Effects of enzyme induction on microsomal benzene metabolism

The effect of induction by phenobarbital (PB), beta-naphthoflavone (BNF), and benzene on benzene metabolism was studied in hepatic microsomes from male Sprague-Dawley rats. Two distinct forms of mixed-function oxidase activity appeared to metabolize benzene. One form was active at all substrate concentrations in microsomes from control, benzene-treated, and BNF-treated animals, and at benzene concentrations of 0.8 mM and below in microsomes from PB-treated animals. It was saturated at benzene concentrations above 0.4 mM, had a pH optimum of approximately 6.6, and was stimulated by fluoride. Pretreatment with benzene, but not BNF, increased benzene metabolism in these preparations. Benzene metabolism in microsomes from PB-induced rats was less active than in controls at benzene concentrations below 0.8 mM, but increased rapidly at higher benzene concentrations. Further characteristics of the PB-induced enzyme activity were that saturation was not observed at benzene concentrations as high as 4 mM, the pH optimum for benzene metabolism in these preparations was 7.1, metabolism was not stimulated by fluoride, and metabolism was inhibited by metyrapone. Both phenol and an unidentified polar component were formed from benzene in all microsomal preparations. Formation of the polar component was increased by PB pretreatment and inhibited by metyrapone, suggesting that formation of the polar component involves a step requiring cytochrome P-450.