Abstract
Nicotine biotransformation affects the smoking habits of addicted individuals and therefore their health risk. Using an improved analytical method, we have discovered that the human UDP-glucuronosyltransferase (UGT) 2B10, a liver enzyme previously unknown to conjugate nicotine or exhibit considerable activity toward any compound, plays a major role in nicotine inactivation by direct conjugation with glucuronic acid at the aromatic nitrogen atom. The Km value of recombinant UGT2B10 for nicotine (0.29 mM) was similar to that determined for human liver microsomes (0.33 mM), whereas the Km value of UGT1A4 for nicotine was almost 10-fold greater (2.4 mM). UGT2B10 was also more active than UGT1A4 in N-glucuronidation of cotinine (oxidative nicotine metabolite), whereas UGT2B7 exhibited only low nicotine glucuronidation activity and was essentially inactive toward cotinine. UGT1A9 did not glucuronidate nicotine or cotinine. Quantitative reverse transcription-polymerase chain reaction showed that UGT2B10 mRNA was exclusively expressed in human liver, whereas UGTs 1A4 and 2B7 were expressed at comparable, although somewhat lower, levels in liver and several other extrahepatic tissues, including kidney and intestine. These findings for UGT2B10 (but not for UGT1A4 and UGT2B7) were mirrored by human tissue activities because nicotine and cotinine glucuronidation rates in intestine microsomes were less than 0.1% that of human liver microsomes. These novel findings solve two seemingly separate questions: which UGT is primarily responsible for nicotine glucuronidation in human liver, and what conjugation reactions are catalyzed by UGT2B10.
Footnotes
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This research was supported by the Academy of Finland (projects 207535 and 210933) and the Sigrid Juselius Foundation. M.H.C. is supported by grants GM61834 and GM74369 from the National Institute of General Medical Sciences, National Institutes of Health, Bethesda, Maryland. L.M.H. is the recipient of a Ruth Kirschstein postdoctoral fellowship training award (F32-AA15647) from the National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, Maryland.
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Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
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doi:10.1124/mol.107.037093.
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ABBREVIATIONS: UGT, UDP-glucuronosyltransferase; HIM, human intestine microsomes; HLM, human liver microsomes; LC-MS, liquid chromatography-mass spectrometry; UDPGA, UDP-glucuronic acid; qRT-PCR, quantitative reverse transcription-polymerase chain reaction; PCR, polymerase chain reaction.
- Received April 13, 2007.
- Accepted June 18, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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