title: Mechanism of inactivation of rat liver microsomal cytochrome P-450c by 2-bromo-4'-nitroacetophenone.
abstract: The mechanism by which 2-bromo-4'-nitroacetophenone (BrNAP) inactivates cytochrome P-450c, which involves alkylation primarily at Cys-292, is shown in the present study to involve an uncoupling of NADPH utilization and oxygen consumption from product formation. Alkylation of cytochrome P-450c with BrNAP markedly stimulated (approximately 30-fold) its rate of anaerobic reduction by NADPH-cytochrome P-450 reductase, as determined by stopped flow spectroscopy. This marked stimulation in reduction rate is highly unusual in that Cys-292 is apparently not part of the heme- or substrate-binding site, and its alkylation by BrNAP does not cause a low spin to high spin state transition in cytochrome P-450c. Under aerobic conditions the rapid oxidation of NADPH catalyzed by alkylated cytochrome P-450c was associated with rapid reduction of molecular oxygen to hydrogen peroxide via superoxide anion. The intermediacy of superoxide anion, formed by the one-electron reduction of molecular oxygen, established that alkylation of cytochrome P-450c with BrNAP uncouples the catalytic cycle prior to introduction of the second electron. The generation of superoxide anion by decomposition of the Fe2+ X O2 complex was consistent with the observations that, in contrast to native cytochrome P-450c, alkylated cytochrome P-450c failed to form a 430 nm absorbing chromophore during the metabolism of 7-ethoxycoumarin. Alkylation of cytochrome P-450c with BrNAP did not completely uncouple the catalytic cycle such that 5-20% of the catalytic activity remained for the alkylated cytochrome compared to the native protein depending on the substrate assayed. The uncoupling effect was, however, highly specific for cytochrome P-450c. Alkylation of nine other rat liver microsomal cytochrome P-450 isozymes with BrNAP caused little or no increase in hydrogen peroxide formation in the presence of NADPH-cytochrome P-450 reductase and NADPH.
mesh_terms: D000098:Acetophenones / Q000494:pharmacology*; D000478:Alkylation; D000818:Animals; D002942:Circular Dichroism; D065607:Cytochrome P-450 Enzyme Inhibitors; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism*; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D011487:Protein Conformation; D051381:Rats; D013481:Superoxides / Q000378:metabolism
chemical_list: D000098:Acetophenones; D065607:Cytochrome P-450 Enzyme Inhibitors; D013481:Superoxides; C018473:2-bromo-4'-nitroacetophenone; D009251:NADPH-Ferrihemoprotein Reductase