ROLE OF HUMAN CYTOCHROME P4502A6 IN C-OXIDATION OF NICOTINE

Nicotine is primarily metabolized to cotinine in humans. In this study , human cytochrome P450 (CYP) isoform involved in cotinine formation w as identified. The formation of cotinine in 10 human liver microsomes was determined with a 50 mu M nicotine concentration and with a cytoso l preparation as a source of aldehyde oxidase. Cotinine formation in h uman liver microsomes significantly correlated with immunochemically d etermined CYP2A6 levels (r=0.663, p<0.05), coumarin 7-hydroxylase acti vities (r=0.831, p<0.01), and cotinine 3'-hydroxylase activities (r=0. 735, p<0.01) that are responsible for CYP2A6. In inhibition studies, c otinine formation in human liver microsomes was inhibited by coumarin and rabbit anti-rat CYP2A1 antibody specifically. When the capability of microsomes of B-lymphoblastoid cells expressing human CYPs to perfo rm biotransformation of nicotine to cotinine was determined, cDNA-expr essed CYP2A6 exhibited the highest cotinine formation. The K-Mapp valu es from microsome expressing CYP2A6 cDNA were similar to the value obt ained from human liver microsomes. The large interindividual variabili ties in cotinine formation and immunochemically determined CYP2A6 leve ls were observed in human liver microsomes, suggesting genetic polymor phism of CYP2A6. Nicotine is a new in vivo probe for phenotyping of CY P2A6 in humans.