Genotoxicity of methyl acrylate and ethyl acrylate and its relationship with glutathione.

Methyl acrylate (MA) and ethyl acrylate (EA) had previously tested positive for mutagenicity in vitro, but in vivo studies were negative. One of the metabolism pathways of alkyl acrylates is conjugation with glutathione. The glutathione availability is restricted in standard in vitro test systems so that they do not reflect the in vivo metabolism in this respect. We investigated whether the addition of glutathione to the in vitro L5178Y/TK+/- mouse lymphoma mutagenicity test prevents alkyl acrylate's mutagenicity in vitro. We also investigated whether the quantitative relationships support the notion that the GSH supplemented in vitro systems reflect the true in vivo activity. Indeed, glutathione concentrations as low as 1 mM completely negate the mutagenicity of MA and EA in the L5178Y/TK+/- mouse lymphoma mutagenicity test up to the highest concentrations of the two acrylates tested, 35 µg/ml, a higher concentration than that previously found to be mutagenic in this test (14 µg MA/ml and 20 µg EA/ml). 1 mM Glutathione reduced the residual MA and EA at the end of the exposure period in the mutagenicity tests by 96-97%, but in vivo up to 100 mg/kg body weight MA and EA left the glutathione levels in the mouse liver and forestomach completely intact. It is concluded that the in-situ levels of glutathione, 7.55 ± 0.57 and 2.84 ± 0.22 µmol/g mouse liver and forestomach, respectively, can efficiently protect against MA and EA-induced mutagenicity up to the high concentration of 100 mg MA and EA/kg body weight and that the negative in vivo mutagenicity tests on MA and EA reflect the true in vivo situation.

N-vinyl compounds: studies on metabolism, genotoxicity, carcinogenicity.

Several N-vinyl compounds are produced in high volumes and are widely employed in the production of copolymers and polymers used in chemical, pharmaceutical, cosmetic and food industry. Hence, information on their genotoxicity and carcinogenicity is requisite. This review presents hitherto available information on the carcinogenicity and genotoxicity of N-vinyl compounds as well as their metabolism potentially generating genotoxic and carcinogenic derivatives. The genotoxicity and carcinogenicity of the investigated N-vinyl compounds vary widely from no observed carcinogenicity tested in lifetime bioassays in two rodent species (up to very high doses) to carcinogenicity in rats at very low doses in the absence of apparent genotoxicity. Despite of the presence of the vinyl group potentially metabolized to an epoxide followed by covalent binding to DNA, genotoxicity was observed for only one of the considered N-vinyl compounds, N-vinyl carbazole. Carcinogenicity was investigated only for two, of which one, N-vinyl pyrrolidone was carcinogenic (but not genotoxic) and ranitidine was neither carcinogenic nor genotoxic. As far as investigated, neither a metabolically formed epoxide nor a therefrom derived diol has been reported for any of the considered N-vinyl compounds. It is concluded that the information collected in this review will further the understanding of the carcinogenic potentials of N-vinyl compounds and may eventually allow approaching their prediction and prevention. A suggestion how to prevent genotoxicity in designing of N-vinyl compounds is presented. However, the available information is scarce and further research especially on the metabolism of N-vinyl compounds is highly desirable.

Xenobiotica-metabolizing enzymes in the lung of experimental animals, man and in human lung models.

The xenobiotic metabolism in the lung, an organ of first entry of xenobiotics into the organism, is crucial for inhaled compounds entering this organ intentionally (e.g. drugs) and unintentionally (e.g. work place and environmental compounds). Additionally, local metabolism by enzymes preferentially or exclusively occurring in the lung is important for favorable or toxic effects of xenobiotics entering the organism also by routes other than by inhalation. The data collected in this review show that generally activities of cytochromes P450 are low in the lung of all investigated species and in vitro models. Other oxidoreductases may turn out to be more important, but are largely not investigated. Phase II enzymes are generally much higher with the exception of UGT glucuronosyltransferases which are generally very low. Insofar as data are available the xenobiotic metabolism in the lung of monkeys comes closed to that in the human lung; however, very few data are available for this comparison. Second best rate the mouse and rat lung, followed by the rabbit. Of the human in vitro model primary cells in culture, such as alveolar macrophages and alveolar type II cells as well as the A549 cell line appear quite acceptable. However, (1) this generalization represents a temporary oversimplification born from the lack of more comparable data; (2) the relative suitability of individual species/models is different for different enzymes; (3) when more data become available, the conclusions derived from these comparisons quite possibly may change.

Correction to: Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models.

This article is published with the incorrect copyright holder name in the HTML article as "© Springer 2018". The correct copyright line should read "The Author(s) 2018" (as it appears in the article PDF).

Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models.

Studies on the metabolic fate of medical drugs, skin care products, cosmetics and other chemicals intentionally or accidently applied to the human skin have become increasingly important in order to ascertain pharmacological effectiveness and to avoid toxicities. The use of freshly excised human skin for experimental investigations meets with ethical and practical limitations. Hence information on xenobiotic-metabolizing enzymes (XME) in the experimental systems available for pertinent studies compared with native human skin has become crucial. This review collects available information of which-taken with great caution because of the still very limited data-the most salient points are: in the skin of all animal species and skin-derived in vitro systems considered in this review cytochrome P450 (CYP)-dependent monooxygenase activities (largely responsible for initiating xenobiotica metabolism in the organ which provides most of the xenobiotica metabolism of the mammalian organism, the liver) are very low to undetectable. Quite likely other oxidative enzymes [e.g. flavin monooxygenase, COX (cooxidation by prostaglandin synthase)] will turn out to be much more important for the oxidative xenobiotic metabolism in the skin. Moreover, conjugating enzyme activities such as glutathione transferases and glucuronosyltransferases are much higher than the oxidative CYP activities. Since these conjugating enzymes are predominantly detoxifying, the skin appears to be predominantly protected against CYP-generated reactive metabolites. The following recommendations for the use of experimental animal species or human skin in vitro models may tentatively be derived from the information available to date: for dermal absorption and for skin irritation esterase activity is of special importance which in pig skin, some human cell lines and reconstructed skin models appears reasonably close to native human skin. With respect to genotoxicity and sensitization reactive-metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the Conclusions section in the end of this review.

Xenobiotic-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models.

The exposure of the skin to medical drugs, skin care products, cosmetics, and other chemicals renders information on xenobiotic-metabolizing enzymes (XME) in the skin highly interesting. Since the use of freshly excised human skin for experimental investigations meets with ethical and practical limitations, information on XME in models comes in the focus including non-human mammalian species and in vitro skin models. This review attempts to summarize the information available in the open scientific literature on XME in the skin of human, rat, mouse, guinea pig, and pig as well as human primary skin cells, human cell lines, and reconstructed human skin models. The most salient outcome is that much more research on cutaneous XME is needed for solid metabolism-dependent efficacy and safety predictions, and the cutaneous metabolism comparisons have to be viewed with caution. Keeping this fully in mind at least with respect to some cutaneous XME, some models may tentatively be considered to approximate reasonable closeness to human skin. For dermal absorption and for skin irritation among many contributing XME, esterase activity is of special importance, which in pig skin, some human cell lines, and reconstructed skin models appears reasonably close to human skin. With respect to genotoxicity and sensitization, activating XME are not yet judgeable, but reactive metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the "Overview and Conclusions" section in the end of this review.

Xenobiotic metabolizing enzyme activities in cells used for testing skin sensitization in vitro.

For ethical and regulatory reasons, in vitro tests for scoring potential toxicities of cosmetics are essential. A test strategy for investigating potential skin sensitization using two human keratinocytic and two human dendritic cell lines has been developed (Mehling et al. Arch Toxicol 86:1273­1295, 2012). Since prohaptens may be metabolically activated in the skin, information on xenobiotic metabolizing enzyme (XME) activities in these cell lines is of high interest. In this study, XME activity assays, monitoring metabolite or cofactor, showed the following: all three passages of keratinocytic (KeratinoSens® and LuSens) and dendritic (U937 und THP-1) cells displayed N-acetyltransferase 1 (NAT1) activities (about 6­60 nmol/min/mg S9-protein for acetylation of para-aminobenzoic acid). This is relevant since reactive species of many cosmetics are metabolically controlled by cutaneous NAT1. Esterase activities of about 1­4 nmol fluorescein diacetate/min/mg S9-protein were observed in all passages of investigated keratinocytic and about 1 nmol fluorescein diacetate/min/mg S9-protein in dendritic cell lines. This is also of practical relevance since many esters and amides are detoxified and others activated by cutaneous esterases. In both keratinocytic cell lines, activities of aldehyde dehydrogenase (ALDH) were observed (5­17 nmol product/min/mg cytosolic protein). ALDH is relevant for the detoxication of reactive aldehydes. Activities of several other XME were below detection, namely the investigated cytochrome P450-dependent alkylresorufin O-dealkylases 7-ethylresorufin O-deethylase, 7-benzylresorufin O-debenzylase and 7-pentylresorufin O-depentylase (while NADPH cytochrome c reductase activities were much above the limit of quantification), the flavin-containing monooxygenase, the alcohol dehydrogenase as well as the UDP glucuronosyl transferase activities.

In vitro mammalian metabolism of the mitosis inhibitor zoxamide and the relationship to its in vitro toxicity.

The in vitro mammalian metabolism of the fungicide zoxamide is related to its in vitro mammalian toxicity. After incubation of zoxamide with rat liver microsomes leading to practically 100% metabolism (mostly hydroxylated zoxamide), the cytotoxicity (methyl thiazole tetrazolium (MTT) test) and the mitosis-inhibiting potential (shown by cell count and by cell cycle analysis) for V79 were not distinguishable from those of zoxamide, demonstrating that the hydroxylation of zoxamide did not change the cytotoxicity or mitosis-inhibiting potential as determined by these assays. After incubation of zoxamide with rat liver S9 predominantly leading to conjugation with glutathione, and after incubation of zoxamide with rat liver slices predominantly leading to the glucuronide of the hydroxylated zoxamide, these activities were eliminated demonstrating that the glutathione conjugate and the glucuronide had lost the activities in these assays due either to no intrinsic potential of these conjugates or to their inability to penetrate the plasma membrane of mammalian cells. It is concluded that the metabolic hydroxylation of zoxamide did not change its activity in the assays used for investigating its influence on cell proliferation, cell cycle and cytotoxicity, while the formation of conjugates with glutathione or glucuronic acid led to the apparent loss of these activities. Thus, with zoxamide as a prototype, it was shown that, in principle, mammalian metabolism and its relationship to mammalian detoxication of fungicidal mitosis inhibitors may be reasonably anticipated from in vitro studies. In addition, the results provide a rational for the observed absence of typically mitosis inhibition-associated toxicities of zoxamide in mammals in vivo.

Contact inhibition of growth of human diploid fibroblasts by immobilized plasma membrane glycoproteins.

The human embryonic fibroblasts used in this study show pronounced inhibition of growth when reaching a critical cell density. High cell density and growth inhibition has previously been mimicked by the addition of glutaraldehyde-fixed cells or of isolated plasma membranes to sparsely seeded proliferating fibroblasts (Wieser, R. J., R. Heck, and F. Oesch, 1985, Exp. Cell Res., 158:493-499). In this report, we describe the successful solubilization of the growth-inhibiting glycoproteins and their covalent coupling to silicabeads (10 microns), which had been derivatized with 3-isothiocyanatopropyltriethoxysilane. The beads, bearing the plasma membrane proteins, were added to sparsely seeded, actively proliferating fibroblasts, and growth was measured by the determination of cell number or of incorporation of [3H]thymidine into DNA. The growth was inhibited in a concentration-dependent manner, whereby 50% inhibition was achieved with 0.3 micrograms of immobilized protein added to 5 X 10(3) cells. Terminal galactose residues of plasma membrane glycoproteins with N-glycosydically bound carbohydrates were responsible for the inhibition of growth. Dense cultures of human fibroblasts are characterized by an accelerated synthesis of procollagen type III. We have found that this cellular response can also be induced by the addition of immobilized plasma membrane glycoproteins to sparsely seeded cells. These observations support the conclusion that the addition of immobilized plasma membrane glycoproteins to sparsely seeded fibroblasts mimics the situation occurring at high cell density. These results show that cell-cell contacts via plasma membrane glycoproteins carrying terminal galactose residues are important for the regulation of the proliferation of cultured human fibroblasts and presumably of the accelerated synthesis of collagen type III.

Isolation and characterization of a 60-70-kD plasma membrane glycoprotein involved in the contact-dependent inhibition of growth.

Previous studies have shown that plasma membrane compounds are involved in the contact-dependent inhibition of growth of human diploid fibroblasts. The purification of the active plasma membrane glycoprotein is described in this report. The glycoprotein has an apparent molecular mass of 60-70 kD and, due to differential sialylation, isoelectric points between pH 5.5. and 6.2. Treatment with sialidase yielded one spot in two-dimensional gel electrophoresis with an isoelectric point of 6.3. After removal of the N-glycosidically linked oligosaccharide chains, the apparent molecular mass is reduced by approximately 22 kD. Treatment was diluted NaOH, which removes the O-glycosidically linked portion of oligosaccharides, resulted in a reduction of the apparent molecular mass by approximately 5 kD. The addition of 50 ng/ml of this glycoprotein-for which the term "contactinhibin" is proposed-in immobilized form to sparsely seeded human fibroblasts resulted in a reversible 70-80% inhibition of growth. The inhibition was not confined to human fibroblasts as other cells were also inhibited, with the exclusion of transformed cells, which are refractory to contactinhibin. The inhibitory activity was abolished by treatment with beta-galactosidase or glycopeptidase F, indicating that the glycan moiety is the biologically active part of the molecule. Confluent cultures treated with antibodies raised against contactinhibin were released from the contact-dependent inhibition of growth. In addition to enhanced saturation density, these cultures exhibited a crisscross growth pattern and the formation of foci. Immunocytochemical studies showed that contactinhibin was associated with vimentin. Furthermore, contactinhibin was found to be not expressed in a species- or organ-specific manner.

Mutagenicity of glutathione and cysteine in the Ames test.

Postmitochondrial supernatant from rat liver and kidney homogenates transformed cysteine into a mutagen that reverted bacteria of the strain Salmonella typhimurium TA100 to histidine independence. Glutathione was also activated by kidney postmitochondrial supernatant but not by liver preparations. Hence, important endogenous compounds of mammals are positive in the most commonly used short-term test for carcinogenicity and mutagenicity. Glutathione is positive in the test even at concentrations found in mammalian tissues.

Inactivation of a diol epoxide by dihydrodiol dehydrogenase but not by two epoxide hydrolases.

The mutagenicity of r-8,t-9-dihydroxy-t-10, 11-oxy-8,9,10,11-tetrahydrobenz[a]anthracene (BA-8,9-diol 10, 11-oxide) toward Salmonella typhimurium TA 100 is not decreased by the presence of large amounts of highly purified microsomal or cytosolic epoxide hydrolase. However, highly purified dihydrodiol dehydrogenase inactivates this diol epoxide, which is a major DNA-binding metabolite of benz[a]anthracene. The K-region epoxide, benz[a]anthracene 5,6-oxide (BA 5,6-oxide) is efficiently inactivated by microsomal epoxide hydrolase, is much less readily inactivated by cytosolic epoxide hydrolase, and is not inactivated by dihydrodiol dehydrogenase. This inactivation of a diol epoxide by dihydrodiol dehydrogenase points to a new significance of this enzyme and a new level of control for diol epoxides.

Effects of an acute exposure to toluene on the DNA repair activity of the human 8-oxoguanine DNA glycosylase 1 (hOGG1) in healthy subjects.

The structure and previous studies on the biotransformation of toluene lead to the suspicion that metabolites may be formed which preferentially react with strongly nucleophilic partners such as sulfhydryl groups of cysteines in proteins. Human 8-oxoguanine DNA glycosylase 1 removes the major oxidative DNA damage and possesses eight cysteines. Its potential inactivation may lead to accumulation of DNA damage by reactive oxygen species formed by exogenous agents or by ubiquitous endogenous processes. The goal of the present investigation was to study the in vivo effect in humans of an acute toluene exposure on hOGG1 activity. Twenty healthy, non-smoking males were exposed to 50 ppm toluene and to filtered air in an exposure chamber for 270 min, using a cross-over design. Before and 30 min after the end of exposure, blood samples were taken and toluene concentrations and the hOGG1 activity were measured. hOGG1 activity was determined in peripheral mononuclear blood cells. Thirty minutes after exposure to toluene, we found a median blood concentration of 0.25 mg toluene/l. Compared with the activity before exposure, upon exposure to toluene a statistically insignificant median increase of hOGG1 activity by +0.4% and upon exposure to air by +2.3% was determined. Thus, no reduction of the hOGG1 repair activity after acute exposure to 50 ppm toluene was observed.

Phosphorylation of xenobiotic-metabolizing cytochromes P450.

The regulation of cytochromes P450 (CYPs) by induction mediated by xenobiotics is well known. Our team has discovered an additional important regulation of xenobiotic-metabolizing CYPs by phosphorylation. Individual CYPs are phosphorylated by different protein kinases, leading to CYP isoenzyme-selective changes in the metabolism of individual substrates and consequent profound changes in the control of mutagenic and cytotoxic metabolites. Some CYPs are phosphorylated by protein kinase C and some by the cyclic adenosine monophosphate (cAMP) dependent protein kinase A. We found that cAMP not only leads to drastic changes in the activity of individual CYPs, but also drastic changes in the nuclear localization of the CYP-related transcription factor Ah receptor (AHR). The consequences are very different from those of AHR nuclear translocation mediated by its classic ligands (such as dioxin and many polycyclic aromatic hydrocarbons) and may represent the long-sought physiological function of the AHR. The disturbance of this physiological function of AHR by extremely persistent high-affinity xenobiotic ligands such as dioxin may represent the most important contributing factor for their potent toxicity.

Density-dependent regulation of cell growth by contactinhibin and the contactinhibin receptor.

BACKGROUND: The number of cells within mammalian tissues is maintained by growth-stimulating and growth-inhibiting mechanisms, with inhibitory signals being superimposed over growth stimuli. This is reflected, in the culture of normal adherent cells, by the phenomenon of density-dependent inhibition of growth: cells cease proliferation after becoming a confluent monolayer. We have shown previously that a plasma membrane glycoprotein, contactinhibin, is a major effector of negative growth regulation. Although transformed cells express contactinhibin in a functionally active form, they are not growth-inhibited, suggesting that the defects that lead to their aberrant growth are located 'downstream' of contactinhibin. RESULTS: Here, we provide evidence that a 92 kD plasma membrane protein, which we call CiR, binds specifically to contactinhibin and acts as a receptor mediating the contact-dependent inhibition of growth of cultured human fibroblasts. When polyclonal antibodies against CiR were introduced into cells using liposomes, confluent cells were released from density-dependent growth control. By contrast, cross-linking CiR that is localized to the plasma membrane, using anti-CiR antibodies, led to growth inhibition, suggesting that CiR is a signalling molecule and implicating CiR oligomerization in signal generation. This conclusion is supported by the finding that binding of contactinhibin by CiR is strongly dependent on the local concentration of both molecules and has a sharp threshold. When CiR was isolated by immuno-precipitation under conditions favouring phosphorylation, it was hyperphosphorylated on serine and threonine residues and had reduced contactinhibin-binding capacity; the binding capacity of CiR was restored after treatment with potato acid phosphatase. Fibroblasts transformed with simian virus 40 had reduced CiR expression, higher CiR phosphorylation levels, and a strongly reduced capacity of CiR to bind to contactinhibin. Phosphatase treatment of the CiR isolated from transformed cells only partially restored its contactinhibin-binding capacity. CONCLUSIONS: Homeostasis is the net result of a highly balanced network of growth-stimulating and growth-inhibitory signals. We have shown that density-dependent inhibition of growth in vitro is mediated by the interaction of contactinhibin with a 92 kD plasma membrane glycoprotein, CiR, the contactinhibin-binding capacity of which is regulated by phosphorylation.

Structure of Aspergillus niger epoxide hydrolase at 1.8 A resolution: implications for the structure and function of the mammalian microsomal class of epoxide hydrolases.

BACKGROUND: Epoxide hydrolases have important roles in the defense of cells against potentially harmful epoxides. Conversion of epoxides into less toxic and more easily excreted diols is a universally successful strategy. A number of microorganisms employ the same chemistry to process epoxides for use as carbon sources. RESULTS: The X-ray structure of the epoxide hydrolase from Aspergillus niger was determined at 3.5 A resolution using the multiwavelength anomalous dispersion (MAD) method, and then refined at 1.8 A resolution. There is a dimer consisting of two 44 kDa subunits in the asymmetric unit. Each subunit consists of an alpha/beta hydrolase fold, and a primarily helical lid over the active site. The dimer interface includes lid-lid interactions as well as contributions from an N-terminal meander. The active site contains a classical catalytic triad, and two tyrosines and a glutamic acid residue that are likely to assist in catalysis. CONCLUSIONS: The Aspergillus enzyme provides the first structure of an epoxide hydrolase with strong relationships to the most important enzyme of human epoxide metabolism, the microsomal epoxide hydrolase. Differences in active-site residues, especially in components that assist in epoxide ring opening and hydrolysis of the enzyme-substrate intermediate, might explain why the fungal enzyme attains the greater speeds necessary for an effective metabolic enzyme. The N-terminal domain that is characteristic of microsomal epoxide hydrolases corresponds to a meander that is critical for dimer formation in the Aspergillus enzyme.

Relevance of environmental alkylating agents to repair protein O6-alkylguanine-DNA alkyltransferase: determination of individual and collective repair capacities of O6-methylguanine.

The repair capacity for O6-methylguanine was determined in cell homogenates of peripheral blood lymphocytes of 35 automobile industry workers, exposed to rubber and tires, and of 35 clinical workers, handling cancer chemotherapeutic agents, compared to control groups. lambda-Phage DNA containing one 32P-labeled O6-methylguanine in each BamHI site was used as substrate for the repair protein O6-alkylguanine-DNA alkyltransferase (AGT). The clinical personnel showed in the mean a highly significantly (P = 0.0014, Wilcoxon U test, Mann and Whitney) reduced activity of the repair enzyme [3.28 +/- 0.28 (SEM) fmol AGT/micrograms DNA] as compared to 37 control persons (4.88 +/- 0.32 fmol AGT/micrograms DNA). The mean AGT value of the automobile industry workers (4.40 +/- 0.28 fmol AGT/micrograms DNA) was not significantly different (P = 0.1303) from the mean of 38 examined controls (5.00 +/- 0.28 fmol AGT/micrograms DNA). By dividing these employees into subgroups according to their different work environments (handling of rubber fittings, tire mounting, and tire storage, respectively) the mean AGT value of the 15 tire storage workers was significantly (P = 0.0270) lower (3.80 +/- 0.36 fmol AGT/micrograms DNA) than the mean value of the controls. The interindividual variations in the activity of AGT were 5.1- and 6.5-fold in the control groups and 5.6-fold for the automobile industry workers; the largest variations were found in the group of the clinical personnel with 12.6-fold. No significant correlations between AGT and age, sex, or smoking behavior were observed in any of the groups examined. The decrease in AGT activity will render the afflicted individuals more susceptible to further exposure to methylating agents.

Rat hepatocyte-mediated bacterial mutagenicity in relation to the carcinogenic potency of benz(a)anthracene, benzo(a)pyrene, and twenty-five methylated derivatives.

7,12-Dimethylbenz(a)anthracene, benz(a)anthracene, and benzo(a)pyrene as well as the 24 monomethylbenzo(a)pyrenes (MBPs) and monomethylbenz(a)anthracenes (MBAs), compounds which differ in carcinogenicity from very potent to apparently inactive, were investigated for mutagenicity (reversion to histidine prototrophy) in Salmonella typhimurium TA100 using either intact or NADPH-fortified homogenized rat hepatocytes for metabolic activation. In both systems, all 27 hydrocarbons showed positive responses. Their mutagenic potency in the homogenate-mediated test varied in a narrow range and did not correlate detectably with their reported activity in carcinogenicity experiments. When the cell homogenate was replaced by intact cells, the maximal mutagenic effects were weaker by factors of 1 to 14, depending on the compound, and were seen only at higher substrate concentrations. The differences between cell- and homogenate-mediated mutagenicity were small with the strong carcinogens 7,12-dimethylbenz(a)anthracene 7-MBA, 12-MBA, benzo(a)pyrene, 1-MBP, and 11-MBP. The differences were large with the apparent noncarcinogens and those weak carcinogens that were strongly mutagenic in the homogenate-mediated test. As a result of this differential reduction in activity, the cell-mediated mutagenicity did not correlate with the homogenate-mediated mutagenicity but correlated approximately with the carcinogenic potency. The lowest effects in the cell-mediated experiments were seen with 7-, 8-, 9- and 10-MBP, 2-MBA, and 3-MBA. In these compounds, the methyl group is attached to a carbon of the terminal angular benzo ring, and therefore bay-region diol-epoxides, if formed at all, additionally would carry a methyl group on one of the oxidized positions. On the other hand, among all the compounds tested 7,12-dimethylbenz(a)anthracene, 12-MBA, and 11-MBP, which have the methyl group attached in the bay-region position opposite the terminal benzo ring, showed the highest mutagenic efficacies in the cell-mediated test, as compared to those observed in the homogenate-mediated test. These structure-activity relationships and the previously reported observation that among various promutagenic benzo(a)pyrene metabolites only the 7,8-dihydrodiol was strongly mutagenic in the cell-mediated test would suggest that in the cell-mediated bacterial mutagenicity test, bay-region diol-epoxides are the ultimate mutagens which are preferentially detected.

Epoxide hydrolase activity in native and in mitogen-stimulated lymphocytes of various human donors.

An assay for epoxide hydrolase activity using benzo(a)-pyrene 4,5-oxide as substrate was modified in such a manner that it allowed the estimation of activities in native and in cultivated mitogen-stimulated human lymphocytes. Their specific activities were about 1000-fold lower than those of human or rat liver microsomes. In native lymphocytes of 28 subjects, the specific activities of epoxide hydrolase varied from 2.0 to 7.2 pmol/min/mg protein. Repeated measurements in the same subjects usually showed reasonable consistency of the activity (variation less than 1.8-fold), but in 3 of 12 subjects the activity varied between 2.5- and 3-fold. This indicates that epigenetic factors can alter the activity. To circumvent effects due to variable exposure to such modulating factors, lymphocytes were cultivated and stimulated by mitogens. In such cells, the variation of enzyme activity clearly decreased. The specific activities in stimulated lymphocytes from 12 donors varied only from 4.4 to 7.0 pmol/min/mg protein. Phenobarbital, pregnenolone-16 alpha-carbonitrile, trans-stilbene oxide, 3-methylcholanthrene, and benzo(a)anthracene in the culture medium did not significantly induce or activate epoxide hydrolase, but beta-naphthoflavone increased the activity. The effect was similar in lymphocytes of 13 different donors, the highest and lowest increases being 1.8- and 2.6-fold. Thus, no genetic heterogeneity of epoxide hydrolase was observed in native or in cultivated and stimulated lymphocytes or in the response to beta-naphthoflavone. Between males and females or smokers and nonsmokers, no significant differences occurred. Inter- and intraindividual variation of epoxide hydrolase activity in native lymphocytes appears to be due almost exclusively to epigenetic factors, e.g., composition of the lymphocyte population or in vivo exposure to unknown modulators of epoxide hydrolase.

Metabolism of benzo(a)pyrene by subcellular fractions of rat liver: evidence for similar patterns of cytochrome P-450 in rough and smooth endoplasmic reticulum but not in nuclei and plasma membrane.

Since our earlier work (P. Stasiecki, F. Oesch, G. Bruder, E.D. Jarasch, and W.W. Franke, Eur. J. Cell Biol., 21: 79-92, 1980) had shown that carcinogen-metabolizing monooxygenase activity was present in almost all investigated cellular membranes, the possibility of differential control of the various metabolic pathways in the individual cellular membranes arose. Using high pressure liquid chromatography we have now studied the benzo(a)pyrene metabolites formed by rough and smooth endoplasmic reticulum, nuclei, and plasma membrane as well as mitochondrial fractions and investigated the metabolic cooperation between the monooxygenases and epoxide hydrolase in these fractions. Since various cytochrome P-450 isozymes catalyze the oxidative attack on the benzo(a)pyrene molecule at defined preferential sites, this analysis also provides an indirect trace of potential differences in the pattern of cytochrome P-450 isozymes present in the individual membranes. The metabolic profiles produced by the two most active fractions, smooth and rough endoplasmic reticulum, were very similar to each other but different from those produced by the other three preparations. The metabolite pattern produced by incubations containing nuclear fractions differed slightly from that produced by the fractions of endoplasmic reticulum, but plasma membrane and mitochondria produced markedly different patterns. Since the similarity of the benzo(a)pyrene metabolite pattern produced by the smooth and rough endoplasmic reticulum suggested similar cytochrome P-450 isozyme patterns in these two subfractions, they were further investigated by the use of selective inducers as well as a broad spectrum substrate, 7-ethoxy-coumarin, in the absence and presence of selective inhibitors. Treatment of animals with trans-stilbene oxide or phenobarbital (a) increased the total amount of metabolites per protein mass and time, (b) changed the pattern of metabolites, but (c) induced a pattern of metabolites which was again very similar in rough and smooth endoplasmic reticulum. Even more distinct changes were found following treatment with 3-methylcholanthrene or beta-naphthoflavone. Both of these compounds (a) preferentially induced the activity of rough endoplasmic reticulum, (b) changed the profile of metabolites, but (c) again did not disturb the similarities of the benzo(a)pyrene metabolite pattern between both fractions.(ABSTRACT TRUNCATED AT 400 WORDS)