Dose dependent induction of rat liver microsomal cytochrome P-450 and microsomal enzymatic activities after inhalation of toluene and dichloromethane

Comparison of genetic damage in brazilian footwear-workers exposed to solvent-based or water-based adhesive

Research has shown that workers employed in footwear manufacture are at increased risk of some cancers, the strongest evidence being for nasal cancer and leukemia. Footwear-workers are routinely exposed to complex mixtures of solvents in degreasers, cleaners, primers, and adhesives used in the production process as toluene, n-hexane, acetone, and possibly dust particles, additives in shoe materials and degradation products of materials. The recognition of the potential health-hazards of solvent-based adhesives (SBAs) has lead to the development of adhesives with no organic solvents, the water-based adhesives (WBA). We investigated footwear-workers (all males) exposed to SBA (n=29) (for 3.98+/-4.13 years), and WBA (n=16), which had spent the six months previous to the study employed in an experimental section which used only water-based adhesives, although they had previously worked in sections which used solvent-based adhesives (for 5.80+/-4.03 years); 25 healthy subjects were used as controls. The Comet assay and the micronucleus test were used as endpoints, while the traditional parameters for assessing exposure to toluene in organic mixtures by measuring the concentration of urinary hippuric acid were also assessed. Our results showed a significantly lower mean concentration of hippuric acid in the control group than found in the SBA (P<0.001) and WBA (P<0.05) groups. The Comet assay results showed that there was a significant increase in the mean damage index for the SBA (P<0.001) group in comparison to the WBA group and control (P<0.05). For the micronucleus test in binucleated lymphocytes and exfoliated buccal cell, the three groups were not statistically different. Our study demonstrated that water-based adhesives are clearly a better option for safeguarding the health of footwear-workers, even with possibility of isocyanate presence, while the positive results observed in SBA group might be explained by chloroprene presence in the adhesive.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.

Induction of cytochrome P450 by toluene

At least six cytochrome P450 (P450) isoenzymes, including CYP1A1/2, CYP2A1, CYP2B1/2, CYP2C6, CYP2C11 and CYP2E1, are involved in the metabolism of toluene in rat liver. Toluene exposure induces CYP1A1/2, CYP2B1/2, CYP2E1 and CYP3A1, but decreases CYP2C11/6 and CYP2A1 in adult males. Both sex and age influence the induction of P450s by toluene: in general, the inductive effect is more prominent in younger than in older animals; in males than in females. Neonatal exposure to toluene causes significant changes in liver microsomal P450 dependent monooxygenase activities during the early stage of life, whereas the effects on the rats of more than 3 weeks of age are small. Although structurally related chemicals of toluene also influence similar hepatic P450 isoenzymes, the degree of CYP2B1/2 induction increases, whilst that of CYP2E1 decreases with increasing molecular weight and aliphatic moieties. Unlike liver, exposure to toluene does not influence the distribution of pulmonary or renal microsomal P450-related enzyme activity in rats. In humans, occupational exposure to toluene is so low that it could not lead to the induction of P450. However, the induction may be seen in toluene sniffers who are exposed to high concentrations.

Monoclonal antibody-directed assessment of toluene induction of rat hepatic cytochrome P450 isozymes

Cytochrome P450 isozymes induced in rat liver by a range of concentrations of toluene were studied with monoclonal antibodies (MAbs) to specific P450 isozymes and by enzyme assays. Nitrosodimethylamine demethylase activity was significantly increased in microsomes from rats exposed to more than 1000 ppm of toluene, an increase that was dose-dependent. Anti-CYP2E1 significantly inhibited the metabolism of toluene to benzyl alcohol (BA) by about 50%, in microsomes from 1000 to 4000 ppm toluene-exposed rats, at low substrate concentration (0.2 mM). With anti-CYP2B1/2, the rate of BA formation was decreased by 15-17% in microsomes from rats of 2000 and 4000 ppm toluene exposures at high substrate concentration (5.0 mM). On the other hand, anti-CYP2C11/6 inhibited the rate of formation of BA in all of the microsomes, but the extent of inhibition was progressively decreased from 55% in control to 33% in 4000 ppm exposure. Immunoblot analysis with anti-CYP2E1 and anti-CYP2B1/2 revealed stronger immunoreactive bands in microsomes from rats exposed to more than 1000 and 2000 ppm of toluene, respectively. Stronger bands were also observed in microsomes from rats of 2000-4000 ppm toluene exposures with anti-CYP3A1/2, but no immunoreactivity appeared with anti-CYP1A1/2. These results suggest that toluene induces CYP2E1, CYP2B1/2 and CYP3A1/2, but reduces CYP2C11/6, and has no effect on CYP1A1/2.

Pharmacokinetic data in the evaluation of the safety of food and color additives

Safety evaluation of food and color additives intended for human use is usually based on toxicity data obtained from animal studies; human data are rarely available. The extrapolation of animal data to humans is often controversial. The important role that pharmacokinetic data could play in the safety evaluation of food and color additives is now widely recognized. This paper reviews the current scientific knowledge concerning the application of properly designed pharmacokinetic studies to the evaluation of the safety of food and color additives. In principle, pharmacokinetic data can be useful not only in designing, interpreting, and extrapolating animal toxicity studies to humans, but also in providing insight into the mechanisms of toxicity. Examples of such applications are provided.

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.

Exposure to toluene increases the urinary excretion of D-glucaric acid

Workers at a printing plant exposed to low concentrations of toluene (43-401 mg/m3, median 155 mg/m3) had increased urinary D-glucaric acid (3.55-5.12 mmol/mol creatinine) excretion at the end of the shift compared with controls (2.45-3.35 mmol/mol creatinine). No increase was found after the summer holiday (1.92-2.89 mmol/mol creatinine) but excretion had increased two weeks later (4.05-5.55 mmol/mol creatinine). These changes in the excretion of D-glucaric acid were not correlated to levels of exposure, to changes of urinary hippuric acid and o-cresol half lives (three to eight hours), nor to o-cresol/hippuric acid concentration ratios when measured at the end of daily exposure. Since a significant intra and interindividual variability of urinary D-glucaric acid was found in all groups, urinary D-glucaric acid excretion is suitable to monitor group but not individual exposure.

Effects of toluene on protein synthesis and the interaction with ethanol in hepatocytes isolated from fed and fasted rats

The effects of three different concentrations (about 10, 100 and 1000 microM) of toluene on protein synthesis were studied in hepatocytes isolated from fed and fasted rats after 60 and 120 min. of incubation. The interaction between ethanol (60 mM) and the low and high toluene concentrations were also tested. To measure protein synthesis, 14C-valine was used as the precursor amino acid. Total valine concentration was 2 mM to ensure near-constant specific radioactivity of precursor. Toluene concentrations were measured by head-space gas chromatography. Protein synthesis was unchanged in the presence of low toluene concentrations. Intermediate toluene concentration decreased protein synthesis by about 20% and high toluene concentration decreased protein synthesis by about 60%. Protein synthesis was similar in cells from fed and fasted rats. Ethanol alone inhibited protein synthesis by 20-30%, more in fasted than in fed rats. Toluene and ethanol in combination inhibited protein synthesis additively. The high toluene concentration with or without ethanol appeared to inhibit synthesis/secretion of export proteins in hepatocytes from fasted rats. In conclusion, our study indicates that toluene in relatively high concentrations inhibits general protein synthesis in isolated rat hepatocytes. Toluene and ethanol seems to inhibit protein synthesis additively.

Influence of cigarette smoking on the toxicokinetics of toluene in humans

To study the influence of cigarette smoking on the toxicokinetics of toluene, 10 habitual smokers who intended to stop smoking were exposed to toluene vapor (3.2 mmol/m3, 4 h) at three different exposure occasions: (I) while the smoking habit was still ongoing, and (II and III) 1 and 3-4 wk, respectively, after the day at which the smoking habit was discontinued. Solvent concentrations in the exhaled air and in the blood as well as hippuric acid concentrations in the urine were measured during the exposure period and for 3 h after the exposure period. The apparent clearance of toluene decreased significantly (p less than 0.05) 3-4 wk after the smoking habit was discontinued. This decrease was not a consequence of the increased body weight noticed in the subjects. No statistically significant differences between the three exposure occasions in the elimination rate of hippuric acid could be demonstrated. Thus, cigarette smoking seems to enhance the elimination rate of toluene from the body, since the apparent clearance was decreased after smoking was stopped.

Increased plasma free fatty acid and triglyceride levels after single administration of toluene in rabbits

Changes of plasma lipids (triglyceride, TG: total cholesterol, Cho; and phospholipids, PL), free fatty acid (FFA), and blood glucose (BG) were studied in male rabbits after toluene administration (0.5 g/kg per os). Hypertriglyceridemia was observed at and after 2 h. Plasma FFA and BG were elevated temporarily during the early stage and lowered gradually thereafter. Initially, plasma Cho and PL were virtually unchanged, but the Cho level increased slowly after 6 h. The hypertriglyceridemia observed may have some adverse effects on heart function.

Effects of subacute toluene exposure on protein phosphorylation levels in rat frontoparietal and striatal membranes

Effects of subacute toluene exposure (80 p.p.m. toluene in air, 5 + 4 days, 6 h day-1) were analysed on calcium (Ca2+)- and cyclic adenosine monophosphate (cAMP)-induced protein phosphorylation levels in membrane preparations from the frontoparietal cortex and the striatum of the adult male rat. After protein separation by gel electrophoresis, the amount of radioactive phosphate incorporated from adenosine 5'-[gamma-32P] triphosphate, tetra-(ethylammonium) salt ([32P]ATP) was measured indirectly by autoradiography. The 21 most phosphorylated protein bands were then analysed by computerized image analysis. In the frontoparietal cortex no protein bands were significantly affected after cAMP-induced back phosphorylation, while after Ca2+ stimulation there was a decreased incorporation of [32P]ATP in a 22,000 protein band. In the striatum there was a reduced incorporation of [32P]ATP in a 26,000 protein band after cAMP-induced back phosphorylation, and in four bands of 20,000, 21,000, 52,000 and 134,000, respectively, after Ca2+ stimulation. The reduced incorporation of [32P]ATP in these proteins indicated increased original phosphorylation levels after toluene exposure. A comparison between the frontoparietal cortex and the striatum showed a selective vulnerability of phosphorylation processes in striatal membrane protein bands. In conclusion, toluene exposure at low doses augments membrane protein phosphorylation levels in the rat forebrain and especially in the striatum, probably leading to changes in information handling and/or metabolic changes.

The increasing and decreasing effects of aromatic hydrocarbon solvents on pulmonary and hepatic cytochrome P-450 in the rat

The effects of pretreatment with benzene and various methylbenzenes, ethyl- and propylbenzene, cumene and styrene on hepatic and pulmonary microsomal enzymes were studied in male rats. In the lungs, all the substituted benzenes, but not benzene itself, decreased cytochrome P-450 concentration, and most of them also decreased 7-ethoxycoumarin O-deethylase activity, whereas 7-ethoxyresorufin O-deethylase activity was increased by the same treatment. The change in aryl hydrocarbon hydroxylase activity was negligible. Neither NADPH-cytochrome c reductase activity, nor cytochrome b5 concentration were changed after hydrocarbon treatment. In the liver, all the compounds studied, except for benzene, increased 7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase, and most of them also cytochrome P-450, aryl hydrocarbon hydroxylate and NADPH-cytochrome c reductase. The effect on cytochrome b5 in the liver was less marked. In the liver, all the monooxygenases studied seemed to be inducible by alkylbenzenes and styrene, whereas the effect was selective in the lung; depending on the monooxygenase, the activity can increase, decrease or remain unchanged.

The hepatotoxic potential of combined toluene-chronic ethanol exposure

The hepatotoxic properties of concurrent chronic oral ethanol ingestion and acute toluene inhalation were evaluated. Male rats were maintained on ethanol-containing or control liquid diets for 29 days. Animals of each group were subjected to five 20-min exposures to 10 000 ppm toluene with 30 min of room air inhalation between exposures on days 22, 24, 26, and 28 of liquid diet feeding. Some of the ethanol-fed animals were withdrawn from ethanol 14 h before exposure. Ethanol-withdrawn animals displayed an increased sensitivity to the narcotic action of toluene. Animals were sacrificed and assays performed on day 29. Stress markers (plasma corticosterone, free fatty acid, and glucose) were not affected by treatments. A modest elevation in plasma aspartate aminotransferase occurred in non-withdrawn animals receiving both ethanol and toluene. Ethanol-toluene exposure increased both relative liver weight and liver triglycerides. Toluene antagonized the hypertriglyceridemia associated with chronic ethanol ingestion. This study indicates that combined ethanol and toluene exposure has minor potential to induce acute liver injury, but results in altered deposition of hepatic triglycerides.

A comparative study of the effects of benzene, toluene, and xylenes on their in vitro metabolism and drug-metabolizing enzymes in rat liver

Male Sprague-Dawley rats were injected ip with benzene, toluene, or a mixture of xylene isomers at 20 mmol hydrocarbon/kg daily for 3 days. The effects of administration of these hydrocarbons upon their own in vitro metabolism, as well as upon cytochrome P-450, NADPH-cytochrome c reductase, aminopyrine N-demethylase, aniline hydroxylase, glutathione, glutathione S-transferase, and UDPglucuronyltransferase in liver were studied. Each hydrocarbon studied increased its own in vitro metabolism. Benzene had no effect on the metabolism of toluene or xylenes. Toluene and xylenes increased the metabolism of benzene, toluene, and xylenes. Cytochrome P-450 was elevated by toluene and xylenes, but was not affected by benzene. NADPH-cytochrome c reductase was induced by all three hydrocarbons. Aminopyrine N-demethylase and aniline hydroxylase were induced by toluene and xylenes and were not affected by benzene. Glutathione was elevated by benzene, decreased by xylenes, and not affected by toluene. Glutathione S-transferase was induced differentially by these hydrocarbons toward various substrates: toward 1-chloro-2,4-dinitrobenzene by benzene and toluene, toward 1,2-dichloro-4-nitrobenzene by benzene and xylenes, and no effect toward 1,2-epoxy-3-(p-nitrophenoxy)propane by any hydrocarbons. UDPglucuronyltransferase was induced by benzene and toluene when o-aminophenol and phenol were used as the substrate. Xylenes had no effect. Benzene was more effective at inducing conjugation enzymes. Xylenes were more effective at inducing cytochrome P-450 dependent enzymes. Toluene was equipotent at inducing both types of enzymes. The results indicate that the addition of methyl groups to the aromatic ring affects the inductive pattern of these monocyclic aromatic hydrocarbons.

Neonatal exposure to toluene: effects on the development of liver microsomal cytochrome P-450 and serum hormone levels in the rat

Lactating Sprague-Dawley rats and their pups were exposed on postnatal days 1-7, 6 h/day, to 80, 500 and 1000 ppm toluene, respectively, by inhalation. Exposure to 80 ppm toluene decreased the liver microsomal AHH activity and the rate of 7 alpha- and 6 beta-hydroxylation of androstenedione in 8-day-old-pups. On the other hand, neonatal exposure to 500 or 1000 ppm toluene resulted in a significant increase in AHH and 7-ethoxyresorufin O-deethylase activities and in the formation of 16-oxygenated metabolites of androstenedione in 8-day-old animals. Exposure to toluene increased the cytochrome P-450 content at all 3 dose levels in male but not in female pups. Twenty-one days after neonatal exposure no such effects were seen in young animals of either sex. In 56-day-old male rats, however, neonatal exposure to 80 ppm toluene resulted in a decreased rate of 6 beta-hydroxylation of androstenedione and a reduced AHH activity. No such effects were seen in female rats of the same age. Neonatal exposure to toluene affected the body and liver weights in 8-day-old pups of both sexes but had no effect on these parameters in 21-day-old animals of either sex. Exposure to 80 ppm toluene during the neonatal period gave a significantly increased body weight of 56-day-old male but not of female rats of the same age although this treatment increased liver weight in both sexes at this age. Serum testosterone levels were decreased in 21-day-old male rats following neonatal exposure to 80 or 500 ppm toluene and in 56-day-old male rats exposed neonatally to 1000 ppm toluene. In conclusion, exposure to toluene during the first week of life caused significant changes in various liver microsomal cytochrome P-450 dependent enzyme activities in 8-day-old pups, whereas the long-term effects on liver metabolism of the adult animal were small.

A physiological model for the pharmacokinetics of methylene chloride in B6C3F1 mice following i.v. administrations

A physiologic mathematical model was developed to describe the time course of 14C-methylene chloride (14CH2Cl2) distribution and elimination in mice following single i.v. administrations of 10 and 50 mg/kg. A whole-body model was used to simulate 14CH2Cl2 concentrations in blood and tissues, pulmonary clearance of unchanged 14CH2Cl2, and metabolic conversion to 14CO2 and 14CO as monitored by the appearances of these metabolites in expired breath. This diffusion-limited model was identified via a sequential optimization scheme using hybrid models for each compartment. Pulmonary elimination of unchanged 14CH2Cl2 was modeled as a linear process while hepatic metabolism of 14CH2Cl2 to the compounds 14CO2 and 14CO was described by a saturable metabolic rate term. The model adequately described the dose dependence in methylene chloride distribution and metabolism when simulations were compared to experimental data.

Effects of pretreatment with toluene, phenobarbital and 3-methylcholantrene on the in vivo metabolism of toluene and on the excretion of hippuric acid in the rat

The limit of the capacity of male adult Sprague-Dawley rats to metabolize toluene and excrete it as hippuric acid was reached at a dose of 10 mmol toluene/kg. At this dose of benzoic acid or hippuric acid, the excretion of hippuric acid was 1.5-fold and 4-fold, respectively, greater than that following this dose of toluene. When toluene was given after phenobarbital pretreatment the excretion rate of hippuric acid increased 3-4-fold, but 3-methylcholanthrene pretreatment had no effect. Consecutive toluene exposures increased hippuric acid excretion on the third day by about 2-fold. It seems that the side-chain oxidation is the rate-limiting step in toluene metabolism of uninduced rats, and toluene accelerates its own metabolism by inducing cytochrome P-450 dependent microsomal monooxygenases.

Age-and sex-related differences in rat liver microsomal enzymes and their inducibility by toluene

In the liver microsomes of toluene-treated and control Sprague-Dawley rats (n = 148, males and females aged 13-93 days), the contents of cytochrome P-450 and b5 and the activities of NADPH-cytochrome c reductase and four monooxygenases were studied. In male control rats, cytochrome contents and NADPH-cytochrome c reductase, aminopyrine N-demethylase and aniline hydroxylase activities increased to the age of one month, and after a slight decrease in cytochrome concentrations, the average adult level was reached by the age of two months. Aniline hydroxylase and 7-ethoxycoumarine O-deethylase activities decreased to about half at the same age period. In control female rats, the activities of aminopyrine N-demethylase and aniline hydroxylase decreased after the age of one month, and they remained at a considerably lower level in adult females than in males. The sex-dependence of other enzymes was negligible. Toluene induction was already well developed in the youngest age group of both sexes; in most cases the induced enzyme levels in young rats were as high or higher than in adults. In adult female rats, toluene induction of all enzymes was weaker than in males. In male rats, the toluene-induced level of aniline hydroxylase and 7-ethoxycoumarine O-deethylase showed deep minima at the age of 43-53 days, at the puberty of rats.

Effects of xylene exposure on the metabolism of antipyrine in vitro and in vivo in the rat

Exposure of male rats to different concentrations of xylene for 3 days induced, in a dose-dependent way, the in vitro liver microsomal metabolism of antipyrine. The degree of induction was statistically significant at an exposure level of 250 ppm and maximal (2.5-fold increase) at 2000 ppm. This increase was of the same magnitude as after phenobarbital treatment. Female rats had a lower basal antipyrine metabolism than males, but exhibited a greater relative increase in antipyrine metabolism following xylene exposure. Cytochrome P-450 isozymes, purified from xylene- and phenobarbital-treated animals, were efficient catalysts of antipyrine metabolism, with turnover numbers of 33.3 and 21.1, respectively. A reduction in the half-life of antipyrine to 39% of preexposure values occurred after exposure of male rats to 1000 ppm of xylene for 3 days. Exposure to lower xylene levels did not produce significant alterations in antipyrine elimination half-life. In vitro, xylene was shown to be a non-competitive metabolic inhibitor of antipyrine. Experiments in vivo indicated that inhibition is not important at relatively low xylene exposure levels. It is concluded that induction of hepatic monooxygenases by xylene can be demonstrated, with antipyrine as a test drug, both in vitro and in vivo.