Kinetic studies on toluene metabolism in ethanol- and phenobarbital-induced rat liver microsomes in vitro.

In vitro metabolism of toluene was investigated at substrate concentrations of 0.03-6.25 mM in liver microsomes from control and ethanol- and phenobarbital (PB)-treated rats. Three metabolites, benzylalcohol (BA), o- and p-cresol, were measured by high-performance liquid chromatograph. BA was the main metabolite of toluene, whereas o- and p-cresol contributed only 1.1-1.5% and 1.7-2.8% of total metabolites, respectively, in microsomes from control rats. Ethanol treatment showed little effect on the percentages of three metabolites, but PB increased the percentages of o- and p-cresol to as high as 5.5% and 8.0%, respectively, following the increase in toluene concentration. There were two different isozymes with different Km involved in the side-chain hydroxylation of toluene in microsomes from control and ethanol-treated rats. One had a low Km value (0.13-0.17 mM) and could be greatly induced with ethanol treatment. The other was a high Km isozyme (0.60-0.87 mM). PB-induced isozyme showed a similar Km value to that of the high Km isozyme existing in microsomes from control and ethanol-treated rats. Two isozymes were involved in the formation of p-cresol in microsomes of control rats: the low-Km type had a similar value (0.15 mM) to the low isozyme of BA formation, but the high Km isozyme had a larger value (2.04 mM) than the high isozyme of BA. Only one enzyme responsible for o-cresol formation was detected in microsomes of control rats, and had a similar Km (2.11 mM) to that of the high Km isozyme of p-cresol.(ABSTRACT TRUNCATED AT 250 WORDS)