In vitro-in vivo correlations of human ( s)-nicotine metabolism

The profile of ( S)-nicotine metabolism in human liver microsomes was examined at concentrations approaching in vivo conditions (10 μM). At such concentrations, no ( S)-nicotine N-1′-oxygenation was seen, and thus C-oxidation to the ( S)-nicotine Δ 1′,5′-iminium ion was the sole product observed in...

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Bibliographic Details
Published in:Biochemical pharmacology 1995-08, Vol.50 (4), p.565-570
Main Authors: Berkman, Clifford E., Park, Sang B., Wrighton, Steven A., Cashman, John R.
Format: Article
Language:English
Subjects:
( S)-cotinine
( S)-nicotine N-1′-oxide
Adult
Aldehyde Oxidase
Aldehyde Oxidoreductases - metabolism
Barbiturates - administration & dosage
Biological and medical sciences
Biotransformation
Cotinine - metabolism
Cyclic N-Oxides - metabolism
CYP 2A6
Female
human ( S)-nicotine metabolism
human liver flavin-containing monooxygenase
Humans
Kinetics
Male
Medical sciences
Microsomes, Liver - enzymology
Microsomes, Liver - metabolism
Middle Aged
Nicotine - analogs & derivatives
Nicotine - metabolism
Nicotine - urine
Stereoisomerism
Tobacco, tobacco smoking
Toxicology
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Summary:The profile of ( S)-nicotine metabolism in human liver microsomes was examined at concentrations approaching in vivo conditions (10 μM). At such concentrations, no ( S)-nicotine N-1′-oxygenation was seen, and thus C-oxidation to the ( S)-nicotine Δ 1′,5′-iminium ion was the sole product observed in the metabolic profile in the presence of the human liver microsomes. For simplicity of analysis, the ( S)-nicotine Δ 1′,5′-iminium ion formed was converted to ( S)-cotinine in the presence of exogenously added aldehyde oxidase. To explain the lack of ( S)-nicotine N-1′-oxygenation at low ( S)-nicotine concentrations, inhibition of flavin-containing monooxygenase (FMO) activity by ( S)-cotinine was examined. Although ( S)-cotinine was observed to inhibit pig FMO1 ( K i = 675 μM), partially purified cDNA-expressed adult human liver FMO3 was not inhibited by ( S)-cotinine. We therefore concluded that the kinetic properties of the nicotine N′- and C-oxidases were responsible for the metabolic product profile observed. Kinetic constants were determined for individual human liver microsomal preparations from low (10 μM) and high (500 μM) ( S)-nicotine concentrations by monitoring ( S)-cotinine formation with HPLC. The mean K m app and V max for formation of ( S)-cotinine by the microsomes examined were 39.6 μM and 444.3 pmol· min −1 · (mg protein) −1, respectively. The formation of ( S)-cotinine was strongly dependent on the previous drug administration history of each subject, and among the highest rates for ( S)-cotinine formation were those of the barbiturate-pretreated subjects. The rate of ( S)-cotinine formation at low (10 μM) concentration correlated well with immunoreactivity for cytochrome P450 2A6 ( r = 0.89). In vitro-in vivo correlation of the results suggests that the low amount of ( S)-nicotine N-1′-oxygenation and the large amount of ( S)-cotinine formed in human smokers (i.e. 4 and 30% of a typical dose, respectively) are determined primarily by the kinetic properties of the human monooxygenase enzyme systems. However, additional non-hepatic monooxygenase(s) contributes to ( S)-nicotine metabolism.
ISSN:0006-2952
1873-2968
DOI:10.1016/0006-2952(95)00168-Y