N-oxidation and N-demethylation of methylephedrine by rat-liver microsomes.

1. The metabolism of methylephedrine was studied using rat-liver microsomes; methylephedrine N-oxide and ephedrine were isolated as metabolites. 2. The formation of methylephedrine N-oxide was inhibited by methimazole or by preincubation of microsomes, but that of ephedrine was not; whereas the formation of ephedrine was inhibited by either SKF 525-A or CO, but the formation of the N-oxide was not. 3. Pretreatment of rats with phenobarbital increased N-demethylation, but decreased N-oxidation. Pretreatment with 3-methylcholanthrene did not influence either reaction. 4. The results indicate that the formation of the N-oxide was mediated by the flavin-containing monooygenase system and that of ephedrine was mediated by the cytochrome P-450 system.