Does cobalt pretreatment of mice induce a phenobarbitone-type cytochrome P-450?

1. Pretreatment of male C57BL/6JHan mice with 40 mg/kg cobaltous chloride for two days, or three days pretreatment with 80 mg/kg phenobarbitone led to an increase of biphenyl-4-hydroxylation of similar magnitude. 2-Hydroxylation remains unaffected in both cases. 2. The time course shows an equivalent decrease in 2- and 4-hydroxylation for 6 h, when microsomal Co concn, reaches its maximum. Thereafter 4-hydroxylation increases to reach the enhanced values. 3. Kinetic analysis of biphenyl 2- and 4-hydroxylation reveals distinct differences. The apparent Km for 4-hydroxylation decreases in Co-pretreated mice but remains constant in phenobarbitone-pretreated animals. Also, the ratio of 4- to 2-hydroxylation in microsomes from phenobarbitone-treated animals remains unchanged for substrate concn. of 10(-5) to 2 X 10(-3) m, but for Co-pretreatment this ratio increases markedly from 2 to 5, with increasing substrate concn. 4. Increasing concn. of the competitive inhibitor, metyrapone, reveal a greater susceptibility of microsomal cytochrome P-450 from Co-treated mice than normal or phenobarbitone-induced animals. In contrast, deprivation of reducing equivalents in vitro in the presence of metyrapone shows similarities between microsomes from cobalt- and phenobarbitone-pretreated mice.