2′-Hydroxylation of nicotine by cytochrome P450 2A6 and human liver microsomes: Formation of a lung carcinogen precursor

Abstract

Smokers or people undergoing nicotine replacement therapy excrete approximately 10% of the nicotine dose as 4-oxo-4-(3-pyridyl)butanoic acid (keto acid) and 4-hydroxy-4-(3-pyridyl)butanoic acid (hydroxy acid). Previously, these acids were thought to arise by secondary metabolism of the major nicotine metabolite cotinine, but our data did not support this mechanism. Therefore, we hypothesized that nicotine is metabolized by 2 -hydroxylation, which would ultimately yield keto acid and hydroxy acid as urinary metabolites. This pathway had not been established previously in mammalian systems and is potentially significant because the product of nicotine 2 -hydroxylation, 4-(methylamino)-1-(3-pyridyl)-1-butanone (aminoketone), can be converted to the potent tobacco-specific lung carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. Incubation of nicotine with cytochrome P450 2A6 and cofactors did indeed produce aminoketone, which was identified as its N-benzoyl derivative by GC-MS. The rate was 11% of that of cotinine production. Incubation of human liver microsomes with nicotine gave keto acid by using aminoketone as an intermediate; keto acid was not formed from cotinine. In 10 human liver samples, rates of formation of keto acid were 5.7% of those of cotinine and production of these metabolites correlated. These results provide definitive evidence for mammalian 2 -hydroxylation of nicotine and elucidate a pathway by which endogenous formation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone could occur in humans.

Nicotine and 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone Binding and Access Channel in Human Cytochrome P450 2A6 and 2A13 Enzymes

“…The dominant oxidation reaction for both enzymes is 5Ј-hydroxylation to form 5Ј-hydroxynicotine (31), which is in equilibrium with the nicotine ⌬ 1Ј(5Ј) -iminium ion and then further oxidized to cotinine and eliminated in urine (32). However, both enzymes can also oxidize nicotine on the methyl to ultimately generate nornicotine (31,33), and at least CYP2A6 is also able to oxidize the 2Ј position to generate the aminoketone precursor of NNK (34). In the CYP2A13 complex with nicotine (Fig.…”

OPRM1 genetic polymorphisms are associated with the plasma nicotine metabolite cotinine concentration in methadone maintenance patients: a cross sectional study

“…To avoid recall bias, it is important to find a reliable biomarker for nicotine exposure. 17 Cotinine is a metabolite derived from the 5 0 -hydroxylation metabolic pathway of nicotine 18 and has been reported by the Society for Research on Nicotine and Tobacco Subcommittee on Biochemical Verification as a reliable biomarker in estimating an average nicotine exposure over an extended period of time. 17 Plasma cotinine concentration offers a window of 5-7 days for detection of nicotine exposure.…”

4-(Methylnitrosamino)-1-(3-Pyridyl)-1-Butanone from Cigarette Smoke Stimulates Colon Cancer Growth via β-Adrenoceptors

“…NNK is formed during the processing and curing of tobacco plants (9). It is also believed that NNK could be converted endogenously from nicotine, a major alkaloid in cigarette smoke (10). This tobaccospecific nitrosamine has a high affinity for h-adrenergic receptors because of its structural resemblance to classical h-adrenergic agonists (11).…”