Biological activation of 1,3-butadiene to vinyl oxirane by rat liver microsomes and expiration of the reactive metabolite by exposed rats

In vivo roles of conjugation with glutathione and O6-alkylguanine DNA-alkyltransferase in the mutagenicity of the bis-electrophiles 1,2-dibromoethane and 1,2,3,4-diepoxybutane in mice

Several studies with bacteria and in vitro mammalian systems have provided evidence of the roles of two thiol-based conjugation systems, glutathione (GSH) transferase and O(6)-alkylguanine DNA-alkyltransferase (AGT), in the bioactivation of the bis-electrophiles 1,2-dibromoethane and 1,2,3,4-diepoxybutane (DEB), the latter an oxidation product of 1,3-butadiene. The in vivo relevance of these conjugation reactions to biological activity in mammals has not been addressed, particularly with DEB. In this work, we used transgenic Big Blue mice, utilizing the cII gene, to examine the effects of manipulation of conjugation pathways on liver mutations arising from dibromoethane and DEB in vivo. Treatment of the mice with butathionine sulfoxime (BSO) prior to dibromoethane lowered hepatic GSH levels, dibromoethane-GSH DNA adduct levels (N(7)-guanyl), and the cII mutation frequency. Administration of O(6)-benzylguanine (O(6)-BzGua), an inhibitor of AGT, did not change the mutation frequency. Depletion of GSH (BSO) and AGT (O(6)-BzGua) lowered the mutation frequency induced by DEB, and BSO lowered the levels of GSH-DEB N(7)-guanyl and N(6)-adenyl DNA adducts. Our results provide evidence that the GSH conjugation pathway is a major in vivo factor in dibromoethane genotoxicity; both GSH conjugation and AGT conjugation are major factors in the genotoxicity of DEB. The latter findings are considered to be relevant to the carcinogenicity of 1,3-butadiene.

(32)P-postlabelling analysis of 1,3-butadiene-induced DNA adducts in vivo and in vitro

Butadiene monoepoxide (BMO), epoxybutanediol (EBD) and diepoxybutane (DEB) are reactive metabolites of 1,3-butadiene (BD), an important industrial chemical classified as a probable human carcinogen. The covalent interactions of these metabolites with DNA lead to the formation of DNA adducts which may induce mutations or other types of DNA damage, resulting in tumour formation. In the present study, two pairs of diastereomeric N-1-BMO-adenine adducts were identified in the reaction of BMO with 2´-deoxyadenosine-5´-monophosphate (5´-dAMP). The major products formed by reacting EBD with 2´-deoxyguanosine-5´-monophosphate (5´-dGMP) were characterized as diastereomeric N-7-(2´,3´,4´-trihydroxybut-1´-yl)-5´-dGMP by UV and electrospray mass spectrometry. The formation of N-7-BMO-guanine adducts (1´-carbon, 60; 2´carbon, 54/10(4) nucleotides) in BMO-treated DNA was about four times higher than that of N-1-BMO-adenine adducts (1´-carbon, 20; 2´-carbon, 8.7/10(4) nucleotides). However, the recovery of N-1-BMO-adenine adducts in DNA (45 ± 5%) was two times higher than that of N-7-guanine adducts (20 ± 4%) by 32P-postlabelling analysis. Using the 32P-postlabelling/ HPLC assay, N-1-BMO-adenine, N-7-BMO-guanine and N-7-EBDguanine adducts were detected in BMO- or DEB-treated DNA and in liver DNA of rats exposed to BD by inhalation. The amount of N-7-EBD-guanine adducts (11/10(8) nucleotides) in rat liver was about three-fold higher than N-7-BMO-guanine adducts (4.0/10(8) nucleotides). The novel finding of N-1-BMO-adenine adducts formed in vivo may contribute to the understanding of the mechanisms of BD carcinogenic action.

Toxicology of 1,3-butadiene, chloroprene, and isoprene

The diene monomers, 1,3-butadiene, chloroprene, and isoprene, respectively, differ only in substitution of a hydrogen, a chlorine, or a methyl group at the second of the four unsaturated carbon atoms in these linear molecules. Literature reviewed in the preceding sections indicates that these chemicals have important uses in synthesis of polymers, which offer significant benefits within modern society. Additionally, studies document that these monomers can increase the tumor formation rate in various organs of rats and mice during chronic cancer bioassays. The extent of tumor formation versus animal exposure to these monomers varies significantly across species, as well among strains within species. These studies approach, but do not resolve, important questions of human risk from inhalation exposure. Each of these diene monomers can be activated to electrophilic epoxide metabolites through microsomal oxidation reactions in mammals. These epoxide metabolites are genotoxic through reactions with nucleic acids. Some of these reactions cause mutations and subsequent cancers, as noted in animal experiments. Significant differences exist among the compounds, particularly in the extent of formation of highly mutagenic diepoxide metabolites, when animals are exposed. These metabolites are detoxified through hydrolysis by epoxide hydrolase enzymes and through conjugation with glutathione with the aid of glutathione S-transferase. Different strains and species perform these reactions with varying efficacy. Mice produce these electrophilic epoxides more rapidly and appear to have less adequate detoxification mechanisms than rats or humans. The weight of evidence from many studies suggests that the balance of activation versus detoxification offers explanation of differing sensitivities of animals to these carcinogenic actions. Other aspects, including molecular biology of the many processes that lead through specific mutations to cancer, are yet to be understood. Melnick and Sills (2001) compared the carcinogenic potentials of these three dienes, along with that of ethylene oxide, which also acts through an epoxide intermediate. From the number of tissue sites where experimental animal tumors were detected, butadiene offers greatest potential for carcinogenicity of these dienes. Chloroprene and then isoprene appear to follow in this order. Comparisons among these chemicals based on responses to external exposures are complicated by differences among studies and of species and tissue susceptibilities. Physiologically based pharmacokinetic models offer promise to overcome these impediments to interpretation. Mechanistic studies at the molecular level offer promise for understanding the relationships among electrophilic metabolites and vital genetic components. Significant improvements in minimization of industrial worker exposures to carcinogenic chemicals have been accomplished after realization that vinyl chloride caused hepatic angiosarcoma in polymer production workers (Creech and Johnson 1974; Falk et al. 1974). Efforts continue to minimize disease, particularly cancer, from exposures to chemicals such as these dienes. Industry has responded to significant challenges that affect the health of workers through efforts that minimize plant exposures and by sponsorship of research, including animal and epidemiological studies. Governmental agencies provide oversight and have developed facilities that accomplish studies of continuing scientific excellence. These entities grapple with differences in perspective, objectives, and interpretation as synthesis of knowledge develops through mutual work. A major challenge remains, however, in assessment of significance of environmental human exposures to these dienes. Such exposure levels are orders of magnitude less than exposures studied in experimental or epidemiological settings, but exposures may persist much longer and may involve unknown but potentially significant sensitivities in the general population. New paradigms likely will be needed for toxicological evaluation of these human exposures, which are ongoing but as yet are not interpreted.

The "Tuebingen desiccator" system, a tool to study oxidative stress in vivo and inhalation toxicokinetics

The "Tuebingen desiccator," a gas-tight all-glass closed chamber system (CCS), has been established in Herbert Remmer's Institute of Toxicology, University of Tuebingen, to investigate the mechanisms underlying the exhalation of endogenous volatile hydrocarbons in rats under oxidative stress. Remmer and associates confirmed the former view that ethane and n-pentane were derived from polyunsaturated fatty acids, and they demonstrated that propane, n-butane and isobutane were released from amino acids. Hydrocarbons exhaled following acute ethanol treatment of rats resulted predominantly from ethanol-dependent inhibition of their metabolism and partly from oxidation of proteins. Exhalation of alkanes in carbon tetrachloride exposed rats did not reflect liver damage, which was, however, directly linked to the amount of carbon tetrachloride metabolized. As has first been shown in Herbert Remmer's institute by investigating the fate of inhaled vinyl chloride in rats, the CSS proved to be also an excellent tool for studying toxicokinetics of inhaled gaseous xenobiotics by means of gas uptake experiments. Based on results gained by such studies, it was recently demonstrated that knowledge of compound-specific physicochemical and species-specific physiological parameters are often sufficient to predict important toxicokinetic properties of inhaled chemicals such as tissue burdens at steady state. By means of the CCS, not only kinetics of a parent gaseous substance but also of gaseous metabolites can be investigated in vivo, as exemplified for ethylene oxide and 1, 2-epoxy-3-butene, metabolites of ethylene and 1,3-butadiene, respectively. Gas uptake studies in closed chamber systems are now worldwide used for determining toxicokinetic parameters relevant for physiological toxicokinetic modeling.

N7-guanine adducts of the epoxy metabolites of 1,3-butadiene in mice lung

Epoxy metabolites of 1,3-butadiene are electrophilic and can bind to nucleophilic sites in DNA forming DNA adducts. In this study, guanine N7 adducts of epoxy butene and guanine N7 adducts of epoxy butanediol were measured in lung tissues of mice inhalation exposed to various concentrations of 1,3-butadiene. 32P-postlabeling of DNA adducts were used to demonstrate that the DNA adducts derived from epoxybutene and epoxybutanediol were formed in a dose dependent manner. More than 98% of all adducts detected were formed from epoxybutanediol. Enantiomeric distribution of the adducts formed in vivo differs from that of in vitro experiments demonstrated before. In the case of epoxybutene most of the adducts were formed to the terminal carbon of the S-epoxybutene enantiomer. Most of the adducts derived from epoxybutanediol were formed from the 2S-3R enantiomer. The data demonstrates that enzymatic processes involved with activation and/or detoxification of the metabolites are enantiospecific and/or DNA repair machinery repairs the damage with stereochemical considerations. These are the crucial factors if interspecies differences in tumor sensitiveness is concerned.

Specific DNA adducts induced by some mono-substituted epoxides in vitro and in vivo

Alkyl epoxides are important intermediates in the chemical industry. They are also formed in vivo during the detoxification of alkenes. Alkyl epoxides have shown genotoxicity in many toxicology assays which has been associated with their covalent binding to DNA. Here aspects of the formation and properties of DNA adducts, induced by some industrially important alkenes and mono-substituted epoxides are discussed. These include propylene oxide, epichlorohydrin, allyl glycidyl ether and the epoxy metabolites of styrene and butadiene. The major DNA adducts formed by epoxides are 7-substituted guanines, 1- and 3-substituted adenines and 3-substituted cytosines. In addition, styrene oxide and butadiene monoepoxide are able to modify exocyclic sites in the DNA bases, the sites being in the case of styrene oxide N(2)- and O(6)-positions of guanine, N(6)-adenine as well as N(4)-and O(2)-cytosine. In vivo the main adduct is the 7-substituted guanines. The 1-substituted adenines have also shown marked levels, and these adducts should also be targets in biomonitoring of human exposures. Due to its low mutagenicity, 7-substituted guanines are considered as a surrogate marker for other mutagenic lesions, e.g. those of 1-adenine or 3-uracil adducts.

A review of the genetic and related effects of 1,3-butadiene in rodents and humans

In this paper, the metabolism and genetic toxicity of 1,3-butadiene (BD) and its oxidative metabolites in humans and rodents is reviewed with attention to newer data that have been published since the latest evaluation of BD by the International Agency for Research on Cancer (IARC). The oxidative metabolism of BD in mice, rats and humans is compared with emphasis on the major pathways leading to the reactive intermediates 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBdiol). Results from recent studies of DNA and hemoglobin adducts indicate that EBdiol may play a more significant role in the toxicity of BD than previously thought. All three metabolites are capable of reacting with macromolecules, such as DNA and hemoglobin, and have been shown to induce a variety of genotoxic effects in mice and rats as well as in human cells in vitro. DEB is clearly the most potent of these genotoxins followed by EB, which in turn is more potent than EBdiol. Studies of mutations in lacI and lacZ mice and of the Hprt mutational spectrum in rodents and humans show that mutations at G:C base pairs are critical events in the mutagenicity of BD. In-depth analyses of the mutational spectra induced by BD and/or its oxidative metabolites should help to clarify which metabolite(s) are associated with specific mutations in each animal species and which mutational events contribute to BD-induced carcinogenicity. While the quantitative relationship between exposure to BD, its genotoxicity, and the induction of cancer in occupationally exposed humans remains to be fully established, there is sufficient data currently available to demonstrate that 1,3-butadiene is a probable human carcinogen.

Butadiene: species comparison for metabolism and genetic toxicology

The synthetic monomer 1,3-butadiene and its metabolites have been reviewed in various in vitro and in vivo metabolic studies and in genetic toxicology assays. The species differences have been compared.

Comparison of the disposition of butadiene epoxides in Sprague-Dawley rats and B6C3F1 mice following a single and repeated exposures to 1,3-butadiene via inhalation

1,3-Butadiene (BD), a compound used extensively in the rubber industry, is a potent carcinogen in mice and a weak carcinogen in rats in chronic carcinogenicity bioassays. While many chemicals are known to alter their own metabolism after repeated exposures, the effect of exposure prior to BD on its in vivo metabolism has not been reported. The purpose of the present research was to examine the effect of repeated exposure to BD on tissue concentrations of two mutagenic BD metabolites, butadiene monoepoxide (BDO) and butadiene diepoxide (BDO2). Concentrations of BD epoxides were compared in several tissues of rats and mice following a single exposure or ten repeated exposures to a target concentration of 62.5 ppm BD. Female Sprague-Dawley rats and female B6C3F1 mice were exposed to BD for 6 h or 6 h x 10 days. BDO and BDO2 were quantified in blood and several other tissues following preparation by cryogenic vacuum distillation and analysis by multidimensional gas chromatography-mass spectrometry. Blood and lung BDO concentrations did not differ significantly (P < or = 0.05) between the two exposure regimens in either species. Following multiple exposures to BD, BDO levels were 5- and 1.6-fold higher (P < or = 0.05) in mammary tissue and 2- and 1.4-fold higher in fat tissue of rats and mice, respectively, as compared with single exposures. BDO2 levels also increased in rat fat tissue following multiple exposures to BD. However, in mice, levels of this metabolite decreased by 15% in fat, by 28% in mammary tissue and by 34% in lung tissue following repeated exposures to BD. The finding that the mutagenic epoxide BDO, which is the precursor to the highly mutagenic BDO2, accumulates in rodent fat may be important in assessing the potential risk to humans from inhalation of BD.

Disposition of butadiene epoxides in Sprague-Dawley rats

1,2-Epoxybutene (BMO) and diepoxybutane (BDE) are metabolic products of 1,3-butadiene in rodents. Both BMO and BDE are suspect in the development of tumors in rats and mice. To understand the distribution and elimination of these compounds in the absence of the rate-limiting production from butadiene, the pharmacokinetics of BMO and BDE in blood were determined in adult male Sprague-Dawley rats following intravenous administration. All animals were dually cannulated in these studies. For the BMO studies, rats were dosed with 71, 143, or 286 mumol/kg BMO (n = 3 for each dose group). For the BDE studies, rats were dosed with 523 mumol/kg BDE (n = 3). All animals tolerated the BMO and BDE doses without grossly observable adverse effects. Blood was drawn at predetermined time points and extracted in methylene chloride. BDE and BMO concentrations were quantitated by gas chromatography or gas chromatography/mass spectrometry. The BMO distribution half-lives were short and ranged from 1.4 min at the lowest dose to 1.8 min at the highest dose. Volume of distribution at steady state ranged from 0.53 +/- 0.17 to 0.59 +/- 0.31 l/kg. Systemic clearances ranged from 67 +/- 17 to 114 +/- 20 ml/min per kg. The terminal elimination half-lives were also short and ranged from 5.7 to 8.5 min among the doses. The pharmacokinetic parameters after an i.v. dose of 523 mumol/kg BDE were a distribution half-life of 2.7 min, terminal elimination T1/2 of 14 min, volume of distribution at steady state of 0.73 +/- 0.06 l/kg, and systemic clearance of 76 +/- 8 ml/min per kg. These pharmacokinetic parameters demonstrate the similarity between disposition of the two epoxides in rats, that include a rapid distribution after i.v. administration into a small extravascular body compartment as well as a rapid elimination from blood. These pharmacokinetic data provide useful blood clearance information for assessing the critical physiological and biochemical determinants underlying the disposition of butadiene epoxides.

Toxicokinetics of isoprene in rodents and humans

A physiological toxicokinetic model (PT model) was developed for inhaled isoprene in mouse, rat and man. Partition coefficients blood:air and tissue:blood were determined in vitro by a headspace method. Parameters of a saturable isoprene metabolism in B6C3F1 mice, Sprague-Dawley rats and volunteers were obtained from gas uptake experiments in closed systems, analyzed by means of a two-compartment model. Incorporation of these parameters into the PT model revealed that isoprene was metabolized not only in the liver but also in extrahepatic organs. Endogenous production of isoprene in man was quantified from experiments with volunteers breathing into a closed system. The PT model was validated for mice, rats and humans by comparing simulated values with data determined by other authors.

Biochemistry of 1,3-butadiene metabolism and its relevance to 1,3-butadiene-induced carcinogenicity

Recently, the roles of specific P450 isoforms, myeloperoxidase (MPO), GSH-S-transferase and epoxide hydrolase in the metabolism of 1,3-butadiene, and its major oxidative metabolite, butadiene monoxide (BM), were investigated. The results provided evidence for P450s 2A6 and 2E1 being major catalysts of 1,3-butadiene oxidation in human liver microsomes. cDNA-expressed human P450s 2E1, 2A6, and 2C9 catalyzed BM oxidation to meso- and (+/-)-diepoxybutane (DEB), but the rates of BM oxidation in mouse, rat, or human liver microsomes were much lower than the rates of 1,3-butadiene oxidation in these tissues. Human MPO catalyzed 1,3-butadiene oxidation to BM, but MPO incubations with BM did not yield DEB. Rates of BM formation in mouse and human liver microsomes were similar and were nearly 3.4-fold higher than that obtained with rat liver microsomes. However, rat liver epoxide hydrolase activity was nearly 2-fold higher than that of mouse liver microsomes. Rat and mouse liver GSH-S-transferases exhibited similar BM conjugation kinetics, but rats excreted more BM-mercapturic acids compared to mice given low equimolar doses of BM. BM reacted with guanosine and adenosine to yield N7-, N2-, and N1-guanosinyl and N6-adenosinyl adducts, respectively. These results may contribute to a better understanding of the biochemical basis of 1,3-butadiene-induced carcinogenicity.

Sister-chromatid exchanges, glutathione S-transferase theta deletion and cytogenetic sensitivity to diepoxybutane in lymphocytes from butadiene monomer production workers

The magnitude of health risks to workers associated with current and past exposures to butadiene has been the subject of considerable recent debate. Butadiene is metabolized in-vivo and in-vitro to the genotoxic intermediates 3,4-epoxybutene and diepoxybutane. Studies in animals and in-vitro systems have clearly demonstrated that 1,3-butadiene is a genotoxin and a potent inducer of sister-chromatid exchanges (SCEs). Data on the genotoxicity of butadiene in humans is, however, limited. Epidemiologic data indicate that butadiene is a probable human carcinogen. Recent work has further demonstrated that cultured lymphocytes from the approximately 20% of the Caucasian population that lack the glutathione S-transferase class theta gene (GSTT1) are relatively sensitive to the induction of cytogenetic damage by butadiene metabolites. In order to test whether butadiene exposure was associated with increases in SCE frequencies in peripheral blood lymphocytes and whether any increase observed could be affected by the DEB sensitivity-GSTT1 deletion, we studied 40 workers employed in the production of butadiene. In these workers baseline frequencies of SCEs, diepoxybutane-induced SCE frequencies and GSTT1 deletion status were assessed. Questionnaires were administered to each worker and exposure to 1,3-butadiene was determined using three separate approaches. Industrial hygiene personal sampling was used to measure breathing zone butadiene exposure and urine was collected to use in measurement of the urinary butadiene metabolite 1,2-dihydroxy-4-(N-acetylcysteinyl-S-)-butane (M1). Exposure to butadiene was generally below 2 ppm. The urinary metabolite M1 was found in all workers, but it did not correlate significantly with exposure. Six of 40 of the workers were GST theta-deleted DEB sensitive. No measure of acute or chronic exposure to butadiene was associated with an increase in SCE frequency. However, smoking and DEB sensitivity-GSTT1 null status were each significantly associated with elevations in baseline SCE frequency.

Heritable translocations induced by inhalation exposure of male mice to 1,3-butadiene

Previously, we reported that dominant lethal mutations were induced in spermatids after inhalation exposure of male (102/El x C3H/El)F1 mice to 1300 ppm of 1,3-butadiene on 5 days for 6 h per day (exposure dose 39,000 ppm h). The same inhalation exposure was given to male C3H/El inbred mice which were mated to inbred line 102/El females 8-14 d after the end of exposure. Male and female F1 hybrid progeny were tested for the presence of heritable translocations by observation of litter sizes and by cytogenetic analyses in meiotic and somatic cells. 1,3-Butadiene induced heritable translocations in late spermatids. The translocation frequency after 1,3-butadiene exposure to 39,000 ppm h was 2.7% (16 translocation heterozygotes among 559 F1 offspring). This frequency is 54 times higher than the historical control frequency (0.05%; 5 translocation heterozygotes among 9500 F1 offspring). Thus, 1,3-butadiene causes heritable germ cell effects in mice.

Chromosome aberrations and response to gamma-ray challenge in lymphocytes of workers exposed to 1,3-butadiene

An integrated population monitoring study was initiated to investigate whether occupational exposure to current low levels of butadiene is mutagenic to workers. Ten exposed workers (mean production area concentration of 3.5 ppm) and 10 matched plant controls (mean exposure to 0.03 ppm) were selected and blood samples were collected for our study. The standard cytogenetic assay was used to determine chromosome aberration frequencies. In addition, a challenge assay was used to determine response to gamma-rays as an indication of DNA repair deficiencies. In the latter assay, cells were exposed to gamma-rays at the G1 phase of the cell cycle in vitro and the frequencies of chromosome aberrations in the first post-irradiation metaphase cells were quantitated. Based on results of the cytogenetic assay, the exposed group had a higher frequency of cells with chromosome aberrations and higher chromatid breaks per 100 cells compared with the control. However, the difference was not significant (p > 0.1). With the challenge assay, the exposed group had a higher frequency of aberrant cells (p < 0.04), chromatid breaks (p < 0.05), deletions (p < 0.07), and dicentrics (p < 0.02) than the controls. In addition, the dicentric frequencies from workers were significantly correlated with the presence of a butadiene metabolite [1,2-dihydroxy-4-(N-acetylcysteinyl-S)butane] in urine with a correlation of coefficient of 0.6 (p < 0.01). Two outliers were identified and our interpretation of their responses will be discussed. This study indicates that the workers had exposure-induced mutagenic effects. Together with the observation of gene mutation in a subset of the present population, this study indicates that the current occupational exposure to butadiene may not be safe to workers.

Evaluation of DNA damage by alkaline elution technique after inhalation exposure of rats and mice to 1,3-butadiene

The alkaline filter elution technique was used to evaluate single strand breaks (SSB), DNA-DNA (DDCL) and DNA-protein cross-links (DPCL) in liver and lung of male rats (Sprague-Dawley) and male mice (B6C3F1) after exposure to 2000 ppm 1,3-butadiene (BD) for 7 days (7 h/day and/or to 100, 250, 500, 1000) 2000 ppm BD for 7 h. SSB were detected in liver DNA of both species at 2000 ppm. Cross-links are more pronounced in mouse lung than in mouse liver. Elution rates of lung DNA from mice exposed for 7 h to different concentrations of BD revealed an increase in cross-links between 250 and 500 ppm, and a further increase in cross-links up to 2000 ppm. No such signs of genotoxicity could be observed for the lung of rats. Our data support the involvement of reactive metabolites (epoxybutene and especially diepoxybutane) in butadiene-induced carcinogenesis in the mouse but not to that extent in the rat.

In vitro biotransformation of 2-methylpropene (isobutene): epoxide formation in mice liver

Until now, no data are available concerning the biotransformation and toxicity of 2-methylpropene (or isobutene), a gaseous alkene widely used in industry (rubber, fuel additives, plastic polymers, adhesives, antioxidants). In this work, the biotransformation of 2-methylpropene (MP) has been studied, using total liver homogenates of mice, supplemented with a NADPH-generating system. In analogy to other olefins, 2-methylpropene is metabolized to its epoxide 2-methyl-1,2-epoxypropane (MEP), as proved by the identification by gas chromatography coupled with mass spectrometry. The epoxidation is cytochrome P-450 dependent, as shown by experiments in the absence of the NADPH-generating system and in the presence of various concentrations of metyrapone and SKF 525-A, two known inhibitors of the mono-oxygenases. A simple gas chromatographic headspace method has been developed for the quantitative determination of the epoxide formed. The formation of MEP is never linear in function of time and it reaches a maximum after 20 min. Thereafter is decreases continuously to undetectable levels. This observation can be explained by the immediate action of epoxide hydrolase and glutathione S-transferase, converting the epoxide to 2-methyl-1,2-propanediol and to the glutathione conjugate respectively. The involvement of both enzymes has been demonstrated by the addition of 3,3,3-trichloropropene oxide and indomethacin. These inhibitors of, respectively, epoxide hydrolase and glutathione S-transferase increase the epoxide formation in a significant way. The actual concentration of MEP is therefore not only dependent on its formation by cytochrome P-450 dependent mono-oxygenases, but also on its conversion by epoxide hydrolase and glutathione S-transferase, both very active in liver tissue.

Mechanisms of 1,3-butadiene oxidations to butadiene monoxide and crotonaldehyde by mouse liver microsomes and chloroperoxidase

NADPH-dependent oxidation of 1,3-butadiene by mouse liver microsomes or H2O2-dependent oxidation by chloroperoxidase produced both butadiene monoxide and crotonaldehyde; methyl vinyl ketone and 2,3- and 2,5- dihydrofuran were not detected. The crotonaldehyde to butadiene monoxide ratio remained constant over time in both the microsomal and the chloroperoxidase reactions; however, much more crotonaldehyde was produced by chloroperoxidase than microsomes; crotonaldehyde was not detected when reference samples of butadiene monoxide were used in control incubations containing NADPH and microsomes or H2O2 and chloroperoxidase. Moreover, incubations of 1,3-butadiene with horseradish peroxidase and H2O2, or microsomes and H2O2 or arachidonic acid did not result in the oxidation of 1,3-butadiene. In microsomes, metabolite formation was dependent on incubation time, NADPH, and protein concentrations and did not change when the 1,3-butadiene pressure was varied between 24 and 52 cm Hg. Inclusion of the cytochrome P450 inhibitor 1-benzylimidazole inhibited 1,3-butadiene metabolism, but inclusion of KCN, catalase, or superoxide dismutase had no effect. These results support the role of cytochrome P450 in 1,3-butadiene oxidation by mouse liver microsomes. The formation of crotonaldehyde but not methyl vinyl ketone by cytochrome P450 or chloroperoxidase indicates regioselectivity in the oxygen transfer from the hemoproteins to 1,3-butadiene. The intermediates formed may undergo either ring closure to form butadiene monoxide or a hydrogen shift to form 3-butenal which tautomerizes to produce crotonaldehyde. Evidence for this tautomerization was obtained by the finding that 3-buten-1-ol, an alternative precursor of 3-butenal, was oxidized to crotonaldehyde under incubation conditions similar to that used for 1,3-butadiene.

Enzyme specific kinetics of 1,2-epoxybutene-3 in microsomes and cytosol from livers of mouse, rat, and man

Kinetics of the metabolism of 1,2-epoxybutene-3 (butadiene monoxide) were investigated in liver fractions of mouse, rat, and man. In these species similar enzyme characteristics were found. In microsomes, no NADPH-dependent metabolism of butadiene monoxide was detectable. Epoxide hydrolase activity was found only in microsomes. The Vmax [nmol butadiene monoxide/(mg protein x min)] was 19 in mouse, 17 in rat, and 14 in man and the apparent Km (mmol butadiene monoxide/l incubate) was 1.5 in mouse. 0.7 in rat, and 0.5 in man. Glutathione S-transferase activity was found in cytosol only, revealing first order kinetics in the measured range. The ratio Vmax/Km [(nmol butadiene monoxide x 1)/(mg protein x min x mmol of butadiene monoxide)] was 15 in mouse, 11 in rat, and 8 in man. The data obtained were used to extrapolate on the total rate of butadiene monoxide metabolism for each species in vivo: it was calculated to be 1.3 times higher in mice and 2.3 times lower in man compared to rats, when corrected for body weight.

Hemoglobin adducts and urinary mercapturic acids in rats as biological indicators of butadiene exposure

Binding of 1,2-epoxy-3-butene, the primary metabolite of butadiene, to hemoglobin (Hb) and excretion of its mercapturic acid in urine were studied as potential indicators of butadiene exposure. Four groups of Wistar rats were exposed to butadiene at 0, 250, 500 and 1000 ppm 6 h/day, 5 days/week, during 2 weeks. Blood was collected at the end of exposure and 17 days later for analysis of hemoglobin adducts and adduct stability. Urine was collected each day during exposure (afternoon samples) and in between exposures (morning samples). Adducts of 1,2-epoxy-3-butene to N-terminal valine in Hb were measured using the N-alkyl Edman procedure and GC/MS of the thiohydantoin derivatives. The corresponding mercapturic acid was analysed, after deacetylation, through derivatization with phthaldialdehyde and HPLC with fluorescence detection. The Hb adducts proved to be stable and are therefore useful for dosimetry of long-term exposure to butadiene. The adduct levels increased linearly with exposure dose up to 1000 ppm (3 nmol/g Hb at 1000 ppm). The increase with exposure dose of the mercapturic acid concentration in urine was also compatible with a linear dose response up to 1000 ppm. The sensitivity of both analytical methods needs to be improved for their application to human samples.

Assessment of the potential risk to workers from exposure to 1,3-butadiene

The available epidemiologic data provide equivocal evidence that 1,3-butadiene is carcinogenic in humans; some available studies suggest that the lymphopoietic system is a target, but there are inconsistencies among studies in the types of tumors associated with 1,3-butadiene exposure, and there is no evidence of a relationship between length of exposure and cancer risk, as one might expect if there was a true causal relationship between 1,3-butadiene exposure and cancer risk. The available chronic animal studies, however, show an increase in tumor incidence associated with exposure to high concentrations of 1,3-butadiene. In addition to the general uncertainty of the relevance of animal data to humans, there are several additional reasons why the National Toxicology Program's mouse study may not be appropriate for assessing possible human risks. These include: a) the possible involvement of a species-specific tumor virus (MuLV) in the response in mice; b) apparent differences between mice and humans in the rate of metabolism of 1,3-butadiene to reactive epoxides that may be proximate carcinogens; c) use of high dose levels that caused excess early mortality; and d) exposure of animals to 1,3-butadiene for only about half their lifetime. While recognizing the uncertainty in using the available animal data for risk assessment, we have performed low-dose extrapolation of the data to examine the implications of the data if humans were as sensitive as rats or mice to 1,3-butadiene, and to examine how the predictions of the animal data compare to that observed in the epidemiologic studies. With the mouse data, because the study was of less than lifetime duration, we have used the Hartley-Sielken time-to-tumor model to permit estimation of lifetime risk from the less than lifetime exposure of the study. With the rat data, we have used three plausible models for assessing low-dose risk: the multistage model, the Weibull model, and the Mantel-Bryan probit model. With both the rat and mouse data, we used information on how much 1,3-butadiene is retained by animals exposed to various concentrations of the chemical. This improves the accuracy of the low-dose extrapolation. When extrapolated to low-dose levels, mice appear to be at greater risk (by a factor of 5-fold to 40-fold) than rats. Some of this difference (a factor 3-fold to 5-fold) may be due to the faster rate of metabolism of 1,3-butadiene to, and higher blood levels of, epoxide derivatives in mice than in rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of hemoglobin adduct formation in mice and rats after administration of [14C]butadiene or [14C]isoprene

Occupational exposures to 1,3-butadiene or isoprene occur through their use in the manufacture of rubber and other related polymer products. The purpose of this study was to determine if butadiene or isoprene administration would result in the formation of adducts with blood hemoglobin (Hb), and if such adducts can be used as a measure of previous exposure(s). Male B6C3F1 mice and male Sprague-Dawley rats were injected intraperitoneally with 1, 10, 100, or 1000 mumol [14C]butadiene or 0.3, 3.0, 300, 1000, or 3000 mumol [14C]isoprene per kilogram body weight. Animals were killed 24 hr later. Globin was isolated from blood samples and was analyzed for 14C by liquid scintillation spectroscopy. Hb adduct formation was linearly related to administered doses up to 100 mumol [14C]butadiene or 500 mumol [14C]isoprene per kilogram body weight for mice and rats, respectively. For [14C]butadiene, the efficiency of Hb adduct formation in mice and rats within the linear response range was 0.177 +/- 0.003 and 0.407 +/- 0.019 (pmol of 14C-adducts/mg globin)/(mumol of retained [14C]butadiene/kg body wt), respectively (mean +/- SE; n = 18). For [14C]isoprene, these values for mice and rats were 0.158 +/- 0.035 and 0.079 +/- 0.016 (pmol of 14C-adducts/mg globin)/(mumol of retained [14C]isoprene/kg body wt), respectively (mean +/- SE; n = 12). Hb adducts also accumulated linearly after repeated daily administration of 100 mumol [14C]butadiene or 500 mumol [14C]isoprene per kilogram body wt to mice and rats, respectively, for 3 days. [14C]Butadiene-derived Hb adducts in blood showed lifetimes of approximately 24 and approximately 65 days for mice and rats, respectively, which correlate with the reported lifetimes for red blood cells in these rodent species. Thus, levels of butadiene- or isoprene-derived adducts on Hb in circulating blood may be a useful measure of prior repeated exposures to these compounds.

Concentration-dependent depletion of non-protein sulfhydryl (NPSH) content in lung, heart and liver tissue of rats and mice after acute inhalation exposure to butadiene

The effects of different exposure concentrations of butadiene on the cellular non-protein sulfhydryl (NPSH) content of liver, lung and heart tissue were investigated in B6C3F1 mice and Sprague-Dawley rats. Groups of male animals of both species were exposed for 7 h to 10, 50, 100, 250, 500, 1000 and 2000 ppm butadiene. Immediately after exposure, NPSH content of liver, lung and heart tissue was determined according to a modified Ellman procedure. A comparison of both species shows that a dose-dependent NPSH depletion can be observed in mice for all tissues examined. In rats, liver NPSH content shows a major reduction at high exposure concentrations only. In mice, depletion of NPSH content of liver, lung and heart tissue starts at exposure concentrations of about 250 ppm butadiene. A reduction in NPSH content of about 80% is observed for lung tissue at 1000 ppm and for liver and heart tissue at exposure concentrations of 2000 ppm butadiene. The data on tissue concentrations of NPSH obtained after exposure of rats and mice to butadiene reflect the quantitative differences in butadiene metabolism and in biological effectivity of reactive butadiene intermediates between both species.

Cytochrome P-450-catalyzed asymmetric epoxidation of simple prochiral and chiral aliphatic alkenes: species dependence and effect of enzyme induction on enantioselective oxirane formation

The enantioselectivity of the in vitro conversion of simple prochiral and chiral aliphatic alkenes into oxiranes by liver microsomes of untreated or induced (phenobarbital) rats, of untreated or induced (phenobarbital, benzo[a] pyrene) mice, and of humans was determined by complexation gas chromatography. The enantiomeric excess (ee) of the epoxides extends from 0 (trimethyloxirane) to 50% (ethyloxirane). The configuration (R or S) of the enantiomers formed in excess is consistent for homologous oxiranes but is species dependent and in some cases influenced by enzyme induction. Enantioselectivity differences of aliphatic alkene epoxidation by human liver microsomes of four individuals are negligible.

Metabolism of 1,3-butadiene by lung and liver microsomes of rats and mice repeatedly exposed by inhalation to 1,3-butadiene

1,3-Butadiene, a colorless gas widely used as an intermediate in the production of synthetic rubber, is carcinogenic in rats and mice. Species differences exist in the sensitivity to inhaled 1,3-butadiene and the target tissue specificity for tumor formation. We examined whether repeated inhalation exposure of rats and mice to 1,3-butadiene would affect the rate of metabolism of 1,3-butadiene by lung and liver microsomes in these species. Male Sprague-Dawley rats and B6C3F1 mice were exposed nose-only to air (control) or 7600 +/- 170 ppm 1,3-butadiene (13,600 +/- 300 micrograms/l) and 740 +/- 10 ppm 1,3-butadiene (1300 +/- 20 micrograms/l), respectively, for 6 h/day for 5 days. After the last exposure, nasal tissue (rats only), lungs and livers were removed from the animals and microsomes were prepared. Microsomes from the different tissues were incubated with 6 mumol 1,3-butadiene and 10 mumol NADPH for 30 min and the rate of disappearance of 1,3-butadiene from the reaction flasks was quantitated. There was a statistically significant (P less than 0.05) depression in the rate of 1,3-butadiene metabolism (50%) in microsomes from lungs of both rats and mice that were exposed repeatedly to 1,3-butadiene compared to control animals. There was no effect of repeated 1,3-butadiene exposure on liver or nasal tissue (rats only) metabolism of 1,3-butadiene in rats or mice. The data from these studies indicate that it is unlikely that species differences in sensitivity or tissue susceptibility are due to an inductive or inhibitory effect of 1,3-butadiene on its own metabolism in the tissues examined.

Species differences in butadiene metabolism between mouse and rat

Studies on inhalation pharmacokinetics of 1,3-butadiene were conducted in mice (B6C3F1) and rats (Sprague-Dawley) to investigate the considerable differences in the susceptibility of both species to butadiene-induced carcinogenesis. In rats and mice metabolism of 1,3-butadiene to 1,2-epoxybutene-3 follows saturation kinetics. "Linear" (first-order) pharmacokinetics apply at exposure concentrations below 1000 ppm 1,3-butadiene. Saturation of butadiene metabolism is observed at atmospheric concentrations of about 2000 ppm butadiene. In the lower concentration range where first-order metabolism applies, metabolic clearance of inhaled 1,3-butadiene per kg body weight was 7300 ml (gas volume) x hr-1 for mice and 4500 ml x hr-1 for rats. The calculated maximal metabolic elimination rates (Vmax - conditions) were 400 mumol x hr-1 x kg-1 for mice and 220 mumol x hr-1 x kg-1 for rats. This shows that 1,3-butadiene is metabolized by mice at about twice the rate of rats, under conditions of both low and high exposure concentrations.

Depletion of hepatic non-protein sulfhydryl content during exposure of rats and mice to butadiene

B6C3F1 mice, Sprague-Dawley and Wistar rats were exposed to 1,3-butadiene in a closed exposure system. Exposure concentrations were kept above 2000 ppm to ensure saturation of butadiene metabolism in both species (Vmax conditions). Hepatic non-protein sulfhydryl (NPSH) content was determined in butadiene-exposed animals (and air-exposed controls) after exposures for 0, 7 and 15 h. Depletion of hepatic NPSH content was different for the species and strains investigated. In mice, hepatic NPSH content declined to about 20% after 7 h and was further depleted to about 4% at 15 h when signs of acute toxicity were observed. After a 7 h exposure of rats to butadiene, hepatic NPSH content was depleted to about 65% (Wistar) or 80% (Sprague-Dawley) of the corresponding controls but remained practically stable after a 15 h exposure to butadiene. The time-courses of depletion by butadiene of hepatic NPSH support previous findings on differences in butadiene metabolism between rats and mice and offer an additional explanation for the considerable species differences observed in the toxicity and carcinogenicity of this compound.

Inhalation pharmacokinetics of 1,2-epoxybutene-3 reveal species differences between rats and mice sensitive to butadiene-induced carcinogenesis

Comparative investigations of inhalation pharmacokinetics of 1,2-epoxybutene-3 (vinyl oxirane, the primary reactive intermediate of butadiene) revealed major differences in metabolism of this compound between rats and mice. Whereas in rats no indication of saturation kinetics of epoxybutene metabolism could be observed up to exposure concentrations of 5000 ppm, in mice saturation of epoxybutene metabolism becomes apparent at atmospheric concentrations of about 500 ppm. The estimated maximal metabolic rate (Vmax) in mice for epoxybutene was only 350 mumol X h-1 X kg-1 (rats: greater than 2600 mumol X h-1 X kg-1). In the lower concentration range where first order metabolism applies (up to about 500 ppm) epoxybutene is metabolized by mice at higher rates compared to rats (metabolic clearance per kg body weight, mice: 24,900 ml X h-1, rats: 13,400 ml X h-1). Under these conditions the steady state concentration of epoxybutene in the mouse is about 10 times that in the rat. When mice are exposed to high concentrations of butadiene (greater than 2000 ppm; conditions of saturation of butadiene metabolism; closed exposure system) epoxybutene is exhaled by the animals, and its concentration in the gas phase increases with exposure time. At about 10 ppm epoxybutene signs of acute toxicity are observed. When rats are exposed to butadiene under similar conditions, the epoxybutene concentration reaches a plateau at about 4 ppm. Under these conditions hepatic non-protein sulfhydryl compounds are virtually depleted in mice but not in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and genotoxicity of acetoxyoxirane, the epoxide of vinyl acetate

Acetoxyoxirane, the epoxide of vinyl acetate and a potential reactive intermediate, was synthesized and characterized by 13C-nuclear magnetic resonance (13C-NMR) and mass spectroscopy. The compound induced lesions (endonuclease-sensitive and alkali-labile sites) in supercoiled PM2 DNA in vitro and was directly mutagenic toward Salmonella typhimurium TA100. The mutagenicity of the epoxide in phosphate buffer (pH 7.4, 37 degrees C) decreased, with an initial half-life of 2.8 minutes, and mutagenicity was completely abolished by addition of S-9 mix. Acetoxyoxirane did not induce unscheduled DNA synthesis on incubation with Syrian hamster embryo fibroblasts (SHE cells). These findings may possibly be explained by an effective inactivation of acetoxyoxirane by esterases when these are present in the biological system. This view is consistent with the lack of acetoxyoxirane detected in rat liver microsomal incubations of vinyl acetate.

Species differences in butadiene metabolism between mice and rats evaluated by inhalation pharmacokinetics

Metabolism of 1,3-butadiene to 1,2-epoxybutene-3 in rats follows saturation kinetics. Comparative investigation of inhalation pharmacokinetics in mice also revealed a saturation pattern. For both species "linear" pharmacokinetics apply at exposure concentrations below 1000 ppm 1,3-butadiene; saturation of butadiene metabolism is observed at atmospheric concentrations of about 2000 ppm. For mice metabolic clearance per kg body weight in the lower concentration range where first order metabolism applies was 7300 ml X h-1 (rat: 4500 ml X h-1. Maximal metabolic elimination rate (Vmax) was 400 mumol X h-1 X kg-1 (rat: 220 mumol X h-1 X kg-1. This shows that 1,3-butadiene is metabolized by mice at higher rates compared to rats. Based on these investigations, the metabolic elimination rates of butadiene in both species were calculated for the exposure concentrations applied in two inhalation bioassays with rats and with mice. The results show that the higher rate of butadiene metabolism in mice when compared to rats may only in part be responsible for the considerable difference in the susceptibility of both species to butadiene-induced carcinogenesis.

Alkylation of nuclear proteins and DNA after exposure of rats and mice to [1,4-14C]1,3-butadiene

B6C3F1 mice and Wistar rats were exposed to [1,4-14C]1,3-butadiene in a closed exposure system. Based on body weight, mice metabolized the test compound at about twice the rate, compared to rats. Nucleoproteins and DNA were isolated from the livers of the animals and covalent binding of [14C]-butadiene-derived radioactivity was determined. In both species comparable amounts of radioactivity were covalently bound to liver DNA. Covalent binding to mouse-liver nucleoproteins was twice as high as in rats and thus it paralleled the higher metabolic rate for butadiene in this species.

Epoxidation of 1,3-butadiene in liver and lung tissue of mouse, rat, monkey and man

When 1,3-butadiene is incubated with liver postmitochondrial fractions from mouse, rat, monkey or man and a NADPH-regenerating system the formation rate of butadiene monoxide is different in the four species. With the exception of rhesus monkey the amount of epoxide is proportional to the monooxygenase activity. The sequence of epoxide formation is B6C3F1-mouse, Sprague Dawley rat, man, rhesus monkey. The relation between mouse and monkey was about 7:1. When 1,3-butadiene is incubated with homogenates from lung tissue, only tissues from mouse and rat produces measurable butadiene monoxide concentrations. The monooxygenase activity in lung tissue of the mouse was only 1/30 that in mouse liver. By contrast, lung tissue formed the epoxide concentrations comparable to those formed by liver tissue, whereas monkey and human lung tissue did not produce any measurable levels of butadiene monoxide. The data might suggest that the results of recent rodent inhalation studies with 1,3-butadiene could not automatically be extrapolated to man.

Hepatic microsomal metabolism of isoprene in various rodents

Microsomal monooxygenases of various rodents metabolise isoprene to the corresponding monoepoxides, 3,4-epoxy-3-methyl-1-butene and 3,4-epoxy-2-methyl-1-butene. The kinetic constants (Km and Vmax) for the formation of the major products were determined by gas-liquid chromatography (GLC). The minor product was further epoxidised to the mutagenic isoprene dioxide by the microsomes of all rodents studied. The Km and Vmax for this subsequent epoxidation were determined and phenobarbital was found to be a good inducer in all species.

Species differences in the formation of butadiene monoxide from 1,3-butadiene

When 1,3-butadiene is incubated with liver postmitochondrial fractions from mouse, rat, monkey or man and a NADPH-regenerating system, the formation rate of butadiene monoxide is different in the four species. With the exception of the rhesus monkey, the amount of epoxide is proportional to the monooxygenase activity. The sequence of epoxide formation is B6C3F1 mouse, Sprague Dawley rat, man, rhesus monkey. The ratio between mouse and monkey was about 7:1. When 1,3-butadiene is incubated with homogenates from lung tissue, only tissues from mouse and rat produce measurable butadiene monoxide concentrations. The monooxygenase activity in lung tissue of the mouse was only 1/30 that in mouse liver. By contrast, lung tissue formed epoxide concentrations comparable to those formed by liver tissue, whereas monkey and human lung tissue did not produce any measurable levels of butadiene monoxide. The data might suggest that the results of recent rodent inhalation studies with 1,3-butadiene could not automatically be extrapolated to man.

Isoprene metabolism by liver microsomal mono-oxygenases

Mouse-liver microsomal mono-oxygenases metabolize isoprene to the corresponding mono-epoxides. The reaction was NADPH- and O2-dependent and was inhibited by CO, SKF525-A and metyrapone. 3,4-Epoxy-3-methyl-1-butene was the major metabolite of isoprene, and the kinetic constants (Km and Vmax) for this epoxidation were determined by analysing the corresponding diol by g.l.c. in incubations with microsomes from control or pretreated mice. 3,4-Epoxy-2-methyl-1-butene was a minor metabolite (approx. 20%). 3,4-Epoxy-2-methyl-1-butene was epoxidated further to the mutagenic isoprene dioxide by microsomes from control or pretreated mice. The Km and Vmax were determined and phenobarbital shown to be an inducer of this epoxidation.

Covalent interaction of reactive metabolites with cytosolic coenzyme A as mechanism of haloethylene-induced acetonemia

Previous experiments have shown that a number of xenobiotics such as halogenated ethylenes cause an experimental acetonemia. In addition, under exposure of rats to vinylidene fluoride (one of the agents producing this effect), the urinary excretion rates of acetoacetate and 3-hydroxybutyrate are enhanced. The enhanced formation of ketone bodies is theoretically explained by a covalent interaction of reactive metabolites of the applied xenobiotic with hepatic cytosolic coenzyme A. This theory is further corroborated by the following experiments: Microsomal incubations of [14C]vinyl chloride and [3H]coenzyme A lead to one metabolite containing 2 moles vinyl chloride/mole coenzyme A and two other with equimolar ratios of both components. Exposure of rats to vinyl chloride leads to a progressive depletion of hepatic cytosolic CoASH, but not of CoASH in mitochondria. In the cytosol acetyl-CoA is also diminished after vinyl chloride exposure. These changes may cause secondary effects in lipid metabolism which are regarded as responsible for the enhancement of ketone bodies.

Multiple organ carcinogenicity of 1,3-butadiene in B6C3F1 mice after 60 weeks of inhalation exposure

Groups of 50 male and 50 female B6C3F1 mice were exposed 6 hours per day, 5 days per week, for 60 to 61 weeks to air containing 0, 625, or 1250 parts per million 1,3-butadiene. These concentrations are somewhat below and slightly above the Occupational Safety and Health Administration standard of 1000 parts per million for butadiene. The study was designed for 104-week exposures but had to be ended early due to cancer-related mortality in both sexes at both exposure concentrations. There were early induction and significantly increased incidences of hemangiosarcomas of the heart, malignant lymphomas, alveolar-bronchiolar neoplasms, squamous cell neoplasms of the forestomach in males and females and acinar cell carcinomas of the mammary gland, granulosa cell neoplasms of the ovary, and hepatocellular neoplasms in females. Current workplace standards for exposure to butadiene should be reexamined in view of these findings.

The Reproductive Effects Assessment Group's report on the mutagenicity of 1,3-butadiene and its reactive metabolites

A major data gap for assessing heritable risk from exposure to 1,3-butadiene is the lack of mammalian mutagenicity data. The data base on the mutagenic potential of 1,3-butadiene is limited to three bacterial studies from the same laboratory. Two of these studies were positive only in the presence of liver S9 mix from chemically pretreated animals. In vitro data suggest that 1,3-butadiene is metabolized to two epoxide intermediates. 3,4-Epoxybutene, one potential reactive metabolite of 1,3-butadiene, is a monofunctional alkylating agent and is a direct-acting mutagen in bacteria. In addition, unpublished data suggest that 3,4-epoxy-butene induces DNA damage and chromosomal aberrations in mice. Another potential reactive metabolite, 1,2:3,4-diepoxybutane, is a bifunctional alkylating agent and is mutagenic in a wide variety of organisms (bacteria, fungi, and the germ cells of Drosophila). This metabolite also induces DNA damage in mice and in cultured hamster cells, is clastogenic in fungi and cultured rat cells, and produces chromosome damage/breakage in Drosophila germ cells. These data, when combined with evidence that 1,3-butadiene is carcinogenic in rodent gonadal tissues and is associated with gonadal atrophy in mice, constitute suggestive evidence that 1,3-butadiene may be a human germ cell mutagen. However, because the mutagenicity of 1,3-butadiene has been studied only in bacteria, studies in mammalian test systems are needed to further characterize the mutagenic potential of 1,3-butadiene.

Inhalation pharmacokinetics based on gas uptake studies. V. Comparative pharmacokinetics of ethylene and 1,3-butadiene in rats

The pharmacokinetics of ethylene and 1,3-butadiene were studied in male Sprague-Dawley rats by use of a closed inhalation chamber system. Both compounds showed saturable metabolism when untreated rats were used. "Linear" pharmacokinetics applied at exposure concentrations below 800 ppm ethylene and below 1,000 ppm 1,3-butadiene. A constant elimination rate, indicative of metabolic saturation, occurred at concentrations higher than 1,000 ppm ethylene or 1,500 ppm 1,3-butadiene. Pretreatment with aroclor 1254 (polychlorinated biphenyls) increased Vmax for both compounds. For 1,3-butadiene, no saturation of metabolic capacity was observed with exposure concentrations up to 12,000 ppm when the rats were pretreated with aroclor 1254. A comparison with previous studies on ethane and n-pentane suggested that introduction of a double bond into a saturated aliphatic hydrocarbon increased the rate of metabolism under conditions in vivo.

Inhalation pharmacokinetics based on gas uptake studies. VI. Comparative evaluation of ethylene oxide and butadiene monoxide as exhaled reactive metabolites of ethylene and 1,3-butadiene in rats

When ethylene oxide or butadiene monoxide is added to the atmosphere of a closed inhalation chamber occupied by Sprague-Dawley rats, a first-order elimination pattern is observed. When either of these compounds is IP injected into rats which are subsequently placed in the closed chamber, the course of epoxide in the atmosphere follows Bateman exponential functions. From the experimental data, the kinetic parameters for distribution and metabolic elimination of ethylene oxide and butadiene monoxide can be derived. When ethylene or 1,3-butadiene was added to the closed exposure systems and kept at atmospheric concentrations which assured maximal metabolic turnover of the olefin (i.e., concentrations above 1,000 ppm ethylene or 1,500 ppm 1,3-butadiene), exhalation of the appropriate epoxide occurred and led finally to a constant (plateau) concentration of the reactive metabolite in the system's atmosphere. Although the initial time-course was different between butadiene monoxide and ethylene oxide (with a high initial increase of ethylene oxide and a subsequent decrease) an analysis at steady-state (plateau concentrations) revealed that only 29% of the amounts of both epoxides which in theory are formed as primary metabolites from the parent olefins are systematically available (i.e., distributed in the entire organism). The discrepancy is probably related to first pass elimination of the epoxide.