Changes of n-hexane metabolites in urine of rats exposed to various concentrations of n-hexane and to its mixture with toluene or MEK

Abstract

It is well known that n-hexane produces peripheral neuropathy, and 2,5-hexanedione, one of the metabolites of n-hexane, is thought to be the main causative agent. Recently, the metabolites of n-hexane in urine have been measured by gas chromatography, and 2,5-hexanedione was proved to be useful for the biological monitoring of n-hexane exposure. In the present experiment, we intended to clarify the change of n-hexane metabolites in the urine of rats exposed to various concentrations of n-hexane and to its mixture with toluene or MEK. In the first experiment, five separate groups of five rats each were exposed to 100, 500, 1000, or 3000 ppm of n-hexane, or fresh air respectively in an exposure chamber for 8 h a day. Urinary samples were gathered during exposure, 16, 24, and 40 h after exposure. Half of each sample was analyzed by gas chromatography after hydrolysis with acid and enzymes, and the other half was analyzed without hydrolysis. 2,5-Dimethylfuran, MBK, 2-hexanol, 2,5-hexanedione, and γ-valerolactone could be identified as n-hexane metabolites in the urine. The main metabolites were 2-hexanol and 2,5-hexanedione. 2-Hexanol was mostly excreted during exposure, while most of the 2,5-hexanedione was excreted after the end of exposure. The amount of metabolites in the urine correlatively increased with the concentration of n-hexane from 100 to 1000 ppm, but the amount of metabolites scarcely increased when the concentration of n-hexane increased from 1000 to 3000 ppm. The maximum concentration of the excreted metabolites in urine was delayed when the concentration of n-hexane increased. These findings may indicate that the capacity of hydroxylation in rat liver microsomes was saturated when exposed to high concentrations of n-hexane. Free (not conjugated) metabolites were also excreted in the urine in all concentrations during exposure and 16 h after exposure. In the second experiment, four separate groups of five rats each were exposed to 1000 ppm of n-hexane, 1000 ppm of n-hexane plus 1000 ppm of toluene, 1000 ppm of n-hexane plus 1000 ppm of MEK, or fresh air. The analyzing procedure was the same as that in the first experiment. Distributions of n-hexane metabolites in the mixed exposure group were almost similar to that of the n-hexane-alone group. The amount of metabolites decreased to about one-sixth of that in the n-hexane group by co-exposure with toluene and to about one-fourth by co-exposure with MEK. From these results, it can be considered that toluene decreases the neurotoxicity of n-hexane by the inhibition of n-hexane metabolism. However, the reason the neurotoxicity of n-hexane is increased by co-exposure with MEK cannot be clearly explained.