Benzene inhalation produces leukemia in mice

Comparative metabolism of benzene and trans,trans-muconaldehyde to trans,trans-muconic acid in DBA/2N and C57BL/6 mice

Our laboratory recently identified trans,trans-muconaldehyde (MUC), a six-carbon diene dialdehyde, as a hematotoxic microsomal metabolite of benzene (Latriano et al., Proc Natl Acad Sci USA 83: 8356-8360, 1986). We also showed that MUC is metabolized in vitro to trans,trans-muconic acid (MA), a six-carbon diene dicarboxylic acid and known urinary metabolite of benzene. To elucidate further the role of ring-opened metabolites in benzene toxicity, the metabolism of benzene and MUC was examined in the benzene sensitive DBA/2N mouse strain and the less benzene sensitive C57BL/6 strain. A sensitive assay for urinary MA analysis was developed. The percent of benzene dose excreted as urinary MA within the first 24 hr after treatment decreased with an increase in benzene dose, i.e. from 9.8 to 0.4% in DBA/2N mice and from 17.6 to 0.2% in C57BL/6 mice treated with 0.5 to 880 mg/kg benzene. DBA/2N mice excreted significantly (P less than or equal to 0.05) more MA compared with C57BL/6 mice after treatment with hematotoxic benzene doses (220-880 mg/kg). At low benzene doses (0.5 to 2.5 mg/kg), C57BL/6 mice excreted significantly (P less than or equal to 0.05) more MA compared with DBA/2N mice. There were no significant differences in the metabolism of MUC to MA between the two strains after treatment with 0.5 to 3.0 mg/kg. Furthermore, mice from both strains excreted similar amounts of muconic acid when treated with 0.7 to 7.1 mg/kg MA. These results are consistent with the hypothesis that reactive ring-opened metabolites such as trans,trans-muconaldehyde play a role in benzene hematotoxicity. Sensitivity towards benzene may be due, in part, to increased metabolism to ring-opened compounds.

Purification and characterization of a benzene hydroxylase from rat liver mitochondria

An enzyme has been purified to electrophoretic homogeneity from rat liver mitoplasts which metabolizes benzene to phenol. The enzyme has a Mr of 52,000 and requires NADPH, adrendoxin, and adrenodoxin reductase for activity. Benzene hydroxylase activity could be inhibited by carbon monoxide and SKF-525A, and by specific inhibitors of microsomal benzene metabolism. The purified enzyme also oxidized phenol to catechol. The data suggest that a cytochrome P-450 of mitochondrial origin is involved in benzene metabolism, and provide another example of a role for the mitochondrion in xenobiotic activation.

Subclinical effects of groundwater contaminants. III. Effects of repeated oral exposure to combinations of benzene and toluene on immunologic responses in mice

Toxicity of environmental pollutants may be expressed as combined effects of a chemicals. Benzene, a proven hematotoxic agent, frequently occurs with toluene in cocontaminated groundwater. Groups of CD-1 male mice were exposed continuously for 4 weeks to benzene (166 mg/l), toluene (80 and 325 mg/l), and combinations of benzene (166 mg/l) + toluene (80 mg/l or 325 mg/l) in drinking water. Benzene-induced anemia was alleviated by simultaneous toluene treatment. Leukopenia and lymphopenia were observed in the case of benzene only and benzene + toluene (80 mg/l)-treated mice. The cytopenia, however, was less severe in the benzene + toluene (325 mg/l)-treated group. Immunotoxicity induced by benzene treatment alone was characterized by involution of thymic mass and suppressions of both B- and T-cell mitogeneses, mixed lymphocyte culture response to alloantigens, the tumor lytic ability of cytotoxic T-lymphocytes as determined by 51Cr-release assay, and antibody production response to T-dependent antigen (sheep red blood cells). IL-2 secretion by Con A-stimulated mouse T-cells was decreased in the benzene-treated group. Toluene (325 mg/l) completely inhibited these adverse effects when it was coadministered with benzene, while the low dose of toluene (80 mg/l) did not protect against benzene-induced depressions of immune functions. Toluene administered alone at levels up to 325 mg/l showed no obvious immunotoxic effects. Results of this study demonstrated that toluene, in sufficient amounts, has an antagonistic effect on benzene immunotoxicity.

Effect of repeated benzene inhalation exposures on benzene metabolism, binding to hemoglobin, and induction of micronuclei

Metabolism of benzene is thought to be necessary to produce the toxic effects, including carcinogenicity, associated with benzene exposure. To extrapolate from the results of rodent studies to potential health risks in man, one must know how benzene metabolism is affected by species, dose, dose rate, and repeated versus single exposures. The purpose of our studies was to determine the effect of repeated inhalation exposures on the metabolism of [14C]benzene by rodents. Benzene metabolism was assessed by characterizing and quantitating urinary metabolites, and by quantitating 14C bound to hemoglobin and micronuclei induction. F344/N rats and B6C3F1 mice were exposed, nose-only, to 600 ppm benzene or to air (control) for 6 hr/day, 5 days/week for 3 weeks. On the last day, both benzene-pretreated and control animals were exposed to 600 ppm, 14C-labeled benzene for 6 hr. Individual benzene metabolites in urine collected for 24 hr after the exposure were analyzed. There was a significant decrease in the respiratory rate of mice (but not rats) pretreated with benzene which resulted in lower levels of urinary [14C]benzene metabolites. The analyses indicated that the only effects of benzene pretreatment on the metabolite profile in rat or mouse urine were a slight shift from glucuronidation to sulfation in mice and a shift from sulfation to glucuronidation in rats. Benzene pretreatment also had no effect, in either species, on formation of [14C]benzene-derived hemoglobin adducts. Mice and rats had similar levels of hemoglobin adduct binding, despite the higher metabolism of benzene by mice. This indicates that hemoglobin adduct formation occurs with higher efficiency in rats. After 1 week of exposure to 600 ppm benzene, the frequency of micronucleated, polychromatic erythrocytes (PCEs) in mice was significantly increased. Exposure to the same level of benzene for an additional 2 weeks did not further increase the frequency of micronuclei in PCEs. These results indicate that repeated exposures to benzene, such as might be encountered by humans as a result of occupational or environmental exposures, are not likely to change or increase benzene metabolism.

Pharmacokinetics of benzene

A physiologically based pharmacokinetic model was developed and used to describe the pharmacokinetics of benzene in three species: mice, rats, and humans. For each species, the body was divided into five anatomical compartments, consisting of liver, fat, bone marrow, and muscle, and organs such as brain, heart, kidney, and viscera, connected by the arterial and venous blood flow pathways. Metabolism of benzene followed Michaelis-Menten (nonlinear) kinetics in all species and occurred primarily in the liver compartment and, to a lesser extent, in the bone marrow. Comparison of model results with empirical data on inhalation, gavage, and intraperitoneal and subcutaneous injection in mice, rats, and humans, demonstrates the utility of a physiological pharmacokinetic model in describing the pharmacokinetics of benzene in three species across multiple routes of exposure.

Risk assessment of leukaemia and occupational exposure to benzene

Experimental toxicological studies have offered clear evidence that benzene induces haematopoietic neoplasms, and it is generally accepted that exposure to benzene is a risk factor for leukaemia, in particular for acute non-lymphatic leukaemia. Quantitative aspects of benzene risk assessment are still a matter of controversy, however. In several risk assessments an estimated 50 deaths from leukaemia per 1000 deaths would arise from exposures to benzene of 10 ppm during a working life of 30 years. The assessment presented in this paper leads to lower estimates, which are more in agreement with the weak toxicological data. Furthermore, an approach is presented to incorporate the results of low exposure epidemiological studies into the process of quantitative risk assessment.

Effect of exposure concentration, exposure rate, and route of administration on metabolism of benzene by F344 rats and B6C3F1 mice

To determine the effect of exposure concentration and the route of administration on benzene metabolism, male F344/N rats and B6C3F1 mice were orally exposed to 1, 10, and 200 mg benzene/kg, and by inhalation for 6 hr to 5, 50, and 600 ppm benzene vapor. The effect of different exposure rates on the metabolism of benzene was determined by exposing rodents over different time intervals to the same total amount of benzene [constant concentration X time factor (C X T) = 300 ppm.hr]. Water-soluble metabolites constituted greater than 90% of the metabolite dose to the tissues and were used as a measure of the metabolism of benzene via different pathways. Water-soluble metabolites were measured in the blood, urine, liver, lung, and bone marrow from animals killed following oral exposures and during and following inhalation exposures. The total "dose" to the tissue of individual metabolites was determined by the area under the curve (AUC). The results indicated a shift in metabolism from putative toxification pathways to detoxification pathways as the exposure concentration or oral dose increased. In mice, hydroquinone glucuronide and muconic acid (markers of toxification metabolic pathways) represented a greater percentage of the administered dose at low doses than at high doses. At high doses, phenylglucuronide and prephenylmercapturic acid (detoxification products) increased as a percentage of the administered dose. This same metabolic shift was observed in rats, except that hydroquinone glucuronide was a minor metabolite of benzene at all concentrations. The AUC of phenylsulfate (detoxification pathway) was proportional to the exposure concentration in both species. Within the range of C X T factors studied, the rate of the inhalation exposure to benzene did not affect the AUC of metabolites in tissues of rats; however, a high dose rate (600 ppm 0.5 hr) in mice caused a shift in metabolism to phenyl conjugates. The comparison of oral and 6-hr inhalation exposures indicated that, in terms of metabolite dose to tissues, there is no simple relationship between these two routes of administration. An oral dose and an inhalation exposure concentration which produce an equal dose of one metabolite produce very different doses of another metabolite. These studies demonstrated a species difference in benzene metabolism, as well as a metabolic shift in benzene metabolic pathways as the exposure concentration was increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetically based risk assessment of workplace exposure to benzene

Cancer risk from exposure to benzene for a working lifetime was estimated from data obtained in studies with rodents. Cancers of the Zymbal gland and the blood-forming system were selected as endpoints for the assessment because of their consistent occurrence. The combined metabolites were judged from toxicological data to be the best representative of the reactive agent. Because of similarity in the percentages of lifetime exposed in the rodent studies and in the occupational setting, the amount metabolized/day as a result of exposures 5 days a week for a lifetime was judged to be an appropriate dose paradigm for this assessment. Derived Michaelis-Menton constants were used to convert the doses of combined metabolites from the pharmacokinetic studies to the doses used in the bioassays. Scaling across species was based on allometric relationships. Experimental data were used to scale doses across species with body weight ratios raised to the exponents of 0.74 for the inhalation route and 1.0 for the oral route. The occupational lifetime cancer risk estimated from rodent data was 6 to 14 cases/1000 workers, which is consistent with the 9.5 to 174 leukemia cases/1000 estimated by others from epidemiological data. Implications of these estimates and uncertainties associated with making them are discussed.

Chromosome abnormalities and sister chromatid exchanges in children with acute intoxication due to inhalation of volatile substances

Deliberate inhalation of volatile substances is a common and harmful practice among young persons worldwide. Recently, we described chromosome damage in children who chronically inhale volatile agents. Clinical and cytogenetic studies were performed for 15 "sniffing" children (13 boys and 2 girls), the purpose of which was to define the chromosomal effect of the acute intoxication. A significant increase in the rate of chromosome abnormalities and in the frequency of sister chromatid exchanges (SCEs) was found in sniffers vs. controls. The values were also higher in children who were acutely intoxicated than in those who chronically inhaled volatile agents. Clinical, socioeconomic, and cytogenetic findings are also discussed.

Carcinogenic risk assessment: some comparisons between risk estimates derived from human and animal data

Carcinogenic risk assessment may be based on epidemiological as well as experimental data; the aim of the present study is to compare these two procedures and to verify the efficiency of some animal bioassays for the prediction of tumor incidence and relative risk in man. The predictability obtained from animal models has been found to be fairly consistent with the data obtained from epidemiological studies.

The carcinogenicity of discontinuous inhaled benzene exposures in CD-1 and C57Bl/6 mice

Groups of male C57Bl and CD-1 mice were exposed to benzene via inhalation using two different exposure protocols. One protocol consisted of repetitive week-long exposures to 300 ppm benzene (6 h/d x 5 d/wk) interrupted by 2 weeks of non-exposure. The exposure pattern (1 week of exposure followed by 2 weeks of non-exposure) was continued until the death of the last exposed animal. The second protocol consisted of exposures to 1200 ppm benzene (6 h/d x 5 d/wk) for 10 weeks. Exposures were then terminated and the animals allowed to live out their lives. For each protocol, appropriate age-matched control mice received comparable exposures to filtered, conditioned air. The discontinuous exposure patterns mimic the patterns of exposure often encountered in the workplace and, in addition, prolong the survival of exposed animals so as to maximize potential tumorigenic responses. Both exposure protocols were markedly hematotoxic to both mouse strains as measured by peripheral blood counts. Both strains of mice responded to the intermittent 300 ppm benzene exposures with elevated incidences of malignant tumors. Particularly noteworthy was a 35% incidence of zymbal gland tumors in the C57Bl mice. In contrast, only the CD-1 mice responded to the 1200 ppm benzene exposures delivered over 10 weeks with elevated tumor incidences. A 46% incidence of lung adenoma was particularly striking in these mice. Neither of the benzene exposure protocols induced elevated incidences of leukemia/lymphoma in either strain.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of exposure regimen and duration on benzene-induced bone-marrow damage in mice. I. Sex comparison in DBA/2 mice

In the mouse, the concurrent evaluation of micronuclei frequencies in peripheral blood polychromatic erythrocytes (PCE) and normochromatic erythrocytes (NCE) permits an assessment of both recently-induced and chronically-accumulated bone-marrow damage. This assay system was used to evaluate on a weekly basis the effect of exposure duration (1-13 weeks, 6 h per day) and exposure regimen (Regimen 1:5 exposure days per week; Regimen 2:3 exposure days per week) on the ability of 300 ppm benzene to induce genotoxic damage in the bone marrow of male and female DBA/2 mice. In addition, an analysis of the percentage of PCE in peripheral blood was used to evaluate benzene-induced alterations in the rate of erythropoiesis. Exposure to benzene induced a marked increase in the frequency of micronucleated PCE (MN-PCE), an effect which was considerably greater in male mice than in female mice. In both sexes, the induction of MN-PCE was independent of exposure regiment and of exposure duration. Exposure to benzene also resulted in an exposure duration-dependent increase in the frequency of MN-NCE. The frequency of MN-NCE increased more slowly in female than in male mice and, within each sex, more slowly in Regimen 2 animals. Apparent steady-state conditions for MN-NCE frequencies were attained by about the fifth week of exposure in female mice exposed by either regimen and in male mice exposed by Regimen 2. Steady-state conditions for MN-NCE frequencies in male mice exposed to benzene by Regimen 1 did not occur during the duration of the study. An analysis of %PCE data revealed an initial severe depression in the rate of erythropoiesis in both sexes, with the return in the production of PCE to control levels being dependent on both sex and exposure regimen. Suppression of PCE production occurred throughout the course of the study in Regimen 2 males, while the percentage of PCE returned to control levels sporadically after 5 weeks in Regimen 1 males and within 5 weeks in females, regardless of regimen. Thus, while the sex-dependent induction of genotoxic damage by multiple exposures to benzene over a 13-week period was independent of exposure regimen and duration, the induction of cytotoxic damage was both sex- and regimen-dependent. The most severe depression of erythropoiesis occurred in male DBA/2 mice exposed to benzene by the more intermittent regimen (i.e., 3 days/week versus 5 days/week).(ABSTRACT TRUNCATED AT 400 WORDS)

Differences in the metabolism and disposition of inhaled [3H]benzene by F344/N rats and B6C3F1 mice

Benzene is a potent hematotoxin and has been shown to cause leukemia in man. Chronic toxicity studies indicate that B6C3F1 mice are more susceptible than F334/N rats to benzene toxicity. The purpose of the studies presented in this paper was to determine if there were metabolic differences between F344/N rats and B6C3F1 mice which might be responsible for this increased susceptibility. Metabolites of benzene in blood, liver, lung, and bone marrow were measured during and following a 6-hr 50 ppm exposure to benzene vapor. Hydroquinone glucuronide, hydroquinone, and muconic acid, which reflect pathways leading to potential toxic metabolites of benzene, were present in much greater concentrations in the mouse than in rat tissues. Phenylsulfate, a detoxified metabolite, and an unknown water-soluble metabolite were present in approximately equal concentrations in these two species. These results indicate that the proportion of benzene metabolized via pathways leading to the formation of potentially toxic metabolites as opposed to detoxification pathways was much higher in B6C3F1 mice than in F344 rats, which may explain the higher susceptibility of mice to benzene-induced hematotoxicity and carcinogenicity.

A high pressure liquid chromatographic method for the separation and quantitation of water-soluble radiolabeled benzene metabolites

The glucuronide and sulfate conjugates of benzene metabolites as well as muconic acid and pre-phenyl- and phenylmercapturic acids were separated by ion-pairing HPLC. The HPLC method developed was suitable for automated analysis of a large number of tissue or excreta samples. p-Nitrophenyl [14C]glucuronide was used as an internal standard for quantitation of these water-soluble metabolites. Quantitation was verified by spiking liver tissue with various amounts of phenylsulfate or glucuronides of phenol, catechol, or hydroquinone and analyzing by HPLC. Values determined by HPLC analysis were within 10% of the actual amount with which the liver was spiked. The amount of metabolite present in urine following exposure to [3H]benzene was determined using p-nitrophenyl [14C]glucuronide as an internal standard. Phenylsulfate was the major water-soluble metabolite in the urine of F344 rats exposed to 50 ppm [3H]benzene for 6 h. Muconic acid and an unknown metabolite which decomposed in acidic media to phenylmercapturic acid were also present. Liver, however, contained a different metabolic profile. Phenylsulfate, muconic acid, and pre-phenylmercapturic acids as well as an unknown with a HPLC retention time of 7 min were the major metabolites in the liver. This indicates that urinary metabolite profiles may not be a true reflection of what is seen in individual tissues.

Neutrophil-derived oxidants as mediators of chemical activation in bone marrow

Neutrophil-derived oxidants have been implicated in both damage to biomolecules and the metabolic activation of xenobiotics. Since the bone marrow is a relatively neutrophil-rich tissue which is subject to xenobiotic toxicity, we have characterized the oxidant generating capability of neutrophilic cells isolated from femurs of male C57BL/6J mice. Addition of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to neutrophil preparations (70 +/- 5% ring neutrophils and metamyelocytes) elicited superoxide anion generation, as indicated by superoxide dismutase (SOD)-inhibitable acetylated cytochrome c reduction, and oxidant-dependent chemiluminescence (CL) from luminol or lucigenin. The interaction of benzo[a]pyrene-7,8-dihydrodiol (BP-diol), a proximate carcinogenic metabolite of benzo[a]pyrene (BP), with TPA-stimulated bone marrow neutrophils resulted in azide-inhibitable CL (90%) indicative of its myeloperoxidase-dependent oxidation to an excited-state intermediate. Covalent binding of [3H]BP-diol to exogenous DNA was similarly increased 3-fold in the presence of TPA-stimulated bone marrow neutrophils. Recently, our laboratory has shown that in addition to CL, TPA-stimulated human polymorphonuclear leukocytes can activate BP-diol to an intermediate which covalently binds to DNA and elicits mutagenicity in Salmonella typhimurium TA100. These observations combined with our current results suggest a possible role for neutrophil-derived oxidants in the mechanisms of chemically-induced bone marrow toxicity.

Pyridine prevents the clastogenicity of benzene but not of benzo[a]pyrene or cyclophosphamide

Pyridine has been shown to be a much more potent inhibitor than other solvents of the metabolism and therefore the clastogenicity of benzene. In this report, pyridine prevented benzene-derived micronucleus formation in the bone marrow of ICR Swiss mice at much lower levels than xylene did. Time-course experiments did not indicate any delay in the peak micronucleus response to benzene caused by either pyridine or xylene. Similar experiments using pyridine with benzo[a]pyrene and pyridine with cyclophosphamide indicated that the effect of pyridine was specific for benzene. Benzo[a]pyrene (150 mg/kg) was inhibited by pyridine only at levels of 100 mg/kg or more, compared to inhibition of benzene (440 or 880 mg/kg) by pyridine at levels of 5 mg/kg. Cyclophosphamide was not inhibited at any level, and micronucleus formation was increased at lower ratios of pyridine to cyclophosphamide. These results provide indirect conformation of the work by others indicating that benzene is activated in part by a cytochrome P450 isozyme different from those activating benzo[a] pyrene or cyclophosphamide. Since DBA/2 mice (AHH-non-inducible) are more sensitive to benzene than C57Bl/6 mice (AHH-inducible), single and multiple treatments with benzene were compared by micronucleus response in these two strains. DBA mice were more responsive in all cases. Pretreatment with methylcholanthrene caused a greater response to benzene in DBA/2 mice, suggesting that the cytochrome P450 isozyme involved is one of the forms induced by methylcholanthrene independent of the high-affinity Ah receptor. It is hypothesized that more efficient activation of benzene by the unique cytochrome P450 isozyme, perhaps combined with relatively less conjugation, may result in a greater sensitivity of the bone marrow versus the liver, and of DBA/2 versus C57Bl/6 mice.

Induction of chromosome aberrations in lymphocytes of mice after subchronic exposure to benzene

The induction of chromosome aberrations in lymphocytes of mice after subchronic exposure to benzene was investigated. 4 groups of 5 Swiss (ICR) male mice were given orally a solution of benzene every day for 14 days except days 5 and 10. The daily doses were 0, 36.6, 73.2 and 146.4 mg/kg. Mice were sacrificed on day 15, lymphocytes were obtained by perfusion of the spleen and the cells were cultured in RPMI 1640 medium. After 48 h of culture, cells were harvested for cytogenetic analysis. A significant dose-dependent increase in the frequency of cells with chromatid aberrations were found (p less than 0.001). A significant increase in polyploid cells were also observed (p less than or equal to 0.05). This study represents the first report on the induction of chromosome aberrations and polyploid cells in lymphocytes of mice after subchronic exposure to benzene. Such dual activity of benzene suggests that benzene may be responsible for more human health problems than currently estimated.

Cytogenetic study in children chronically habituated to inhalation of volatile substances

Deliberate inhalation of volatile substances is a common and harmful phenomenon among young persons around the world. Clinical and cytogenetic studies were performed in 35 children (33 boys and 2 girls) chronically habituated to inhalation of volatile agents, with the aim to define whether this practice is associated with chromosome abnormalities and if so to what extent. A significant increase in the rate of chromosome abnormalities and in the frequency of sister chromatid exchanges (SCE) was found in sniffers over the values in controls (p less than 0.001). Clinical, socioeconomic, and cytogenetic findings are discussed.

Chemical carcinogens. A review and analysis of the literature of selected chemicals and the establishment of the Gene-Tox Carcinogen Data Base. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on 506 selected chemicals has been evaluated for evidence that these chemicals induce tumors in experimental animals and this assessment comprises the Gene-Tox Carcinogen Data Base. Three major sources of information were used to create this evaluated data base: all 185 chemicals determined by the International Agency for Research on Cancer to have Sufficient evidence of carcinogenic activity in experimental animals, 28 selected chemicals bioassayed for carcinogenic activity by the National Toxicology Program/National Cancer Institute and found to induce tumors in mice and rats, and 293 selected chemicals which had been evaluated in genetic toxicology and related bioassays as determined from previous Gene-Tox reports. The literature data on the 239 chemicals were analyzed by the Gene-Tox Carcinogenesis Panel in an organized, rational and consistent manner. Criteria were established to assess individual studies employing single chemicals and 4 categories of response were developed: Positive, Negative, Inconclusive (Equivocal) and Inconclusive. After evaluating each of the individual studies on the 293 chemicals, the Panel placed each of the 506 chemicals in an overall classification category based on the strength of the evidence indicating the presence or absence of carcinogenic effects. An 8-category decision scheme was established using a modified version of the International Agency for Research on Cancer approach. This scheme included two categories of Positive (Sufficient and Limited), two categories of Negative (Sufficient and Limited), a category of Equivocal (the evidence of carcinogenicity from well-conducted and well-reported lifetime studies had uncertain significance and was neither clearly positive nor negative), and three categories of Inadequate (the evidence of carcinogenicity was insufficient to make a decision, however, the data suggested a positive or negative indication). Of the 506 chemicals in the Gene-Tox Carcinogen Data Base, 252 were evaluated as Sufficient Positive, 99 as Limited Positive, 40 as Sufficient Negative, 21 as Limited Negative, 1 as Equivocal, 13 as Inadequate with the data suggesting a positive indication, 32 as Inadequate with the data suggesting a negative indication, and 48 Inadequate with the data not suggesting any indication of activity. This data base was analyzed and examined according to chemical class, using a 29 chemical class scheme.(ABSTRACT TRUNCATED AT 400 WORDS)

Recent advances in the metabolism and toxicity of benzene

Benzene is a heavily used industrial chemical, a petroleum byproduct, an additive in unleaded gas, and a ubiquitous environmental pollutant. Benzene is also a genotoxin, hematotoxin, and carcinogen. Chronic exposure causes aplastic anemia in humans and animals and is associated with increased incidence of leukemia in humans and lymphomas and certain solid tumors in rodents. Bioactivation of benzene is required for toxicity. In the liver, the major site of benzene metabolism, benzene is converted by a cytochrome P-450-mediated pathway to phenol, the major metabolite, and the secondary metabolites, hydroquinone and catechol. The target organ of benzene toxicity, the hematopoietically active bone marrow, metabolizes benzene to a very limited extent. Phenol is metabolized in the marrow cells by a peroxidase-mediated pathway to hydroquinone and catechol, and ultimately to quinones, the putative toxic metabolites. Benzene and its metabolites appear to be nonmutagenic, but they cause myeloclastogenic effects such as micronuclei, chromosome aberrations, and sister chromatid exchange. It is unknown whether these genomic changes, or the ability of the quinone metabolites to form adducts with DNA, are involved in benzene carcinogenicity. Benzene, through its active metabolites, appears to exert its hematological effects on the bone marrow stromal microenvironment by preventing stromal cells from supporting hemopoiesis of the various progenitor cells. Recent advances in our understanding of the mechanisms by which benzene exerts its genotoxic, hematotoxic, and carcinogenic effects are detailed in this review.

Peroxidase-catalyzed-3-(glutathion-S-yl)-p,p'-biphenol formation

Oxidation of p,p'-biphenol with horseradish peroxidase (HRP)-hydrogen peroxide in the presence of bovine serum albumin or with bone marrow cell homogenate-hydrogen peroxide resulted in the formation of reactive products that conjugate with protein. Glutathione prevented the protein binding. Glutathione readily reacted with p,p'-biphenoquinone, the principal oxidation product of p,p'-biphenol in the HRP-hydrogen peroxide system and resulted in the formation of several glutathione conjugates, p,p'-biphenol and small amounts of oxidized glutathione. The major glutathione conjugate was identified as 3-(glutathion-S-yl)-p,p'-biphenol by high field nuclear magnetic resonance and fast atom bombardment mass spectrometry. The same conjugate was formed in the bone marrow homogenate-hydrogen peroxide system. p,p'-Biphenoquinone reduction by glutathione to p,p'-biphenol without glutathione oxidation was explained by the rapid reduction of p,p'-biphenoquinone by 3-(glutathion-S-yl)-p,p'-biphenol.

Metabolism of benzene and phenol in macrophages in vitro and the inhibition of RNA synthesis by benzene metabolites

Benzene may affect hemopoiesis by damaging the bone marrow stroma that provides the microenvironment for hemopoiesis. A possible target of benzene toxicity in the stroma is the macrophage, which is a major source of protein factors required for the proliferation and differentiation of progenitor cells. As an initial approach towards understanding whether benzene inhibits hemopoietic factor production in bone marrow stroma, the metabolism of benzene and phenol has been studied and the effect of benzene and its metabolites on macrophage RNA synthesis has been examined. Benzene is not metabolized in macrophages but phenol, the major metabolite of benzene in bone marrow, is converted by peroxidase in the macrophage to both free metabolites and species which covalently bind to cellular macromolecules. Benzene and its metabolites inhibited RNA synthesis in a dose-dependent manner, with 50% inhibitory concentrations of 5 X 10(-3) M for benzene, 2.5 X 10(-3) M for phenol, 2.5 X 10(-5) M for hydroquinone, and 6 X 10(-6) M for p-benzoquinone; this inhibition was not attributable to loss of cell viability. Benzene, possibly by an inhibition of uridine transport into macrophages, and phenol, by its conversion to covalently binding species, inhibit RNA synthesis in macrophages and thus may inhibit the synthesis of colony stimulating factors required for hemopoiesis.

Induction of cytogenetic damage in rodents after short-term inhalation of benzene

Experiments were designed to investigate both the induction of sister chromatid exchanges (SCEs) in peripheral blood lymphocytes (PBLs) and micronuclei (MN) in bone marrow polychromatic erythrocytes (PCEs) of mice and rats after inhalation of benzene (BZ). Male DBA/2 mice (17-19 weeks old) were exposed to target concentrations of either 0, 10, 100, or 1,000 ppm BZ for 6 hr. Male Sprague-Dawley rats (11-14 weeks old) were exposed to target concentrations of either 0, 0.1, 0.3, 1, 3, 10, or 30 ppm BZ for 6 hr. Blood was obtained by cardiac puncture 18 hr after exposure, and PBLs were cultured in the presence of lipopolysaccharide (mouse B cells, 60 micrograms/ml) or concanavalin A (rat T cells, 30 micrograms/ml) to stimulate blastogenesis for SCE analysis. Femoral bone marrow smears from both species were analyzed for MN in PCEs 18 hr after BZ exposure. Mouse PBLs revealed a significant concentration-related increase in the SCE frequency over controls at 10, 100, or 1,000 ppm BZ. Mouse bone marrow showed a significant concentration-dependent increase in MN over controls after exposure to 10, 100, or 1,000 ppm BZ. Rat PBLs showed a significant increase in the SCE frequency after exposure to 3, 10, or 30 ppm BZ. The statistical significance of the 1 ppm BZ result was borderline and dependent on the statistical test chosen. Rat cells revealed a significant concentration-related increase in MN after inhalation of either 1, 3, 10, or 30 ppm BZ. PBLs from treated mice showed significant concentration-dependent decreases in mitotic indices; however, cell cycle kinetics and leucocyte counts remained unaffected. Rat PBLs showed significant decreases in mitotic activity only after exposure to 3 and 30 ppm BZ, whereas cell cycle kinetics and leucocyte counts were unaffected. These results show that BZ can induce statistically significant cytogenetic effects in PBLs and PCEs of both mice and rats after a 6-hr inhalation of BZ at low concentrations.

Effects of ethylbenzene, toluene, and xylene on the induction of micronuclei in bone marrow polychromatic erythrocytes of mice

Genotoxic effects of five widely used aromatic industrial solvents, ethylbenzene, methylbenzene (toluene), o-, m-, and p-dimethylbenzene (xylene), on bone marrow cells of male NMRI mice were studied using micronucleus test. Each compound was given to animals by IP administration of two similar doses 24 h apart. Increased formation of micronuclei within polychromatic erythrocytes of femoral bone marrow 30 h after the first injection was conducted to be due to the clastogenic effect of the test compound. Of the chemicals tested, only toluene gave a dose-dependent increase in the frequency of micronucleated polychromatic erythrocytes. This genotoxic activity of toluene was confirmed in male B6C3F1 mice.

Inhibition of RNA synthesis and interleukin-2 production in lymphocytes in vitro by benzene and its metabolites, hydroquinone and p-benzoquinone

The effects of benzene and its metabolites, hydroquinone and p-benzoquinone (PBQ) on RNA synthesis in mouse spleen lymphocytes in vitro were studied. Benzene and the quinones were shown to inhibit RNA synthesis in a dose-dependent manner at concentrations which had no significant effect on lymphocyte viability. Furthermore, 5 microM PBQ, the putative toxic metabolite of benzene, was shown to inhibit the formation of the T-cell growth factor IL-2. These results suggest that inhibition of RNA synthesis in lymphocytes by benzene may prevent the production of factors required for hemopoiesis and thus contribute to the aplastic anemia caused by benzene.

Quinones as toxic metabolites of benzene

Occupational exposure to benzene has long been associated with toxicity to the blood and bone marrow, including lymphocytopenia, pancytopenia, aplastic anemia, acute myelogenous leukemia, and possibly lymphoma. A variety of studies have established that benzene itself is not the toxic species but requires metabolism to reactive intermediates. The bioactivation of benzene is complex. Both primary and secondary oxidation of benzene and its metabolites are mediated via cytochrome P-450 in the liver, although the role of secondary metabolism in the bone marrow is not clear. Toxicity is associated with the dihydroxy metabolites, hydroquinone and catechol, which concentrate in bone marrow. Hydroquinone and its terminal oxidation product, p-benzoquinone, have been demonstrated to be potent suppressors of cell growth in culture. Suppression of lymphocyte blastogenesis by these compounds is a sulfhydryl-dependent process and occurs at concentrations that do not result in cell death, or in detectable alterations in energy metabolism, intracellular glutathione concentration, or protein synthesis. Recent studies suggest that these compounds and other membrane-penetrating sulfhydryl alkylating agents, such as N-ethylmaleimide and cytochalasin A, and endogenous regulatory molecules, such as soluble immune response suppressor (SIRS), interfere with microtubule assembly in vitro and selectively interfere with microtubule-dependent cell functions at identical concentrations. These agents appear to react with nucleophilic sulfhydryl groups essential for guanosine triphosphate binding to tubulin that are particularly sensitive to sulfhydryl-alkylating agents.

Benzene hematotoxicity and leukemogenesis

Eight-to-twelve-week-old male and female C57B1/6 BNL mice were exposed to air or benzene vapor in air at a concentration of 10, 25, 100, 300, or 400 ppm. Benzene at concentrations of 100 ppm or higher for 10 exposures of 6 hours per day 5 days a week produced a reduction in bone marrow cellularity and the number of pluripotent stem cells in the bone marrow. The fraction of stem cells in DNA synthesis was also increased. Exposure to 300 ppm 6 hours a day 5 days a week for 2, 4, 8, and 16 weeks produced a diminution in the stem cell levels in bone marrow which returned to those of controls 2 weeks after benzene exposure for 2 and 4 weeks, 16 weeks after exposure for 8 weeks, and to 92% of controls 25 weeks after 16 weeks of exposure. There was a more rapid return of blood lymphocytes to the control level. Mice exposed to 300 ppm for 6 hours/day, 5 days per week for 16 weeks began dying at 330 days of age, whereas no deaths were observed in sham-exposed mice until 440 days of age. The benzene-exposed mice died in two waves: the first was from 330-390 days of age, with a second wave commencing at 570 days of age. The first wave of mortality was due primarily to thymic lymphomata. The second wave was due to a mixture of nonthymic lymphomata and solid tumors.