Identification of N-(hydroxymethyl) norcotinine as a major product of cytochrome P450 2A6, but not cytochrome P450 2A13-catalyzed cotinine metabolism

Chem Res Toxicol. 2005 Dec;18(12):1792-8. doi: 10.1021/tx0501381.


Cotinine formation is the major pathway of nicotine metabolism in smokers, and the primary pathway of cotinine metabolism is trans-3'-hydroxylation. trans-3'-Hydroxycotinine and its glucuronide conjugate account for up to 50% of the nicotine metabolites excreted by smokers. Minor metabolites of cotinine excreted by smokers include norcotinine and cotinine N-oxide, each of which account for <5% of the nicotine dose. It has been reported that P450 2A6 is the catalyst of cotinine metabolism. However, we report here that the major product of P450 2A6-catalyzed cotinine metabolism is N-(hydroxymethyl)norcotinine, a previously unknown human metabolite of cotinine. N-(Hydroxymethyl)norcotinine was chemically synthesized, and its stability under the conditions of the enzyme reactions was confirmed. The products of P450 2A6-catalyzed [5-3H]cotinine metabolism were quantified by radioflow HPLC. The identification of N-(hydroxymethyl)norcotinine as the major metabolite was based on HPLC analysis on three unique systems and coelution with N-(hydroxymethyl)norcotinine standard. 5'-Hydroxycotinine and trans-3'-hydroxycotinine were minor products of P450 2A6-catalyzed cotinine metabolism, accounting for 14 and 8% of the total cotinine metabolites, respectively. N-(Hydroxymethyl)norcotinine was a product of cotinine metabolism by the extrahepatic P450, 2A13, but it was a minor one. The major product of P450 2A13-catalyzed cotinine metabolism was 5'-hydroxycotinine, which was formed at twice the rate of trans-3'-hydroxycotinine. The identification of all cotinine metabolites formed by both enzymes was confirmed by LC/MS/MS analysis. Kinetic parameters for cotinine metabolism were determined for P450 2A6 and P450 2A13. This work has confirmed that the major metabolite of cotinine in smokers, trans-3'-hydroxycotinine, is only a minor metabolite of P450 2A6-catalyzed cotinine metabolism.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Chromatography, High Pressure Liquid
  • Cotinine / analogs & derivatives*
  • Cotinine / chemistry
  • Cotinine / metabolism*
  • Cytochrome P-450 Enzyme System / chemistry
  • Cytochrome P-450 Enzyme System / metabolism*
  • Escherichia coli / enzymology
  • Humans
  • Microsomes, Liver / chemistry
  • Microsomes, Liver / metabolism


  • 5'-hydroxycotinine-N-oxide
  • N-hydroxymethylnorcotinine
  • Cytochrome P-450 Enzyme System
  • Cotinine