A
complex mixture of
polycyclic aromatic hydrocarbons (PAH) extracted from
coal tar, the Standard Reference Material (
SRM) 1597, was recently shown to decrease the levels of
DNA binding of the 2 strong
carcinogens benzo[a]pyrene (BP) and
dibenzo[a,l]pyrene (DBP) in the
human mammary carcinoma-derived cell line MCF-7 (Mahadevan et al., Chem Res Toxicol 2005;18:224-231). The present study was designed to further elucidate the biochemical mechanisms involved in this inhibition process. We examined the effects of
SRM 1597 on the metabolic activation of BP and DBP toward
DNA-binding derivatives in Chinese hamster cells expressing either human
cytochrome P450 (
CYP) 1A1 or CYP1B1.
SRM 1597 inhibited BP-
DNA adduct formation through the entire exposure time in cells expressing human
CYP1A1, while it significantly inhibited adduct formation only up to 48 hr when co-treated with DBP. Conversely, human CYP1B1-expressing cells were unable to catalyze
PAH-DNA adduct formation on treatment with
SRM 1597 alone, and on co-treatment with BP or DBP. The data obtained from biochemical experiments revealed that
SRM 1597 competitively inhibited the activity of both human
enzymes as analyzed by
7-ethoxyresorufin O-deethylation assays. While the Michaelis-Menten constant (K(M)) was <0.4 microM in the absence of
SRM 1597, this value increased up to 1.12 (
CYP1A1) or 4.45 microM (CYP1B1) in the presence of 0.1 microg/ml
SRM 1597. Hence the inhibitory effects of the
complex mixture on human CYP1B1 were much stronger when compared to human
CYP1A1. Taken together, the decreases in
PAH-DNA adduct formation on co-treatment with
SRM 1597 revealed inhibitory effects on the CYP
enzymes that convert carcinogenic PAH into
DNA-binding metabolites. The implications for the tumorigenicity of complex environmental PAH mixtures are discussed.