Microbial models of mammalian metabolism: involvement of cytochrome P450 in the N-demethylation of N-methylcarbazole by Cunninghamella echinulata.
1. As previously reported (Yang and Davis 1992), N-methylcarbazole (NMC) is converted to N-hydroxymethylcarbazole (NHMC), and 3-hydroxy-N-hydroxymethylcarbazole (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 beta-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.
Yang
W
W
Division of Medicinal and Natural Products Chemistry, College of Pharmacy, University of Texas, Austin 78712.
Jiang
T
T
Acosta
D
D
Davis
P J
PJ
D002227:Carbazoles / Q000378:metabolism*; D002273:Carcinogens / Q000378:metabolism*; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000378:metabolism*; D005656:Fungal Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D008745:Methylation; D008954:Models, Biological; D009090:Mucorales / Q000201:enzymology*; D010100:Oxygen / Q000378:metabolism; D010103:Oxygen Isotopes; D014316:Tritium
D002227:Carbazoles; D002273:Carcinogens; D065607:Cytochrome P-450 Enzyme Inhibitors; D005656:Fungal Proteins; D010103:Oxygen Isotopes; D014316:Tritium; C038545:N-methylcarbazole; D003577:Cytochrome P-450 Enzyme System; D010100:Oxygen