Changes of n-hexane neurotoxicity and its urinary metabolites by long-term co-exposure with MEK or toluene

Blood 2,5-dimethylfuran as a sensitive and specific biomarker for cigarette smoking

OBJECTIVE

We evaluated the validity of blood 2,5-dimethylfuran (DMF) for determining smoking status using population-based data.

METHODS

We obtained blood DMF concentrations and smoking status from National Health and Nutrition Examination Survey 2003-2006 and computed sensitivity, specificity and Kappa statistic.

RESULTS

Self-reported smoking showed very high agreement (Kappa = 92.8-93.3%) in daily smokers and fair agreement in non-daily smokers (Kappa = 33.7-36.4%). Coffee intake did not influence the detection of blood DMF.

CONCLUSIONS

Blood DMF has comparable sensitivity and specificity with serum cotinine for identifying current daily smokers, which may make it a useful biomarker in epidemiologic studies.

Predicted toxicity of the biofuel candidate 2,5-dimethylfuran in environmental and biological systems

Although not mutagenic by Ames test, 2,5-dimethylfuran (DMF), a leading biofuel candidate, was found to induce chromosomal damage in cultured murine cells, suggesting that it may be genotoxic. We sought to prioritize the environmental and biological impacts of using DMF as a combustible biofuel. First, we assessed DMF and its combustion intermediates for potential persistence, bioaccumulation, and aquatic toxicity (PBT) using PBT profiler. Our findings predict DMF to have moderate-level aquatic toxicity; however, a greater subset of the combustion intermediates is predicted to have moderate- and high-level aquatic toxicity with bioaccumulation and persistence concerns. Second, we assessed the biological impact of DMF by testing for statistically significant chemical-disease associations. No direct associations for DMF were found; however, indirect associations were identified from two structurally similar analogs. Curated associations between furfuryl alcohol to kidney neoplasm and adenoma, and significant inferred associations between furan to lung neoplasm, drug-induced liver injury, and experimentally induced liver cirrhosis were found, based on 21 furan-gene interactions. Nine of 49 DMF combustion intermediates analyzed, including benzene and 1,3-butadiene, were found to have associations with 26 tumors and systemic diseases. Although inadequate for a stand-alone risk assessment, our data suggest that DMF combustion intermediates pose a much broader range of hazards than DMF itself, and that both should be further investigated.

Changes of cytoskeletal proteins in nerve tissues and serum of rats treated with 2,5-hexanedione

To investigate the mechanisms and biomarker of the neuropathy induced by 2,5-hexanedione (HD), male Wistar rats were administrated HD at dosage of 200 or 400mg/kg for 8 weeks (five-times per week). All rats were sacrificed after 8 weeks of treatment and the cerebrum cortex (CC), spinal cord (SC) and sciatic nerves (SN) were dissected, homogenized and used for the determination of cytoskeletal proteins by western blotting. The levels of neurofilaments (NFs) subunits (NF-L, NF-M and NF-H) in nerve tissues of 200 and 400mg/kg HD rats significantly decreased in both the supernatant and pellet fractions. Furthermore, significant negative correlations between NFs levels and gait abnormality were observed. As for microtubule (MT) and microfilament (MF) proteins, the levels of alpha-tubulin, beta-tubulin and beta-actin in the supernatant and pellet fraction of SN significantly decreased in 200 and 400mg/kg HD rats and correlated negatively with gait abnormality. However, the contents of MT and MF proteins in CC and SC were inconsistently affected and had no significant correlation with gait abnormality. The levels of NF-L and NF-H in serum significantly increased, while NF-M, alpha-tubulin, beta-tubulin and beta-actin contents remain unchanged. A significant positive correlation (R=0.9427, P<0.01) was observed between gait abnormality and NF-H level in serum as the intoxication went on. These findings suggested that HD intoxication resulted in a progressive decline of cytoskeletal protein contents, which might be relevant to the mechanisms of HD-induced neuropathy. NF-H was the most sensitive index, which may serve as a good indicator for neurotoxicity of n-hexane or HD.

Physiologically based modeling of n-hexane kinetics in humans following inhalation exposure at rest and under physical exertion: impact on free 2,5-hexanedione in urine and on n-hexane in alveolar air

We used a modified physiologically based pharmacokinetic (PBPK) to describe/predict n-hexane (HEX) alveolar air concentrations and free 2,5-HD urinary concentrations in humans exposed to n-HEX by inhalation during a typical workweek. The effect of an increase in workload intensity on these two exposure indicators was assessed and, using Monte Carlo simulation, the impact of biological variability was investigated. The model predicted HEX alveolar air concentrations at rest of 19.0 ppm (25 ppm exposure) and 38.7 ppm (50 ppm exposure) at the end of the last working day (day 5), while free 2,5-HD urinary concentrations of 3.4 micromol/L (25 ppm) and 6.3 micromol/L (50 ppm) were predicted for the same period (last 4.5 hours of Day 5). Monte Carlo simulations showed that the range of values expected to occur in a group of 1000 individuals exposed to 50 ppm of HEX (95% confidence interval) for free 2,5-HD (1.7-14.7 micromol/L) is much higher compared with alveolar air HEX (33.4-46 ppm). Simulations of exposure at 50 ppm with different workloads predicted that an increase in workload intensity would not greatly affect both indicators studied. However, the alveolar air HEX concentration is more sensitive to modifications of workload intensity and time of sampling, after the end of exposure, compared with 2,5-HD. The PBPK model successfully described the HEX alveolar air concentrations and free 2,5-HD urinary concentrations measured in human volunteers and is the first, to our knowledge, to describe the excretion kinetics of free 2,5-HD in humans over a 5-day period.

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.

Relationship between 2,5-hexanedione concentrations in nerve, serum, and urine alone or under co-treatment with different doses of methyl ethyl ketone, acetone, and toluene

To ascertain the relationship among 2,5-hexanedione (2,5-HD) concentrations in nerve, serum and urine, rats were injected subcutaneously with 2.6 mmol/kg 2,5-HD alone, or together with 2.6 or 13.0 mmol/kg of methyl ethyl ketone, acetone and toluene. 2,5-HD concentrations in sciatic nerve (NC), serum (SC) and urine (UC) were determined, and the linear regression between each two of NC, SC, and UC were calculated. There was good correlation between NC and SC, SC and UC in the 2,5-HD alone group, and good correlation between NC and SC in the co-treated groups. Co-treatment solvent had little effect on the relationship between SC and NC. 13.0 mmol/kg co-treated solvent tended to decrease the regression coefficients compared with 2.6 mmol/kg co-treated solvent. These results show that SC can be used in estimating NC in the 2,5-HD alone or co-treated groups, and UC can be used in estimating SC in the 2,5-HD alone group.

Metabolic interaction of n-hexane and methyl ethyl ketone in vitro in a head space rat liver S9 vial equilibration system

Methyl ethyl ketone pretreatment induced rat liver cytochrome P450 and increased significantly the in vitro metabolism of n-hexane and the formation of 2,5-hexanedione in rat liver S9. No significant changes were, however, found in the levels of the intermediate metabolites 2-hexanol, 2,5-hexanediol or methyl n-butyl ketone. Methyl ethyl ketone added in vitro to untreated (non-induced) liver S9 inhibited in a non-competitive pattern the metabolism of n-hexane and decreased significantly and in a dose-dependent way the levels of methyl n-butyl ketone and 2,5-hexanedione. When methyl ethyl ketone and n-hexane were added in vitro to in vivo methyl ethyl ketone pretreated (induced liver S9, the significant increase in the formation of 2,5-hexanedione was maintained, an increase which was only to a minor extent influenced by the in vitro addition of methyl ethyl ketone. These findings are in agreement with an in vivo induction by methyl ethyl ketone of key enzyme(s) in a generally minor metabolic pathway for the conversion of n-hexane to 2,5-hexanedione in rat liver, a pathway which is not influenced by the presence of methyl ethyl ketone itself. The results obtained in this study indicate that the head space equilibration technique is well suited for screening studies of metabolic interactions between organic solvents.

Function of the auditory and visual systems, and of peripheral nerve, in rats after long-term combined exposure to n-hexane and methylated benzene derivatives. II. Xylene

Rats were exposed to xylene, to n-hexane, or to xylene together with n-hexane, each solvent 1000 p.p.m. (1000 + 1000 p.p.m. in mixed exposure), 18 hr/day, 7 days/week during 61 days. Neurophysiological recordings were made 2 days, 4 months, and 10 months after the end of exposure. Exposure to n-hexane alone, or xylene alone, caused a slight loss of auditory sensitivity as recorded by auditory brainstem response 2 days after the exposure. Exposure to n-hexane together with xylene caused persistent loss of auditory sensitivity (7-17 dB; P < 0.05) which was non-additively enhanced (P < 0.01). The latencies of the flash evoked potentials in the group exposed to n-hexane alone were prolonged (re C group) 2 days after exposure, while smaller prolongations were found in the group exposed to xylene together with n-hexane. Exposure to n-hexane alone caused a marked decrease in nerve conduction velocity, while simultaneous exposure to xylene inhibited n-hexane-induced velocity reduction in peripheral nerve (P < 0.01).

Function of the auditory and visual systems, and of peripheral nerve, in rats after long-term combined exposure to n-hexane and methylated benzene derivatives. I. Toluene

Rats were exposed to n-hexane, toluene, or toluene together with n-hexane, each solvent 1000 p.p.m. (1000 + 1000 p.p.m. in combined exposure), 21 hr/day, 7 days/week during 28 days. Neurophysiological recordings were made 2 days, 3 months, and one year after end of exposure. A reduction in auditory sensitivity, recorded by click evoked auditory brainstem response, was observed 2 days after exposure to toluene alone, or to toluene together with n-hexane, but not after exposure to n-hexane alone. The reduction lasted one year after the exposure. Three months after combined exposure, a synergistic enhancement of loss of auditory sensitivity was observed in the mixed exposure group. One amplitude in the flash evoked potentials was lowered in the n-hexane exposed group 2 days after exposure. No such reduction was seen after simultaneous exposure to toluene. Exposure to n-hexane alone caused a marked decrease in peripheral nerve conduction velocity 2 days and 3 months after exposure, while exposure to n-hexane together with toluene had only a small effect on this velocity.

Modification of metabolism and neurotoxicity of hexane by co-exposure of toluene

The effects of co-exposure of hexane and toluene were investigated in field surveys and animal experiments. One field survey suggested that increase of hexane content in adhesives might have caused an outbreak of polyneuropathy in a vinyl sandal manufacture in Japan. The animal experiments proved that co-exposure of hexane and toluene decrease hexane neurotoxicity and urinary excretion of hexane metabolites in rats. The results also suggested that toluene might inhibit metabolism of hexane. Another recent field survey indicated that the ratio of urinary 2,5-hexanedione to hexane exposure in the workers co-exposed to hexane and toluene decreased in parallel with in more crease of toluene concentration. The results indicated that urinary excretion of 2,5-hexanedione could be depressed by co-exposure of toluene even in the workers exposed to relatively low concentrations. These above-mentioned results suggest that co-exposure of hexane and toluene could inhibit hexane metabolism and decrease hexane neurotoxicity in both experimental animals and workers. Although metabolism of hexane could be easily modified by toluene or other solvents and might not be a good indicator for hexane exposure in mixed exposure, urinary 2,5-hexanedione might be a good indicator for neurotoxicity of hexane even in mixed exposure.

Determination of urinary 2,5-hexanedione concentration by an improved analytical method as an index of exposure to n-hexane

2,5-Hexanedione is a main metabolite of n-hexane and is considered as the cause of n-hexane polyneuropathy. Therefore, it is useful to measure 2,5-hexanedione for biological monitoring of exposure to n-hexane. The analytical methods existing for n-hexane metabolites, however, were controversial and not established enough. Hence, a simple and precise method for determination of urinary 2,5-hexanedione has been developed. Five ml of urine was acidified to pH 0.5 with concentrated hydrochloric acid and heated for 30 minutes at 90-100 degrees C. After cooling in water, sodium chloride and dichloromethane containing internal standard were added. The sample was shaken and centrifuged. 2,5-Hexanedione concentration in an aliquot of dichloromethane extract was quantified by gas chromatography using a widebore column (DB-1701). Urinary concentration of 2,5-hexanedione showed a good correlation with exposure to n-hexane (n = 50, r = 0.973, p less than 0.001). This method is simple and precise for analysis of urinary 2,5-hexanedione as an index of exposure to n-hexane.

Effects of MEK on kinetics of n-hexane metabolites in serum

The neurotoxicity of n-hexane is thought to be caused ultimately by 2,5-hexanedione (2,5-HD), one of the n-hexane metabolites. The potentiation of n-hexane neurotoxicity by co-exposure with MEK, therefore, is suspected to be related to kinetics of 2,5-HD in blood. To clarify the kinetics of n-hexane metabolites in the mixed exposure of n-hexane and MEK, rats were exposed to 2000 ppm n-hexane or a mixture of 2000 ppm n-hexane and 2000 ppm MEK, and the time courses of serum n-hexane metabolites were determined. 2,5-HD in serum increased until 2 h after the end of exposure, when serum 2,5-HD concentration reached a peak of 16.35 micrograms/ml in the n-hexane-alone group. In contrast, 2,5-HD in the mixed exposure group increased much more slowly during and after exposure than in the n-hexane-alone group. It reached a peak of 2.12 micrograms/ml at 8 h after the end of exposure. Serum MBK, a precursor of 2,5-HD in the co-exposure group, was about half in the n-hexane-alone group during exposure. However, MBK decreased more slowly in the co-exposure group than in the n-hexane-alone group after the end of the exposure. The results suggest that co-exposed MEK might inhibit oxidation of n-hexane and decrease clearance of n-hexane metabolites. Co-exposed MEK did not increase serum 2,5-HD, which was considered a main neurotoxic metabolite. Therefore the enhancement of neurotoxicity could not be attributed to increased serum 2,5-HD in the co-exposed group.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in urinary n-hexane metabolites by co-exposure to various concentrations of methyl ethyl ketone and fixed n-hexane levels

To make clear how the n-hexane metabolism is modified by co-exposure with MEK, rats were exposed to various concentrations of MEK mixed with a fixed concentration of n-hexane. Twenty-four male Wistar rats were divided into four equal groups. Each group was exposed for 8 h to 2000 ppm n-hexane, 2000 ppm n-hexane plus 200 ppm MEK, 2000 ppm n-hexane plus 630 ppm MEK and 2000 ppm n-hexane plus 2000 ppm MEK, respectively. Free metabolites and the sum of free and conjugated metabolites of n-hexane were analyzed by gas chromatography. The main metabolite was 2-hexanol during the exposure and 2,5-hexanedione (2,5-HD) after the exposure in any group. The main metabolites, 2-hexanol and 2,5 HD, decreased in inverse proportion to the co-exposed MEK concentrations. The results suggest that augmentation of n-hexane neurotoxicity by MEK co-exposure could not be explained only by 2,5-HD. In addition, 2,5-HD is recommended as an index for biological monitoring of n-hexane exposure. However, one should be careful to evaluate the exposed n-hexane concentration by urinary 2,5-HD, because n-hexane metabolism could be largely modified by co-exposure with MEK.

Effects of methyl ethyl ketone pretreatment on hepatic mixed-function oxidase activity and on in vivo metabolism of n-hexane

1. Male Fischer-344 rats were given methyl ethyl ketone (MEK; 1.87 ml/kg), a potentiator of the neurotoxicity of n-hexane, by gavage for 4 days prior to a single inhalation exposure to n-hexane (1000 ppm). 2. Samples of blood, liver, testis and sciatic nerve were obtained and analysed for n-hexane, MEK and their metabolites by g.l.c.-mass spectrometry. 3. Pretreatment with MEK increased the concentrations of 2,5-hexanedione (2,5-HD; the proximal neurotoxin) in blood, sciatic nerve and testis relative to concentrations in the tissues in sham-treated controls. 4. Concentrations of 2,5-dimethylfuran, a metabolite of 2,5-HD, were increased in all four tissues tested. 5. After 1-7 days treatment with MEK, the activity of 7-ethoxycoumarin O-deethylase was increased (up to 500%), but benzphetamine N-demethylase activity was virtually unaffected. 6. Hence, the potentiating effects of MEK on the neurotoxicity of n-hexane appear to arise, at least in part, from the activating effects of MEK on selected microsomal enzymes responsible for n-hexane activation.

Testicular atrophy and loss of nerve growth factor-immunoreactive germ cell line in rats exposed to n-hexane and a protective effect of simultaneous exposure to toluene or xylene

Testicular and germ cell line morphology in rats were studied 2 weeks, 10 months and 14 months after cessation of a 61-day inhalation exposure to 1000 ppm n-hexane. Androgen biosynthetic capacity of testis, testosterone blood concentration, vas deferens morphology and noradrenaline (NA) concentration, epididymal sperm morphology, and fertility were also studied. Severe testicular atrophy involving the seminiferous tubules with loss of the nerve growth factor (NGF) immunoreactive germ cell line was found. Total loss of the germ cell line was found in a fraction of animals up to 14 months post-exposure, indicating permanent testicular damage. No impairment of androgen synthesis or androgen dependent accessory organs was observed. Simultaneous administration of 1000 ppm n-hexane and 1000 ppm toluene, or 1000 ppm n-hexane and 1000 ppm xylene, did not cause germ cell line alterations or testicular atrophy. Toluene and xylene were thus found to protect from n-hexane induced testicular atrophy.