CYP1A1 genotype-selective inhibition of benzo[a]pyrene activation by quercetin

Epidemiological studies suggest that food rich in quercetin and naringin may protect against certain types of lung cancer, and that genotype dependent inhibition of cytochrome P450 1A1 (CYP1A1)-mediated bioactivation of procarcinogens could be the underlying mechanism. We studied the inhibitory effects of quercetin and naringin on the terminal bioactivation step of benzo[a]pyrene (B[a]P), a member of the major class of lung carcinogens. This reaction (epoxidation of (+/-)-trans-7,8-dihydro-7,8-dihydroxy-B[a]P to the ultimate carcinogenic product, (+/-)-B[a]P-r-7,t-8-dihydrodiol-t-9,10-epoxide (diolepoxide 2)) was examined using three of the most common allelic variants of human CYP1A1, namely wild-type CYP1A1.1, CYP1A1.2, and CYP1A1.4. Quercetin potently inhibited diolepoxide 2 formation by all CYP1A1 types with IC(50) values between 1.6 and 7.0 microM. The differences between the wild-type enzyme and the variants were statistically highly significant (P < 0.01). Enzyme kinetics revealed quercetin as a mixed-type inhibitor of CYP1A1.1, CYP1A1.2, and CYP1A1.4 with K(i) values of 2.0, 6.4, and 9.3 microM, respectively. Naringin inhibited diolepoxide 2 formation only slightly. Our data support the hypothesis that quercetin may have a stronger chemopreventive effect in individuals carrying wild-type compared with variant CYP1A1 genes. Future studies should consider the influence of P450 polymorphisms on both procarcinogen activation and its inhibition to facilitate the development of genotype-specific chemoprevention regimes.