Tyndale, “Characterization and comparison of nicotine and cotinine metabolism in vitro and

DBA/2 and C57BL/6 are two commonly used mouse strains that differ in response to nicotine. Previous studies have shown that the nicotine-metabolizing enzyme CYP2A5 differs in cou-marin metabolism between these two strains, suggesting dif-ferences in nicotine metabolism. Nicotine was metabolized to cotinine in vitro by two enzymatic sites. The high-affinity sites exhibited similar parameters (Km, 10.7 4.8 versus 11.4 3.6 M; Vmax, 0.58 0.18 versus 0.50 0.07 nmol/min/mg for DBA/2 and C57BL/6, respectively). In vivo, the elimination half-lives of nicotine (1 mg/kg, s.c.) were also similar between DBA/2 and C57BL/6 mice (8.6 0.4 versus 9.2 1.6 min, respectively); however, cotinine levels were much higher in DBA/2 mice. The production and identity of the putative cotin-ine metabolite 3-hydroxycotinine in mice was confirmed by liquid chromatography/mass spectrometry/mass spectrome-try. The in vivo half-life of cotinine (1 mg/kg, s.c.) was signifi-cantly longer in the DBA/2 mice compared with the C57BL/6 mice (50.2 4.7 versus 37.5 9.6 min, respectively, p 0.05). The in vitro metabolism of cotinine to 3-hydroxycotinine wa