In vivo and in vitro binding of benzene to nucleic acids and proteins of various rat and mouse organs.

Benzene binds to macromolecules of various organs in the rat and mouse in vivo. Labelling of RNA and proteins is higher (1 order of magnitude) than DNA labelling, which is low in many organs (liver, spleen, bone marrow and kidney), and negligible in lung; no difference between labelling of rat and mouse organs was found. The covalent binding index (CBI) value was about 10, i.e. typical of genotoxic carcinogens classified as weak initiators. In vitro binding of benzene to nucleic acids and proteins is mediated by hepatic microsomes, but not by microsomes from kidney, spleen and lung, or by cytosol from whatever organ. Nucleic acid binding can be induced by pretreatment with phenobarbitone (PB) and suppressed in the presence of SKF 525-A, of cytosol and/or GSH or of heat-inactivated microsomes. Labelling of exogenous DNA is low and is similar in the presence of rat or mouse microsomes in agreement with the low interaction with DNA measured in vivo.

In vivo and in vitro binding of epichlorohydrin to nucleic acids.

Epichlorohydrin (EC) binds to macromolecules of biological relevance in vivo: DNA is less labelled than RNA and proteins, rat organs interact more than mouse organs, stomach is the most labelled organ with liver, kidney and lung involved in decreasing order. Based on the Covalent Binding Index (CBI), EC is a weak-moderate oncogen, just as other chlorinated hydrocarbons such as 1,2-dichloroethane and carbon tetrachloride. An interaction of EC with nucleic acids (DNA and polyribonucleotides) occurs also in vitro. It is mediated either by chemical reactivity per se of the molecule (near-UV (NUV) irradiation does not photoactivate EC) and by enzymatic (microsomal and/or cytosolic) fractions, whose relative effectiveness is variable in relation to the organ tested. The best substrates for interaction are poly(G) and poly(A) when using microsomal and cytosolic fractions, respectively, whereas the labelling of double-stranded DNA is always lower. On the whole, the picture of enzyme (microsome + cytosol)-mediated in vitro interaction is similar to the pattern of in vivo binding, with the exception of rat stomach enzymes which are inactive in vitro.

Molecular non-genetic biomarkers of effect related to methyl thiophanate cocarcinogenesis: organ- and sex-specific cytochrome P450 induction in the rat.

We used selective biochemical markers of effect to evaluate some non-genotoxic cocarcinogenic properties of methyl thiophanate (MTH) associated with cytochrome P450 (CYP) changes. Several CYP-dependent reactions were monitored in the liver, kidney and lung microsomes of male and female Sprague-Dawley rats treated (i.p.) with a single (285 or 570 mg/kg body weight) or repeated (daily 285 or 570 mg/kg body weight for three consecutive days) doses of this pesticide. No significant changes in absolute or relative liver, kidney and lung weights were observed after MTH injection. Highly specific substrates were used as probes of different isoforms, such as CYP1A1, 1A2, 2B1, 2E1 and 3A. A complex pattern of CYP induction, including organ- and sex-related differences, was observed, particularly in the liver (CYP3A, 2B1), kidney (CYP1A1, 2E1) and lung (CYP3A, 1A1). In the liver, an increase up to 29-fold in the 2B1-like activity, probed by the O-dealkylation of pentoxyresorufin, was observed at lower dose in both sexes, and the induction of CYP 1A2-mediated methoxyresorufin O-demethylase activity (up to 3.6-fold) was recorded at the higher dose in males. In the kidney, the O-deethylation of ethoxyresorufin (CYP1A1-linked) was increased up to 28.2-fold and the CYP2E1-dependent p-nitrophenol hydroxylases were enhanced up to 6.3-fold in females receiving higher multiple MTH administration. In the lung, the CYP3A-associated activity was the most induced oxidases, as exemplified by the marked increase in the O-demethylation of aminopyrine (up to 3.6-fold) in males. A weak, although significant, reduction of CYP2B1-linked oxidases was also observed in repeated treatment in the kidney (males) and lung (females). These results suggest that the induction of CYP-catalyzed drug metabolism by prolonged exposure to MTH may result in accelerated metabolism of coadministered drugs with important implications for their disposition Together with an alteration of endogenous metabolism, the adverse effects associated with CYP changes such as toxicity/cotoxicity, cocarcinogenicity and promotion may also have clinical consequences.

Molecular non-genetic biomarkers of effect related to acephate cocarcinogenesis: sex- and tissue-dependent induction or suppression of murine CYPs.

The aim of this work was to study the ability of the organophosphate insecticide acephate to alter some biochemical markers of effect related non-genetic cocarcinogenesis. For this purpose, selective CYP-dependent reactions have been examined in liver, kidney and lung microsomes of male and female Swiss albino CD1-mice treated (i.p.) with a 125 or 250 mg/kg b.w. dose of this pesticide. High specific substrates were used as a probe of various isozymes, such as CYP 1A1, 1A2, 2B1, 2E1 and 3A. Maked organ- and sex-related differences in either inducive or suppressive response by acephate depict a complex pattern of CYP modulation with the kidney being more responsive to 3A induction (up to 6.9-fold increase, male) and the lung to 2B1 suppression (up to 70% loss, mainly female). In the liver, a 2.7-fold increase in the 3A-like activity, probed by the O-demethylation of aminopyrine, in the O-deethylation of phenacetin (1.8-fold increase, 1A2), as well as in the hydroxylation of p-nitrophenol (1.6-fold increase, 2E1) was observed in male animals at a lower dose. In contrast, a marked reduction of CYP 1A1-mediated ethoxyresorfin O-deethylase activity ranging from 43% (lower dose) to 44% loss (higher dose) in female and male mice, respectively, and of CYP 2B1-mediated pentoxyresorufin O-dealkylase (3% loss, female) was achieved. In the kidney, an increase in the 'mixed' ethoxycoumarin O-deethylase (up to 2-fold) as well as in the 2B1-like activity (up to 2.8-fold) was also recorded in males at 250 mg/kg. Once again, in the lung, a different behaviour on 3A isoforms between female (approximately 2-fold increase) and male (44% loss) was seen at a lower dose. The specificity of CYP changes was corroborated by means of Western immunoblotting analysis using rabbit polyclonal antibodies, anti-CYP 3A1/2 and 2E1. Taken together, these data indicate a possible toxic/cotoxic, cocarcinogenic and promoting potential of acephate.

Results of animal studies suggest a nonlinear dose-response relationship for benzene effects.

Considering the very large industrial usage of benzene, studies in risk assessment aimed at the evaluation of carcinogenic risk at low levels of exposure are important. Animal data can offer indications about what could happen in humans and provide more diverse information than epidemiological data with respect to dose-response consideration. We have considered experiments investigating metabolism, short-term genotoxicity tests, DNA adduct formation, and carcinogenicity long-term tests. According to the different experiments, a saturation of benzene metabolism and benzene effects in terms of genotoxicity seems evident above 30 to 100 ppm. Below 30 to 60 ppm the initiating effect of benzene seems to be linear for a large interval of dosages, at least judging from DNA adduct formation. Potential lack of a promoting effect of benzene (below 10 ppm) could generate a sublinear response at nontoxic levels of exposure. This possibility was suggested by epidemiological data in humans and is not confirmed or excluded by our observations with animals.

Benzene adducts with rat nucleic acids and proteins: dose-response relationship after treatment in vivo.

The dose-response relationship of the benzene covalent interaction with biological macromolecules from rat organs was studied. The administered dose range was 3.6 x 10(7) starting from the highest dosage employed, 486 mg/kg, which is oncogenic for rodents, and included low and very low dosages. The present study was initially performed with tritium-labeled benzene, administered by IP injection. In order to exclude the possibility that part of the detected radioactivity was due to tritium incorporated into DNA from metabolic processes, 14C-benzene was then also used following a similar experimental design. By HPLC analysis, a single adduct from benzene-treated DNA was detected; adduct identification will be attempted in the near future. Linear dose-response relationship was observed within most of the range of explored doses. Linearity was particularly evident within low and very low dosages. Saturation of benzene metabolism did occur at the highest dosages for most of the assayed macromolecules and organs, especially in rat liver. This finding could be considered as indicative of the dose-response relationship of tumor induction and could be used in risk assessment.

In vivo and in vitro binding of 1,2-dibromoethane and 1,2-dichloroethane to macromolecules in rat and mouse organs.

The comparative interaction of equimolar amounts of 1,2-dichloroethane and 1,2-dibromoethane with rat and mouse nucleic acids was studied in both in vivo (liver, lung, kidney and stomach) and in vitro (liver microsomal and/or cytosolic fractions) systems. In vivo, liver and kidney DNA showed the highest labeling, whereas the binding to lung DNA was barely detectable. Dibromoethane was more highly reactive than dichloroethane in both species. With dichloroethane, mouse DNA labeling was higher than rat DNA labeling whatever the organ considered: the opposite was seen for the bioactivation of dibromoethane. RNA and protein labelings were higher than DNA labeling, with no particular pattern in terms of organ or species involvement. In vitro, in addition to a low chemical reactivity towards nucleic acids shown by haloethanes per se, both compounds were bioactivated by either liver microsomes and cytosolic fractions to reactive forms capable of binding to DNA and polynucleotides. UV irradiation did not photoactivate dibromoethane and dichloroethane. The in vitro interaction with DNA mediated by enzymatic fractions was PB-inducible (one order of magnitude, using rat microsomes). In vitro bioactivation of haloethanes was mainly performed by microsomes in the case of dichloroethane and by cytosolic fractions in the case of dibromoethane. When microsomes plus cytosol were used, rat enzymes were more efficient than mouse enzymes in inducing a dibromoethane-DNA interaction: the opposite situation occurred for dichloroethane-DNA interaction, and this is in agreement with the in vivo pattern. In the presence of both metabolic pathways, addition or synergism occurred. Dibromoethane was always more reactive than dichloroethane. An indication of the presence of a microsomal GSH transferase was achieved for the activation of dibromoethane. No preferential binding in vitro to a specific polynucleotide was found. Polynucleotide labeling was higher than (or equal to) DNA binding. The labeling of microsomal RNA and proteins and of cytosolic proteins was many times lower than that of DNA or polynucleotides. The in vivo and in vitro data reported above give an unequivocal indication of the relative reactivity of the haloethanes examined with liver macromolecules from the two species and agree, on the whole, with the relative genotoxicity (DNA repair induction ability, mutagenicity and carcinogenicity) of the chemicals.

Evaluation of genotoxic effects of the herbicide dicamba using in vivo and in vitro test systems.

The genotoxic effects of the herbicide dicamba have been studied by measuring 1) the unwinding rate of liver DNA from intraperitoneally (i.p.) treated rats (fluorimetric assay); 2) DNA repair as unscheduled DNA synthesis (UDS) induced in cultured human peripheral blood lymphocytes (HPBL); and 3) sister chromatid exchanges (SCE) in HPBL. Results show that dicamba is capable of inducing DNA damage since it significantly increases the unwinding rate of rat liver DNA in vivo and also induces UDS in HPBL in vitro in the presence of exogenous metabolic activation (S-9 mix). Furthermore, dicamba causes a very slight increase in SCE frequency in HPBL in vitro.

The covalent interaction of 1,4-dibromobenzene with rat and mouse nucleic acids: in vivo and in vitro studies.

1,4-Dibromobenzene (1,4-DBB) was covalently bound to DNA from liver, kidney, lung and stomach of mice after intraperitoneal administration. The covalent binding index (CBI) value (23 in mouse liver) was typical of weak initiators. On the contrary, no interaction with DNA from rat organs was observed (CBI detection limit: 1.3-2.6). The in vitro interaction of 1,4-DBB with calf thymus DNA was mediated mainly by microsomes, especially those from liver of both species and from mouse lung. Mouse subcellular fractions were more active then rat subcellular fractions. Unlike liver cytosol, subcellular cytosolic fractions from lung, kidney and stomach were capable of bioactivating 1,4-DBB, although to a lesser extent than liver microsomes. Both cytochrome P-450 and GSH-transferases are involved in 1,4-DBB bioactivation.

Inhibitory activity of vitamin E and alpha-naphthoflavone on beta-carotene-enhanced transformation of BALB/c 3T3 cells by benzo(a)pyrene and cigarette-smoke condensate.

We previously found that beta-carotene (betaCT) can act as a co-carcinogenic agent enhancing the cell transforming activity of powerful carcinogens such as benzo(a)pyrene (B(a)P) and cigarette-smoke condensate (TAR) in an in vitro medium-term ( approximately 8 weeks) experimental model utilizing BALB/c 3T3 cells (Mutat. Res. 440 (1999) 83-90). Here, we investigated whether vitamin E (VitE) and alpha-naphthoflavone (alphaNF) are able to affect the co-carcinogenic activity of betaCT in terms of inhibiting B(a)P and TAR cell transforming potential. The following experimental schedules were performed: (i) cultures treated for 72 h with chemicals in various experimental combinations (acute treatment); (ii) cultures grown in presence of tester agents for the whole period of the assay (chronic treatment) to more closely mimic human exposure. While the co-carcinogenic potential of betaCT was confirmed on both B(a)P and TAR, the latter being ineffective by itself, we found in repeated experiments that the presence of VitE or alphaNF significantly reduced the betaCT's enhancing effect in the formation of transformation foci by B(a)P and TAR. The mechanism of the inhibition could be explained by the known ability of alphaNF to inhibit cytochrome P450-linked B(a)P-bioactivating monooxygenases, while VitE may contrast the prooxidant activity of betaCT (e.g., oxygen radicals overgeneration). While highlighting the importance of increasing knowledge of the role of single provitamins, vitamins and micronutrients, our findings also underline the potential advantages of combining several dietary supplements in in vitro preventive investigations.

beta-carotene as enhancer of cell transforming activity of powerful carcinogens and cigarette-smoke condensate on BALB/c 3T3 cells in vitro.

We report the ability of beta-carotene (betaC) to affect the cell transforming activity of 3-methylcholanthrene (3-MCA), benzo(a)pyrene (B(a)P) and cigarette-smoke condensate (TAR) in an in vitro medium-term (approximately 8 weeks) experimental model utilizing BALB/c 3T3 cells. Different experimental schedules were performed either in the presence or absence of betaC: (i) cultures treated for 72 h with each chemical (acute treatment), (ii) cultures grown in presence of each chemical for the whole period of the experiment (chronic treatment). These procedures suggested a possible cocarcinogenic potential of the carotenoid following interactions with other chemicals mimicking continuous human exposition to several xenobiotics. Although the pigment did not show any cell transforming potential when tested alone either in acute or chronic treatment, it did augment that of other tested agents. Induction of cell transformation by B(a)P was markedly enhanced by the presence of this carotenoid in either acute or chronic treatment. Only in presence of betaC, was TAR able to significantly act as a cell transforming agent in prolonged, chronic treatment of cultures. Enhanced cell transformation activity could be due to the boosting effect of betaC on P450 apparatus. Indeed, elsewhere we have found that the latter increased the ratio of formation of diol epoxide carcinogenic metabolites of B(a)P as well as other carcinogens present in TAR. By contrast, no differences of cell transforming activity of 3-MCA, an ultimate carcinogen, were seen either in the presence or absence of betaC under the various experimental conditions. These data, which are in keeping with the cocarcinogenic potential of betaC, may help to explain the unexpected lung cancer increases obtained in chemoprevention trials in heavy smokers supplemented with the isoprenoid. Our findings also highlight the potential risk to humans derived from interactions among xenobiotics present in the environment.

Cytotoxic and cell transforming activities of the fungicide methyl thiophanate on BALB/c 3T3 cells in vitro.

Cytotoxic and cell-transforming activities of methyl thiophanate a systemic fungicide capable of entering plant cells and thus controlling fungal diseases that have already started were studied in an in vitro medium-term (6-8 weeks) experimental model utilizing BALB/c 3T3 cells. Cells were exposed to the chemical, dissolved in dimethyl sulfoxide, in the absence or presence of an exogenous metabolizing system derived from rat livers supplemented with cofactors (S9 mix). In the absence of metabolic activation, methyl thiophanate exerted cytotoxic activity, evidenced through the formation of cell colonies, at low doses (> 10 micrograms/ml). However, the cytotoxic activity was greatly reduced by the S9 mix-induced metabolic activation of the chemical. Without bioactivation, cell-transforming potential, evidenced through the induction of transformation foci, was observable only at the highest (weakly toxic) dose employed (25 micrograms/ml). On the contrary, in the presence of metabolic activation, the cell-transforming activity was detectable at all tested doses (i.e. from 20 to 200 micrograms/ml) and it was particularly evident in a level-II transformation amplification test when the cells were allowed to perform active proliferative activity. These results, providing further information on the activity of methyl thiophanate in multistep carcinogenesis as possible genotoxic and/or co-carcinogenic agent, may contribute to better evaluate the oncogenic risk to man.

A database for evaluating the toxicological risk of pesticides.

This study of Overtox-DB, a computerized database for managing chemical toxicity data, is a product of the application of typical methodologies regarding information science and computer technology. The methodology applied can be reduced to three-basic elements: the collection of requirements, design, and achievement. Overtox-DB was developed by defining technological elements for managing data and its structure and by identifing the procedures and methodologies for data storage, retrieval, distribution, and standardization of many kinds of test data stored in the same format. The program stores data about chemical identification, physical and chemical properties, toxicological tests, mutagenicity, teratogenicity, carcinogenicity, and a bibliography of chemical compounds. Overtox-DB consists of five modules: experimental and bibliographic, data collection, molecular data collection, data search, and data report. The Overtox-DB user responds to a simplified set of query commands and boolean operators that interact with the system to retrieve different toxicological data (the majority of fields are defined as search fields and identify the test system, results of the assays, administration route, dose, etc.). The collected information provides an analytical characterization of biological activities for many compounds and identifies evidence possibly lacking in experimental approaches. Indeed, this database could permit a comparative evaluation with other substances and can be used for structure-activity relationship studies.

Levels of the inhibitor of PMN-elastase in venous blood reflowing from chronically affected veins: the role of venous stasis.

BACKGROUND: Attention has recently been paid to the cell and biochemical disorders involved in chronic venous insufficiency (CVI) and to their possible relationship to the endothelium. METHODS: In the present study, carried out in 14 patients with CVI, we evaluated the levels of the inhibitor of elastase (I-EL) generated by polymorphonucleate cells in the blood reflowing from affected superficial veins of legs both at rest and after prolonged venous stasis (1 hour in standing position). RESULTS: We evaluated the I-EL both as percentage of activity (baseline 82.3+/-24.5%; after stasis 100.7+/-37.8%) and as absolute values (0.67+/-0.26 U/ml; after stasis 0.79+/-0.39 U/ml). In blood samples taken after venous stasis we found a tendency toward a trapping of white blood cells and an increase of the haematocrit over baseline. The difference in the percentages of activity of I-EL was statistically significant, but only a trend was observed for the absolute values. CONCLUSIONS: We believe that the typical haemodynamic disorders of patient with CVI increased by prolonged venous stasis can modify the function of white blood cells, which are closely linked with venous hypertension, thus playing an important role in the pathogenesis of skin ulcers.

The different genotoxicity of p-dichlorobenzene in mouse and rat: measurement of the in vivo and in vitro covalent interaction with nucleic acids.

Twenty-two hours after i.p. injection to male Wistar rats and BALB/c mice para-dichlorobenzene (p-DCB) is bound covalently to DNA from liver, kidney, lung and stomach of mice but not of rats. DNA adducts in mouse liver are repaired in seventy-two hours. The covalent binding index value, calculated on the labelling of mouse liver DNA, classifies p-DCB as a weak initiator with an oncogenic activity lower than that of chlorobenzene. The labelling of RNA and proteins from the different organs of both species is, however, low. In vitro interaction with calf thymus DNA mediated by mouse and rat microsomes from liver and lung did occur. Binding extent was strongly reduced by addition of 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF 525-A) to the microsomal standard incubation mixture, whereas it was enhanced by adding GSH. Cytosolic fractions from kidney and lung were able to induce binding of p-DCB to DNA to a lower extent with respect to microsome-mediated binding. These results indicate that microsomal mixed function oxidase system and microsomal GSH-transferases can be involved in overall activating metabolism whereas cytosolic GSH-transferases play a minor role. This study, which is a part of a structure-activity relationship approach on benzene and its haloderivatives, provides the first evidence of genotoxicity of p-DCB in mammalian cell. It allows to partly explain variations of susceptibility of different species to hepatocarcinogenesis and of hepatotoxicity of different isomers.

Chloroform bioactivation leading to nucleic acids binding.

Chloroform was bound covalently to DNA, RNA and proteins of rat and mouse organs in vivo after i.p. injection. Covalent Binding Index values of rat and mouse liver DNA classify chloroform as a weak initiator. Labelings of RNA and proteins from various organs of both species were higher than that of DNA. In an in vitro cell-free system, chloroform was bioactivated by cytochrome P450-dependent microsomal fractions, by cytosolic GSH-transferases from rat and mouse liver, and particularly by the latter enzymes from mouse lung. This observation suggests that GSH plays a role in the binding of chloroform metabolites to DNA. The presence of both microsomal and cytosolic enzymatic systems in the standard incubation mixture generally led to an additive or synergistic bioactivating effect for rat and mouse, respectively.

[Early diagnosis of progressive systemic sclerosis: the role of oro-facial phenomena]. / La diagnosi precoce della sclerosi sistemica progressiva. Il ruolo delle manifestazioni oro-facciali.

BACKGROUND: The study considers the oro-facial manifestations occurring in a group of patients suffering from progressive systemic sclerosis, evaluating their predictive role and monitoring the systemic disease. METHODS: 34 patients with systemic sclerosis were examining: 30 women, 4 men, at the Department of Dental Sciences, Palermo University, from July 1998 to March 2001. All patients were positive for the diagnostic criteria of scleroderma.1 For each patient we considered: observation of any changes in soft tissues of the oral cavity; evaluation of possible xerostomia; evaluation of any fungal infections; presence of prosthetic sores in patients wearing dentures; DMFS evaluation; evaluation of periodontal health; evaluation of bone resorption. RESULTS: One of the most interesting results was that: in 4 patients, medical history contained past trigeminal sensory neuropathy characterised by the slow and gradual numbness of the face muscles, which developed into pain and paresthesia, weakening or disappearance of the sense of taste and in one case loss of sensitivity at the oro-pharynx (this progression of symptoms preceded by three years the onset of systemic manifestations, and consequently diagnosis); 14 patients as first sign of the disease had a "hardening" of the skin of the face with consequent microstomia with marked difficulty in speaking. CONCLUSIONS: The most significant result emerging from this study was the predictive role of progressive systemic sclerosis, that can be played by some oro-facial manifestations; an important element which has recently changed the life expectation for scleroderma patients is the possibility of diagnosing the disease in an early phase and thus bringing to bear all the therapeutic measures necessary to hinder development of the disease and its consequences.

In vivo and in vitro covalent binding of chlorobenzene to nucleic acids.

At 22 hr after ip injection into male Wistar rats and BALB/c mice, chlorobenzene was covalently bound to DNA, RNA and proteins of the liver, kidney and lung, as has been found with various weak carcinogens. A microsome-mediated interaction with DNA occurred in vitro. The interaction was enhanced by pretreatment in vivo with phenobarbitone but was suppressed by addition of 2-diethylaminoethyl-2,2-diphenylvalerate HC1 in vitro. These results indicate the involvement of cytochrome P-450. Liver microsomes were efficient bioactivators, whereas cytosol was ineffective. The extent of in vitro interaction of chlorobenzene with synthetic polyribonucleotides was of the same order as that with DNA. Finally, ultraviolet irradiation (lambda = 254 nm or lambda max = 365 nm) activated this environmental contaminant to forms capable of interacting with DNA. The results represent evidence for genotoxicity of chlorobenzene.

In vitro microsome- and cytosol-mediated binding of 1,2-dichloroethane and 1,2-dibromoethane with DNA.

Metabolic activation of 1,2-dichloroethane (DCE) and 1,2-dibromoethane (DBE) to forms able to bind covalently with DNA occurs in vitro either by way of microsomal or cytosolic pathways. The involvement of these two pathways is variable with respect to species or compound tested. Rat enzymes are generally more efficient than mouse enzymes in bioactivating haloalkanes and DBE is more reactive than DCE. This parallels both the previous report on in vivo comparative interaction and the higher genotoxicity of DBE.

1,1,2-Trichloroethane: evidence of genotoxicity from short-term tests.

At 22 hr after ip injection into adult male Wistar rats, 1,1,2-trichloroethane was covalently bound to DNA, RNA and proteins of the liver, kidney, lung and stomach, as has been found with various weakly carcinogenic halo compounds. The extent of interaction of 1,1,2-trichloroethane with mouse liver DNA was much higher than that with rat liver DNA. This result provides evidence of a correlation between adducts formation and species susceptibility to hepatocarcinogenesis (only the mouse is sensitive to the oncogenic effect of this compound). Interaction with DNA mediated by murine liver microsomes occurred in vitro. No particular differences between the two species were found. In vitro binding was enhanced (approximately 5-fold) by pretreatment in vivo with phenobarbitone but was suppressed by addition of 2-diethylamino-ethyl-2,2-diphenylvalerate X HCl in vitro. Cytochrome P-450 was, therefore, involved in the interaction process. Glutathione suppressed the microsome-mediated interaction, acting as a "scavenger" of reactive intermediate(s). Murine lung microsomes were less effective bioactivators than liver microsomes for the interaction with DNA and microsomal RNA, but not microsomal protein. Kidney and stomach microsomes were ineffective, as were cytosolic fractions from all of the assayed organs of the two species. The extent of in vitro interaction of 1,1,2-trichloroethane with synthetic polyribonucleotides was of the same order of magnitude as that with DNA. The results represent further clear evidence for genotoxicity of 1,1,2-trichloroethane.