title: On the mechanism of the inactivation of the major phenobarbital-inducible isozyme of rat liver cytochrome P-450 by chloramphenicol.
abstract: The mechanism of the inactivation of the major phenobarbital-inducible isozyme of rat liver cytochrome P-450 (P-450 PB-B2) by chloramphenicol has been investigated. Preparations of the enzyme from animals treated in vivo with chloramphenicol (CAP PB-B2) have been isolated, and their catalytic, spectral, and physical properties have been compared with those of the native PB-B2. The CAP PB-B2 exhibited: 1) a 60-70% loss in the rate of NADPH-supported monooxygenase activity with the substrates benzphetamine, 7-ethoxycoumarin, and p-nitroanisole; 2) a 60% decrease in the extent of enzymatic P-450 reduction catalyzed by NADPH-cytochrome P-450 reductase under both aerobic and anaerobic conditions; 3) a 60% decrease in the steady-state level of the ferrous dioxygen complex in the presence of substrates; 4) a 60% decrease in the magnitude of the type I spectral change induced by benzphetamine; and 5) a shift in the wavelength maximum for the chemically reduced ferrous-carbonyl complex from 450 to 451.5 nm. On the other hand, the ability of the CAP PB-B2 to catalyze the iodosobenzene-supported metabolism of 7-ethoxycoumarin and p-nitroanisole was unaltered. The results are consistent with a scheme whereby the binding of metabolites of chloramphenicol to amino acid residues in the PB-B2 close to the heme moiety blocks electron transport from NADPH-cytochrome P-450 reductase, thereby leading to a loss of monooxygenase activity.
mesh_terms: D015660:7-Alkoxycoumarin O-Dealkylase; D000818:Animals; D001589:Benzphetamine / Q000494:pharmacology; D002701:Chloramphenicol / Q000494:pharmacology*; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis; D004790:Enzyme Induction; D007527:Isoenzymes / Q000037:antagonists & inhibitors* / Q000096:biosynthesis; D007700:Kinetics; D008297:Male; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D010105:Oxygenases / Q000378:metabolism; D010634:Phenobarbital / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013053:Spectrophotometry; D013997:Time Factors
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007527:Isoenzymes; D001589:Benzphetamine; D002701:Chloramphenicol; D003577:Cytochrome P-450 Enzyme System; D010105:Oxygenases; D015660:7-Alkoxycoumarin O-Dealkylase; D009251:NADPH-Ferrihemoprotein Reductase; D010634:Phenobarbital