Molecular dimensions of the substrate binding site of cytochrome P-448

The Safety Evaluation of Drugs and Chemicals by the Use of Computer Optimised Molecular Parametric Analysis of Chemical Toxicity (COMPACT)

The historical development of the safety evaluation of drugs and chemicals is critically reviewed, and failures of the present approach using experimental animals are examined. The roles of the cytochromes P450 in the detoxication of drugs and chemicals, and in their activation to mutagens, carcinogens and neoantigens, are described, and the importance of the selective induction of the cytochromes P450 in the manifestation of chemical toxicity/carcinogenicity is highlighted. The computer graphic procedure of COMPACT, which relates chemical structures to metabolism by individual cytochromes P450, and hence to their potential toxicity/carcinogenicity, is described, and the advantages and disadvantages of this method of safety evaluation, which does not use experimental animals, are considered.

Murine precision-cut lung slices exhibit acute responses following exposure to gasoline direct injection engine emissions

Gasoline direct injection (GDI) engines are increasingly prevalent in the global vehicle fleet. Particulate matter emissions from GDI engines are elevated compared to conventional gasoline engines. The pulmonary effects of these higher particulate emissions are unclear. This study investigated the pulmonary responses induced by GDI engine exhaust using an ex vivo model. The physiochemical properties of GDI engine exhaust were assessed. Precision cut lung slices were prepared using Balb/c mice to evaluate the pulmonary response induced by one-hour exposure to engine-out exhaust from a laboratory GDI engine operated at conditions equivalent to vehicle highway cruise conditions. Lung slices were exposed at an air-liquid interface using an electrostatic aerosol in vitro exposure system. Particulate and gaseous exhaust was fractionated to contrast mRNA production related to polycyclic aromatic hydrocarbon (PAH) metabolism and oxidative stress. Exposure to GDI engine exhaust upregulated genes involved in PAH metabolism, including Cyp1a1 (2.71, SE=0.22), and Cyp1b1 (3.24, SE=0.12) compared to HEPA filtered air (p<0.05). GDI engine exhaust further increased Cyp1b1 expression compared to filtered GDI engine exhaust (i.e., gas fraction only), suggesting this response was associated with the particulate fraction. Exhaust particulate was dominated by high molecular weight PAHs. Hmox1, an oxidative stress marker, exhibited increased expression after exposure to GDI (1.63, SE=0.03) and filtered GDI (1.55, SE=0.04) engine exhaust compared to HEPA filtered air (p<0.05), likely attributable to a combination of the gas and particulate fractions. Exposure to GDI engine exhaust contributes to upregulation of genes related to the metabolism of PAHs and oxidative stress.

In vitro metabolism of polychlorinated biphenyls and cytochrome P450 monooxygenase activities in dietary-exposed Greenland sledge dogs

The in vitro metabolism of a polychlorinated biphenyl (PCB) mixture was examined using hepatic microsomes of dietary-exposed Greenland sledge dogs (Canis familiaris) to an organohalogen-rich diet (Greenland minke whale blubber: EXP cohort) or a control diet (pork fat: CON cohort). The associations between in vitro PCB metabolism, activity of oxidative hepatic microsomal cytochrome P450 (CYP) isoenzymes and concentrations of PCBs and hydroxylated metabolites were investigated. The CON dogs exhibited a 2.3-fold higher depletion percentage for the PCB congeners having at least two pairs of vicinal meta-para Cl-unsubstituted carbons (PCB-18 and -33) relative to the EXP dogs. This depletion discrepancy suggests that there exist substrates in liver of the organohalogen-contaminated EXP dogs that can competitively bind and/or interfere with the active sites of CYP isoenzymes, leading to a lower metabolic efficiency for these PCBs. Testosterone (T) hydroxylase activity, determined via the formation of 6beta-OH-T, 16alpha-OH-T, 16beta-OH-T and androstenedione, was strongly correlated with the depletion percentages of PCB-18 and -33 in both cohorts. Based on documented hepatic microsomal CYP isoenzyme substrate specificities in canines, present associations suggest that primarily CYP2B/2C and CYP3A were inducible in sledge dogs and responsible for the in vitro metabolism of PCB-18 and -33.

Quantitative structure-activity relationships (QSARs) within the cytochrome P450 system: QSARs describing substrate binding, inhibition and induction of P450s

Quantitative structure-activity relationships (QSARs) within substrates, inducers and inhibitors of cytochromes P450 involved in xenobiotic metabolism are reported, together with QSARs associated with induction, inhibition and metabolic rate. The importance of frontier orbitals and shape descriptors, such as planarity (estimated by the area/depth(2) parameter) and rectangularity (estimated by the length/width parameter) is discussed, particularly in the context of the COMPACT system which discriminates between several P450 families associated with the activation and detoxication of xenobiotics. The use of parameters, particularly those derived from homology modelling of mammalian (especially human) P450s that are involved in exogenous metabolism, in generating QSARs for P450 substrates is discussed in the context of explaining differences in the binding affinities of human P450 substrates which are pharmacologically active.

Cell bioassays for detection of aryl hydrocarbon (AhR) and estrogen receptor (ER) mediated activity in environmental samples

In vitro cell bioassays are useful techniques for the determination of receptor-mediated activities in environmental samples containing complex mixtures of contaminants. The cell bioassays determine contamination by pollutants that act through specific modes of action. This article presents strategies for the evaluation of aryl hydrocarbon receptor (hereafter referred as dioxin-like) or estrogen receptor mediated activities of potential endocrine disrupting compounds in complex environmental mixtures. Extracts from various types of environmental or food matrices can be tested by this technique to evaluate their 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents or estrogenic equivalents and to identify contaminated samples that need further investigation using resource-intensive instrumental analyses. Fractionation of sample extracts exhibiting significant activities, and subsequent reanalysis with the bioassays can identify important classes of contaminants that are responsible for the observed activity. Effect-directed chemical analysis is performed only for the active fractions to determine the responsible compounds. Potency-balance estimates of all major compounds contributing to the observed effects can be calculated to determine if all of the activity has been identified, and to assess the potential for interactions such as synergism or antagonism among contaminants present in the complex mixtures. The bioassay approach is an efficient (fast and cost effective) screening system to identify the samples of interest and to provide basic information for further analysis and risk evaluation.

Indications for the involvement of a CYP3A-like iso-enzyme in the metabolism of chlorobornane (Toxaphene) congeners in seals from inhibition studies with liver microsomes

The different isoforms of the cytochrome P450 (CYP) system can metabolise a suite of classes of lipophilic, anthropogenic compounds. The bioaccumulative potential as well as the toxicity of xenobiotics may be significantly altered in the process. To compare the metabolic ability of different wildlife species, it is important to identify the different iso-enzymes of CYP, which are responsible for the metabolism of different classes of compounds. This can be achieved with in vitro incubation assays. In the present study, preparations of hepatic microsomes of a harbour seal (Phoca vitulina) and a grey seal (Halichoerus grypus) demonstrated that the chlorobornane (CHB) congeners CHB-32 and -62 were metabolised enzymatically to their hydroxylated derivatives. These derivatives were partially characterised by their NCI mass-spectra. Inhibition studies were carried out to identify the specific CYP isoform(s) responsible for the metabolism of CHB-32 and -62. Ketoconazole has been shown to inhibit CYP3A enzymes in human and rat studies. In this study, ketoconazole caused concentration-dependent inhibition of metabolism of CHB-32 and -62, reaching 80% at the 1.0 microM treatment level. Ellipticine (1.0 microM), which has been shown to inhibit CYP1A1/2, also inhibited CHB-32 and -62 metabolism in the microsomes of grey seal, but to a much lower degree of less than 10 and 24%, respectively. In the same experiment the metabolism of 4,4'-dichlorobiphenyl was already inhibited 70% by ellipticine treatment at the same concentration. This non-ortho substituted PCB congener can easily attain a planar molecular configuration, and therefore served as a model CYP1A substrate. Inhibition of chlorobornane metabolism was not observed after the addition of goat anti-rat CYP2B antibodies or Aldrin, which is a model CYP2B substrate in rat. Cautious interpretation is advised for results obtained with so-called selective competitive inhibitors. Regardless, these studies indicated for the first time the possible involvement a CYP3A isoform in the mediation of chlorobornane metabolism in seals. The immunochemical cross-reactivity of mouse, rabbit or sheep anti-rat antibodies in the hepatic microsomes of harbour seal confirmed the presence of CYP1A1/2, CYP1A1, CYP2B1/2, CYP3A and CYP4A isoenzymes. Enantioselective metabolism by the microsomes of harbour seal was observed for both CHB-32 and -62. Stereochemical preferences of biotransformation enzymes can have an influence on the environmental distribution of both enantiomers of optically active compounds.

Orientation of caffeine within the active site of human cytochrome P450 1A2 based on NMR longitudinal (T1) relaxation measurements

Longitudinal (T1) relaxation studies were performed in order to examine the interaction of caffeine with the heme of human P450 1A2. Addition of caffeine to this P450 resulted in a small, incomplete conversion of the heme from high spin to low spin, as shown by changes in the optical spectrum. Determination of a relatively large dissociation constant (Ks = 2.6 mM) as well as the relative instability of the P450 after 2 h at room temperature necessitated the performance of these experiments at high concentrations (25 mM) of caffeine. The relaxation measurements on the three sets of methyl hydrogens led to the determination of the corresponding distances between the iron and the methyl groups on the bound caffeine as well as the position and orientation of caffeine within the active site of P450 1A2. The three methyl groups were found to be nearly equidistant from the iron (> or = 4.79-4.89 A), with slight preference for the N-3 position, and thus, the average position of caffeine was parallel to the heme. In vitro incubations with P450 1A2 and 5 mM caffeine led primarily to paraxanthine formation (N-3 demethylation), as expected. However, with 25 mM substrate, the overall extent of oxidation was doubled and there was more equivalent oxidation at each of the four potential sites on caffeine. This latter observation was consistent with the lack of selective positioning of the N-3 methyl group of caffeine relative to the heme.

Cell bioassays for detection of aryl hydrocarbon (AhR) and estrogen receptor (ER) mediated activity in environmental samples

In vitro cell bioassays are useful techniques for the determination of receptor-mediated activities in environmental samples containing complex mixtures of contaminants. The cell bioassays determine contamination by pollutants that act through specific modes of action. This article presents strategies for the evaluation of aryl hydrocarbon receptor (AhR)-(hereafter referred as dioxin-like) or estrogen receptor (ER)-mediated activities of potential endocrine disrupting compounds (EDCs) in complex environmental mixtures. Extracts from various types of environmental or food matrices can be tested by this technique to evaluate their 2,3,7,8-tetrachlorodibenzop-dioxin equivalents (TCDD-EQs) or estrogenic equivalents (E(2)-EQs) and to identify contaminated samples that need further investigation using resource-intensive instrumental analyses. Fractionation of sample extracts exhibiting significant activities, and subsequent reanalysis with the bioassays can identify important classes of contaminants that are responsible for the observed activity. Effect-directed chemical analysis is performed only for the active fractions to determine the responsible compounds. Mass-balance estimates of all major compounds contributing to the observed effects can be calculated to determine if all of the activity has been identified, and to assess the potential for interactions such as synergism or antagonism among contaminants present in the complex mixtures. The bioassay approach is an efficient (fast and cost effective) screening system to identify the samples of interest and to provide basic information for further analysis and risk evaluation.

Preferential inhibition of CYP1A enzymes in hepatic microsomes by mexiletine

We examined the inhibitory behavior of theophylline oxidations and a variety of cytochrome P450 (P450)-dependent metabolism in the presence of mexiletine (MEX), using hepatic microsomes from both control mice and mice exposed to beta-naphthoflavone (beta-NF). Theophylline metabolism, which is mainly catalyzed by CYP1A2, was susceptible to competitive inhibition by MEX. The calculated inhibition constants (Ki) for theophylline 3-demethylation and its 8-hydroxylation were 4.3 microM and 8.3 microM, respectively, which are comparable to the recommended therapeutic serum range for MEX. The inhibitory potency of MEX on cytochrome P450-dependent enzyme activities diverged among the several metabolic reactions, which were probes for CYP1A, 2A, 2C, 2D, 2E and 3A subfamilies. The Ki value (6.7 microM) for methoxyresorufin O-demethylation mediated by CYP1A2 agreed with those from theophylline oxidations. These metabolic reactions exhibited the smallest Ki values, 1-3 orders of magnitude lower than activities of other constitutive cytochrome P450 species. Similar degrees of inhibition were observed in CYP1A1, a beta-NF-inducible isoform with a relatively high conformity to CYP1A2. These results indicate that MEX acts as a selective and potent inhibitor of the CYP1A enzymes responsible for oxidative biotransformation of chemicals such as theophylline. This evidence provides a fundamental explanation for the pharmacokinetic interactions experienced in clinical practice.

A combined COMPACT and HazardExpert study of 40 chemicals for which information on mutagenicity and carcinogenicity is known, including the results of human epidemiological studies

The COMPACT approach for defining structural criteria for substrates and inducers of cytochrome P450 (CYP) enzymes which mediate the formation of reactive intermediates is discussed in the context of prediction of potential carcinogenicity. This is broadened to encompass structural studies on mammalian P450s, including those relevant to genetic polymorphism in man. The use of the COMPACT system, in parallel with the structure alert program HazardExpert (now incorporated into the Pallas system), for evaluating human carcinogenicity data is reported, as an example of the possible employment of a battery of short-term test procedures for safety evaluation. In particular, the importance of using the log P value (as a measure of compound lipophilicity) to assess the likelihood of a potentially toxic compound reaching the site of activation, is emphasized by the finding that most procarcinogens requiring metabolic activation by P450s are lipophilic in nature.

Further evaluation of COMPACT, the molecular orbital approach for the prospective safety evaluation of chemicals

The molecular dimensions and electronic structures of the first group of 100 US NCI/NTP miscellaneous chemicals, evaluated for potential carcinogenicity by computer-optimized molecular parametric analysis for chemical toxicity (COMPACT) have been re-determined. Using improved criteria for cytochrome P450 (CYP) substrate specificity, re-defined for CYP1 as having a COMPACT radius [square root of (deltaE - 9.5)2 + (a/d(2) - 7.8)2] of < 6.5, and for CYP2E as having a collision diameter of 6.5 angstroms or less and deltaE < 15.5, the likely substrates of CYP1 and CYP2E, which are regarded as potential carcinogens, have been identified. In addition, log P values have been taken into account; those chemicals with log P < 0 are non-lipophilic substrates unlikely to reach the activating cytochrome enzymes, and have been regarded as non-carcinogens. The second group of 100 US NCI/NTP chemicals have also now been categorized by COMPACT into CYP1 and CYP2E substrates, and their potential carcinogenicities evaluated. Of the 203 chemicals in the 2 groups, those positive in the rodent two-species life-span carcinogenicity study (rodent assay) were 53%, those positive in the Ames test (mutagenicity) were 48%, and those positive in the COMPACT programme (carcinogenicity, mutagenicity, cytotoxicity) were 54%. Concordance between the COMPACT prediction of carcinogenicity/cytotoxicity and rodent two species life-span carcinogenicity data for the 203 chemicals is 69%, and correlation of COMPACT with Ames test data is 61%. The sensitivity of COMPACT for predicting rodent carcinogenicity is 72%, whereas the sensitivity of the Ames test for predicting carcinogenicity for the 203 chemicals was only 57%. The degree (severity) of rodent carcinogenicity also showed correlation with the COMPACT predictive evaluations of the chemicals.

Diaminonaphthalenes and related aminocompounds: mutagenicity, CYP1A induction and interaction with the Ah receptor

Using 1- and 2-aminonaphthalene as model substrates, we investigated the effect of insertion of a second amino group on mutagenicity, binding to the cytosolic Ah receptor and CYP1A inducibility, and the effects were compared to those elicited by 3,3'-diaminobenzidine and 1-naphthylethylenediamine. 1,5- and 1,8-diaminonaphthalene were effective inducers of CYP1A activity, more potent than 1-aminonaphthalene. 2,3-Diaminonaphthalene was also an inducer of CYP1A, but the effect was similar to that elicited by 2-aminonaphthalene. In contrast, 3,3'-diaminobenzidine and 1-naphthylethylenediamine did not induce CYP1A activity. All aminonaphthalenes displaced [3H]TCDD from the Ah receptor, whereas 3,3'-diaminobenzidine and 1-naphthylethylenediamine failed to do so. The latter two compounds did not elicit a mutagenic response in the Ames test. Introduction of a second amino group at the 3-position of 2-aminonaphthalene did not modulate its mutagenicity. In the case of the non-mutagenic 1-aminonaphthalene, introduction of a second amino group at position 5 had no effect but when it was incorporated at position 8, mutagenic potential was conferred to the molecule. Computer modelling of the putative active site of CYP1A2 revealed that 1,5-diaminonaphthalene is orientated so that the distance of the second amino group from the iron-oxene is 4.037 A while in the case of 1,8-diaminonaphthalene the distance is shorter, 2.744 A, favouring its activation through N-hydroxylation. Of the compounds studied, 1,8-diaminonaphthalene and, to a lesser extent, 2,3-diaminonaphthalene autoinduced their activation. It is concluded that insertion of a second amino group at the 5- or 8-position of 1-aminonaphthalene may enhance biological activity but in the case of 2-aminonaphthalene insertion of a second amino group at position 3 had no major effect.

Intermolecular interactions of antimicrobial fluoroquinolones with purified rat liver CYP1A2 studied by proton nuclear magnetic resonance spectroscopy

1. Binding and inhibition of antimicrobial fluoroquinolones towards liver CYP1A2 purified from 3-methylcholanthrene-treated rats were investigated using proton nuclear magnetic resonance (nmr) and phenacetin metabolism. 2. The proton nmr longitudinal relaxation rate study indicated that the paramagnetic effects of the haem iron of CYP1A2 were observed in protons of enoxacin with a 1,8-naphthyridine skeleton and its 4'-nitrogen atom on the 7-piperazine ring probably participated in specific binding to the haem iron. These data suggest a facile accessibility and strong binding of enoxacin to the active site of the enzyme. On the contrary, the binding region of norfloxacin with a quinoline skeleton could not be specified, and an 8-fluorinated derivative (AT-3970) had much lower paramagnetic effects and no specific binding region. 3. In a reconstituted CYP1A2 system, enoxacin exhibited the most potent inhibition of phenacetin O-deethylation. The metabolism was less inhibited by norfloxacin, and AT-3970 had a weak inhibitory activity. 4. The binding ability of the fluoroquinolones to the CYP1A2 active site is likely to determine their inhibitory activity against phenacetin metabolism.

Structure-related inhibitory effect of antimicrobial enoxacin and derivatives on theophylline metabolism by rat liver microsomes

Enoxacin, an antimicrobial fluoroquinolone with a 7-piperazinyl-1, 8-naphthyridine skeleton, is a potent inhibitor of cytochrome P-450-mediated theophylline metabolism. The present study was designed to clarify, using seven enoxacin derivatives, the molecular characteristics of the fluoroquinolone responsible for the inhibition. Three derivatives with methyl-substituted 7-piperazine rings inhibited rat liver microsomal theophylline metabolism to 1,3-dimethyluric acid to an extent similar to that of enoxacin (50% inhibitory concentrations [IC50s] = 0.39 to 0.48 mM). 7-Piperazinyl-quinoline derivatives, 8-hydroenoxacin (8-Hy) and 1-cyclopropyl-8-fluoroenoxacin (8-F1), which have a hydrogen and a fluorine at position 8, respectively, more weakly inhibited metabolite formation (IC50s = 0.88 and 1.29 mM, respectively). Little inhibition (IC50 > 2 mM) was observed in those with 3'-carbonyl and 4'-N-acetyl groups on the piperazine rings. The substrate-induced difference spectra demonstrated that the affinities of enoxacin, 8-Hy, and 8-F1 to cytochrome P-450 were parallel with their inhibitory activities. The substituent at position 8 was found to determine the molecular conformations of the fluoroquinolones, and the planarity in molecular shape decreased in the same order as the inhibitory activity (enoxacin > 8-Hy > 8-F1). Moreover, the 3'-carbonyl and 4'-N-acetyl groups decreased the basicity of their vicinal 4'-nitrogen atoms when judged from their electrostatic potentials, which showed a remarkably broadened negative charge around the nitrogens. As a result, the planarity of the whole molecule and the basicity of the 4'-nitrogen atom of enoxacin are likely to be dominant factors in the inhibition of theophylline metabolism by cytochrome P-450.

Molecular modelling of CYP1A subfamily members based on an alignment with CYP102: rationalization of CYP1A substrate specificity in terms of active site amino acid residues

1. Using a novel amino acid sequence alignment, proteins of the CYP1A subfamily have been produced from the CYP102 crystal structure template via residue replacement and energy minimization procedures. 2. Known substrates and inhibitors of CYP1A1 and CYP1A2 are shown to fit their respective active sites via key interactions with complementary amino acid residues. Substrates used in the modelling studies include: caffeine, PhIP, oestradiol, 2,4- and 2,5-diaminotoluenes, Glu-P-1, phenacetin, acetanilide, 7-methoxy and 7-ethoxyresorufins, 11-methyl cyclopenta[a]phenanthren-17-one, 7-ethoxycoumarin, aflatoxin B1, benzo[a]pyrene, benzo[a]pyrene-7,8-diol and 1'-hydroxy 3-methylcholanthrene. 3. A number of aspects relating to CYP1A substrate specificity and metabolism can be explained in terms of the enzyme models, as it is found that key interactions with active site amino acid residues direct CYP1A-mediated metabolism in the known positions.

Molecular orbital-generated QSARs in a homologous series of alkoxyresorufins and studies of their interactive docking with P450s

1. Molecular and electronic structural parameters have been determined, by molecular orbital (MO) calculations, for a homologous series of 8 alkoxyresorufins (methoxy- to octoxy-). 2. Quantitative structure-activity relationships (QSARs) between these structural parameters and the rates of metabolism of the alkoxyresorufins in hepatic microsomes from the 3-methylcholanthrene (MC)-, and phenobarbital (PB)-pretreated mouse, and the beta-naphthoflavone (beta NF)-pretreated rat have been established. 3. The most significant single relationship is between beta NF-induction of cytochrome P4501 (CYP1A) and the total nucleophilic superdelocalizability (sigma SN) for the eight compounds in the series. 4. For double regressions, the electronic charge on the alkoxy oxygen, Q(O), or alpha-carbon Q(C), is important when combined with the hydrophobic substituent constant (pi). 5. These findings indicate that the rates of metabolism of these alkoxyresorufins are dependent upon their ability to cross cellular membranes, to fit the relevant CYP1A binding site, and on their ability to accept electrons from a donor nucleophilic species. 6. A different set of parameters correlated with CYP2B activity, namely, parameters of overall shape, which indicates that the way in which the alkoxyresorufins fit the CYP2B site, determines their differences in specificity. 7. Computer graphic interactive docking studies of the alkoxyresorufins with their affinity-specific cytochromes P450, namely, methoxy- with CYP1A2; ethoxy- with CYP1A1; pentoxy- with CYP2B1; and benzyloxy- with CYP3A, have also been undertaken to show the specific interactions of the alkoxyresorufins with the binding sites of the individual P450s.

Induction of hepatic CYP1A2 by the oral administration of caffeine to rats: lack of association with the Ah locus

Caffeine was administered to male Wistar albino rats for two weeks at three concentrations, namely 0.1, 0.2 and 0.3%, and hepatic cytochrome P450-dependent mixed-function oxidase determined. Caffeine administration gave rise to a marked, dose-dependent increase in the O-deethylation of ethoxyresorufin and, to a lesser extent, in the O-depentylation of pentoxyresorufin. Erythromycin N-demethylase, p-nitrophenol hydroxylase and lauric acid hydroxylase activities, as well as total cytochrome P450 content were unaffected by this treatment. Immunoblot analysis revealed that caffeine gave rise to a dose-dependent increase in the hepatic CYP1A2, and at the highest dose only, CYP2B apoprotein levels. Apoprotein levels of CYP3A and CYP2E1 were not modulated by the treatment with caffeine at all dose levels studied. Caffeine could not displace [3H]TCDD from the rat hepatic cytosolic Ah receptor. Computer analysis showed that caffeine is essentially a planar molecule with an area/depth ratio 4.8, characteristic of CYP1A substrates/inducers. Molecular modelling revealed that the caffeine molecule could orientate itself within the putative CYP1A2 active site so as to facilitate demethylation of the N-1, N-3 and N-7 positions. However, at physiological pH, the N-9 nitrogen atom is likely to be partially protonated, allowing it to participate in an electrostatic interaction with the negatively-charged glutamate 318-residue, favouring N-3 demethylation, the major pathway of metabolism in both humans and animals. In conclusion caffeine, being essentially planar, is an inducer of CYP1A2 in rat liver.

CB pattern in the harbour porpoise: bioaccumulation, metabolism and evidence for cytochrome P450 IIB activity

Metabolism of chlorobiphenyls (CBs) was studied in harbour porpoise by comparing patterns of CB-X/CB-153 ratios in blood, brain, liver and blubber with the patterns in herring, the main food source. The CBs were classified in five groups, based on the presence/absence of vicinal H-atoms (vic. Hs) in meta,para (m,p) and/or ortho,meta (o,m) positions and the number of ortho-Cl-atoms (ortho-Cls). Plots of CB-X/CB-153 ratios in porpoise tissue vs the ratios in herring appeared to be linear for each CB group in all tissues. Slopes of these plots (metabolic slopes) were used as quantitative indicators of metabolic activity. In this way, activity of PB-type isozymes of the P450 monooxygenase system was apparent: in contrast to existing literature data, harbour porpoise appears to be able to metabolize congeners with m,p vic. Hs, even in the presence of more than 2 ortho-Cls. The presence of 3-MC-type (MC-type) isozymes was also detected. The metabolic slopes were also used as basis for risk assessment. Due to their metabolism the most toxic non-ortho CBs were not present in the tissues at detectable levels. We suggest a risk assessment approach which takes this into account. It is considered to be an alternative and more reliable basis for risk assessment than the use of toxic equivalent factors. The results support the model of equilibrium distribution of CBs in harbour porpoise and the role of blood as central transport medium. The model has been developed for persistent compounds; it appears to hold for metabolizable CB congeners as well.

Regulation of cytochrome P-450 isozymes CYP1A1, CYP1A2 and CYP2B10 by three benzodioxole compounds

Three benzodioxole (BD) compounds were used to investigate the structural requirement for regulation of the cytochrome P450 isozymes, CYP1A1, CYP1A2 and CYP2B10, in mouse liver. Male mice (C57BL/6) were treated intraperitoneally for 3 days with 5-t-butyl-1,3-benzodioxole (t-BBD), 5-n-butyl-1,3-benzodioxole (n-BBD) and 5-(3-oxobutyl)-1,3-benzodioxole (o-BBD). t-BBD-induced liver microsomes showed the highest pentoxyresorufin O-dealkylation (PROD) activity, while o-BBD induced microsomes showed slightly higher activity in ethoxyresorufin O-deethylation (EROD), benzo[a]pyrene hydroxylation (BaP-OH) and acetanilide hydroxylation (Acet-OH) assays. In vitro enzyme inhibition assays showed that n-BBD inhibited EROD and Acet-OH activities more than either o-BBD or t-BBD, while PROD activity was evenly inhibited by all three compounds. Western and northern blots showed that CYP1A1 was not detectably induced by any of the three BD compounds. The levels of CYP1A2 protein and mRNA were increased in all three treated livers. In addition to CYP1A2 induction, t-BBD also induced the protein and mRNA for CYP2B10.

Computer graphics analysis of the interaction of alkoxy methylenedioxybenzenes with cytochromes P4501

A quantitative structure-activity relationship (QSAR) in a homologous series of alkoxy methylenedioxybenzenes (MDBs) is reported. Measurements of molecular dimensions from computer-generated space-filling structures have provided values for the shape parameter area/depth2. These have been shown to correlate with the extent of inhibition of ethoxyresorufin O-deethylase activity by a series of MDBs. The implication of this is that the MDB nucleus fits the cytochrome P4501 substrate binding site and that this ability decreases with increase in the alkyl chain length of the alkoxy substituent. These findings are in agreement with previous results relating to the spatial dimensions of the cytochrome P4501 binding site, showing that substrate specificity can be rationalized in terms of overall molecular shape.

A retrospective evaluation of COMPACT predictions of the outcome of NTP rodent carcinogenicity testing

The carcinogenic potentials of 40 National Toxicology Program chemicals previously predicted by Computer Optimised Molecular Parametric Analysis for Chemical Toxicity (COMPACT), based on the identification of potential substrates of cytochromes P4501A and 2E (CYP1A and CYP2E), have been compared with new rodent carcinogenicity results. The COMPACT predictions have also been compared with published Ames mutagenicity data and with our own Hazardexpert predictions for carcinogenicity. Concordance evaluations between rodent carcinogenicity (1/4 segments positive) and predictions by COMPACT or Hazardexpert were 64% for COMPACT (CYP1A only), 72% for COMPACT (CYP1A plus CYP2E), 70% for Hazardexpert alone, and 86% for COMPACT (CYP1A plus CYP2E) plus Hazardexpert. Sensitivities of the predictions were for COMPACT, 75%; Hazardexpert, 60%; and Ames, 54%. Positive predictivities were for COMPACT, 75%; Hazardexpert, 78%; and Ames 81%. Negative predictivites were for COMPACT, 62%; Hazardexpert, 52%; and Ames, 42%.

CYP1 induction, binding to the hepatic aromatic hydrocarbon receptor and mutagenicity of a series of 11-alkoxy cyclopenta[a]phenanthren-17-ones: a structure activity relationship

A series of four 11-alkoxy cyclopenta[a]phenanthren-17-ones, ranging from the methoxy to the butoxy derivative, has been synthesised in order to investigate the effect of the size of the 11-substituent on the mutagenicity and ability of these compounds to induce hepatic CYP1 activity in rats. The latter was monitored by using as diagnostic probes methoxy and ethoxy-resorufin, and immunologically in Western blots employing anti-CYP1A1 antibodies. All four members of the series induced both CYP1A1 and CYP1A2 activities and apoprotein levels, but the methoxy- and ethoxy-CPP-17-ones were clearly the most potent. Of the four isomers, only 11-methoxy-CPP-17-one displaced 3H-TCDD from the cytosolic Ah receptor. Similarly only 11-methoxy-CPP-17-one elicited a positive mutagenic response in the Ames test in the presence of an Aroclor 1254-induced activation system. The relevance of these findings to the carcinogenicity of these compounds in the mouse skin painting model is discussed.

The cytochromes P450 and mechanisms of chemical carcinogenesis

This article reviews mechanisms of chemical carcinogenesis, from metabolic activation and generation of reactive oxygen species by cytochromes P4511 and P4502E to DNA damage, activation of protein kinase C and ocogenes, hyperplasia, and proteoglycan changes in the cell glycocalyx and lysosomal enzymes which mediate invasion and metastasis.

Inhibition of rat hepatic aryl hydrocarbon hydroxylase activity by a series of 7-hydroxy coumarins: QSAR studies

1. Molecular orbital calculations, by the Modified Intermediate Neglect of Differential Overlap (MINDO/3) method, of a series of twenty-five 8-acyl-7-hydroxy coumarins show that the inhibition of aryl hydrocarbon hydroxylase (AHH) activities (cytochrome P4501, CYP1 activity, primarily CYP1A1) for 23 of these compounds is related to their structural parameters. The two remaining compounds are the only chlorinated derivatives; these are inactive towards the AHH system and were excluded from the quantitative structure-activity relationship (QSAR) analysis. 2. The results of multiple regression analyses show that AHH activity is dependent on the energy of the highest occupied molecular orbital, E(HOMO), in a single variable expression for the 23 compounds. However, a three-variable expression involving superdelocalizabilities provides a more significant correlation with biological activity. 3. The inactivity of the two chlorinated derivatives can be rationalized in terms of their low degree of molecular planarity, as estimated by the area/depth2 parameter, which presumably precludes them from interaction with CYP1.

Evaluation of the antimutagenic potential of anthracene: in vitro and ex vivo studies

The present study was undertaken in order to evaluate the in vitro and ex vivo antimutagenicity of anthracene against the food-borne carcinogen IQ (2-amino-3-methylimidazo[4,5-f]quinoline). Anthracene caused a marked, concentration-dependent decrease in the mutagenicity of IQ in the Ames test, whether hepatic S9 or isolated microsomes from Aroclor 1254-induced rats served as the activation systems. Anthracene gave rise to a concentration-dependent inhibition of the 0-deethylation of ethoxyresorufin, a diagnostic probe for CYP1A (cytochrome P450 family 1, subfamily A) activity, and of the metabolic activation of Glu-P-1 (2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole, a diagnostic probe for CYP1A2. When microsomal metabolism of IQ was terminated by menadione, incorporation of anthracene into the incubation mixture once again inhibited the mutagenicity of IQ. All the above observations indicate that anthracene owes its antimutagenic response against IQ to: (a) inhibition of its CYP1A-mediated activation and (b) direct interaction between anthracene and the reactive intermediate(s) of IQ leading to their inactivation. Treatment of rats with anthracene did not greatly influence the ability of hepatic preparations to bioactivate IQ to mutagens. Similarly, administration of anthracene 2 h before sacrifice to Aroclor 1254-pretreated rats, did not modulate the hepatic activation of IQ. These findings demonstrate that the in vitro mechanisms of the antimutagenicity of anthracene are not operative in vivo, and further illustrate the inadequacy of in vitro studies, conducted in isolation, in predicting such effects.

The use of computers in the safety evaluation of drugs and other chemicals

The toxicity and carcinogenicity of drugs and other chemicals is, in most cases, mediated by highly reactive intermediates which are generated following metabolism catalysed by the enzymic apparatus of the exposed organisms. These reactive intermediates readily interact covalently with vital cellular components to provoke toxicity and carcinogenicity. The ubiquitous cytochrome P450-dependent mixed-function oxidases are the most important enzyme system in the activation of chemicals. This enzyme system comprises a number of families, each of which contains one or more subfamilies. The CYPIA and CYP2E subfamilies are the most closely associated with the production of reactive intermediates and, consequently, the manifestation of toxicity and carcinogenicity. A computer based molecular structure procedure (COMPACT) has been developed which, via a calculation of the molecular and electronic structure of the chemical, determines whether the chemical will interact with either of these two cytochrome P450 subfamilies and hence be metabolised to form reactive intermediates that manifest toxicity. As the basal levels of these two subfamilies are generally low, the ability of a chemical to induce them selectively, on repeated administration, is an important determinant of its toxic and carcinogenic potential. This inductive capability may be determined in short-term studies (ENACT) using only a small number of animals. Thus the combination of COMPACT and ENACT provides a rapid and inexpensive means for the preliminary screening of chemicals for toxicity and carcinogenicity before undertaking the long-term and expensive rodent lifetime bioassays.

Prediction of carcinogenicity from molecular structure; modification and reinvestigation of the method

The method of Lewis and coworkers for predicting the affinity of molecules for cytochrome P448 is studied. Parameters are modified to clarify their meaning and to simplify their calculation. Additional molecules are involved in the study. Geometric requirements for obtaining reliable parameters and the possibility of predicting carcinogenicity are discussed.

Molecular modelling of cytochrome CYP1A1: a putative access channel explains differences in induction potency between the isomers benzo(a)pyrene and benzo(e)pyrene, and 2- and 4-acetylaminofluorene

The present studies were undertaken to provide a rationale for the observation that benzo(a)pyrene and 2-acetylaminofluorene induce the hepatic CYP1A1 protein, whereas their non-carcinogenic isomers benzo(e)pyrene and 4-acetylaminofluorene are, at best, relatively very weak inducers. Using amino acid sequence alignment, a molecular model of the CYP1A1 was constructed by analogy to CYP101, the bacterial protein for which the 3-dimensional structure is known from X-ray crystallographic analysis. The putative structure of the active site of the CYP1A1 protein shows the presence of two phenylalanine residues preferentially aligned in parallel orientation, presumably functioning as a 'sieve' for planar molecules, the established substrates of CYP1A1. The molecular dimensions of this putative access channel show a width and depth of 8.321 and 3.261 A, respectively. The width of 4-acetylaminofluorene, 8.794 A, and benzo(e)pyrene, 9.153 A, precludes their passage through this channel access in contrast to benzo(a)pyrene and 2-acetylaminofluorene having a width of 7.150 and 5.283 A, respectively, explaining their difference in CYP1A1 induction potential.

Ro 09-1470 is a selective inhibitor of P-450 lanosterol C-14 demethylase of fungi

Ro 09-1470 is a new antifungal agent that belongs to a series of compounds characterized by a tetrahydropyran skeleton with glycine and alkenyl side chains and that inhibits P-450 lanosterol C-14 demethylase (P-450(14DM)) of fungi (Y. Aoki, T. Yamazaki, M. Kondoh, Y. Sudoh, N. Nakayama, Y. Sekine, H. Shimada, and M. Arisawa, J. Antibiot. 45:160-170, 1992; S. Matsukuma, T. Ohtsuka, H. Kotaki, H. Sawairi, T. Sano, K. Watanabe, N. Nakayama, Y. Itezono, M. Fujiu, N. Shimma, K. Yokose, and T. Okuda, J. Antibiot. 45:151-159, 1992). We have studied the compound's mode of interaction with fungal P-450(14DM) and its selectivity for the fungal versus mammalian P-450 enzymes. Ro 09-1470 bound to the Saccharomyces cerevisiae P-450(14DM) by coordinating to the heme with one-to-one stoichiometry. Unlike the azole compounds, it interacted with both ferric and ferrous heme. It was active also against the P-450(14DM) of Candida albicans. Ro 09-1470 preferentially inhibited the yeast P-450(14DM), showing a 50% inhibitory concentration (IC50) of 0.47 to approximately 1.1 microM, which is much lower than the IC50s for rat hepatic P-450s catalyzing cholesterol biosynthesis (IC50 = 341 microM), p-nitroanisol O-demethylation (> 1,000 microM), aniline hydroxylation (> 1,000 microM), and aminopyrine N-demethylation (920 microM). The degree of selectivity for yeast P-450 was higher than that of ketoconazole.

The metabolism and activation of 15,16-dihydrocyclopenta[a]phenanthren-17-one by cytochrome P-450 proteins

The in vitro metabolism and activation to mutagens of 15,16-dihydrocyclopenta[a]phenanthren-17-one (CPP-17-one) were investigated using hepatic preparations from rats pretreated with prototype inducers of the cytochrome P-450-dependent mixed-function oxidases. Aroclor 1254-induced microsomes were the most effective metabolisers of this compound, the major metabolites being oxidation products of the bay region A ring. To a lesser extent hydroxylation of the non-aromatic D ring occurred, the products being the 15- and 16-hydroxyderivatives. Oxidation of the A ring was also achieved with microsomes from benzo[a]pyrene-treated rats but not with those from rats treated with clofibrate, phenobarbitone, isoniazid, dexamethasone and CPP-17-one itself, where the metabolites were primarily the oxidation products of the D ring. When CPP-17-one was used as a promutagen in the Ames test, only microsomes from Aroclor 1254-treated rats could elicit a positive mutagenic response. When 3,4-dihydrodihydroxy-CPP-17-one, the precursor of the ultimate mutagen, was used as the promutagen, a positive response was observed with microsomes from Aroclor 1254- and benzo[a]pyrene-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Validation of a novel molecular orbital approach (COMPACT) for the prospective safety evaluation of chemicals, by comparison with rodent carcinogenicity and Salmonella mutagenicity data evaluated by the U.S. NCI/NTP

The molecular dimensions and electronic structures of 100 chemicals of structural diversity have been determined from molecular orbital calculations and molecular mechanics. From these parameters of molecular structure, those chemicals that are likely substrates of cytochromes P4501 and P4502E have been identified by the computer-optimized molecular parametric analysis of chemical toxicity (COMPACT) programme, and their potential toxicity, mutagenicity and carcinogenicity evaluated. The degree of correlation between COMPACT prediction of toxicity and rodent two species life-span carcinogenicity data is estimated to be 92%, and between COMPACT and Salmonella mutagenicity (Ames test) data is 64%. Anomalous rodent carcinogens are rationalized on the basis of biochemical mechanisms of metabolism, genotoxicity and carcinogenicity. Correlation of the Ames test data with rodent carcinogenicity data was 64%, but correlation of COMPACT plus Ames data versus rodent carcinogenicity data provided the highest correlation of 94%.

Antimutagenicity of ellagic acid towards the food mutagen IQ: investigation into possible mechanisms of action

The ability of the plant phenol ellagic acid to inhibit the mutagenicity of the food mutagen IQ was evaluated using Salmonella typhimurium strain TA98 in the Ames mutagenicity test. Ellagic acid caused a concentration-dependent decrease in the S-9- and microsome-mediated mutagenicity of IQ. The plant phenol did not interact directly with the IQ-derived mutagenic species and did not modify the cytosol-mediated activation of the promutagen. At the concentrations used in the mutagenicity studies, ellagic acid failed to inhibit microsomal mixed-function oxidase activity, including that mediated by the P450I family responsible for the bioactivation of IQ, despite being an essentially planar molecule as indicated by computer-graphic analysis. The inhibitory effect of ellagic acid was independent of its ability to chelate Mg2+. However, pre-incubation of ellagic acid with the bacteria, followed by removal of the plant phenol, did not completely prevent the inhibitory effect of the phenol on the mutagenicity of IQ. Intraperitoneal administration of ellagic acid to rats caused a decrease in total cytochrome P-450 levels and related activities as well as in cytosolic glutathione S-transferase activity. Finally, the possibility that the reported anticarcinogenic action of ellagic acid reflects nothing more than non-selective destruction of hepatic cytochromes P-450, and thus reduced chemical activation, is considered.

Relative stabilities of nitrenium ions derived from polycyclic aromatic amines. Relationship to mutagenicity

The relative energetics of arylamine N-hydroxylation and N-O heterolysis (ArNH2----ArNHOH----ArNH+) for condensed systems of two, three and four rings were calculated using semiempirical AM1 molecular orbital theory. The overall thermodynamics of N-hydroxylation were almost insensitive to the structure of the amine while differences in the energetics of nitrenium ion formation varied from 0 to 35 kcal mol-1. Limited correlations between the latter and the experimental TA98 and TA100 mutagenicities of the amines are presented.

Differential inhibition of individual human liver cytochromes P-450 by cimetidine

Cimetidine binds to cytochrome P-450 and inhibits hepatic metabolism of various drugs in humans. However, cytochrome P-450 is a family of enzymes rather than a single protein, and effects of cimetidine on individual human liver cytochromes P-450 have not been previously characterized. Metabolism of selected substrates and cimetidine-binding assays have been performed using human liver microsomes, purified human liver cytochromes P-450, and cytochrome P-450 complementary DNA-expressed yeast proteins to probe interaction of cimetidine with these individual enzymes. Cimetidine (3.0 mmol/L) in incubations reduced bufuralol hydroxylase activity by 80% and strongly inhibited microsomal nifedipine oxidation (23% +/- 13% of control activity). The same concentration of cimetidine produced intermediate inhibition of cytochrome enzymes responsible for ethoxyresorufin deethylation and aniline hydroxylation (77% +/- 6% and 68% +/- 17% of activity in control microsomal incubations, respectively), but little effect on tolbutamide hydroxylation was observed. Concordantly, the calculated binding constant for the binding of cimetidine to a purified cytochrome P-450 with high tolbutamide hydroxylase activity was 4.4 mmol/L, whereas the calculated binding concentration constant for a purified cytochrome P-450-metabolizing nifedipine was 0.7 mmol/L. These studies show a high variability in the effect of cimetidine on drug metabolism by individual human liver cytochromes P-450. In vitro studies using human liver microsomes and genetically engineered human cytochromes P-450 can be very useful in exploring important clinical questions of hepatic drug metabolism.

Structure-related inhibitory effect of quinolones on alkyl-xanthine elimination in rats

To investigate the relationship between the chemical structures of quinolones, enoxacin (ENX) and its analogues, and their metabolic inhibitory effects on theophylline, a xanthine derivative closely related to theophylline, 1-methyl-3-propylxanthine (MPX), was used as a model of theophylline in rats. The disappearance of MPX from plasma was significantly delayed by treatment with ENX and analogue A (derivatives without substituent group at both 3'- and 5'- carbon atom in the piperazinyl ring): total body clearance of MPX was significantly decreased by approximately 50%. However, analogue A was converted into ENX in the rat body (about 14% of dose). Analogues B and C (derivatives with substituent group at 3'- or 5'-carbon atom in the piperazinyl ring) had little or no effect on MPX disposition. No significant change in the volume of distribution of MPX was observed after coadministration with these quinolones. The results of this study indicate that the substitutions on 3' and 5'-carbon atoms of piperazinyl ring at 7-position of the quinolone molecule may play important role in the inhibition of theophylline metabolism.