Differential metabolism of benzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol by human CYP1A1 variants

Cytochrome P450 1A1 (CYP1A1) plays a key role in the metabolism of carcinog ens, such as benzo[a]pyrene (B[a]P) and metabolites to ultimate carcinogens . Three human allelic variants, namely wild-type (CYP1A1.1), CYP1A1.2 (I452 V) and CYP1A1.4 (T461N), were coexpressed by coinfection of baculovirus-inf ected insect cells with human NADPH-P450 reductase, These recombinant enzym es (in microsomal membranes) were used to analyze whether CYP1A1 polymorphi sms affect catalytic activities towards B[a]P and B[a]P-7,8-dihydrodiol. Th e complete spectrum of phase I metabolites, including the tetrahydrotetrols resulting from hydrolysis of the ultimate carcinogen, B[a]P-7,8-dihydrodio l-9,10-epoxide, was examined by HPLC. Wild-type enzyme showed the highest t otal metabolism of B[a]P, CYP1A1.2 was similar to 50%, and CYP1A1.4 similar to 70%, K-m values for all metabolites with CYP1A1.2 were generally signif icantly lower than with wild-type enzyme (e.g. B[a]P-7,8-diol formation: 13 .8 muM for wild-type, 3.5 muM for CYP1A1.2 and 7.7 muM for CYP1A1.4), Addit ion of epoxide hydrolase markedly increases the relative diol-to-phenol act ivities by all three variants. However, CYP1A1.4 exhibits the greatest effi ciency to produce diol species. Each variant produced the diol epoxides fro m B[a]P-7,8-dihydrodiol. CYP1A1,1 exhibited with 10.4 pmol/min/pmol CYP1A1 the greatest total rate for 7,8-diol metabolites follow-ed by CYPIA1.2 (7.2 pmol/min/pmol CYP1A1) and CYP1A1.4 (5.5 pmol/min/pmol CYP1A1), All enzyme variants produced about three times more diol epoxide 2-derived metabolites than diol epoxide 1-derived ones, whereby both rare allelic variants exhib ited statistically significantly increased formation of diol epoxide 2, Thi s study showed that the three CYP1A1 variants had different enzyme kinetics properties to produce both the diol metabolites from B[a]P and the ultimat e mutagenic species diol epoxide 2 from B[a]P-7,8-dihydrodiol, which must b e considered in the evaluation of individual susceptibility to cancer.