Liver microsomes from uninduced hogs catalyzed the NADPH-dependent N-oxidation of the hydroxylamine (Z)-3-(4-bromophenyl)-N-hydroxy-N-methyl-3-(3-pyridyl)allylamine. The conjugated nitrone N-[(2Z)-3-(4-bromophenyl)-3-(3-pyridyl)-2-propenylidene]methylamin e N-oxide was the principal product and accounted for 90-95% of the total hydroxylamine metabolized by microsomes. The nitrone had a relatively high chemical stability which was investigated at various conditions. Studies of the biochemical mechanisms for N-oxidation of N-hydroxy-norzimeldine showed that the reaction was catalyzed by the flavin-containing monooxygenase. This conclusion was based on studies with the purified hog liver flavin-containing monooxygenase and hog liver microsomes. Purified rat liver cytochrome P-450IIB-1 was ineffective at catalyzing N-oxidation of N-hydroxynorzimeldine.

Animals↗

PubMedDate not supplied

Metabolism of nicotine by human liver microsomes: stereoselective formation of trans-nicotine N'-oxide.

Liver microsomes from humans catalyze the NADPH-dependent oxidation of (S)-nicotine. The principal product is the 5'-carbon atom oxidation product, nicotine delta 1',5'-iminium ion, which is efficiently converted to the gamma-lactam derivative cotinine in the presence of aldehyde oxidase. Another major product is nicotine N'-oxide. In contrast to previous reports describing in vitro or in vivo studies, formation of only trans-nicotine N'-oxide was observed. Demethylation of nicotine was not observed. Studies on the biochemical mechanism of nicotine 5-carbon atom oxidation strongly implicate one major cytochrome P-450 isoenzyme (i.e., P-450 2A6) as largely responsible for delta 1',5'-iminium ion formation. Stereoselective formation of trans-nicotine N'-oxide may be catalyzed in large part by the flavin-containing monooxygenase (form II). These conclusions are based on the effects of alternate substrates for the flavin-containing monooxygenase, heat inactivation studies, immunoblot studies, and selective substrates for cytochromes P-450. The results suggest that (S)-nicotine trans N'-oxygenation and delta 1',5'-iminium ion formation may be selective probes of human liver flavin-containing monooxygenase form II and cytochrome P-450 2A6 activities, respectively, useful for in vivo phenotyping of humans.

Chromatography, High Pressure Liquid↗

PubMedDate not supplied

Chemical, enzymatic, and human enantioselective S-oxygenation of cimetidine.

The S-oxygenation of cimetidine was investigated using achiral chemical and chiral chemical and enzymatic S-oxygenation procedures. The products of the reactions were thoroughly characterized by spectral, chiroptical, chromatographic, and stereochemical correlation methods. S-Oxygenation by the Kagan method or in the presence of pig liver microsomes or pig liver flavin-containing monooxygenase (FMO) (form I) all gave essentially identical enantioselectivity: the average enantiomeric excess was -13.4% and the stereopreference was for formation of (+)-cimetidine S-oxide in a ratio of (+)56.7%:(-)43.3%. The profile of immunoreactivity and the effect of metabolism inhibitors on cimetidine S-oxide formation in the presence of pig liver microsomes were consistent with a role of FMO (form I) in enantioselective (+)-cimetidine S-oxide formation. Administration of cimetidine to seven healthy male volunteers provided pharmacokinetic parameters for cimetidine and cimetidine S-oxide that were typical of those for previously reported studies. The urinary cimetidine S-oxide was isolated and the stereopreference was for formation of (-)-cimetidine S-oxide in a ratio of (+)25.5%:(-)74.5%. In good agreement with the enantiomeric enrichment values observed for the adult human urinary metabolite, the relative configuration of cimetidine S-oxide formed in adult human liver microsomes was (+)-15.8%:(-)-84.2%. Because the enantioselectivity and profile of immunoreactivity and the effect of metabolism inhibitors on cimetidine S-oxygenation in adult human liver microsomes are consistent with a role of FMO (form II) in cimetidine S-oxide formation and because the enantioselectivity of cimetidine S-oxide observed in adult humans is similar, we conclude that in vivo, cimetidine is S-oxygenated principally by FMO (form II).

Adult↗