RT Journal Article
SR Electronic
T1 Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 295
OP 303
DO 10.1124/jpet.105.091306
VO 316
IS 1
A1 Linda B. von Weymarn
A1 Kathryn M. Brown
A1 Sharon E. Murphy
YR 2006
UL http://jpet.aspetjournals.org/content/316/1/295.abstract
AB Nicotine is the major addictive agent in tobacco. The primary catalyst of nicotine metabolism in humans is CYP2A6. However, the closely related enzyme CYP2A13 is a somewhat better catalyst. CYP2A13 is an extrahepatic enzyme that is an excellent catalyst of the metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK). Here we report that both CYP2A6 and CYP2A13 were inactivated during nicotine metabolism. Inactivation of both enzymes was dependent on NADPH and increased with time and concentration. Alternate substrates for CYP2A6 and CYP2A13 protected these enzymes from inactivation. Inactivation of CYP2A13 was irreversible upon extensive dialysis and seems to be mechanism-based. The KI of CYP2A13 inactivation by nicotine was 17 μM, the rate of inactivation, kinact, was 0.1 min-1, and the t1/2 was 7 min. However, the loss in enzyme activity occurred after nicotine metabolism was complete, suggesting that a secondary or possible tertiary metabolite of nicotine may be responsible. [5-3H]Nicotine metabolism by CYP2A13 was monitored by radioflow high-pressure liquid chromatography during the course of enzyme inactivation; the major product was the Δ1′(5′)iminium ion. However, cotinine was a significant metabolite even at short reaction times. The metabolism of the nicotine Δ1′(5′)iminium ion to cotinine did not require the addition of aldehyde oxidase. CYP2A13 catalyzed this reaction as well as further metabolism of cotinine to 5′-hydroxycotinine, trans-3′-hydroxycotinine, and N-(hydroxymethyl)-norcotinine as enzyme inactivation occurred. Studies are on-going to identify the metabolite responsible for nicotine-mediated inactivation of CYP2A13. The American Society for Pharmacology and Experimental Therapeutics