Microbial models of mammalian metabolism: Involvement of cytochrome P450 in the N-demethylation of N-methylcarbazole by Cunningham ella echin ula ta

Abstract

1. As previously reported (Yang and Davis 1992), N-methylcarbazole (NMC) is converted to N-methylcarbazole (NHMC), and 3-hydroxy-N-methylcarbazole (3-OH-NHMC), two relatively stable carbinolamine metabolites by the fungus Cunninghamella echinulata (ATCC 9244). Decomposition of these two carbinolamines yields the corresponding dealkylated metabolites, carbazole and 3-hydroxycarbazole. In the present study, the possible involvement of cytochrome P450 in the requisite N-alkyl hydroxylation reaction was examined.

2. Carbon monoxide, a classical P450 inhibitor, markedly inhibited the formation of NHMC, as did potassium cyanide. 1-Benzylimidazole, piperonyl butoxide and SKF-525A inhibited the formation of both NHMC and 3-OH-NHMC, while β-naphthoflavone (5,6-benzoflavone) induced their formation.

3. The source of the oxygen atom in the metabolite NHMC was examined by GC/MS analysis of NHMC formed during incubation of NMC in H₂¹⁸O-enriched medium which resulted in no incorporation of labelled oxygen into the metabolite.

4. An intermolecular isotope effect was not observed for the formation of NHMC suggesting that C-H bond cleavage is not a rate limiting step in the formation of this metabolite under the conditions examined.

5. It was concluded that P450 enzymes may be involved in the N-demethylation of NMC catalyzed by this fungal model of mammalian metabolism, and provides further support for biochemical and mechanistic parallels between mammalian metabolism and microbial systems catalyzing phase-1 biotransformations.