the relative mRNA content of drug-metabolizing P450s, and metabolic properties, HepaRG cells provide a valuable in vitro model for human drug metabolism studies.

Bioconversion by functional P450 1A9 and P450 1C1 of Anguilla japonica

We indicated that two P450s (1A9 and 1C1) from Japanese eel (Anguilla japonica) metabolized 7-ethoxycoumarin, 7-ethoxyresorufin, and flavanone. At first, we constructed expression vectors for two types of P450 (1A9 and 1C1). The reduced CO-difference spectra of Escherichia coli cells transformed with these plasmids showed Soret peaks (450 nm) that were typical of P450s. We performed bioconversion experiments in which substrates were added directly to incubation medium. The resulting metabolite(s) were extracted and analyzed by high-performance liquid chromatography and spectrofluorometer. Incubation of 50 nmol 7-ethoxyresorufin with P450 1C1 yielded 0.773 nmol of deethylated product, whereas 50 nmol 7-ethoxycoumarin resulted in 4.76 nmol. P450 1A9 metabolized 50 nmol of 7-ethoxyresorufin and 7-ethoxycoumarin to yield 6.54 and 20.9 nmol of deethylated product, respectively. Incubation of 50 nmol flavanone with P450 1C1 yielded 1.46 nmol and 0.69 nmol of products, whereas 50 nmol flavanone with P450 1A9 resulted in 1.10 nmol. In this system, 4'-hydroxy flavanones were formed by P450 1A9 and P450 1C1. P450 1A9 also metabolized 50 nmol of 17 beta-estradiol to yield 4.25 nmol of product. In this system, 2-hydroxy estradiol was formed by P450 1A9 using 17 beta-estradiol as a substrate. This study is the first to identify the substrates that P450 1C1 and 1A9 metabolize.

Bright ideas for chemical biology

Small-molecule fluorescent probes embody an essential facet of chemical biology. Although numerous compounds are known, the ensemble of fluorescent probes is based on a modest collection of modular "core" dyes. The elaboration of these dyes with diverse chemical moieties is enabling the precise interrogation of biochemical and biological systems. The importance of fluorescence-based technologies in chemical biology elicits a necessity to understand the major classes of small-molecule fluorophores. Here, we examine the chemical and photophysical properties of oft-used fluorophores and highlight classic and contemporary examples in which utility has been built upon these scaffolds.

Kinetics of electron transfer in the complex of cytochrome P450 3A4 with the flavin domain of cytochrome P450BM-3 as evidence of functional heterogeneity of the heme protein

We used a rapid scanning stop-flow technique to study the kinetics of reduction of cytochrome P450 3A4 (CYP3A4) by the flavin domain of cytochrome P450-BM3 (BMR), which was shown to form a stoichiometric complex (K(D)=0.48 microM) with CYP3A4. In the absence of substrates only about 50% of CYP3A4 was able to accept electrons from BMR. Whereas the high-spin fraction was completely reducible, the reducibility of the low-spin fraction did not exceed 42%. Among four substrates tested (testosterone, 1-pyrenebutanol, bromocriptine, or alpha-naphthoflavone (ANF)) only ANF is capable of increasing the reducibility of the low-spin fraction to 75%. Our results demonstrate that the pool of CYP3A4 is heterogeneous, and not all P450 is competent for electron transfer in the complex with reductase. The increase in the reducibility of the enzyme in the presence of ANF may represent an important element of the mechanism of action of this activator.

The metabolism of the 5HT3 antagonists, ondansetron, alosetron and GR87442 II: investigation into the in vitro methods used to predict the in vivo hepatic clearance of ondansetron, alosetron and GR87442 in the rat, dog and human

The in vitro clearances of the 5HT3 antagonists, ondansetron, alosetron and GR87442 were investigated. Intrinsic clearances using either metabolite formation or substrate depletion methods were equivalent (R2 = 0.95). Hepatocytes from preclinical species were superior to microsomes for the prediction of hepatic clearance (CL(H)), whereas the predictions from human microsomes and hepatocytes were similar. Using a non-restrictive model, seven of the nine CL(H) predictions using hepatocytes were within 2-fold of the in vivo CL(H) values. If the unbound fraction was included, the clearance of the compounds was generally under-predicted by both in vitro models. However, for the most metabolically stable compound, GR87442, the non-restrictive model over-predicted CLp. This and the possibility of extrahepatic metabolism indicate that the restrictive model is more appropriate for prediction of CL(H). The rank order of metabolic stability correlated with that in vivo. All three compounds were more metabolically stable in human than in the preclinical animal species examined.

Potential impact of steatosis on cytochrome P450 enzymes of human hepatocytes isolated from fatty liver grafts

Liver grafts discarded for transplantation because of macrosteatosis can constitute a valuable source of human hepatocytes for in vitro metabolic and pharmacotoxicological studies or for therapeutic applications. A condition for using hepatocyte suspensions for these purposes is the preservation of their metabolic competence and, particularly, drug-metabolizing enzymes. A reduction in microsomal cytochrome P450 (P450) activities was observed in fatty livers (>40% steatosis) with respect to normal tissue. Similarly, decreased levels of 7-ethoxycoumarin O-deethylation and testosterone metabolism were observed in human hepatocyte cultures prepared from steatotic liver tissue. To clarify the potential impact of lipid accumulation on human hepatic P450 enzymes, we have used an in vitro model of "cellular steatosis" by incubation of cultured hepatocytes with increasing concentrations (0.25-3 mM) of long-chain free fatty acids (FFA). A dose-dependent accumulation of lipids in the cytosol is induced by FFA mixture. Hepatocytes exposed to 1 mM FFA for 14 h showed lower activity values of CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1, and CYP3A4 enzymes than nontreated hepatocytes (about 45-65% reduction). This treatment also produced significant decreases in CYP1A2, CYP2A6, CYP2C9, CYP2D6, CYP2E1, and CYP3A4 mRNA to about 55 to 75% of mRNA levels in control cells. Our results suggest that although human hepatocytes isolated from steatotic liver show reduced P450 activities, they are metabolically competent and can be used for drug metabolism studies.

Cryopreservation of rat, dog and human hepatocytes: influence of preculture and cryoprotectants on recovery, cytochrome P450 activities and induction upon thawing

Several cryopreservation protocols for hepatocytes have been proposed over the past few years, but their effectiveness varies greatly as a function of the characteristics of the method used. One factor in the success of cryopreservation is the quality of cells before freezing. The results suggest that the cryopreservation of hepatocytes in a medium containing polyvinylpyrrolidone (PVP), in addition to DMSO, constitutes a convenient means of long-term storage of hepatocytes for preparing primary cultures to be used in drug metabolism studies. The combined use of the two cryoprotectants is particularly critical for low-viability cell suspensions. An interesting alternative to increase cell viability is the preculture of hepatocytes before cryopreservation. By the use of this procedure, high-quality cells, estimated in terms of post-thaw recovery, viability, adaptation of hepatocytes to culture, drug-metabolizing capability and cytochrome P450 induction, are obtained. Therefore, cryopreserved hepatocytes can provide a regular source of metabolically competent cells for in vitro investigations of the metabolic profile of new drugs and drug-drug interactions in pharmaco-toxicological research.

Kinetic deuterium isotope effects for 7-alkoxycoumarin O-dealkylation reactions catalyzed by human cytochromes P450 and in liver microsomes. Rate-limiting C-H bond breaking in cytochrome P450 1A2 substrate oxidation

7-Ethoxy (OEt) coumarin has been used as a model substrate in many cytochrome P450 (P450) studies, including the use of kinetic isotope effects to probe facets of P450 kinetics. P450s 1A2 and 2E1 are known to be the major catalysts of 7-OEt coumarin O-deethylation in human liver microsomes. Human P450 1A2 also catalyzed 3-hydroxylation of 7-methoxy (OMe) coumarin at appreciable rates but P450 2E1 did not. Intramolecular kinetic isotope effects were used as estimates of the intrinsic kinetic deuterium isotope effects for both 7-OMe and 7-OEt coumarin dealkylation reactions. The apparent intrinsic isotope effect for P450 1A2 (9.4 for O-demethylation, 6.1 for O-deethylation) showed little attenuation in other competitive and noncompetitive experiments. With P450 2E1, the intrinsic isotope effect (9.6 for O-demethylation, 6.1 for O-deethylation) was attenuated in the noncompetitive intermolecular experiments. High noncompetitive intermolecular kinetic isotope effects were seen for 7-OEt coumarin O-deethylation in a baculovirus-based microsomal system and five samples of human liver microsomes (7.3-8.1 for O-deethylation), consistent with the view that P450 1A2 is the most efficient P450 catalyzing this reaction in human liver microsomes and indicating that the C-H bond-breaking step makes a major contribution to the rate of this P450 (1A2) reaction. Thus, the rate-limiting step appears to be the chemistry of the breaking of this bond by the activated iron-oxygen complex, as opposed to steps involved in the generation of the reactive complex. The conclusion about the rate-limiting step applies to all of the systems studied with this model P450 1A2 reaction including human liver microsomes, the most physiologically relevant.

The use of sandwich-cultured rat hepatocytes to determine the intrinsic clearance of compounds with different extraction ratios: 7-ethoxycoumarin and warfarin

The application of sandwich-cultured rat hepatocytes for the identification of the hepatic intrinsic clearance of compounds with widely varying extraction ratios was investigated. We previously showed the applicability of this in vitro system, in combination with a model describing molecular diffusion, hepatocyte/medium partition, and nonsaturated metabolism, which resulted in a successful identification of this parameter for tolbutamide. This approach is further validated using the compounds 7-ethoxycoumarin and warfarin, covering a 100-fold range of extraction ratios. Clearance of these two substrates could be reliably determined, but only if the depletion of the parent compound in medium as well as in the hepatocyte sandwich was measured. Sensitivity analyses showed that the time course of depletion of the parent compound in medium, especially for warfarin, is insensitive to the partition and diffusion parameter values, whereas depletion in the hepatocyte sandwich was far more sensitive. When varying the volumes of collagen in the sandwich culture, it appears that the most reliable kinetic parameters could be obtained by fitting the data with the smaller collagen volume and that these parameters obtained from fitting to data of the larger volumes generally cannot be verified satisfactorily with the data of the smaller volumes. The values of hepatic clearance that were obtained after extrapolation of the intrinsic clearance to the hepatic clearance from blood were comparable within a factor of 2 to hepatic clearance data in the literature. This indicates that this sandwich culture and modeling system can be applied for the identification of the hepatic intrinsic clearance rate of the total range from low to high clearance compounds.

Use of a cocktail of probe substrates for drug-metabolizing enzymes for the assessment of the metabolic capacity of hepatocyte preparations

A cocktail of the following probe substrates for human drug-metabolizing enzymes was used to characterize hepatocyte preparations: phenacetin (for CYP1A2), diclofenac (CYP2C9), diazepam (CYP2C19), bufuralol (CYP2D6), midazolam (CYP3A4/5) and 7-hydroxycoumarin (for glucuronidation and sulphation). The cocktail was incubated with cryopreserved human, dog or minipig hepatocytes or with freshly prepared rat hepatocytes. Sample analysis was performed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in an Open Access environment that allowed less experienced MS operators to login, submit and analyse sample sets using predefined settings without the immediate attendance of an experienced analyst. Intrinsic clearances (CLint) were calculated from the disappearance of the compounds from the incubations. Initially, the cocktail used for human, rat and dog hepatocyte incubations contained 7-ethoxycoumarin instead of 7-hydroxycoumarin. However, 7-ethoxycoumarin had an inhibitory effect on the metabolism of phenacetin. The highest CLint estimated with human and dog hepatocytes was observed for 7-hydroxycoumarin. For rat and minipig hepatocytes, the highest CLint was observed for bufuralol. In incubations with dog and minipig hepatocytes, the lowest CLint was seen with diclofenac, whereas for human and rat hepatocytes, the lowest value was observed with diazepam and phenacetin, respectively. When the cocktail was incubated together with human hepatocytes and 1 microM ketoconazole, the CLint of midazolam was decreased to about 7.5% of the control value, whereas the metabolism of the other cocktail compounds was virtually unaffected by this CYP3A inhibitor. It is suggested that a cocktail of specific human probe substrates for drug-metabolizing enzymes can be used routinely for the determination of the metabolic capacity of hepatocyte preparations in order to ensure the quality and reproducibility of experiments. Moreover, a cocktail of specific probe substrates can also be a useful tool for studies on enzyme inhibition.

Spontaneous apoptosis, necrosis, energy status, glutathione levels and biotransformation capacities of isolated rat hepatocytes in suspension: effect of the incubation medium

Isolated hepatocytes in suspension express most of the functional activities of the intact liver and offer an easy-to-handle in vitro system for investigating both the biotransformation and damaging effects induced after a single exposure to xenobiotics upto 3-4h. There is, however, a general lack of consensus with respect to the choice of a suitable suspension medium. This motivated us to perform a comparative study of the effects of five frequently used bicarbonate-based media (Ca(2+)-containing Krebs-Henseleit buffer (KHB) with or without 25mM HEPES, 10mM glucose and 2% (g/v) BSA supplements, and Williams' E culture medium) on the viability (LDH leakage, caspase-3 processing and activity, Bid/Bax expression) and functionality (energy status, glutathione content, phases I and II biotransformation) of freshly isolated rat hepatocytes in suspension upto 3h. Also included was the bicarbonate-free HEPES buffer that does not require carbogen gassing, and is therefore handled more easily. The results clearly demonstrated that the type of incubation medium profoundly affected the functionality of the suspended hepatocytes, changing their sensitivity and response to exogenous damaging effects. While HEPES buffer and Williams' E medium offered the lowest background of spontaneous cell death, bicarbonate-based buffers and media seemed more suitable for obtaining both phases I and II biotransformation. Williams' E medium ensured a constant glutathione content of the cells and a lower level of oxidative stress.

Optimization of 3-D hepatocyte culture by controlling the physical and chemical properties of the extra-cellular matrices

Hepatocytes are anchorage-dependent cells sensitive to microenvironment; the control of the physicochemical properties of the extra-cellular matrices may be useful to the maintenance of hepatocyte functions in vitro for various applications. In a microcapsule-based 3-D hepatocyte culture microenvironment, we could control the physical properties of the collagen nano-fibres by fine-tuning the complex-coacervation reaction between methylated collagen and terpolymer of hydroxylethyl methacrylate-methyl methacrylate-methylacrylic acid. The physical properties of the nano-fibres were quantitatively characterized using back-scattering confocal microscopy to help optimize the physical support for hepatocyte functions. We further enhanced the chemical properties of the collagen nano-fibres by incorporating galactose onto collagen, which can specifically interact with the asialoglycoprotein receptor on hepatocytes. By correlating a range of collagen nano-fibres of different physicochemical properties with hepatocyte functions, we have identified a specific combination of methylated and galactosylated collagen nano-fibres optimal for maintaining hepatocyte functions in vitro. A model of how the physical and chemical supports interplay to maintain hepatocyte functions is discussed.

Temperature effect on the functional expression of human cytochromes P450 2A6 and 2E1 inEscherichia coli

Human cytochromes P450 (CYP) 2A6 and 2E1 are of great interest because of their important roles in the oxidation of numerous drugs and carcinogens. Bacterial expression systems, especially Escherichia coli cells, have been widely used for the production of various CYP enzymes in order to obtain high yield of proteins. The expression methods usually employ longer culture time (30-72 h) at lower temperature (usually under 30 degrees C). Expression levels of CYPs 2A6 and 2E1 at 37 degrees C were compared to those at 280 degrees C, which is a usual temperature used in most bacterial expression systems for human CYP expression. Within 18 h the expression levels of CYPs 2A6 and 2E1 reached up to 360 and 560 nmol per liter culture at 37 degrees C, respectively, which are compatible with those of 36 h culture at 280 degrees C. The activities of CYPs expressed at 37 degrees C were also comparable to those expressed at 28 degrees C. The present over-expression system can be useful for rapid production of large amounts of active human CYPs 2A6 and 2E1 in E. coli.

Expression and activities of several drug-metabolizing enzymes in LLC-PK1 cells

LLC-PK1 cells are frequently used in toxicology research, but little information is available concerning the capacity of these cells to metabolize xenobiotics. We examined the expression and activities of cytochromes P450 (P450) 1A1/1A2 (CYP 1A1/1A2), 2E1 (CYP 2E1), flavin monooxygenase (FMO), 5-lipoxygenase (5-LO) and prostaglandin H synthase (PHS)-associated cyclooxygenase-1 (COX-1). We prepared S9 fractions from LLC-PK1 cells, rat liver, and rat kidney, and measured enzyme activities using ethoxyresorufin O-deethylation (EROD) for CYP 1A1/1A2 and ethoxycoumarin O-deethylation (ECOD) for CYP 2E1, benzydamine N-oxidation (BNO) for FMO, leukotriene B(4) (LTB(4)) formation for 5-LO, and thromboxane B(2) (TXB(2)) formation for COX-1 activities. To assure that product formation was due to enzymatic activity, we used the following inhibitors: 1-aminobenzotriazole (ABT) for P450, methimazole for FMO, caffeic acid for 5-LO and acetylsalicylic acid (ASA) for COX-1. We also performed Western blot analysis to confirm our observations. All five enzyme activities were demonstrable in rat liver at much greater levels than in rat kidney S9 fractions. Activities in LLC-PK1 cells were significantly lower than activities in rat liver S9 fraction and generally less than activities in rat kidney S9 fraction. Enzyme inhibitors decreased product formation in all three tissues and Western blot analysis supported our observations of low enzyme activity in LLC-PK1 cells. These results indicate that LLC-PK1 cells have very low content of relevant drug-metabolizing enzyme activities.

Establishment of ten strains of genetically engineered Salmonella typhimurium TA1538 each co-expressing a form of human cytochrome P450 with NADPH-cytochrome P450 reductase sensitive to various promutagens

We newly developed 10 Salmonela typhimurium TA1538 strains each co-expressing a form of human cytochrome P450s (P450 or CYP) together with NADPH-cytochrome P450 reductase (CPR) for highly sensitive detection of mutagenic activation of mycotoxins, polycyclic aromatic hydrocarbons, heterocyclic amines, and aromatic amines at low substrate concentrations. Each form of P450 (CYP1A1, CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4 or CYP3A5) expressed in the TA1538 cells efficiently catalyzed the oxidation of a representative substrate. Aflatoxin B1 was mutagenically activated effectively by CYP1A1, CYP1A2, and CYP3A4 and weakly by CYP2A6 and CYP2C8 expressed in S. typhimurium TA1538. CYP1A1 and CYP1A2 were responsible for the mutagenic activation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-acetylaminofluorene. Benzo[a]pyrene was also activated efficiently by CYP1A1 and weakly by CYP1A2, CYP2C9, CYP2C19, and CYP3A4 expressed in TA1538. These results suggest that the newly developed S. typhimurium TA1538 strains are applicable for detecting the activation of promutagens of which mutagenic activation is not or weakly detectable with N-nitrosamine-sensitive YG7108 strains expressing human P450s.

GYKI-47261, a new AMPA [2-amino-3-(3-hydroxymethylisoxazole-4-yl)propionic acid] antagonist, is a CYP2E1 inducer

CYP2E1-inducing capacity of xenobiotics was determined in cultured hepatocytes on the basis of enzyme activities (chlorzoxazone 6-hydroxylation and 7-ethoxycoumarin O-dealkylation) and protein levels. Hepatocytes in culture showed rapid loss of CYP2E1 enzyme during 72 h. CYP2E1 inducers (ethanol, dimethyl sulfoxide, acetone, isopropanol, pyrazole, and imidazole) were able to prevent the fast decrease of the activities and protein levels of CYP2E1 enzyme. Imidazole was found to be the most effective inducer in rat hepatocytes, and it was selected as a reference in subsequent experiments. The effect of GYKI-47261 [6-(4-aminophenyl)-8-chloro-2-methyl-11H-imidazo[1,2c] [2,3]benzodiazepine], a new AMPA [2-amino-3-(3-hydroxymethylisoxazole-4-yl)propionic acid] antagonist drug-candidate, was also tested in the in vitro system. On the basis of enzyme activities and CYP2E1 protein content of rat hepatocytes, GYKI-47261 was considered as a potent CYP2E1 inducer. Furthermore, it was more effective than imidazole, since 10 microM GYKI-47261 produced the maximal induction, whereas 500 microM imidazole brought about the maximal response. Human hepatocytes were more sensitive to GYKI-47261 than were rat cells. In rat hepatocytes, 10 microM caused maximal increase, whereas 0.01 microM produced the highest induction in human cells. Elevation of CYP2E1 gene transcription as the mechanism of induction caused by GYKI-47261 can be excluded. It seems to act mainly on stabilization of CYP2E1 enzyme protein, whereas the role of stabilization of CYP2E1 mRNA can be considered negligible. Although the imidazole part of GYKI-47261 can explain its CYP2E1-inducing capacity, the other part of the molecule must contribute to the final inducing potency.

Comparison of primary human hepatocytes and hepatoma cell line Hepg2 with regard to their biotransformation properties

Cultures of primary hepatocytes and hepatoma cell line HepG2 are frequently used in in vitro models for human biotransformation studies. In this study, we characterized and compared the capacity of these model systems to indicate the presence of different classes of promutagens. Genotoxic sensitivity, enzyme activity, and gene expression were monitored in response to treatment with food promutagens benzo[a]pyrene, dimethylnitrosamine (DMN), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). DNA damage could be detected reliably with the comet assay in primary human hepatocytes, which were maintained in sandwich culture. All three promutagens caused DNA damage in primary cells, but in HepG2 no genotoxic effects of DMN and PhIP could be detected. We supposed that the lack of specific enzymes accounts for their inability to process these promutagens. Therefore, we quantified the expression of a broad range of genes coding for drug-metabolizing enzymes with real-time reverse transcription-polymerase chain reaction. The genes code for cytochromes p450 and, in addition, for a series of important phase II enzymes. The expression level of these genes in human hepatocytes was similar to those previously reported for human liver samples. On the other hand, expression levels in HepG2 differed significantly from that in human. Activity and expression, especially of phase I enzymes, were demonstrated to be extremely low in HepG2 cells. Up-regulation of specific genes by test substances was similar in both cell types. In conclusion, human hepatocytes are the preferred model for biotransformation in human liver, whereas HepG2 cells may be useful to study regulation of drug-metabolizing enzymes.

Expression and regulation of xenobiotic-metabolizing cytochrome P450 (CYP) enzymes in human lung

Pathogenesis of lung diseases, such as lung cancer and chronic obstructive pulmonary disease, is tightly linked to exposure to environmental chemicals, most notably tobacco smoke. Many of the compounds associated with these diseases require an enzymatic activation to exert their deleterious effects on pulmonary cells. These activation reactions are mostly catalyzed by cytochrome P450 (CYP) enzymes. Interindividual differences in the in situ activation and inactivation of chemical toxicants may contribute to the risk of developing lung diseases associated with these compounds. This review summarizes in detail the expression of individual CYP forms in human pulmonary tissue and gives a view on the significance of the pulmonary expression of CYP enzymes. The localization of individual CYP enzymes in various cell types of human lung and the emerging field of regulation of human pulmonary CYP enzymes are discussed. At least CYP1A1 (in smokers), CYP1B1, CYP2B6, CYP2E1, CYP2J2, and CYP3A5 proteins are expressed in human lung, and also other CYP forms are likely to be expressed. Xenobiotic-metabolizing CYP enzymes are mostly expressed in bronchial and bronchiolar epithelium, Clara cells, type II pneumocytes, and alveolar macrophages in human lung, although individual CYP forms have different patterns of localization in pulmonary tissues. Problems in animal to human lung toxicity extrapolation and several specific aspects requiring more detailed assessment are identified.