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

RNA adduction derived from electrophilic species in vitro and in vivo

RNA molecules are essential for cell function by not only serving as genetic materials, but also providing cells with structural support and catalytic functions. Due to nucleophilicity of nucleobases, RNA molecules can react with electrophilic species thus to be "adducted". The electron-deficient agents potentially inducing adduction exist in a variety of natural sources including metabolic products of biomolecules. Although evident and readily detected in human tissue, RNA adduction remains poorly understood for their physiological and pathological function. In this article, we review a collection of exogenous and endogenous molecular species that participate in RNA adduction and elaborates on the chemical nature of their RNA adduction sites. Furthermore, we provide perspectives on the potential of RNA adducts as biomarkers of environmental insults. Finally, we project future investigations that are necessary for understanding the mechanisms of cellular toxicity of RNA adduction.

Investigation of potential early key events and mode of action for 1,2-dichloroethane-induced mammary tumors in female rats

1,2-dichloroethane (DCE or EDC) is a chlorinated hydrocarbon used as a chemical intermediate, including in the synthesis of polyvinyl chloride. Although DCE has induced tumors in both rats and mice, the overall weight-of-evidence suggests a lack of in vivo mutagenicity. The present study was conducted to explore a potential mode of action further for tumor formation in rat mammary tissue. Fischer 344 rats were exposed to target concentrations of 0 or 200 ppm of DCE vapors (6 hours/day, 7 days/week) for at least 28 days; 200 ppm represents a concentration of ~20% higher than that reported to induce mammary tumors. Endpoints examined included DNA damage (via Comet assay), glutathione (reduced, oxidized and conjugated), tissue DNA adducts, cell proliferation and serum prolactin levels. Exposure to DCE did not alter serum prolactin levels with consistent estrous stage, did not cause cell proliferation in mammary epithelial cells, nor result in histopathological alterations in the mammary gland. DNA adducts were identified, including the N⁷ -guanylethyl glutathione adduct, with higher adduct levels measured in liver (nontumorigenic target) compared with mammary tissue isolated from the same rats; no known mutagenic adducts were identified. DCE did not increase the Comet assay response in mammary epithelial cells, whereas DNA damage in the positive control (N-nitroso-N-methylurea) was significantly increased. Although the result of this study did not identify a specific mode of action for DCE-induced mammary tumors in rats, the lack of any exposure-related genotoxic responses further contributes to the weight-of-evidence suggesting that DCE is a nongenotoxic carcinogen.

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.

In Vivo and in Vitro Interaction of 1,2-Dichlorobenzene with Nucleic Acids and Proteins of Mice and Rats

Twenty-two hours after i.p. injection into male Wistar rats and BALB/c mice, 1,2-dichlorobenzene (1,2-DCB) was covalently bound to DNA, RNA, and proteins of liver, kidney, lung and stomach. The covalent binding index to liver DNA was typical of carcinogens classified as weak initiators. The enzyme-mediated in vitro interaction of 1,2-DCB with calf thymus DNA of synthetic polyribonucleotides was carried out by a microsomal mixed-function oxidase system and microsomal GSH-transferases, which seemed to be effective only in liver and lung of rat and mouse. Cytosolic GSH-transferases played a minor role in 1,2-DCB bioactivation. The latter finding provides the first evidence of 1,2-DCB genotoxicity in mammalian cells. The type of halide, the number of halosubstituents and their spatial disposition on the benzene ring are the major determinants of halobenzenes activability to intermediate(s) capable of interacting covalently with DNA and other macromolecules in biologic systems.

Analysis of DNA adducts formed in vivo in rats and mice from 1,2-dibromoethane, 1,2-dichloroethane, dibromomethane, and dichloromethane using HPLC/accelerator mass spectrometry and relevance to risk estimates

Dihaloalkanes are of toxicological interest because of their high-volume use in industry and their abilities to cause tumors in rodents, particularly dichloromethane and 1,2-dichloroethane. The brominated analogues are not used as extensively but are known to produce more toxicity in some systems. Rats and mice were treated i.p. with (14)C-dichloromethane, -dibromomethane, -1,2-dichloroethane, or -1,2-dibromoethane [5 mg (kg body weight)(-1)], and livers and kidneys were collected to rapidly isolate DNA. The DNA was digested using a procedure designed to minimize processing time, because some of the potential dihalomethane-derived DNA-glutathione (GSH) adducts are known to be unstable, and the HPLC fractions corresponding to major adduct standards were separated and analyzed for (14)C using accelerator mass spectrometry. The level of liver or kidney S-[2-(N(7)-guanyl)ethyl]GSH in rats treated with 1,2-dibromoethane was approximately 1 adduct/10(5) DNA bases; in male or female mice, the level was approximately one-half of this. The levels of 1,2-dichloroethane adducts were 10-50-fold lower. None of four known (in vitro) GSH-DNA adducts was detected at a level of >2/10(8) DNA bases from dibromomethane or dichloromethane. These results provide parameters for risk assessment of these compounds: DNA binding occurs with 1,2-dichloroethane but is considerably less than from 1,2-dibromoethane in vivo, and low exposure to dihalomethanes does not produce appreciable DNA adduct levels in rat or mouse liver and kidney of the doses used. The results may be used to address issues in human risk assessment.

The comet assay with multiple mouse organs: comparison of comet assay results and carcinogenicity with 208 chemicals selected from the IARC monographs and U.S. NTP Carcinogenicity Database

The comet assay is a microgel electrophoresis technique for detecting DNA damage at the level of the single cell. When this technique is applied to detect genotoxicity in experimental animals, the most important advantage is that DNA lesions can be measured in any organ, regardless of the extent of mitotic activity. The purpose of this article is to summarize the in vivo genotoxicity in eight organs of the mouse of 208 chemicals selected from International Agency for Research on Cancer (IARC) Groups 1, 2A, 2B, 3, and 4, and from the U.S. National Toxicology Program (NTP) Carcinogenicity Database, and to discuss the utility of the comet assay in genetic toxicology. Alkylating agents, amides, aromatic amines, azo compounds, cyclic nitro compounds, hydrazines, halides having reactive halogens, and polycyclic aromatic hydrocarbons were chemicals showing high positive effects in this assay. The responses detected reflected the ability of this assay to detect the fragmentation of DNA molecules produced by DNA single strand breaks induced chemically and those derived from alkali-labile sites developed from alkylated bases and bulky base adducts. The mouse or rat organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Therefore, organ-specific genotoxicity was necessary but not sufficient for the prediction of organ-specific carcinogenicity. It would be expected that DNA crosslinkers would be difficult to detect by this assay, because of the resulting inhibition of DNA unwinding. The proportion of 10 DNA crosslinkers that was positive, however, was high in the gastrointestinal mucosa, stomach, and colon, but less than 50% in the liver and lung. It was interesting that the genotoxicity of DNA crosslinkers could be detected in the gastrointestinal organs even though the agents were administered intraperitoneally. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative nongenotoxic (Ames test-negative) carcinogens. The Ames test is generally used as a first screening method to assess chemical genotoxicity and has provided extensive information on DNA reactivity. Out of 208 chemicals studied, 117 are Ames test-positive rodent carcinogens, 43 are Ames test-negative rodent carcinogens, and 30 are rodent noncarcinogens (which include both Ames test-positive and negative noncarcinogens). High positive response ratio (110/117) for rodent genotoxic carcinogens and a high negative response ratio (6/30) for rodent noncarcinogens were shown in the comet assay. For Ames test-negative rodent carcinogens, less than 50% were positive in the comet assay, suggesting that the assay, which detects DNA lesions, is not suitable for identifying nongenotoxic carcinogens. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. This assay had a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic noncarcinogens, suggesting that the comet assay can be used to evaluate the in vivo genotoxicity of in vitro genotoxic chemicals. For chemicals whose in vivo genotoxicity has been tested in multiple organs by the comet assay, published data are summarized with unpublished data and compared with relevant genotoxicity and carcinogenicity data. Because it is clear that no single test is capable of detecting all relevant genotoxic agents, the usual approach should be to carry out a battery of in vitro and in vivo tests for genotoxicity. The conventional micronucleus test in the hematopoietic system is a simple method to assess in vivo clastogenicity of chemicals. Its performance is related to whether a chemical reaches the hematopoietic system. Among 208 chemicals studied (including 165 rodent carcinogens), 54 rodents carcinogens do not induce micronuclei in mouse hematopoietic system despite the positive finding with one or two in vitro tests. Forty-nine of 54 rodent carcinogens that do not induce micronuclei were positive in the comet assay, suggesting that the comet assay can be used as a further in vivo test apart from the cytogenetic assays in hematopoietic cells. In this review, we provide one recommendation for the in vivo comet assay protocol based on our own data.

Increased lymphocyte sister chromatid exchange frequency in workers with exposure to low level of ethylene dichloride

The genotoxicity of low-level exposure to ethylene dichloride (EDC) and vinyl chloride monomer (VCM) in humans is not clear. We used lymphocyte sister chromatid exchange (SCE) frequency as a parameter to investigate the genotoxicity of low level EDC and VCM in VCM-manufacturing workers. The SCE frequency was determined for 51 male workers with exposure to VCM and/or EDC and for 20 male workers devoid of such exposure. Epidemiological data were obtained by questionnaire, and included history of smoking, drinking, and any medication taken, as well as a detailed occupational history. Personal- and area-sampling and analysis were conducted in order to calculate the time-weighted average (TWA) contaminant-exposure level corresponding to different job categories. Moderate EDC exposure around 1 ppm corresponded to a significantly greater SCE frequency than was the case for the low EDC exposure group (p<0.01). However, VCM exposure of similar level was not associated with increased SCE. We conclude that EDC may cause genotoxicity at a relatively low level of exposure.

Detection of in vivo genotoxicity of haloalkanes and haloalkenes carcinogenic to rodents by the alkaline single cell gel electrophoresis (comet) assay in multiple mouse organs

The micronucleus test is widely used to assess in vivo clastogenicity because of its convenience, but it is not appropriate for some carcinogenic chemical classes. Halogenated compounds, for example, are inconsistent micronucleus inducers. We assessed the genotoxicity of 7 haloalkanes and haloalkenes carcinogenic to rodents in 7 mouse organs-stomach, liver, kidney, bladder, lung, brain, and bone marrow-using the alkaline single cell gel electrophoresis (SCG) assay. The carcinogens we studied were 1, 2-dibromo-3-chloropropane (DBCP), 1,3-dichloropropene (mixture of cis and trans) (DCP), 1,2-dibromoethane (EDB), 1,2-dichloroethane (EDC), vinyl bromide, dichloromethane, and carbon tetrachloride; only DBCP induces micronuclei in mouse bone marrow. Except for carbon tetrachloride, halocompounds studied are mutagenic to Salmonella typhimurium. Mice were sacrificed 3 or 24 h after carcinogen administration. DCP and EDC induced DNA damage in all of the organs studied. Vinyl bromide yielded DNA damage in all of the organs except for bone marrow. DBCP induced DNA damage in the stomach, liver, kidney, lung, and bone marrow; EDB in the stomach, liver, kidney, bladder, and lung; and dichloromethane in the liver and lung. Since no deaths, morbidity, clinical signs, organ pathology, or microscopic signs of necrosis were observed, the DNA damage was not attributable to cytotoxicity. On the other hand, the positive response in the liver induced by carbon tetrachloride, which was accompanied by necrosis, was considered to be a false positive response. We suggest that the alkaline SCG assay can be used in multiple organs to detect in vivo genotoxicity that is not expressed in bone marrow cells in mice given non-necrogenic doses of halocompounds.

1,2-Dibromoethane as an initiating agent for cell transformation

The two-stage transformation assay increases the sensitivity of cells to chemicals and permits detection of carcinogens acting as initiating agents. 1,2-Dibromoethane, a representative halogenated aliphatic, has been tested in the two-stage BALB/c 3T3 cells transformation test at dosage from 16 microM to 128 microM. This dose range is much lower than those previously found efficient in transforming BALB/c 3T3 cells. Apart from the lowest dose, which induced borderline effects, all the other assayed dosages appeared to induce heritable changes in the target cells. The initiated cells were revealed as fully transformed foci both in the combination with a chronic promoting treatment and also by allowing cells to perform more rounds of cell replication. The results clearly show that 1,2-dibromoethane can act as an initiator of cell transformation.

Effect of inhalation exposure regimen on DNA binding potency of 1,2-dichloroethane in the rat

1,2-Dichloroethane (DCE) was reported to be carcinogenic in rats in a long-term bioassay using gavage in corn oil (24 and 48 mg/kg/day), but not by inhalation (up to 150-250 ppm, 7 h/day, 5 days/week). The daily dose metabolized was similar in the two experiments. In order to address this discrepancy, the genotoxicity of DCE was investigated in vivo under different exposure conditions. Female F-344 rats (183-188 g) were exposed to [1.2-14C]-DCE in a closed inhalation chamber to either a low, constant concentration (0.3 mg/l = 80 ppm for 4 h) or to a peak concentration (0.3 mg/l = 80 ppm for 4 h) or to a peak concentration (up to 18 mg/l = 4400 ppm) for a few minutes. After 12 h in the chamber, the dose metabolized under the two conditions was 34 mg/kg and 140 mg/kg. DNA was isolated from liver and lung and was purified to constant specific radioactivity. DNA was enzymatically hydrolyzed to the 3'-nucleotides which were separated by reverse phase HPLC. Most radioactivity eluted without detectable or with little optical density, indicating that the major part of the DNA radioactivity was due to covalent binding of the test compound. The level of DNA adducts was expressed in the dose-normalized units of the Covalent Binding Index, CBI = mumol adduct per mol DNA nucleotide/mmol DCE per kg body wt.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

In vitro transformation of BALB/c 3T3 cells by 1,1,2,2-tetrachloroethane

1,1,2,2-Tetrachloroethane (1,1,2,2-TTCE) was shown to be capable of inducing in vitro transformation of BALB/c 3T3 cells (clone A-31) either in the presence or in the absence of S9 activating system using an amplification-transformation (level-II) assay by reseeding confluent cells from each treatment and allowing additional rounds of cell replication. In the absence of metabolic activation, the highest assayed dose (1000 micrograms/ml), exerting the highest toxicity, was the only transforming dose. Lower doses of 1,1,2,2-TTCE were capable of transforming BALB/c cells in the presence of S9 activating system, the dose of 500 micrograms/ml exerting the highest transforming activity. The number and size of transformed foci recognized in the level-II plates were a function of the number of cells reseeded in the amplification assay. Foci obtained in the presence of S9 activating systems were larger in size, more deeply basophilic, and exhibited denser multilayering of constituent cells than foci recognized in the absence of exogenous metabolic activation.

Epithelial binding of 1,2-dichloroethane in mice

The metabolism and binding of 14C-labelled 1,2-dichloroethane (DCE) in female C57BL-mice were studied. As shown by whole-body autoradiography with heated and organic solvent-extracted tissue sections of i.v. injected mice, a selective localization of non-volatile and bound DCE-metabolites occurred in the nasal olfactory mucosa and the tracheo-bronchial epithelium. Low levels of metabolites were also present in the epithelia of the upper alimentary tract, vagina and eyelid, and in the liver and kidney. A decreased mucosal and epithelial binding was observed after pretreatment with metyrapone, indicating that the binding might be due to an oxidative metabolism of DCE. The quantitated levels of in vivo binding were considerably lower in mice injected i.p. with DCE, as compared to mice given equimolar doses of 14C-labelled 1,2-dibromoethane. In vitro experiments with 1000 g supernatants from various tissues showed that the nasal mucosa has a marked ability to activate DCE into products that become irreversibly bound to the tissue. It is proposed that the nasal olfactory mucosa is a target tissue for toxicity of DCE.

Covalent binding of 1,1,1,2-tetrachloroethane to nucleic acids as evidence of genotoxic activity

Twenty-two hours after ip administration to male Wistar rats and BALB/c mice, 1,1,1,2-tetrachloroethane (1,1,1,2-TTCE) is bound covalently to DNA, RNA, and proteins of liver, lung, kidney, and stomach. The in vivo reactivity leads to binding values to DNA generally higher in mouse organs than in rat organs. The covalent binding index (CBI) values (82 in mouse liver DNA and 40 in rat liver DNA) classify 1,1,1,2-TTCE as a weak to moderate initiator. Both microsomal and cytosolic enzymatic systems from rat and mouse organs are capable of bioactivating 1,1,1,2-TTCE in vitro. Liver fractions are the most effective. When the activating systems are simultaneously present in the incubation mixture a synergistic effect is observed. Unlike the related chemical 1,1,2,2-tetrachloroethane (1,1,2,2-TTCE), which is bioactivated only through an oxidative route, 1,1,1,2-TTCE metabolism is carried on by oxidative and reductive pathways, both dependent on cytochrome P-450. 1,1,1,2-TTCE is also bioactivated by microsomal GSH-transferases from liver and lung. These data further confirm that correlations exist between structure and genotoxic activity of halocompounds.

Binding of hexachloroethane to biological macromolecules from rat and mouse organs

Hexachloroethane (HCE) binds to macromolecules of rat and mouse both in vivo and in vitro after metabolic activation. The covalent binding index (CBI) to liver DNA in vivo is comparable to that of compounds classified as weak-moderate initiators and is of approximately the same order of magnitude as those of other halocompounds such as 1,2-dichloroethane. HCE is bioactivated in vitro by microsomal enzymatic systems from murine liver and kidney and, to a greater extent, by cytosolic fractions from all assayed organs. HCE is less reactive than 1,1,2,2-tetrachloroethane, which is more toxic and oncogenic. The ability of hexachloroethane and five other chloroethanes to react covalently with mouse liver DNA both in vivo and in vitro parallels the relative oncogenic potency of these hepatocarcinogenic chemicals in mouse liver.

The covalent binding of 1,1,2,2-tetrachloroethane to macromolecules of rat and mouse organs

The in vivo interaction of the hepatocarcinogen 1,1,2,2-tetrachloroethane (1,1,2,2-TTCE) with DNA, RNA, and proteins of male Wistar rats and BALB/c mice was measured 22 hr after i.p. injection. Covalent binding index (CBI) to liver DNA was about 500 and was comparable to those of carcinogens classified as moderate initiators. It was higher than those of other chloroethanes, even than that of 1,2-dichloroethane (1,2-DCE), a symmetrically substituted haloethane whose genotoxicity has been widely demonstrated. In in vitro cell-free systems, 1,1,2,2-tetrachloroethane was bioactivated by mixed-function oxidase(s) and glutathione-S-transferase(s) (GSH-T) from microsomal and cytosolic fractions of rat and mouse liver and, to a lesser extent, of mouse lung. The in vitro activation led to formation of reactive species capable of binding to exogenous DNA and to the subcellular constituents of enzymatic fractions. These data, along with previous literature reports, provide sufficient evidence of 1,1,2,2-TTCE genotoxicity.

Effects of carcinogenic halogenated aliphatic hydrocarbons on [3H]thymidine incorporation into various organs of the mouse. A comparison between 1,2-dibromoethane and 1,2-dichloroethane

The effects of 1,2-dibromoethane (DBE) and 1,2-dichloroethane (DCE) on the incorporation of [3H]thymidine into DNA were evaluated in various tissues of mice. The compounds were given intraperitoneally 24 h before sacrifice in an equimolar dose (293 mumoles/kg body weight). 2 h before the animals were killed, 0.5 mu Ci [3H]thymidine/g body weight was injected intraperitoneally. Both agents inhibited the [3H]thymidine incorporation in the forestomach, a site for their carcinogenic action. Whereas DBE also suppressed the [3H]thymidine incorporation in the nasal mucosa, the thymus, and the "glandular stomach", DCE was inhibitory only in the kidney. The observed difference in the effect of DBE and DCE on the thymus had its counterpart in a DBE-induced decrease of acid-insoluble radioactivity, demonstrated with whole-body autoradiography. The results indicate that in vivo screening of [3H]thymidine incorporation into various organs of an intact experimental animal is a sensitive technique for comparing cyto- and/or genotoxic effects of chemicals with a similar chemical structure.

Interaction of halocompounds with nucleic acids

The binding of epichlorohydrin, 1,2-dichloroethane, 1,2-dibromoethane, chlorobenzene, bromobenzene, and benzene to nucleic acids and proteins of different murine organs was studied in in vivo and in vitro systems. The extent of in vivo enzymatic activation of brominated compounds was higher than that of chlorinated chemicals. Aryl halides were bound mainly to liver DNA whereas interaction of alkyl halides with DNA of liver, kidney, and lung gave rise to similar binding extent. In vitro activation of all chemicals was mediated by microsomal P-450-dependent mixed function oxidase system which is present in rat and mouse liver and, in smaller amount, in mouse lung. Activation of alkyl halides by liver cytosolic GSH-transferases even occurred. The relative reactivity of chemicals in vivo, expressed as Covalent Binding Index (CBI) to rat liver DNA, was: 1,2-dibromoethane greater than bromobenzene greater than 1,2-dichloroethane greater than chlorobenzene greater than epichlorohydrin greater than benzene. On the whole, it agreed with in vitro activation of chemicals, with genotoxicity data from other short-term assays and also with oncogenicity of benzene, epichlorohydrin, 1,2-dichloroethane, and 1,2-dibromoethane. CBI values of chlorobenzene and bromobenzene gave the first clear evidence of genotoxicity and of possible carcinogenicity of these two chemicals.

Chlorinated paraffins: formation of sulphur-containing metabolites of polychlorohexadecane in rats

A uniformly 14C-labelled polychlorinated hexadecane (14C-PCHD; 65% chlorine by wt) was injected into the portal vein in bile duct-cannulated rats (5-6 mg/kg) and the bile was collected for two or three days. Less than 3% of the total amount of radioactivity excreted in the bile was due to unchanged 14C-PCHD. The radioactivity was separated by ion-exchange chromatography into two major fractions: one acidic, the other amphoteric. Comparison with a similar fractionation of propachlor metabolites indicates that the fractions contain 14C-PCHD conjugates of N-acetylcysteine (mercapturic acid) and glutathione, respectively. The tentative 14C-PCHD-mercapturic acid on t.l.c. had an RF value similar to that of a synthetic PCHD-mercapturic acid, and chlorine and divalent sulphur were shown to be present.

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.

The covalent binding of bromobenzene with nucleic acids

The hepatotoxic compound bromobenzene binds to DNA, RNA, and proteins of rat and mouse liver in vivo. Binding to a significant extent is also detected in mouse kidney. The covalent binding index (CBI) of bromobenzene is comparable to CBI values of moderately oncogenic substances. The enzyme-mediated in vitro interaction of bromobenzene with calf thyumus DNA and synthetic polyribonucleotides is effected only by microsomes, especially those from mouse and rat liver. Microsomes from mouse lung are also efficient in bioactivating bromobenzene to interact with DNA. Among polyribonucleotides, poly(G) and poly(A) are the most labeled substrates. The suppression of binding to DNA by SKF 525-A and the induction of microsomal activity by a pretreatment with phenobarbitone in vivo confirm that bromobenzene is bioactivated by a P-450 dependent-microsomal mixed function oxidase system. The covalent binding can be the main event to determine the possible carcinogenicity by genotoxic mechanisms. Bromobenzene is photoactivated by ultraviolet light (lambda = 254 nm) to forms capable of interacting with DNA in vitro; the binding is linear up to time.

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.

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.