Metabolism of methylenedioxyphenyl compounds by rabbit liver preparations. Participation of different cytochrome P450 isozymes in the demethylenation reaction.

The cytochrome P450-mediated oxidative demethylenation of the benzo-1,3-dioxoles (methylenedioxyphenyl compounds, MDPs), methylenedioxybenzene (MDB), methylenedioxyamphetamine (MDA), and methylenedioxymethamphetamine (MDMA), by rabbit liver microsomes and cytochrome P450IIB4 (CYP2B4) was examined. Material balance studies indicated that demethylenation to catechol derivatives is a major metabolic pathway for MDB, MDA and MDMA. The reactions required NADPH and were inhibited by CO/O2 (4:1, v/v). Biphasic double-reciprocal plots of MDMA, MDA and MDB oxidation suggested participation of more than one isozyme of cytochrome P450 in the reaction. Phenobarbital (PB) induction was selective in that the Vmax values for MDB were increased but not those for MDA and MDMA. Exposure of liver microsomes from PB-pretreated animals to phencyclidine (PCP) markedly suppressed MDB oxidation but had little effect on MDA and MDMA demethylenation. Reconstitution experiments with CYP2B4 demonstrated that MDB is a good substrate for the isozyme; but the relative demethylenation activities for MDA and MDMA were 1 and 2% of that for MDB. These results indicate that the PB-inducible isozymes such as CYP2B4 appear to play an important role in MDB demethylenation, whereas MDA and MDMA oxidation is mediated mainly by constitutive isozymes.