Mutagenicity studies of methyl-tert-butylether using the Ames tester strain TA102

Studies on ultrasound-mediated insertion-deletion polymorphisms of DNA and underlying mechanisms based on Ames tester strains

Low-lethality ultrasound technology has received more and more attention in regulating microorganisms of fermentation industry. Herein, two representative Ames tester strains TA97a and TA98 as model organisms were used to explore the effects of ultrasound on insertion-deletion (InDel) polymorphisms of microbial DNA and its underlying mechanisms. Results revealed that a promotion was observed in the reversion mutation of TA98 upon sonication. Sequencing results from 1752 TA98 revertants showed that there was a total of 127 InDels, of which the InDels unique to ultrasound were 36 more than that of the control. Compared with the control, ultrasound-mediated InDels of DNA displayed additional -29 bp deletion and +7 ∼ +43 bp insertions of direct repeat sequences. Combined with the analysis of transcriptomics and prediction of secondary structure of single-stranded DNA from InDels core region (No. 832 ∼ 915 bp) in hisD3052 gene of TA98 strain, ultrasound-mediated "thermal breathing" mechanism was proposed based on the formation of DNA hairpin structure with micro-homologous sequence. This finding implied that low-intensity ultrasound is expected to be developed a new low-lethal mutagenic technology for continuous mutagenesis.

Methyl-tert-butyl ether (MTBE): integration of rat and mouse carcinogenicity data with mode of action and human and rodent bioassay dosimetry and toxicokinetics indicates MTBE is not a plausible human carcinogen

Methyl-tert-butyl ether (MTBE) is a fuel oxygenate used in non-United States geographies. Multiple health reviews conclude that MTBE is not a human-relevant carcinogen, and this review provides updated mode of action (MOA), exposure, dosimetry and risk perspectives supporting those conclusions. MTBE is non-genotoxic and has large margins of exposure between blood concentrations at the overall rat 400 ppm inhalation NOAEL and blood concentrations in typical workplace or general population exposures. Non-cancer and threshold cancer hazard quotients range from a high of 0.046 for fuel-pump gasoline station attendants and are 100-1,000-fold lower for general population exposures. Cancer risks conservatively assuming genotoxicity for these same scenarios are all less than 1 × 10^-6. The onset of MTBE nonlinear toxicokinetics (TK) in rats at inhalation exposures less than 3,000 ppm, a dose that is also not practically achievable in fuel-use scenarios, indicates that high-dose specific male rat kidney and testes (3,000 and 8,000 ppm) and female mouse liver tumors (8000 ppm) are not quantitatively relevant to humans. Mode of action analyses also indicate MTBE male rat kidney tumors, and lesser so female mouse liver tumors, are not qualitatively relevant to humans. Thus, an integrated analysis of the toxicology, exposure/dosimetry, TK, and MOA data indicates that MTBE presents minimal human cancer and non-cancer risks.

Aldehyde dehydrogenase 2 deficiency significantly exacerbates tert-butyl alcohol-induced toxicity in mice

Owing to the use of ethyl tert-butyl ether (ETBE) as a fuel additive, the possible adverse effects of ETBE exposure have become a public concern. Our previous study showed that ETBE-induced toxicity in aldehyde dehydrogenase 2 (Aldh2) gene knockout (KO) mice was caused by its primary metabolite acetaldehyde, which was toxic. However, it is unclear whether tert-butyl alcohol (TBA), another main metabolite of ETBE, plays a role in ETBE-induced toxicity. To investigate this relationship, we analyzed the changes of TBA concentrations in tissues after ETBE exposure, and then evaluated the toxicity after direct TBA treatment in both KO and wild-type (WT) mice. An exposure to 500 ppm ETBE via inhalation resulted in the formation of its three metabolites, TBA, 2-methyl-1,2-propanediol and ethanol, whose concentrations in the liver, brain, fat and testis of male KO mice were significantly higher than the corresponding concentrations observed in male WT mice. Direct treatment to TBA (20 mg/mL of drinking water) caused significant changes in relative organ weights and histopathology, and increased levels of genetic damages in both types of mice. These toxic effects were also seen in KO mice exposed to a lower concentration of TBA (5 mg/mL), which was associated with increased oxidative stress in serum (reduced glutathione and reduced glutathione/oxidized glutathione ratio decreased). Our findings indicate that ALDH2 is involved in the metabolism of ETBE and TBA, and ALDH2 deficiency could greatly increase the sensitivity to TBA-induced toxicity.

Water remediation contaminated with MTBE using a catalytic oxidation process in batch reactor: influence of the cerium loading on the activity and CO2 selectivity

Destruction of methyl tert-butyl ether (MTBE) in liquid phase and in a batch reactor was studied using ruthenium catalysts over alumina support, modified with different cerium loadings. Ce loading increment causes an increase in the particle size from 1.26 nm to 2.3 nm, enhancing the MTBE oxidation (at 150 °C), and the selectivity toward CO_2. The high catalytic activity of Ru/ACe10 is attributed to the species Ce⁴⁺-O^2--M that could favor the oxygen transfer between the catalyst surface and the adsorbed species by a redox mechanism. Thus, CeOx plays an important role in both enhancing the affinity between MTBE and catalyst during MTBE adsorption and promoting the catalytic activity for MTBE oxidation.

Scientific opinion on the evaluation of substances as acceptable previous cargoes for edible fats and oils

Shipping of edible fats and oils into Europe is permitted in bulk tanks, provided that the previous cargo is included in a positive list. The European Commission requested EFSA to evaluate the acceptability as previous cargoes for fats and oils the substances calcium lignosulphonate, methyl acetate, ethyl tert-butyl ether (ETBE) and ammonium sulphate. The evaluation was based on the same criteria as those used for the evaluation of the substances currently on the list in the Annex to Commission Directive 96/3/EC as acceptable previous cargoes for edible fats and oils. Methyl acetate and ETBE meet the criteria for acceptability as previous cargoes. Due to uncertainties, mainly with regard to the composition and toxicity of the low molecular mass fraction, and the fact that the toxicological database is limited to the 40-65 grade and does not cover all grades of calcium lignosulphonate shipped as previous cargoes, the EFSA Panel on Contaminants in the Food Chain (CONTAM Panel) concluded that calcium lignosulphonate does not meet the criteria for acceptability as a previous cargo. Only food-grade ammonium sulphate meets the criteria for acceptability as a previous cargo due to uncertainties about impurities in other (non-food) grades.

Relationship between Methyl Tertiary Butyl Ether Exposure and Non-Alcoholic Fatty Liver Disease: A Cross-Sectional Study among Petrol Station Attendants in Southern China

Methyl tertiary butyl ether (MTBE)—A well known gasoline additive substituting for lead alkyls—causes lipid disorders and liver dysfunctions in animal models. However, whether MTBE exposure is a risk factor for non-alcoholic fatty liver disease (NAFLD) remains uncertain. We evaluate the possible relationship between MTBE exposure and the prevalence of NAFLD among 71 petrol station attendants in southern China. The personal exposure concentrations of MTBE were analyzed by Head Space Solid Phase Microextraction GC/MS. NAFLD was diagnosed by using abdominal ultrasonography according to the guidelines for the diagnosis and treatment of NAFLD suggested by the Chinese Hepatology Association. Demographic and clinical characteristics potentially associated with NAFLD were investigated. Mutivariate logistic regression analysis was applied to measure odds ratios and 95% confidence intervals (CI). The result showed that the total prevalence of NAFLD was 15.49% (11/71) among the study subjects. The average exposure concentrations of MTBE were 292.98 ± 154.90 μg/m³ and 286.64 ± 122.28 μg/m³ in NAFLD and non-NAFLD groups, respectively, and there was no statistically significant difference between them (p > 0.05). After adjusting for age, gender, physical exercise, body mass index (BMI), systolic blood pressure (SBP), diastolic blood pressure (DBP), alanine aminotransferase (ALT), white blood cell (WBC), total cholesterol (TC), triglycerides (TG), low-density lipoprotein (LDL), and high-density lipoprotein (HDL), the odds ratios were 1.31 (95% CI: 0.85–1.54; p > 0.05), 1.14 (95% CI: 0.81–1.32; p > 0.05), 1.52 (95% CI: 0.93–1.61; p > 0.05) in the groups (including men and women) with exposure concentrations of MTBE of 100–200 μg/m³, 200–300 μg/m³, and ≥300 μg/m³, respectively, as compared to the group (including men and women) ≤100 μg/m³. Our investigation indicates that exposure to MTBE does not seem to be a significant risk factor for the prevalence of NAFLD among petrol station attendants in southern China.

Evaluation of toxicity of methyl tert-butyl ether (MTBE) on mouse spermatogenic cells in vitro

As a synthetic organic chemical, methyl tert-butyl ether (MTBE) is the most common fuel oxygenate. The increasing use of MTBE has raised concern over its safety. Previous studies in vivo revealed that MTBE could alter the male reproduction system. Therefore, the current experiments were designed to evaluate whether isolated mice spermatogenic cells in vitro were sensitive to exposure to MTBE at environmental levels, and to evaluate whether spermatogenic cells had undergone changes in morphologic, activity and viability parameters after exposure to MTBE. Spermatogenic cells in vitro were incubated with medium alone (control), 100 ppb, 10 ppm, 1000 ppm, 3000 ppm MTBE, respectively, for 6, 12, 18 h. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazo liumbromide) assay, staining with fluorescein diacetate (FDA) and propidium iodide (PI), and flow cytometric analyses were used to assess MTBE toxicity on cells and DNA. The results showed that there were no significant differences between control and treatments of ≤ 1000 ppm MTBE at the same time point. Although 3000 ppm MTBE could exert toxic effects directly on spermatogenic cells, environmental levels of MTBE did not exert toxic effects on cultured spermatogenic cells.

Reassessment of MTBE cancer potency considering modes of action for MTBE and its metabolites

A 1999 California state agency cancer potency (CP) evaluation of methyl tert-butyl ether (MTBE) assumed linear risk extrapolations from tumor data were plausible because of limited evidence that MTBE or its metabolites could damage DNA, and based such extrapolations on data from rat gavage and rat and mouse inhalation studies indicating elevated tumor rates in male rat kidney, male rat Leydig interstitial cells, and female rat leukemia/lymphomas. More recent data bearing on MTBE cancer potency include a rodent cancer bioassay of MTBE in drinking water; several new studies of MTBE genotoxicity; several similar evaluations of MTBE metabolites, formaldehyde, and tert-butyl alcohol or TBA; and updated evaluations of carcinogenic mode(s) of action (MOAs) of MTBE and MTBE metabolite's. The lymphoma/leukemia data used in the California assessment were recently declared unreliable by the U.S. Environmental Protection Agency (EPA). Updated characterizations of MTBE CP, and its uncertainty, are currently needed to address a variety of decision goals concerning historical and current MTBE contamination. To this end, an extensive review of data sets bearing on MTBE and metabolite genotoxicity, cytotoxicity, and tumorigenicity was applied to reassess MTBE CP and related uncertainty in view of MOA considerations. Adopting the traditional approach that cytotoxicity-driven cancer MOAs are inoperative at very low, non-cytotoxic dose levels, it was determined that MTBE most likely does not increase cancer risk unless chronic exposures induce target-tissue toxicity, including in sensitive individuals. However, the corresponding expected (or plausible upper bound) CP for MTBE conditional on a hypothetical linear (e.g., genotoxic) MOA was estimated to be ∼2 × 10(-5) (or 0.003) per mg MTBE per kg body weight per day for adults exposed chronically over a lifetime. Based on this conservative estimate of CP, if MTBE is carcinogenic to humans, it is among the weakest 10% of chemical carcinogens evaluated by EPA.

Lack of micronucleus induction activity of ethyl tertiary-butyl ether in the bone marrow of F344 rats by sub-chronic drinking-water treatment, inhalation exposure, or acute intraperitoneal injection

Ethyl tertiary-butyl ether (ETBE) is an oxygenated gasoline additive synthesized from ethanol and isobutene that is used to reduce CO2 emissions. To support the Kyoto Protocol, the production of ETBE has undergone a marked increase. Previous reports have indicated that exposure to ETBE or methyl tertiary-butyl ether resulted in liver and kidney tumors in rats and/or mice. These reports raise concern about the effects of human exposure being brought about by the increased use of ETBE. The present study was conducted to evaluate the genotoxicity of ETBE using micronucleus induction of polychromatic erythrocytes in the bone marrow of male and female rats treated with ETBE in the drinking-water at concentrations of 0, 1,600, 4,000 or 10,000 ppm or exposed to ETBE vapor at 0, 500, 1,500 or 5,000 ppm for 13 weeks. There were no significant increases in micronucleus induction in either the drinking water-administered or inhalation-administered groups at any concentration of ETBE; although, in both groups red blood cells and hemoglobin concentration were slightly reduced in the peripheral blood in rats administered the highest concentration of ETBE. In addition, two consecutive daily intraperitoneal injections of ETBE at doses of 0, 250, 500 or 1,000 mg/kg did not increase the frequency of micronucleated bone marrow cells in either sex; all rats receiving intraperitoneal injections of ETBE at a dose of 2,000 mg/kg died after treatment day 1. These data suggest that ETBE is not genotoxic in vivo.

Tertiary-Butanol: a toxicological review

Tert-Butanol is an important intermediate in industrial chemical synthesis, particularly of fuel oxygenates. Human exposure to tert-butanol may occur following fuel oxygenate metabolism or biodegradation. It is poorly absorbed through skin, but is rapidly absorbed upon inhalation or ingestion and distributed to tissues throughout the body. Elimination from blood is slower and the half-life increases with dose. It is largely metabolised by oxidation via 2-methyl-1,2-propanediol to 2-hydroxyisobutyrate, the dominant urinary metabolites. Conjugations also occur and acetone may be found in urine at high doses. The single-dose systemic toxicity of tert-butanol is low, but it is irritant to skin and eyes; high oral doses produce ataxia and hypoactivity and repeated exposure can induce dependence. Tert-Butanol is not definable as a genotoxin and has no effects specific for reproduction or development; developmental delay occurred only with marked maternal toxicity. Target organs for toxicity clearly identified are kidney in male rats and urinary bladder, particularly in males, of both rats and mice. Increased tumour incidences observed were renal tubule cell adenomas in male rats and thyroid follicular cell adenomas in female mice and, non-significantly, at an intermediate dose in male mice. The renal adenomas were associated with alpha(2u)-globulin nephropathy and, to a lesser extent, exacerbation of chronic progressive nephropathy. Neither of these modes of action can function in humans. The thyroid tumour response could be strain-specific. No thyroid toxicity was observed and a study of hepatic gene expression and enzyme induction and thyroid hormone status has suggested a possible mode of action.

Tertiary butyl alcohol in drinking water induces phase I and II liver enzymes with consequent effects on thyroid hormone homeostasis in the B6C3F1 female mouse

Tertiary butyl alcohol (TBA) was administered to groups of 15 female B6C3F1 mice in drinking water at concentrations of 0, 2.0 or 20 mg TBA ml(-1), for 14 days, for assessment of gross and histological changes in the liver and thyroid, thyroid hormones (T3, T4, and TSH), total hepatic cytochrome P450 (Cyp) content, specific Cyp activities and quantitative PCR analysis of specific Cyp enzymes (Cyp1a1, Cyp2b9, Cyp2b10, Cyp3a11), sulfuryltransferases (ST1a1, ST2a2, and STn) and glucuronyltransferases (UGT1a1, UGT2b1, and UGT2b5). Phenobarbital (PB) was administered to a positive control group by oral gavage at a daily dose of 80 mg kg(-1). TBA caused, on day 14, a reduction in circulating T3 (12-15% decrease) and a dose-dependent reduction in T4 (13-22% decrease), with no evidence of thyroid pathology. Two of five livers examined in the 20 mg TBA ml(-1) dose group showed mild, diffuse centrilobular hypertrophy. On day 14, Cyp 7-benzoxyresorufin-O-debenzylase activity was significantly induced 12-fold by TBA at 20 mg ml(-1), and 1.8-fold at the 2.0 mg TBA ml(-1) concentration. Cyp 7-pentoxyresorufin-O-dealkylase activity was slightly induced (2.1-fold) by 20 mg TBA ml(-1) on day 14. Quantitative PCR analysis of gene transcripts showed a significant induction of Cyp2b10 and ST1a1 with both TBA concentrations, and a slight induction of Cyp2b9 at 20 mg TBA ml(-1) only. PB induced all phase I and phase II gene transcripts except for Cyp1a1 and Cyp2b9. These findings suggest that TBA, at and below doses used in chronic studies, is an inducer of phase I and phase II liver enzymes, with resulting decreases in circulating thyroid hormones in B6C3F1 mice.

Differential toxic effects of methyl tertiary butyl ether and tert-butanol on rat fibroblasts in vitro

Methyl tertiary butyl ether (MTBE) is the most widely used motor vehicle fuel oxygenate since it reduces harmful emissions due to gasoline combustion. However, the significant increase in its use in recent years has raised new questions related to its potential toxicity. In fact, although available data are somehow conflicting, there is evidence that MTBE is a toxic substance that may have harmful effects on both animals and humans and an unresolved problem is the role played by MTBE metabolites, especially tertiary butyl alcohol (TBA), in determining toxic effects due to MTBE exposure. In this study, the toxic effects of MTBE have been analyzed on a normal diploid rat fibroblast cell line (Rat-1) and compared to the effects of TBA. The results obtained suggest that both MTBE and TBA inhibit cell growth in vitro but with different mechanisms in terms of effects on the cell cycle progression and on the modulation of cell cycle regulatory proteins. In fact, MTBE caused an accumulation of cells in the S-phase of the cell cycle, whereas TBA caused an accumulation in the G0/G1-phase with different effects on the expression of cyclin D1, p27Kip1, and p53. Moreover, both MTBE and TBA were also shown to induce DNA damage, as assessed in terms of oxidative DNA damage and nuclear DNA fragmentation, that appeared to be susceptible of repair by the cell DNA-repair machinery. In conclusion, these findings suggest that both MTBE and TBA can exert, by acting through different molecular mechanisms, important biological effects on fibroblasts in vitro. Further studies are warranted to shed light on the mechanisms responsible for the observed effects and on their potential significance for the in-vivo exposure.

Methyltertiary-Butyl Ether: Studies for Potential Human Health Hazards

When methyl tertiary-butyl ether (MTBE) in gasoline was first introduced to reduce vehicle exhaust emissions and comply with the Clean Air Act, in the United States, a pattern of complaints emerged characterised by seven "key symptoms." Later, carefully controlled volunteer studies did not confirm the existence of the specific key symptoms, although one study of self-reported sensitive (SRS) people did suggest that a threshold at about 11-15% MTBE in gasoline may exist for SRSs in total symptom scores. Neurobehavioral and psychophysiological studies on volunteers, including SRSs, found no adverse responses associated with MTBE at likely exposure levels. MTBE is well and rapidly absorbed following oral and inhalation exposures. Cmax values for MTBE are achieved almost immediately after oral dosing and within 2 h of continuous inhalation. It is rapidly eliminated, either by exhalation as unchanged MTBE or by urinary excretion of its less volatile metabolites. Metabolism is more rapid humans than in rats, for both MTBE and tert-butyl alcohol (TBA), its more persistent primary metabolite. The other primary metabolite, formaldehyde, is detoxified at a rate very much greater than its formation from MTBE. MTBE has no specific effects on reproduction or development, or on genetic material. Neurological effects were observed only at very high concentrations. In carcinogenicity studies of MTBE, TBA, and methanol (included as an endogenous precursor of formaldehyde, without the presence of TBA), some increases in tumor incidence have been observed, but consistency of outcome was lacking and even some degree of replication was observed in only three cases, none of which had human relevance: alpha(2u)-globulin nephropathy-related renal tubule cell adenoma in male rats; Leydig-cell adenoma in male rats, but not in mice, which provide the better model of the human disease; and B-cell-derived lymphoma/leukemia of doubtful pathogenesis that arose mainly in lungs of orally dosed female rats. In addition, hepatocellular adenomas were significantly higher in female CD-1 mice and thyroid follicular-cell adenomas were increased in female B6C3F1 mice treated with TBA, but these results lack any independent confirmation, which would have been possible from a number of other studies.

Effect of oral methyl-t-butyl ether (MTBE) on the male mouse reproductive tract and oxidative stress in liver

MTBE is found in water supplies used for drinking and other purposes. These experiments follow up on earlier reports of reproductive tract alterations in male mice exposed orally to MTBE and explored oxidative stress as a mode of action. CD-1 mice were gavaged with 400-2000 mg/kg MTBE on days 1, 3, and 5, injected i.p. with hCG (2.5 IU/g) on day 6, and necropsied on day 7. No effect was seen in testis histology or testosterone levels. Using a similar dosing protocol, others had initially reported disruption of seminiferous tubules in MTBE-gavaged mice, although later conclusions published were consistent with our findings. Another group had also reported testicular and other reproductive system abnormalities in male BALB/c mice exposed for 28 days to 80-8000 microg/ml MTBE in drinking water. We gave these MTBE concentrations to adult mice for 28 days and juvenile mice for 51 days through PND 77. Evidence of oxidative stress was examined in liver homogenates from the juvenile study using MDA, TEAC and 8OH2hG as endpoints. MTBE exposures at the levels examined indicated no significant changes in the male mouse reproductive tract and no signs of hepatic oxidative stress. This appears to be the first oral MTBE exposure of juvenile animals, and also the first to examine potential for MTBE to cause oxidative stress in vivo using a typical route of human exposure.

Effects of tert-butyl acetate on maternal toxicity and embryo-fetal development in Sprague-Dawley rats

This study investigated the potential adverse effects of tert-butyl acetate (TBAc) on maternal toxicity and embryo-fetal development after maternal exposure of pregnant rats from gestational days 6 through 19. TBAc was administered to pregnant rats by gavage at 0, 400, 800, and 1,600 mg/kg/day. All dams were subjected to a Caesarean section on day 20 of gestation, and their fetuses were examined for any morphological abnormalities. At 1,600 mg/kg, maternal toxicity manifested as increases in the incidence of clinical signs and death, lower body weight gain and food intake, increases in the weights of adrenal glands and liver, and a decrease in thymus weight. Developmental toxicity included a decrease in fetal weight, an increase in the incidence of skeletal variation, and a delay in fetal ossification. At 800 mg/kg, only a minimal developmental toxicity, including an increase in the incidence of skeletal variation and a delay in fetal ossification, were observed. In contrast, no adverse maternal or developmental effects were observed at 400 mg/kg. These results show that a 14-day repeated oral dose of TBAc is embryotoxic at a maternally toxic dose (i.e., 1,600 mg/kg/day) and is minimally embryotoxic at a nonmaternally toxic dose (i.e., 800 mg/kg/day) in rats. However, no evidence for the teratogenicity of TBAc was noted in rats. It is concluded that the developmental findings observed in the present study are secondary effects to maternal toxicity. Under these experimental conditions, the no-observed-adverse-effect level of TBAc is considered to be 800 mg/kg/day for dams and 400 mg/kg/day for embryo-fetal development.

Ethyl tertiary-butyl ether: a toxicological review

A number of oxygenated compounds (oxygenates) are available for use in gasoline to reduce vehicle exhaust emissions, reduce the aromatic compound content, and avoid the use of organo-lead compounds, while maintaining high octane numbers. Ethyl tertiary-butyl ether (ETBE) is one such compound. The current use of ETBE in gasoline or petrol is modest but increasing, with consequently similar trends in the potential for human exposure. Inhalation is the most likely mode of exposure, with about 30% of inhaled ETBE being retained by the lungs and distributed around the body. Following cessation of exposure, the blood concentration of ETBE falls rapidly, largely as a result of its metabolism to tertiary-butyl alcohol (TBA) and acetaldehyde. TBA may be further metabolized, first to 2-methyl-1,2-propanediol and then to 2-hydroxyisobutyrate, the two dominant metabolites found in urine of volunteers and rats. The rapid oxidation of acetaldehyde suggests that its blood concentration is unlikely to rise above normal as a result of human exposure to sources of ETBE. Single-dose toxicity tests show that ETBE has low toxicity and is essentially nonirritant to eyes and skin; it did not cause sensitization in a maximization test in guinea pigs. Neurological effects have been observed only at very high exposure concentrations. There is evidence for an effect of ETBE on the kidney of rats. Increases in kidney weight were seen in both sexes, but protein droplet accumulation (with alpha(2u)-globulin involvement) and sustained increases in cell proliferation occurred only in males. In liver, centrilobular necrosis was induced in mice, but not rats, after exposure by inhalation, although this lesion was reported in some rats exposed to very high oral doses of ETBE. The proportion of liver cells engaged in S-phase DNA synthesis was increased in mice of both sexes exposed by inhalation. ETBE has no specific effects on reproduction, development, or genetic material. Carcinogenicity studies have been conducted with ETBE, TBA, and ethanol (included in this review as an endogenous precursor of acetaldehyde in the absence of TBA). A single experiment with ETBE in rats and several experiments with ethanol in rats and mice were not considered adequate for an evaluation of ETBE carcinogenicity. In male rats only, TBA induced alpha(2u)-globulin nephropathy-related renal tubule adenomas. These are generally considered to have no human relevance. In addition, increases in thyroid follicular cell adenoma incidence were associated with TBA treatment in female mice. This result lacks independent confirmation and is not supported by experiments in which similar or higher internal doses of TBA were delivered.

Methyl tert-butyl ether (MTBE)-induced cytotoxicity and oxidative stress in isolated rat spermatogenic cells

Methyl tert-butyl ether (MTBE) is a class of synthetic organic chemical. In the USA, MTBE pollution is regarded as a serious environmental problem. The objective of the present study was to investigate the cytotoxic effects and oxidative stress induced by MTBE in isolated rat spermatogenic cells. In cytotoxic experiments, spermatogenic cells isolated from the testes of adult Sprague-Dawley rats by a mechanical procedure without the use of trypsin were incubated with medium alone (control), 0.5, 5, 50 mm MTBE, respectively, for 6, 12 and 18 h. MTT assay, staining with fluorescein diacetate (FDA) and propidium iodide (PI) and flow cytometric analyses were used. In oxidative stress experiments, the spermatogenic cells were incubated with medium alone (control) and with 0.5, 50 microm, 5 mm MTBE. For 1, 2, 6, 12, 18 h incubation, ROS production was tested using a 2',7'-dichlorofluorescein diacetate (DCHF-DA) probe; for 1, 3, 6, 12, 18 h incubation, cytosolic superoxide dismutase (SOD) and extracellular SOD (SOD(EX)) activity was assessed; and for 18 h incubation, lipid peroxidation was assessed. The results showed that MTBE at high doses significantly decreased the spermatogenic cell viability and increased plasma membrane damage and the ratio of necrotic cells compared with the control. Assessment of the MTBE-induced oxidative stress revealed that MTBE increased the production of reactive oxygen species (ROS) and enhanced lipid peroxidation. In addition, although SOD(EX) activity increased at a high dose level, cytosolic SOD activity decreased. These results suggest that an increase of MTBE-induced ROS production and an enhancement of membrane lipid peroxidation may play an important role in its cytotoxicity in isolated rat spermatogenic cells.

Effect of methyl tert-butyl ether in standard tests for mutagenicity and environmental toxicity

Methyl tert-butyl ether (MTBE) is a synthetic compound that is used as a technological solution to problems created by air pollution from vehicle emissions. An important source of MTBE in the environment is leakage from underground storage tanks at gasoline stations or accidents during gasoline transport. The aim of this study was to evaluate the risk of MTBE leakage for the environment using the Microtox (Vibrio fischeri) toxicity test, Lactuca sativa seed germination test, and Ames bacterial mutagenicity test with Salmonella typhimurium his(-) strains TA98, TA100, YG1041, and YG1042, using both standard plate and preincubation protocols. The result of Microtox expressed as EC(50) was 33 mg (MTBE)/L. The effect of all tested MTBE concentration (0.05, 0.50, and 1.00% v/v) on Lactuca sativa roots elongation was negative and proved its toxicity. The highest tested concentration of MTBE that could be tested in Ames test was 3 mg (MTBE)/plate, because of cytotoxicity. No mutagenic response was observed at this or lower concentrations in any of the four strains used.

Methyl tertiary-butyl ether mode of action for cancer endpoints in rodents

There are no reports of studies that evaluate if methyl tertiary-butyl ether (MTBE) exposure causes cancer in humans. This evaluation of MTBE carcinogenicity is based on the results of animal studies. A weak tumorigenic response was reported for both MTBE and TBA in one tumor type (kidney) in male rats, for MTBE in one other tumor type (testicular) in male rats, for MTBE in one tumor type (liver) in female mice, and for TBA in one tumor type (thyroid) in female mice. The weight of the evidence does not support a genotoxic mode of action (MOA). Non-genotoxic MOAs have been demonstrated or suggested that correspond to the weak tumorigenic responses. These MOAs either do not occur in humans or humans are much less susceptible to these effects. It is, therefore, unlikely that humans would be exposed to sufficient levels of MTBE to cause these tumorigenic responses.

Acute toxicity and cancer risk assessment values for tert-butyl acetate

tert-Butyl acetate (TBAc) is an industrial chemical with potential uses as a degreaser and in architectural coatings. Limited chronic toxicity data exist for TBAc. However, acute inhalation exposure data are available for TBAc. Additionally, TBAc has been demonstrated to be substantially metabolized to tert-butanol (TBA) in rats, and a positive TBA genotoxicity study suggests that TBA may cause oxidative DNA damage. TBA has been shown to induce tumors in both rats and mice, and the Office of Environmental Health Hazard Assessment has calculated an oral cancer potency factor (CSF) for TBA of 3 x 10(-3)(mg/kg-day)(-1). Therefore, TBAc should be considered to pose a potential cancer risk to humans because of the metabolic conversion to TBA. An acute 1-h reference exposure level of 1 mg/m3 can be calculated from the extrapolated no observed adverse effect level of 50 mg/m3. A CSF of 0.002(mg/kg-day)(-1) can be derived for TBAc, assuming 100% metabolism of TBAc to TBA. An inhalation unit risk value for TBAc of 4 x 10(-7)(microg/m(3))(-1) can then be derived from the CSF value for TBAc by assuming a human breathing rate of 20 m3/day, 70% fractional absorption, and an average human body weight of 70 kg.

The mutagenicity testing of tertiary-butyl alcohol, tertiary-butyl acetate™ and methyl tertiary-butyl ether in Salmonella typhimurium

Tertiary-Butyl alcohol (TBA), tertiary-butyl acetate (TBAc) and methyl tertiary-butyl ether (MTBE) are chemicals to which the general public may be exposed either directly or as a result of their metabolism. There is little evidence that they are genotoxic; however, an earlier publication reported that significant results were obtained in Salmonella typhimurium TA102 mutagenicity tests with both TBA and MTBE. We now present results of testing these chemicals and TBAc against S. typhimurium strains in two laboratories. The emphasis was placed on testing with S. typhimurium TA102 and the use of both dimethyl sulphoxide and water as vehicles. Dose levels up to 5000 microg/plate were used and incubations were conducted in both the presence and absence of liver S9 prepared from male rats treated with either Arochlor 1254 or phenobarbital-beta-naphthoflavone. The experiments were replicated, but in none of them was a significant mutagenic response observed, thus the current evidence indicates the TBA, TBAc and MTBE are not mutagenic in bacteria.

The genotoxicity of ambient outdoor air, a review: Salmonella mutagenicity

Mutagens in urban air pollution come from anthropogenic sources (especially combustion sources) and are products of airborne chemical reactions. Bacterial mutation tests have been used for large, multi-site, and/or time series studies, for bioassay-directed fractionation studies, for identifying the presence of specific classes of mutagens, and for doing site- or source-comparisons for relative levels of airborne mutagens. Early research recognized that although carcinogenic PAHs were present in air samples they could not account for the majority of the mutagenic activity detected. The mutagenicity of airborne particulate organics is due to at least 500 identified compounds from varying chemical classes. Bioassay-directed fractionation studies for identifying toxicants are difficult to compare because they do not identify all of the mutagens present, and both the analytical and bioassay protocols vary from study to study. However, these studies show that the majority of mutagenicity is usually associated with moderately polar/highly polar classes of compounds that tend to contain nitroaromatic compounds, aromatic amines, and aromatic ketones. Smog chamber studies have shown that mutagenic aliphatic and aromatic nitrogen-containing compounds are produced in the atmosphere when organic compounds (even non-mutagenic compounds) are exposed to nitrogen oxides and sunlight. Reactions that occur in the atmosphere, therefore, can have a profound effect on the genotoxic burden of ambient air. This review illustrates that the mutagenesis protocol and tester strains should be selected based on the design and purpose of the study and that the correlation with animal cancer bioassay results depends upon chemical class. Future emphasis needs to be placed on volatile and semi-volatile genotoxicants, and on multi-national studies that identify, quantify, and apportion mutagenicity. Initial efforts at replacing the Salmonella assay for ambient air studies with some emerging technology should be initiated.

Potential ecotoxicological implication of methyl tert-butyl ether (MTBE) spills in the environment

Streptomyceticidal activity of Methyl tert-butyl ether (MTBE) elucidated for the first time. Adverse effect of MTBE, the gasoline additive, against 11 soil inhabitant Streptomyces spp. isolates was investigated. MTBE, an octane enhancer is added to gasoline to reduce atmospheric concentrations of carbon monoxide and ozone. It contaminates soil and groundwater by fuel leaks and spills. Streptomyces spp. are of the major contributors to the biological buffering of soils by exerting beneficial and antagonistic activity against wide range of bacteria and fungi. To evaluate anti-streptomycetidal activity of MTBE, it was tested against 11 soil isolates of Streptomyces isolates and also a plant-root bacterial pathogen, Erwinia carotovora and a plant-root fungal pathogen, Fusarium solani. MTBE did not reveal any growth inhibitory activity against E. carotovora and F. solani, but showed strong inhibitory effect against Streptomyces isolates. The Minimum Inhibitory Concentration (MIC) on Streptomyces isolates was 1/800 of the original MTBE. Fuel leaks and spills have the potential to suppress or eliminate the Streptomyces role in the soil causing alteration in the balance of soil micro flora. This change can promote the domination of microorganisms with adverse biological or ecotoxicological effects.

A yeast-based method for the detection of cyto and genotoxicity

A miniaturized short-term in vivo genotoxicity screening assay based on genetically modified yeast (Saccharomyces cerevisiae) cells was performed to explore the capacity of this eukaryotic organism to detect the presence of genotoxic compounds. An increased general sensitivity of yeast cells to toxic compounds was obtained by using a strain being deleted in the prominent pleiotropic drug resistance mediating efflux transporters PDR5, SNQ2 and YOR1. In order to detect genotoxic effects, a yeast optimized version of the green fluorescent protein (GFP) was fused to the RAD54 promoter that is activated upon DNA damage. Various model substances including the oxygenated fuel additive methyl tertiary-butyl ether (MTBE) and the direct acting genotoxins methyl-N-nitro-N-nitrosoguanidine (MNNG) and 4-nitroquinoline-1-oxide (4-NQO) were tested. All model substances were in parallel examined for chronic cytotoxicity. The results point out the sufficiency of both the sensitivity of the yeast cells to detect chronic cytotoxicity and the intensity of the fluorescence signal for the assessment of genotoxic effects. Thus, the test enables simultaneous detection of cytotoxic and genotoxic effects. By partial automation and implementation of the test in the microtitre scale this bioassay allows parallel sensitive pre-screening of numerous samples.

Toxicology and human health effects following exposure to oxygenated or reformulated gasoline

In order to replace antiknock leaded derivatives in gasoline, legislations were enacted in the United States and other countries to find safer additives and to reduce CO, O3, and volatile organic compounds (VOCs) in non-attainment areas. Oxygenates commonly used include various alcohols and aliphatic ethers. Methyl tert-butyl ether (MTBE) is the most widely used and studied ether oxygenate and is added to gasoline at concentrations up to 15% by volume. Inhalation of fumes while fueling automobiles is the main source of human exposure to MTBE. Humans are also exposed when drinking water contaminated with MTBE. Epidemiological, clinical, animal, metabolic and kinetic studies have been carried out to address human health risks resulting from exposure to MTBE. MTBE is an animal carcinogen, but its human carcinogenic potential remains unclear. Because MTBE functions as a non-traditional genotoxicant, several mechanisms were suggested to explain its mode of action, such as, functioning as a cytotoxic as opposed to a mitogenic agent; involvement of hormonal mechanisms; or operating as a promoter instead of being a complete carcinogen. Some studies suggested that carcinogenicity of MTBE might be due to its two main metabolites, formaldehyde or tributanol. A role for DNA repair in MTBE carcinogenesis was recently unveiled, which explains some, but not all effects. The totality of the evidence shows that, for the majority of the non-occupationally exposed human population, MTBE is unlikely to produce lasting adverse health effects, and may in some cases improve health by reducing the composition of emitted harmful VOCs and other substances. A small segment of the population (e.g. asthmatic children, the elderly, and those with immunodeficiency) may be at increased risk for toxicity. However, no studies have been conducted to investigate this hypothesis. Concern over ground and surface water contamination caused by persistent MTBE has lead the Environmental Protection Agency (EPA) to proposed reducing or eliminating its use as a gasoline additive. The major potential alternatives to MTBE are other forms of ethers such as ethyl tert-butyl ether (ETBE) or tert-amyl methyl ether (TAME), and alcohols such as ethanol. More definitive studies are needed to understand the mechanism(s) by which aliphatic ethers may pose health and environmental impacts. The switch from MTBE to ethanol is not without problems. Ethanol costs more to produce, poses challenges to the gasoline distribution system, extends the spread of hydrocarbons through ground water in gasoline plumes, and in the short-term is unlikely to be available in sufficient quantity. Moreover, its metabolite acetaldehyde is a possible carcinogen that undergoes a photochemical reaction in the atmosphere to produce the respiratory irritant peroxylacetate nitrate (PAN). Congress is addressing whether the Clean Air Act Amendments (CAA) provisions concerning reformulated gasoline (RFG) should be modified to allow refineries to discontinue or lessen the use of oxygenates.