Partitioning of benzene in blood: influence of hemoglobin type in humans and animals

Earlier studies have shown that air/blood partition coefficients (PCs) for many volatile organic chemicals (VOCs) are much higher in rat blood than in human blood. It has been suggested that the discrepancy could be attributed to the fact that hemoglobin (Hb) in rat blood exists in a quasi-crystalline form of hydrophobicity greater than that of normal human Hb (HbA) and thus has a higher carrying capacity for VOCs. In the present study, we used benzene as a prototypic VOC to examine its relative partitioning into human and animal blood. Additionally, we sought to ascertain whether the water-insoluble form of hemoglobin (HbS) found in subjects with homozygous sickle cell (SC) disease has a greater VOC-carrying capacity than does HbA blood. At a low-O(2) tension, HbS switches to water-insoluble polymers, which physically deforms the red blood cells (RBCs) to the sickle shape. We equilibrated HbA, HbS, Hartley guinea pig, CD1 mouse, and rat (F-344, Wistar, and Sprague-Dawley) blood and their respective fractions with benzene vapor (80 or 400 ppm) for 3 hr at 37 degrees C in air-tight vials. We introduced benzene vapor into the vial head space that contained air or respiratory mixtures of venous-type (low-O(2)) or arterial-type (high-O(2)) gases. The blood measurements included the PC, Hb, partial pressures of O(2)(pO(2)) and CO(2)(pCO(2), pH, and percentage of SCs. The benzene concentration had no effect on these parameters, and the high- and low-O(2) gas mixtures produced the expected changes in pO(2), pCO(2), and pH. At equilibrium, the low-O(2) HbS blood had approximately 85% SCs compared with roughly 15% with air or high-O(2) gas. PCs for rat and mouse blood were about 100% higher than those for human and guinea pig blood, but the PC for deoxygenated HbS blood was only slightly higher than that for HbA or oxygenated HbS blood. Benzene showed higher affinities for RBCs in the deoxygenated HbS, rat, and mouse blood and higher affinity for plasma in the guinea pig blood. There was no evidence of disproportionate partitioning of benzene into oxygenated HbS or into HbA blood forms. These data suggest that the water solubility of Hb alone appears to have little effect on the VOC-carrying capacity of blood and that the influence of species is large in comparison. These latter differences in partitioning may depend on the number of hydrophobic sites on the surface of the plasma/heme proteins and thus be unique to the species.

Comparative metabolism of [14C]benzene to excretable products and bioactivation to DNA-binding derivatives in maternal and neonatal mice

Lactating adult female mice treated with a single dose of 880 mg/kg i.p. [14C]benzene, and their 2-day-old sucklings similarly treated or nursed by their treated dams were compared in terms of their ability to metabolize benzene to urinary products or reactive intermediates as assessed by covalently-bound benzene derivatives in whole blood or liver DNA. Six metabolite fractions were identified in the urine of sucklings by high performance liquid chromatographic (HPLC) analysis at 5 h following intraperitoneal (direct) treatment with benzene. Three of the metabolite fractions co-chromatographed with authentic phenol, phenyl glucuronide, and muconic acid, and contributed 11, 6.9 and 0.6%, respectively, to the total urinary benzene metabolites. Two of the fractions were unidentified. The sixth and most polar fraction consisted of multiple metabolites, 21% of which were conjugates, and accounted for 72% of the total urinary metabolites. A similar metabolite profile was observed in 24-h urine samples from treated dams with the exception that one of the unidentified fractions in the sucklings was absent and levels of the metabolites were quantitatively higher than those observed in sucklings 5 h following their treatment with benzene. Furthermore, 78% of the most polar fraction from the dams consisted of conjugates compared with 21% of that from the sucklings. The metabolite pattern in urine of sucklings nursed by treated dams was qualitatively similar to, but quantitatively different from the pattern in treated dams. Five hours following intraperitoneal treatment with benzene, covalent binding of the compound to DNA (expressed as pmol benzene equivalents/mg DNA) in sucklings was slightly higher in whole blood (1.15+/-0.07) than in liver (0.77+/-0.07), whereas in the dam, it was slightly lower in whole blood (0.88+/-0.48) than in liver (1.63+/-0.61). Twenty four hours following benzene exposure in sucklings of benzene-treated dams, DNA binding by the compound in whole blood (3.85+/-1.05) and liver (0.11+/-0.03) was higher and lower, respectively than the binding observed in benzene-injected sucklings 5 h following the injection. Our results show that excretable as well as reactive metabolites of benzene are formed substantially by the neonatal mouse, and that the extent of bioactivation of the compound is comparable in the adult and the suckling mouse. The results show also that sucklings of benzene-exposed mothers are exposed to substantial levels of the compound and are potentially susceptible to its toxic effects.

Urine trans,trans-muconic acid as a biomarker for benzene exposure in gas station attendants in Bangkok, Thailand

The toxicity of benzene, a chemical used in many industrial processes, involves bone marrow depression and leukemogenesis and is associated with damage to multiple classes of hematopoietic cells and hematopoietic functions. Environmental exposure to benzene causes an increased body burden, which is reflected in several biomarkers, eg, urine trans,trans-muconic acid (ttMA). Associated with the industrialization of Thailand, a developing country in Southeast Asia, workers in many occupations have acquired substantial risks of benzene exposure. In this study, benzene exposure was monitored by high-performance liquid chromatography (HPLC) of urine ttMA in 79 persons, including 49 controls and 30 gas station attendants. In controls, urine ttMA concentration averaged 0.12 (SD +/- 0.03) mg/g creatinine; in gas station attendants, urine ttMA concentration averaged 4.00 (SD +/- 12.49) mg/g creatinine (p < 0.05). Based on these findings, wider use of urine ttMA determination is recommended as a biomarker for occupational exposure to benzene.

A PBPK modeling-based approach to account for interactions in the health risk assessment of chemical mixtures

The objectives of the present study were: (1) to develop a risk assessment methodology for chemical mixtures that accounts for pharmacokinetic interactions among components, and (2) to apply this methodology to assess the health risk associated with occupational inhalation exposure to airborne mixtures of dichloromethane, benzene, toluene, ethylbenzene, and m-xylene. The basis of the proposed risk assessment methodology relates to the characterization of the change in tissue dose metrics (e.g., area under the concentration-time curve for parent chemical in tissues [AUCtissue], maximal concentration of parent chemical or metabolite [Cmax], quantity metabolized over a period of time) in humans, during mixed exposures using PBPK models. For systemic toxicants, an interaction-based hazard index was calculated using data on tissue dose of mixture constituents. Initially, the AUCtarget tissue (AUCtt) corresponding to guideline values (e.g., threshold limit value [TLV]) of individual chemicals were obtained. Then, the AUCtt for each chemical during mixed exposure was obtained using a mixture PBPK model that accounted for the binary and higher order interactions occurring within the mixture. An interaction-based hazard index was then calculated for each toxic effect by summing the ratio of AUCtt obtained during mixed exposure (predefined mixture) and single exposure (TLV). For the carcinogenic constituents of the mixture, an interaction-based response additivity approach was applied. This method consisted of adding the cancer risk for each constituent, calculated as the product of q*tissue dose and AUCtt. The AUCtt during mixture exposures was obtained using an interaction-based PBPK model. The approaches developed in the present study permit, for the first time, the consideration of the impact of multichemical pharmacokinetic interactions at a quantitative level in mixture risk assessments.

Pharmacokinetic studies in Tg.AC and FVB mice administered [14C] benzene either by oral gavage or intradermal injection

Chronic benzene toxicity has been demonstrated to result in either aplastic anemia or acute myelogenous leukemia, a form of granulocytic leukemia, in exposed people (Snyder and Kalf, Crit. Rev. Toxicol. 24, 177-209, 1994). Aplastic anemia has been demonstrated in animal models following benzene exposure but, heretofore, it has not been possible to replicate benzene-induced granulocytic leukemia in animals. The Tg.AC mouse appears to be the first animal model in which a granulocytic leukemia was produced by treatment with benzene (Tennant et al., The Use of Short- and Medium-Term Tests for Carcinogenic Hazard Evaluation, 1999; French and Saulnier, J. Toxicol. Environ. Health 61, 377-379, 2000). Leukemia was observed in Tg.AC mice to which benzene was administered dermally. Neither orally dosed Tg.AC mice or mice of the parental FVB strain treated by either route of exposure developed leukemia. It is well established that benzene metabolism is required to produce benzene toxicity. To determine whether metabolic differences arising from differences in route of exposure or strain of mouse directed the development of leukemia, the pharmacokinetics of benzene were compared between the two strains and between the two routes of administration. Regardless of the route of exposure or the strain of mouse, seven major metabolites plus unmetabolized benzene were detected in most samples at most time points. Few differences were observed between the two strains following either route of administration. These results suggest that the genetic modification in the Tg.AC mouse, i.e., insertion of the v-Ha-ras construct into the genome, did not disrupt any major pathways involved in determining the pharmacokinetics of benzene. Two significant differences were observed between the two routes of exposure: first, benzene was absorbed more slowly after intradermal injection than after oral gavage, and second, the intradermally dosed mice produced more conjugates of hydroquinone than did the orally dosed mice. These differences in metabolism may be involved in the previously observed differences in hematotoxicity between the two routes of exposure.

Cytochromes P450 involved with benzene metabolism in hepatic and pulmonary microsomes

Benzene is an occupational hazard and environmental toxicant found in cigarette smoke, gasoline, and the chemical industry. The major health concern associated with benzene exposure is leukemia. The toxic effects of benzene are dependent on its metabolism by the cytochrome P450 enzyme system. Previous research has identified CYP2E1 as the primary P450 isozyme responsible for benzene metabolism at low concentrations, whereas CYP2B1 is involved at higher concentrations. Our studies using microsomal preparations from human, mouse, and rat indicate that CYP2E1 is the P450 isozyme primarily responsible for benzene metabolism in lung and in liver. CYP2B isozymes have little involvement in benzene metabolism by either lung or liver. Our results also indicate that isozymes of the CYP2F subfamily may play a role in benzene metabolism by lung.

Comparative xenobiotic metabolism between Tg.AC and p53+/- genetically altered mice and their respective wild types

The use of transgenic animals, such as v-Ha-ras activated (TG:AC) and p53+/- mice, offers great promise for a rapid and more sensitive assay for chemical carcinogenicity. Some carcinogens are metabolically activated; therefore, it is critical that the altered genome of either of these model systems does not compromise their capability and capacity for metabolism of xenobiotics. The present work tests the generally held assumption that xenobiotic metabolism in the TG:AC and p53+/- mouse is not inherently different from that of the respective wild type, the FVB/N and C57BL/6 mouse, by comparing each genotype's ability to metabolize benzene, ethoxyquin, or methacrylonitrile. Use of these representative substrates offers the opportunity to examine arene oxide formation, aromatic ring opening, hydroxylation, epoxidation, O-deethylation, and a number of conjugation reactions. Mice were treated by gavage with (14)C-labeled parent compound, excreta were collected, and elimination routes and rates, as well as (14)C-derived metabolite profiles in urine, were compared between relevant treatment groups. Results of this study indicated that metabolism of the 3 parent compounds was not appreciably altered between either FVB/N and TG:AC mice or C57BL/6 and p53+/- mice. Further, expression of CYP1A2, CYP2E1, CYP3A, and GST-alpha in liver of naive genetically altered mice was similar to that of corresponding wild-type mice. Thus, these results suggest that the inherent ability of TG:AC and p53+/- mice to metabolize xenobiotics is not compromised by their altered genomes and would not be a factor in data interpretation of toxicity studies using either transgenic mouse line.

A review of quantitative studies of benzene metabolism

Benzene is a ubiquitous, highly flammable, colorless liquid that is a known hematotoxin, myelotoxin, and human leukemogen. Benzene-induced toxicity in animals is clearly mediated by its metabolism. The mechanisms of acute hemato- and myelotoxicity in humans are almost certainly the same as in animals, and there is compelling evidence that metabolism is requisite for the induction of leukemia in humans. A very large number of experimental investigations of benzene metabolism have been conducted with animals, both in vivo and in vitro. There have also been many investigations of benzene metabolism in humans and with human tissues, Although the blood or tissue concentrations of benzene metabolites in humans resulting from benzene exposure have never been measured. Further, a number of mathematical models of benzene metabolism and dosimetry have been developed. In this article, we consider results from both experimental and mathematical modeling research, with particular emphasis on the last decade, and discuss the factors that are likely to be most influential in the metabolism of benzene.

A model for the induction of aplastic anemia by subcutaneous administration of benzene in mice

Long-term exposure to benzene vapors is associated with hematological diseases such as leukemia, lymphoma and aplastic anemia. CD(1) male mice were randomly assigned to six groups: 1B(10), 1B(15), 1B(20), 2B(10), 2B(15), and 2B(20.) 1B mice were administered 2 ml/kg (1940 mg/kg) subcutaneous injection (in the dorsal region) of benzene 5 days a week, and 2B mice were exposed 3 days a week (Monday, Wednesday and Friday) until a total of 10, 15 and 20 doses were completed. About 48 h after treatment completion, leukocyte, erythrocyte, and bone marrow cells were counted, and spleen histopathology was analyzed. 1B(15) and 1B(20) mice showed lethargy and irritability, 80% body and 42% spleen weight loss (P<0.001), while body and spleen weight loss were less severe in 2B mice (12 and 48%, respectively). After exposure to 20 benzene doses, 1B(20) and 2B(20) mice showed decreased hemoglobin concentrations, and erythrocyte, leukocyte and bone marrow cell counts (37, 34, 80 and 50%, respectively in group 1B(20); P<0.001; and 12, 48, 62 and 62%, respectively in group 2B(20)). Thrombocytopenia occurred only in group 2B. Both benzene-treatment schemes caused aplastic anemia, however, the disease was masked by spleen toxicity in group 1B. Scheme 2 allowed mice survival and caused less non-hematological effects. We establish here a reproducible and inexpensive experimental model to induce aplastic anemia in mice by subcutaneous injection of 2 ml/kg benzene, using two short-term treatment schemes.

Physiologically based pharmacokinetic modeling of benzene metabolism in mice through extrapolation from in vitro to in vivo

Benzene (C6H6) is a highly flammable, colorless liquid. Ubiquitous exposures result from its presence in gasoline vapors, cigarette smoke, and industrial processes. Benzene increases the incidence of leukemia in humans when they are exposed to high doses for extended periods; however, leukemia risks in humans at low exposures are uncertain. The exposure-dose-response relationship of benzene in humans is expected to be nonlinear because benzene undergoes a series of metabolic transformations, detoxifying and activating, in the liver, resulting in multiple metabolites that exert toxic effects on the bone marrow. We developed a physiologically based pharmacokinetic model for the uptake and elimination of benzene in mice to relate the concentration of inhaled and orally administered benzene to the tissue doses of benzene and its key metabolites, benzene oxide, phe nol, and hydroquinone. As many parameter values as possible were taken from the literature; in particular, metabolic parameters obtained from in vitro studies with mouse liver were used since comparable parameters are also available for humans. Parameters estimated by fitting the model to published data were first-order rate constants for pathways lacking in vitro data and the concentrations of microsomal and cytosolic protein, which effectively alter overall enzyme activity. The model was constrained by using the in vitro metabolic parameters (maximum velocities, first-order rate constants, and saturation parameters), and data from multiple laboratories and experiments were used. Despite these constraints and sources of variability, the model simulations matched the data reasonably well in most cases, showing that in vitro metabolic constants can be successfully extrapolated to predict in vivo data for benzene metabolism and dosimetry. Therefore in vitro metabolic constants for humans can subsequently be extrapolated to predict the dosimetry of benzene and its metabolites in humans. This will allow us to better estimate the risks of adverse effects from low-level benzene exposures.

Xylem conduits of a resurrection plant contain a unique lipid lining and refill following a distinct pattern after desiccation

The axial and radial refilling with water of cut dry branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia was studied in both acro- and basipetal directions by using 1H-NMR imaging. NMR measurements showed that the conducting elements were not filled simultaneously. Axial water ascent occurred initially only in a cluster of a very few conducting elements. Refilling of the other conducting elements and of the living cells was mainly achieved by radial extraction of water from these initial conducting elements. With time, xylem elements in a few further regions were apparently refilled axially. Radial water spread through the tissue occurred almost linearly with time, but much faster in the acropetal than in the basipetal direction. Application of hydrostatic pressure (up to 16 kPa) produced similar temporal and spatial radial refilling patterns, except that more conducting elements were refilled axially during the first phase of water rise. The addition of raffinose to the water considerably reduced axial and radial spreading rates. The polarity of water climbing was supported by measurements of the water rise in dry branches using the 'light refraction'(and, sometimes, the 'leaf recurving') method. Basipetal refilling of the xylem conduit exhibited biphasic kinetics; the final rise height did not exceed 20-30 cm. Three-cm-long branch pieces also showed a directionality of water climbing, ruling out the possibility that changes in the conducting area from the base to the apex of the branches were responsible for this effect. The polarity of water ascent was independent of gravity and also did not change when the ambient temperature was raised to c. 40 degrees C. At external pressures of 50-100 kPa the polarity disappeared, with basipetal and acropetal refill times of the xylem conduit of tall branches becoming comparable. Refilling of branches dried horizontally (with a clinostat) or inverted (in the direction of gravity) showed a pronounced reduction of the acropetal water rise to or below basipetal water climbing level (which was unaffected by this treatment). Unlike water, benzene and acetone climbing showed no polarity. In the case of benzene, the rise kinetics (including the final heights) were comparable with those measured acropetally for water, whereas with acetone the rise height was less. Transmission electron microscopy of dry branches demonstrated that the inner surfaces of the conducting tracheids and vessels were lined with a continuous osmiophilic (lipid) layer, as postulated by the kinetic analysis and light microscopy studies. The thickness of the layer varied between 20 and 80 nm. The parenchymal and intervessel pits as well as numerous tracheid corners contained opaque inclusions, presumably also consisting of lipids. Electron microscopy of rehydrated plants showed that the lipid layer was either thinned or had disintegrated and that numerous vesicle-like structures and lipid bodies were formed (together with various intermediate structural elements). These, many other data and the physical-chemical literature imply that several (radial) driving forces (such as capillary condensation, Marangoni forces, capillary, osmotic and turgor pressure forces) operate when a few conducting elements become axially refilled with water. These forces apparently lead to an avalanche-like radial refilling of most of the conducting elements and living cells, and thus to the removal of the 'internal cuticle' and of the hydrophobic inclusions in the pits. The polarity of water movement presumably results from high resistances in the basipetal direction, which are created by local gradients in the thickness of the lipid film as a result of draining under gravity in response to drought. There are striking similarities in morphology and function between the xylem-lining lipid film and the lung surfactant film lining the pulmonary air spaces of mammals.

Physiologically based modeling of the maximal effect of metabolic interactions on the kinetics of components of complex chemical mixtures

The objective of this study was to predict and validate the theoretically possible, maximal impact of metabolic interactions on the blood concentration profile of each component in mixtures of volatile organic chemicals (VOCs) [dichloromethane (DCM), benzene (BEN), trichloroethylene (TCE), toluene (TOL), tetrachloroethylene (PER), ethylbenzene (EBZ), styrene (STY), as well as para, ortho-, and meta-xylene (p-XYL, o-XYL, m-XYL)] in the rat. The methodology consisted of: (1) obtaining the validated, physiologically based toxicokinetic (PBTK) model for each of the mixture components from the literature, (2) substituting the Michaelis-Menten description of metabolism with an equation based on the hepatic extraction ratio (E) for simulating the maximal impact of metabolic interactions (i.e., by setting E to 0 or 1 for simulating maximal inhibition or induction, respectively), and (3) validating the PBTK model simulations by comparing the predicted boundaries of venous blood concentrations with the experimental data obtained following exposure to various mixtures of VOCs. All experimental venous blood concentration data for 9 of the 10 chemicals investigated in the present study (PER excepted) fell within the boundaries of the maximal impact of metabolic inhibition and induction predicted by the PBTK model. The modeling approach validated in this study represents a potentially useful tool for screening/identifying the chemicals for which metabolic interactions are likely to be important in the context of mixed exposures and mixture risk assessment.

Validation of a physiological modeling framework for simulating the toxicokinetics of chemicals in mixtures

The objective of this study was to investigate the usefulness of a physiologically based toxicokinetic (PBTK) modeling framework for simulating the kinetics of chemicals in mixtures of varying complexities and composition. The approach involved the simulation of the kinetics of components in two situations: (i) when one of the mixture components was substituted with another (i.e., benzene in the benzene (B)-toluene (T)-ethyl benzene (E)-m-xylene (X) mixture was substituted with dichloromethane (D)), and (ii) when another chemical was added to the existing four-chemical mixture model (i.e., when D was added to the existing BTEX mixture model). In both cases, differing compositions of mixtures were used to obtain simulations and to generate experimental data on kinetics for validation purposes. Since the quantitative and qualitative mechanisms of interaction among B, T, E, and X have already been established, the mechanisms of binary interactions between D and the BTEX components (e.g., competitive, noncompetitive, or uncompetitive metabolic inhibition) were investigated in the present study. The analysis of rat blood kinetic data (4-h inhalation exposures, 50-200 ppm each) to all binary combinations (D-B, D-T, D-E, and D-X) investigated in the present study was suggestive of competitive metabolic inhibition as the plausible interaction mechanism. By incorporating the newly estimated values of metabolic inhibition constant (K(i)) for each of these binary combinations within the five-chemical PBTK model (i.e., for the DBTEX mixture), the model adequately predicted the venous blood kinetics of chemicals in rats following a 4-h inhalation exposure to various mixtures (mixture 1:100 ppm of D and 50 ppm each of T, E, and X; mixture 2: 100 ppm each of D, T, E, and X; mixture 3: 100 ppm of D and 50 ppm each of B, T, E, and X; mixture 4: 100 ppm each of D, B, T, E, and X). The results of the present study suggest that the PBTK model framework is useful for conducting extrapolations of the kinetics of chemicals from one mixture to another differing in complexity and composition, based on mechanistic considerations of interactions elucidated at the binary level.

Lack of specificity of trans,trans-muconic acid as a benzene biomarker after ingestion of sorbic acid-preserved foods

The benzene metabolite, trans,trans-muconic acid (MA), has been shown to be a sensitive and specific biomarker for ambient benzene exposure levels as low as approximately 0.5 ppm. However, at lower exposure levels, the use of MA as a benzene biomarker is complicated by the fact that it is also a metabolite of the food preservative, sorbic acid. To better assess the extent of this interference, MA was measured in sequential spot urine samples over a 2-day study period from eight volunteers (four adults and two parent-children pairs) who consumed two sorbic acid-preserved foods. Large increases in MA concentration were seen after ingestion of both foods. Individual peaks ranged as high as 1673.7 ng/ml (705.3 ng/mg creatinine) in adults and 1752.1 ng/mg creatinine (1221.3 ng/ml) in children. Ratios of peak to baseline values varied from 2.5 to 60. The average peak in the seven subjects who showed an increase in MA after ingestion of the first sorbic acid-containing food was 531.1 ng/ml (693.2 ng/mg creatinine). The average in the seven participants who ingested the second food was 1102.1 ng/ml (795.3 ng/mg creatinine). Twenty-four-hour personal air benzene levels were all low (< or = 5.6 ppb). Substantial variation in MA results were seen in some males related to creatinine adjustment. These data indicate that sorbic acid-preserved foods have the potential to cause substantial interference with MA as a biomarker for both occupational and environmental benzene exposure in populations, such as in the United States, where consumption of preserved foods is common. Development of methods to minimize and/or assess sorbic acid interference will improve MA specificity in such populations.

Utilization of breath analysis for exposure estimates of benzene associated with active smoking

This study included three different experiments for benzene exposures associated with active smoking. In the first experiment, the mean exhaled breath benzene concentrations measured 1 min after an active smoke ranged from 58.1 to 81.3 microgram/m(3), depending on the commercial cigarette brand, while those measured prior to an active smoke ranged from 15.9 to 19.2 microgram/m(3). The postexposure breath concentrations were much higher than the mean breath concentrations reported by some previous studies whose exposure conditions and postsampling times were not controlled. Similar to some previous decay studies conducted for different volatile organic compounds in different microenvironments, our second experiment showed that there was a rapid fall in the breath concentration and thereafter the decrease was much slower. One-compartment half-lives ranged from 30.1 to 57.8 min. Two-compartment half-lives ranged from 3.2 to 25.7 min for the first half-life and from 67 to 462 min for the second half-life. In the final repeated smoke experiment conducted with two specified time intervals, the breath concentrations showed increasing trends for both the pre- and the post exposure concentrations, with few exceptions. However, none of the changes were statistically significant at P<0.05.

[Study on the absorption of environmental contaminants in low-level exposure by pharmacokinetic analysis]

A dynamic generating toxic gas system and a nose-only exposure system were used for the pharmacokinetic study of inhaled environmental contaminants for benzene, toluene, xylene, ethylbenzene, chlorobenzene, styrene, isopropyl benzene, tetrachloroethylene, nonane and methylcyclohexane in male guinea pig. The change of these substances in blood with time was determined simultaneously by solid phase micro-extraction(SPME) gas chromatography (GC). The results showed that the fraction of absorption of benzene at low (121 micrograms/m3) exposure was 4.8 times higher than that at high(12.1 mg/m3) exposure. The pharmacokinetics of these substances were evaluated by using linear compartment models. The data showed that more styrene was absorbed than tetrachloroethylene at low-exposure. The metabolic elimination of these compounds at various exposure concentrations was extrapolated by using estimated pharmacokinetic parameters. Moreover, not only should the differences in absorption quantities be considered in evaluation of potential risk assessment, the metabolic elimination rates should also be considered although the exposure concentrations in gas for all chemicals were equal. The data presented in this paper was fundamental data used for risk assessment.

Stability of hemoglobin and albumin adducts of benzene oxide and 1,4-benzoquinone after administration of benzene to F344 rats

The stability of cysteinyl adducts of benzene oxide (BO) and mono-S-substituted cysteinyl adducts of 1,4-benzoquinone (1,4-BQ) was investigated in both hemoglobin (Hb) and albumin (Alb) following administration of a single oral dose of 400 mg [U-14C/13C6]benzene/kg body weight to F344 rats. Total radiobound adducts to Hb were stable, as were adducts formed by the reaction of [13C6]BO with cysteinyl residues on Hb. In both cases adduct stability was indicated by zero-order kinetics with decay rates consistent with the lifetime of rat erythrocytes. Hb adducts of 1,4-BQ were not detected, possibly due to the production of multi-S-substituted adducts within the erythrocyte. Regarding Alb binding, total radiobound adducts decayed more rapidly than expected (half-life of 0.4 days), suggesting that uncharacterized benzene metabolites were noncovalently bound or formed unstable adducts with Alb. Although adducts from reactions of BO and 1,4-BQ with Alb both decayed with rates consistent with those of Alb turnover in the rat, the half-life for 1,4-BQ-Alb (2.5 days) was shorter than that for BO-Alb (3.1 days), suggesting some instability of 1,4-BQ-Alb. Assuming similar rates of adduct instability in humans and rats, the 1,4-BQ-Alb adducts would be eliminated with a half-life of approximately 8 days, compared with BO-Alb, which would be expected to turnover with Alb (half-life of approximately 21 days).

Cytochromes P450 involved with benzene metabolism in hepatic and pulmonary microsomes

Benzene is an occupational hazard and environmental toxicant found in cigarette smoke, gasoline, and the chemical industry. The major health concern associated with benzene exposure is leukemia. The toxic effects of benzene are dependent on its metabolism by the cytochrome P450 enzyme system. Previous research has identified CYP2E1 as the primary P450 isozyme responsible for benzene metabolism at low concentrations, whereas CYP2B1 is involved at higher concentrations. Our studies using microsomal preparations from human, mouse, and rat indicate that CYP2E1 is the P450 isozyme primarily responsible for benzene metabolism in lung and in liver. CYP2B isozymes have little involvement in benzene metabolism by either lung or liver. Our results also indicate that isozymes of the CYP2F subfamily may play a role in benzene metabolism by lung.

An air pollution model for use in epidemiological studies: evaluation with measured levels of nitrogen dioxide and benzene

The aim of the study was to evaluate the predictions derived from the Danish Operational Street Pollution Model (OSPM) when the input data are obtained by simple methods that could be used in large-scale epidemiological studies. The model calculations were thus compared with passive sampler measurements of nitrogen dioxide and benzene at 103 street locations in Copenhagen, Denmark, and at 101 locations in rural areas. Data on traffic and street configuration were collected by means of a simple registration scheme in which forms were filled out by local municipal authorities. Meteorological data were derived from routine measurements at Copenhagen airport, and data on background air pollution were based on a simple empirical model. Differences in air pollution levels between rural areas and Copenhagen and differences in nitrogen dioxide concentrations at various locations in Copenhagen were well reproduced by the OSPM. The correlation coefficients (r) between the measured and the predicted half-year average concentrations of nitrogen dioxide in Copenhagen were between 0.75 and 0.80 for various degrees of precision of the input data for the model. The results indicate that the OSPM used with the presented methods for generation of input data might be useful in assessing long-term exposure to air pollutants in epidemiological studies.

Benzene accumulation in horticultural crops

In this study apple, blackberry and cucumber crops were exposed to elevated levels of benzene under controlled conditions. Benzene was retained in fruits of all crops, but only accumulated in leaves of blackberries and apples. The retention by cucumber fruits is suggested to result from the longer pathway for the desorption of benzene as a consequence of their increased tissue depth compared to leaves. The process of accumulation in blackberry and apple leaves is unknown. The ingestion of benzene via the food-chain pathway on the basis of this study is concluded not to be significant.

Adsorption of BTEX from aqueous solution by macroreticular resins

Theoretical and experimental investigations were conducted on the adsorption of benzene, toluene, ethylbenzene and xylene (BTEX) by macroreticular resins. A mass transfer model based on the squared-driving force principle is presented for describing the BTEX transfer between the aqueous and solid phases. Also proposed is a theoretical model for describing the BTEX breakthrough curves of the adsorption column. While the mass transfer model involves only an overall mass transfer coefficient, the column adsorption model has two model parameters. Those parameters are conveniently estimated using the observed mass transfer and breakthrough data. The predictions using the proposed models were found to compare well with the experimental data of batch and column BTEX adsorption tests.