Interaction of ligands with cytochrome P-450. On the 442 nm spectral species generated during the oxidative metabolism of pyridine.
When added to aerobic rabbit liver microsomal fractions fortified with an NADPH-generating system, pyridine initially produces a type II difference spectrum such as is observed with other aromatic amines. There is a time-dependent conversion of this perturbation into a new spectral species characterized by an absorbance maximum at 442 nm and a minor peak at 389 nm. Experiments with inhibitors of the cytochrome P-450-dependent electron-transport chain suggest that these species originate from binding to the haemoprotein of metabolic intermediate(s) derived from the amine substrate. Analysis of the incubation mixtures by t.l.c., high-pressure liquid chromatography, u.v.- and mass-spectrometry reveals the presence of a single metabolite arising from cytochrome P-450-catalysed oxidation of the heteroaromatic tertiary amine, which was identified as pyridine N-oxide, obviously accounting for adduct formation. This view is supported by comparative studies on the spectral changes generated by exogenous amine oxide with NADPH-reduced cytochrome P-450. Moreover, dithiothreitol, a potent N-oxidase inhibitor, strongly suppresses development of the 442 nm and 389 nm complexes. The ability of forming low-spin adducts with ferrous cytochrome P-450 absorbing around 440 nm appears to be an inherent property of different types of N-oxides. Considering the dipole character of the N+-O- function, a co-ordinate iron-oxygen bond is proposed to be formed in these complexes.
D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004229:Dithiothreitol / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D010084:Oxidation-Reduction; D010634:Phenobarbital / Q000494:pharmacology; D011725:Pyridines / Q000378:metabolism*; D011817:Rabbits; D013053:Spectrophotometry
D008024:Ligands; D011725:Pyridines; D003577:Cytochrome P-450 Enzyme System; C023666:pyridine; D004229:Dithiothreitol; D010634:Phenobarbital