Relationship of naphthalene and 2-methylnaphthalene metabolism to pulmonary bronchiolar epithelial cell necrosis

Development of a Novel AOP for Cyp2F2-Mediated Lung Cancer in Mice

Traditional methods for carcinogenicity testing rely heavily on the rodent bioassay as the standard for identification of tumorigenic risk. As such, identification of species-specific outcomes and/or metabolism are a frequent argument for regulatory exemption. One example is the association of tumor formation in the mouse lung after exposure to Cyp2F2 ligands. The adverse outcome pathway (AOP) framework offers a theoretical platform to address issues of species specificity that is consistent, transparent, and capable of integrating data from new approach methodologies as well as traditional data streams. A central premise of the AOP concept is that pathway progression from the molecular initiating event (MIE) implies a definable “response-response” (R-R) relationship between each key event (KE) that drives the pathway towards a specific adverse outcome (AO). This article describes an AOP for lung cancer in the mouse from an MIE of Cyp2F2-specific reactive metabolite formation, advancing through KE that include protein and/or nucleic acid adducts, diminished Club Cell 10 kDa (CC10) protein expression, hyperplasia of CC10 deficient Club cells, and culminating in the AO of mixed-cell tumor formation in the distal airways. This tumor formation is independent of route of exposure and our AOP construct is based on overlapping mechanistic events for naphthalene, styrene, ethyl benzene, isoniazid, and fluensulfone in the mouse. This AOP is intended to accelerate the explication of an apparent mouse-specific outcome and serve as a starting point for a quantitative analysis of mouse-human differences in susceptibility to the tumorigenic effects of Cyp2F2 ligands.

A cell culture analogue of rodent physiology: Application to naphthalene toxicology

The difficulties of large-scale animal testing of compounds has spurred development of in vitro testing methods and physiologically based pharmacokinetic models (PBPK). In existing in vitro methods, tissue interactions occurring in vivo are not reproduced accurately and in PBPKs the a priori prediction of metabolism is difficult. Through development of a multicompartmental, multiple cell type bioreactor system these limitations can be circumvented. A cell culture analogue (CCA) of a PBPK was developed. The CCA contains multiple chambers, each of which represents a tissue or group of similar tissues as specified in the PBPK. Proof-of-concept experiments were done using naphthalene as a model. Naphthalene is converted into naphthalene oxide and the circulation of this reactive metabolite from the liver to lung is a possible mechanism for lung injury. A CCA with liver, lung and other tissue compartments was constructed. This system was used in conjunction with cultured H4IIE rat hepatoma cells and L2 rat lung cells to study the importance of circulated naphthalene metabolites (presumably naphthalene oxides) on lung cell toxicity in rodents. By increasing the number of cells and/or inducing cytochrome P-450 activity in the liver compartment, lung cell mortality was increased. Glutathione depletion in the lung and liver cells was also observed. These results indicate that the CCA is a potentially useful concept for studying the action of compounds with reactive metabolites.

Functional analysis of two distinct bronchiolar progenitors during lung injury and repair

Air spaces of the mammalian lung are lined by a specialized epithelium that is maintained by endogenous progenitor cells. Within bronchioles, the abundance and distribution of progenitor cells that contribute to epithelial homeostasis change as a function of maintenance versus repair. It is unclear whether functionally distinct progenitor pools or a single progenitor cell type maintain the epithelium and how the behavior is regulated in normal or disease states. To address these questions, we applied fractionation methods for the enrichment of distal airway progenitors. We show that bronchiolar progenitor cells can be subdivided into two functionally distinct populations that differ in their susceptibility to injury and contribution to repair. The proliferative capacity of these progenitors is confirmed in a novel in vitro assay. We show that both populations give rise to colonies with a similar dependence on stromal cell interactions and regulation by TGF-β. These findings provide additional insights into mechanisms of epithelial remodeling in the setting of chronic lung disease and offer hope that pharmacologic interventions may be developed to mitigate tissue remodeling.

An adjustment factor for mode-of-action uncertainty with dual-mode carcinogens: the case of naphthalene-induced nasal tumors in rats

The U.S. Environmental Protection Agency (USEPA) guidelines for cancer risk assessment recognize that some chemical carcinogens may have a site-specific mode of action (MOA) involving mutation and cell-killing-induced hyperplasia. The guidelines recommend that for such dual MOA (DMOA) carcinogens, judgment should be used to compare and assess results using separate "linear" (genotoxic) versus "nonlinear" (nongenotoxic) approaches to low-level risk extrapolation. Because the guidelines allow this only when evidence supports reliable risk extrapolation using a validated mechanistic model, they effectively prevent addressing MOA uncertainty when data do not fully validate such a model but otherwise clearly support a DMOA. An adjustment-factor approach is proposed to address this gap, analogous to reference-dose procedures used for classic toxicity endpoints. By this method, even when a "nonlinear" toxicokinetic model cannot be fully validated, the effect of DMOA uncertainty on low-dose risk can be addressed. Application of the proposed approach was illustrated for the case of risk extrapolation from bioassay data on rat nasal tumors induced by chronic lifetime exposure to naphthalene. Bioassay data, toxicokinetic data, and pharmacokinetic analyses were determined to indicate that naphthalene is almost certainly a DMOA carcinogen. Plausibility bounds on rat-tumor-type-specific DMOA-related uncertainty were obtained using a mechanistic two-stage cancer risk model adapted to reflect the empirical link between genotoxic and cytotoxic effects of the most potent identified genotoxic naphthalene metabolites, 1,2- and 1,4-naphthoquinone. Bound-specific adjustment factors were then used to reduce naphthalene risk estimated by linear extrapolation (under the default genotoxic MOA assumption), to account for the DMOA exhibited by this compound.

Naphthalene metabolism in relation to target tissue anatomy, physiology, cytotoxicity and tumorigenic mechanism of action

This report provides a summary of deliberations conducted under the charge for members of Module C Panel participating in the Naphthalene State-of-the-Science Symposium (NS(3)), Monterey, CA, October 9-12, 2006. The panel was charged with reviewing the current state of knowledge and uncertainty about naphthalene metabolism in relation to anatomy, physiology and cytotoxicity in tissues observed to have elevated tumor incidence in these rodent bioassays. Major conclusions reached concerning scientific claims of high confidence were that: (1) rat nasal tumor occurrence was greatly enhanced, if not enabled, by adjacent, histologically related focal cellular proliferation; (2) elevated incidence of mouse lung tumors occurred at a concentration (30 ppm) cytotoxic to the same lung region at which tumors occurred, but not at a lower and less cytotoxic concentration (tumorigenesis NOAEL=10 ppm); (3) naphthalene cytotoxicity requires metabolic activation (unmetabolized naphthalene is not a proximate cause of observed toxicity or tumors); (4) there are clear regional and species differences in naphthalene bioactivation; and (5) target tissue anatomy and physiology is sufficiently well understood for rodents, non-human primates and humans to parameterize species-specific physiologically based pharmacokinetic (PBPK) models for nasal and lung effects. Critical areas of uncertainty requiring resolution to enable improved human cancer risk assessment were considered to be that: (1) cytotoxic naphthalene metabolites, their modes of cytotoxic action, and detailed low-dose dose-response need to be clarified, including in primate and human tissues, and neonatal tissues; (2) mouse, rat, and monkey inhalation studies are needed to better define in vivo naphthalene uptake and metabolism in the upper respiratory tract; (3) in vivo validation studies are needed for a PBPK model for monkeys exposed to naphthalene by inhalation, coupled to cytotoxicity studies referred to above; and (4) in vivo studies are needed to validate a human PBPK model for naphthalene. To address these uncertainties, the Panel proposed specific research studies that should be feasible to complete relatively promptly. Concerning residual uncertainty far less easy to resolve, the Panel concluded that environmental, non-cytotoxic exposure levels of naphthalene do not induce tumors at rates that can be predicted meaningfully by simple linear extrapolation from those observed in rodents chronically exposed to far greater, cytotoxic naphthalene concentrations.

In vitro toxicity of naphthalene, 1-naphthol, 2-naphthol and 1,4-naphthoquinone on human CFU-GM from female and male cord blood donors

In animal models, naphthalene toxicity has been studied in different target organs and has been shown to be gender-dependent and metabolism related. In humans, it is readily absorbed and is metabolised by several cytochrome P450's. Naphthalene and its metabolites can cross the placental barrier and consequently may affect foetal tissues. The aim of this study was to compare the in vitro toxicity of naphthalene and its metabolites, 1-naphthol, 2-naphthol and 1,4-naphthoquinone, on human haematopoietic foetal progenitors (CFU-GM) derived from newborn male and female donors. The mRNA expression of Cyp1A2 and Cyp3A4 was also evaluated. Naphthalene did not affect CFU-GM proliferation, while 1-naphthol, 2-naphthol and particularly 1,4-naphthoquinone strongly inhibited the clonogenicity of progenitors, from both male and female donors. mRNA of Cyp1A2 and Cyp3A4 was not expressed neither at the basal level, nor after naphthalene treatment, while treatment with 1,4-naphthoquinone induced expression of both enzymes in both genders, with Cyp1A2 being expressed four times more than Cyp3A4. Female CFU-GM was significantly more sensitive to 1,4-naphthoquinone than male and after treatment both enzymes were expressed twice as much as in the male precursors. These results suggest that a gender-specific 1,4-naphthoquinone metabolic pathway may exist, which gives rise to unknown toxic metabolites.

Racemic and chiral lactams as potent, selective and functionally active CCR4 antagonists

A series of racemic and chiral, nonracemic lactams that display high binding affinities, functional chemotaxis antagonism, and selectivity toward CCR4 are described. Compound 41, which provides reasonably high blood levels in mice when dosed intraperitoneally, was identified as a useful pharmacological tool to explore the role of CCR4 antagonism in animal models of allergic disease.

Predictive value of comparative molecular field analysis modelling of naphthalene inhibition of human CYP2A6 and mouse CYP2A5 enzymes

The objects of this study were first to compare how well the recently constructed structure-inhibition activity relationship models of mouse CYP2A5 and human CYP2A6 predict the interaction of naphthalene in liver microsomes and secondly to study if these CYP enzymes actually oxidize naphthalene. The CoMFA model of CYP2A5 predicted the IC(50) value of naphthalene to be 42 microM (18-115 microM 95% CL) whereas in the in vitro experiment the result was 74 microM (65-83 microM) with the corresponding values for CYP2A6 being 41 microM (18-112 microM) and 25 microM (21-30 microM), respectively. Naphthalene appeared to be a competitive inhibitor both for mouse and human liver microsomal coumarin 7-hydroxylase, which is the specific probe activity for CYP2A5 and CYP2A6. The K(i)-value for the mouse enzyme was between 12-26 microM and for the human enzyme 1.2-5.6 microM. A 1-h in vitro incubation of naphthalene with human and pyrazole treated mouse liver microsomes produced more 1-naphthol than 2-naphthol. Antibody against the purified CYP2A5 inhibited 50-60% of the formation of 1-naphthol and 30-40% of the formation of 2-naphthol. These results indicate that in silico CoMFA models predict relatively well the interaction of naphthalene with CYP2A5 and CYP2A6 and that these CYPs actually oxidize naphthalene in vitro. CoMFA CYP2A5 and CYP2A6 models are thus useful as a technique for elucidating the interaction and potency of untested chemicals with these CYPs.

Neurotoxicity of polycyclic aromatic hydrocarbons and simple chemical mixtures

Polycyclic aromatic hydrocarbons (PAHs) are a major class of environmental pollutants. These chemicals are the products of incomplete combustion and are present in every compartment of the environment. While the carcinogenic potential of these chemicals has been investigated in numerous studies, very little is known about the potential of these chemicals to produce damage to neural cells. The objective of this study was to investigate the toxicity of several model PAHs and binary mixtures of these chemicals in neural cells. Chemicals tested included benzo[a]pyrene (BaP), chrysene, anthracene, and pentachlorophenol (PCP). Four end points, including amino acid incorporation, total protein, total cell count, and viable cells (trypan dye exclusion), were measured in SY5Y human neuroblastoma cells and C6 rat glioma cells. The most sensitive measure of PAH toxicity in neural cells was amino acid incorporation into proteins. BaP was the most toxic of all PAHs tested, and anthracene failed to produce a toxic response at any concentration tested. Without metabolic activation, BaP induced a significant cytotoxic response at a concentration of 30 microM. With activation (0.25% S9), BaP induced a response at concentration levels of 3 microM and 30 microM. Minimal toxicity was observed with chrysene at the highest concentration tested, and anthracene failed to produce a toxic response at any concentration tested. With mixtures of PAHs the majority of samples induced additive responses. The minimum concentration required to induce a significant response was reduced for the mixture of chrysene and BaP when compared to BaP alone. In addition, PCP appeared to increase the inhibition of acetylcholinesterase by mipafox. The data suggest that PAHs are capable of producing damage to neural cells only at concentrations that are near their solubility limits.

Preliminary in vitro toxicological evaluation of a series of 2-pyridylcarboxamidrazone candidate anti-tuberculosis compounds III

We have evaluated the cytotoxicity of a series of novel anti-tubercular 2-pyridyl carboxamidrazones through incubation with human mononuclear leucocytes (MNL), with and without a rat microsomal metabolising system. Isoniazid (INH), the closest structurally related agent, was used as a positive control. Incubation of the 3-benzyloxy-benzylidene, dimethylpropyl-benzylidene and 4-phenyl-benzylidene with MNL showed no significant toxicity in comparison with either INH or DMSO vehicle control. However, the 4-N,N-dimethylamino-1-naphthylidene derivative exerted more than sevenfold greater toxicity compared with INH, while the 4-N,N-dimethylamino-1-naphthylidene, 2-benzyloxy-3-methoxy-benzylidene, 2-t-butylthio-benzylidene and 4-i-propyl-benzylidene derivatives showed toxicity which ranged from five to fourfold that of INH. In the presence of either rat microsomes with or without NADPH, the 3-benzyloxy-benzylidene, dimethylpropyl-benzylidene and 4-phenyl-benzylidene derivatives showed no metabolically-mediated cytotoxicity. The latter two derivatives showed a combination of low toxicity and considerabe efficacy against Mycobacteria tuberculosis in vitro and show promise for future development.

Hepatic and pulmonary enzyme activities in horses

OBJECTIVE

To determine hepatic and pulmonary phase-I and phase-II enzyme activities in horses.

SAMPLE POPULATION

Pulmonary and hepatic tissues from 22 horses that were 4 months to 32 years old.

PROCEDURE

Pulmonary and hepatic tissues from horses were used to prepare cytosolic (glutathione S-transferase and soluble epoxide hydrolase) and microsomal (cytochrome P450 monooxygenases) enzymes. Rates of microsomal metabolism of ethoxyresorufin, pentoxyresorufin, and naphthalene were determined by high-performance liquid chromatography. Activities of glutathione S-transferase and soluble epoxide hydrolase were determined spectrophotometrically. Cytochrome P450 content was determined by carbon monoxide bound-difference spectrum of dithionite-reduced microsomes. Activity was expressed relative to total protein concentration.

RESULTS

Microsomal protein and cytochromeP450 contents were detectable in all horses and did not vary with age. Hepatic ethoxyresorufin metabolism was detected in all horses; by comparison, pulmonary metabolism of ethoxyresorufin and hepatic and pulmonary metabolism of pentoxyresorufin were detected at lower rates. Rate of hepatic naphthalene metabolism remained constant with increasing age, whereas rate of pulmonary naphthalene metabolism was significantly lower in weanlings (ie, horses 4 to 6 months old), compared with adult horses. Hepatic glutathione S-transferase activity (cytosol) increased with age; however, these changes were not significant. Pulmonary glutathione S-transferase activity (cytosol) was significantly lower in weanlings than adult horses. Hepatic and pulmonary soluble epoxide hydrolase did not vary with age of horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Activity of cytochrome P450 isoforms that metabolize naphthalene and glutathione S-transferases in lungs are significantly lower in weanlings than adult horses, which suggests reduced ability of young horses to metabolize xenobiotics by this organ.

CYP1A1 and CYP1B1, two hydrocarbon-inducible cytochromes P450, are constitutively expressed in neonate and adult goat liver, lung and kidney

The ontogeny of cytochrome P-450 isozymes (P450) in goat liver, lung and kidney was studied using anion exchange HPLC separation of solublized microsomal proteins and Western immunoblotting. Comparison of the overall HPLC profile of goat P450 isozymes between liver, lung and kidney showed that while the P450's of goat liver were equally separated into five peaks of isozyme(s), only two peaks constitute the majority of P450 isozyme(s) in lung and kidney, thus demonstrating the tissue specific differences in P450 isozyme distribution in goats. Immunoblotting analysis using polyclonal antibodies against rat CYP1B1, and mouse CYP1B1, polyaromatic hydrocarbon-regulated P450's, revealed that goat orthologs of CYP1A1 and CYP1B1 are expressed constitutively in goats. The CYP1A1 was expressed in goat liver and lung as early as 1st day of age, and the levels of its expression in adult lung and liver were, respectively, 1.3 and 5.5 pmol per mg microsomal proteins. CYP1B1 was expressed in goat livers in substantial levels as of 1 week of age and increased thereafter to reach approximately 4.5 pmol per mg microsomal proteins in adult livers, while low level was detectable only in adult but not neonate lung tissues. Furthermore, polyclonal antibodies against rat CYP1A2 detected very high levels of CYP1A2 in livers of adult and 6 week old goats. The Ah receptor which controls the expression of CYP1A1/1A2 and CYP1B1, was detected in cytosolic fractions from these tissues as a 104 kDa and a minor level of the 106 kDa form. These are potentially very important findings in light of the role of CYP1A1/1A2 and CYP1B1 in activation of polyaromatic hydrocarbons, heterocyclic amines and nitroaromatic hydrocarbons to genotoxic metabolites, and the health consequences of these metabolites on humans, as consumers of goat milk and meat. Using polyclonal antibodies against rat hepatic CYP2B1 and CYP3A1, the goat CYP2B and CYP3A forms were not detectable in livers of goats at any age, but lungs of adult and 6 week old goats expressed these two CYPs in levels equivalent to the livers of phenobarbital-induced rats. On the other hand, anti-rat CYP2C6 antibodies specifically detected two goat ortholog forms which were expressed in all three tissues and exhibited age-dependent changes. In conclusion, results from both immunoblot and HPLC analyses confirmed that, as in other species, the expression of P450 isozymes in goat is under both developmental- and tissue-specific regulatory factors.

Metabolism and toxicity of diisopropylnaphthalene as compared to naphthalene and monoalkyl naphthalenes: a minireview

Detailed knowledge does exist on the toxicological safety of diisopropylnaphthalene (DIPN). Its metabolism is the key to understanding its very low toxicity. The metabolic pathway of 2,6-DIPN in rats was found to proceed almost exclusively through oxidation of the isopropyl side-chain. This has decisive toxicological implications, which could be demonstrated by comparing the lung-specific toxic effects of naphthalenes in mouse: the lack of ring oxidation correlates well with lack of lung toxicity while, vice versa, the extent of enzymatic oxidative attack at the aromatic ring structure results in a toxic pattern that is observed with naphthalene and its monomethyl derivatives. It is concluded that DIPN and other highly alkylated naphthalenes are supposed to offer favourable safety properties because of their 'alkyl character' and therefore must not be compared with the toxic properties of naphthalene and closely related aromatic compounds.

A preliminary physiologically based pharmacokinetic model for naphthalene and naphthalene oxide in mice and rats

Naphthalene is a toxicant with unusual species and tissue specificity that has been the subject of in vitro studies. We describe a preliminary physiologically based pharmacokinetic (PBPK) model for naphthalene constructed solely from in vitro data for comparison to animal data without the use of adjustable parameters. The prototypical PBPK model containing five lumped tissue compartments was developed to describe the uptake and metabolism of naphthalene by mice and rats dosed intraperitoneally (i.p.) and orally (po). The model incorporates circulation and biotransformation of the semistable reactive intermediate, naphthalene oxide, as well as the parent compound naphthalene. Circulation is included because the toxic action of naphthalene has been proposed to be caused by the formation of a reactive metabolite in one organ (liver) and its circulation to another organ (lung) being adversely affected by the metabolite. The model allows conversion of naphthalene oxide into dihydrodiol, glutathione (GSH) conjugates, 1-naphthol (non-enzymatically) and covalently bound adducts with proteins. Model simulations are compared with previously reported in vivo measurements of glutathione depletion, mercapturic acid formation, and covalently bound protein formation. The mouse model predicts accurately the amount of mercapturates excreted, the effect of various pretreatments, and the extent of covalent binding in the lung and liver resulting from ip administration, including the sharp increase in binding between 200 and 400 mg/kg.

A mechanism for the development of Clara cell lesions in the mouse lung after exposure to trichloroethylene

Female CD-1 mice exposed to trichloroethylene (6 h/day) at concentrations from 20-2000 ppm developed a highly specific lung lesion after a single exposure, characterised by vacuolation of the Clara cells, the number of cells affected increasing with increasing dose level. At the highest dose levels pyknosis of the Clara cells was apparent. After 5 days of repeated exposures the lesion had resolved but exposure of mice following a 2-day break resulted in recurrence of the lesion. The changes in mouse lung Clara cells were accompanied by a marked loss of cytochrome P-450 activities. No morphological changes were seen in the lungs of rats exposed to either 500 or 1000 ppm trichloroethylene. Isolated mouse lung Clara cells were shown to metabolize trichloroethylene to chloral, trichloroethanol and trichloroacetic acid. Chloral was the major metabolite. Trichloroethanol glucuronide was not detected. In comparative experiments using mouse hepatocytes the major metabolites were trichloroethanol and its glucuronide conjugate. The activity of UDP-glucuronosyltransferase was compared in mouse lung Clara cells and hepatocytes using two phenolic substrates and trichloroethanol. Hepatocytes readily formed glucuronides from all three substrates whereas Clara cells were only active with the two phenolic substrates. The three major metabolites of trichloroethylene, chloral, trichloroethanol and trichloroacetic acid were each dosed to mice and of these metabolites, only chloral had an effect on mouse lung causing a lesion (Clara cell) identical to that seen with trichloroethylene. It is proposed that the failure of Clara cells to conjugate trichloroethanol leads to an accumulation of chloral which results in cytotoxicity. The known genotoxicity of chloral suggests that this lesion may be related to the development of lung tumours in mice exposed to trichloroethylene by inhalation.

Morphologic and morphometric techniques for the detection of drug- and toxin-induced changes in lung

The lung is one of the main target organs of drug-induced toxicity. An assemblage of quantitative techniques is available to make precise determinations of structural effects. While stereology is the principal technique, particularly in its application to the parenchyma, other compartments such as the airways and vasculature demand modifications or different methods altogether. The new methods of molecular biology can now be used to uncover the mechanisms underlying drug toxicity and with a more rational use of image analysis are likely to yield quantitative data. Established techniques that quantify structural change combined with more novel approaches that utilize molecular interventions may emerge as exciting integrated approaches in this important field.

Effects of naphthalene and naphthalene metabolites on the in vitro humoral immune response

Naphthalene-induced pulmonary and renal toxicity and polycyclic aromatic hydrocarbon-induced carcinogenesis are known to be mediated by their reactive metabolites. Subchronic exposure (90 d) of mice to naphthalene does not alter humoral and cellular-mediated immune responses, whereas polycyclic aromatic hydrocarbons, such as benzo[a]pyrene and 7,12-dimethylbenzanthracene, are known to be immunosuppressive. To understand these differences, the antibody-forming cell (AFC) responses of splenocyte cultures exposed to naphthalene (2, 20, and 200 microM) were evaluated. At these concentrations, the antibody-forming cell response to sheep red blood cells (RBC) was not affected. To determine if reactive metabolites of naphthalene were immunosuppressive, splenocytes were exposed to naphthalene metabolites by direct addition or through the use of a metabolic activation system. The addition of 1-naphthol (70 and 200 microM) and 1,4-naphthoquinone (2, 7, and 20 microM) resulted in a decreased antibody-forming cell response. Suppression of AFC responses was also obtained by culturing splenocytes with liver S9 and naphthalene. Since splenic metabolism of naphthalene to nonimmunosuppressive metabolites may account for the absence of immunotoxicity, the types of naphthalene metabolites generated by splenic microsomes were determined. It was observed that splenic microsomes were unable to generate any detectable naphthalene metabolites, whereas liver microsomes were able to generate both 1,2-naphthalene diol and 1-naphthol. Thus, the absence of an immunosuppressive effect by naphthalene exposure may be related to the inability of splenocytes to metabolize naphthalene. Moreover, the concentration of naphthalene metabolites generated within the liver that may diffuse to the spleen may be inadequate to produce immunotoxicity.

Localization, distribution, and induction of xenobiotic-metabolizing enzymes and aryl hydrocarbon hydroxylase activity within lung

The metabolism of xenobiotics within lung often leads to toxicity, although certain pulmonary cells are more readily damaged than others. This differential susceptibility can result from cell-specific differences in xenobiotic activation and detoxication. The localization and distribution of xenobiotic-metabolizing enzymes (cytochromes P-450, NADPH-cytochrome P-450 reductase, epoxide hydrolase, glutathione S-transferases, UDP-glucuronosyltransferases, and a sulfotransferase) and of aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity determined immunohistochemically and histochemically, respectively, within lung are discussed. Findings reveal that xenobiotics can be metabolized in situ, albeit to different extents, by bronchial epithelial cells, Clara and ciliated bronchiolar epithelial cells, and type II pneumocytes and other alveolar wall cells and that enzymes and activities are not necessarily induced uniformly among these cells.