Functional characterization of 9 CYP2A13 allelic variants by assessment of nicotine C-oxidation and coumarin 7-hydroxylation.

Cytochrome P450 2A13 (CYP2A13) is responsible for the metabolism of chemical compounds such as nicotine, coumarin, and tobacco-specific nitrosamine. Several of these compounds have been recognized as procarcinogens activated by CYP2A13. We recently showed that CYP2A13*2 contributes to inter-individual variations observed in bladder cancer susceptibility because CYP2A13*2 might cause a decrease in enzymatic activity. Other CYP2A13 allelic variants may also affect cancer susceptibility. In this study, we performed an in vitro analysis of the wild-type enzyme (CYP2A13.1) and 8 CYP2A13 allelic variants, using nicotine and coumarin as representative CYP2A13 substrates. These CYP2A13 variant proteins were heterologously expressed in 293FT cells, and the kinetic parameters of nicotine C-oxidation and coumarin 7-hydroxylation were estimated. The quantities of CYP2A13 holoenzymes in microsomal fractions extracted from 293FT cells were determined by measuring reduced carbon monoxide-difference spectra. The kinetic parameters for CYP2A13.3, CYP2A13.4, and CYP2A13.10 could not be determined because of low metabolite concentrations. Five other CYP2A13 variants (CYP2A13.2, CYP2A13.5, CYP2A13.6, CYP2A13.8, and CYP2A13.9) showed markedly reduced enzymatic activity toward both substrates. These findings provide insights into the mechanism underlying inter-individual differences observed in genotoxicity and cancer susceptibility.