Received for publication June 7, 2005.
Revised October 19, 2005.
Accepted for publication October 19, 2005.

In Vitro Metabolism of Naphthalene by Human Liver Microsomal Cytochrome P450 Enzymes

Abstract

The polycyclic aromatic hydrocarbon naphthalene is an environmental pollutant, a component of jet fuel and, since 2000, has been reclassified as a potential human carcinogen. Few studies of the in vitro human metabolism of naphthalene are available and these focus on lung metabolism. The current studies were performed to characterize naphthalene metabolism by human cytochromes P450 (CYP). Naphthalene metabolites from pooled human liver microsomes (pHLM) were trans-1,2-dihydro-1,2-naphthalenediol (dihydrodiol), 1-naphthol, and 2-naphthol. Metabolite production generated Kms of 23, 40, and 116 µM and Vmaxs of 2860, 268, and 22 pmol/mg protein/min., respectively. CYP isoform screening of naphthalene metabolism identified CYP1A2 as the most efficient isoform for producing dihydrodiol and 1-naphthol and CYP3A4 as the most effective for 2-naphthol production. Metabolism of the primary metabolites of naphthalene was also studied to identify secondary metabolites. While 2-naphthol was readily metabolized by pHLM to produce 2,6- and 1,7-dihydroxynaphthalene, dihydrodiol and 1-naphthol were inefficient substrates for pHLM. A series of human CYP isoforms was used to further explore the metabolism of dihydrodiol and 1-naphthol. 1,4-naphthoquinone and four minor unknown metabolites from 1-naphthol were observed, and 1A2 and 2D6*1 were identified as the most active isoforms for the production of 1,4-naphthoquinone. Dihydrodiol was metabolized by CYP isoforms to three minor unidentified metabolites with CYP3A4 and CYP2A6 having the greatest activity toward this substrate. The metabolism of dihydrodiol by CYP isoforms was lower than that of 1-naphthol. These studies identify primary and secondary metabolites of naphthalene produced by pHLM and CYP isoforms. The dihydrodiol is a potential biomarker of human exposure to naphthalene.

Key words: CYP1A, CYP2B, CYP3A, cytochrome P450, cytochrome P450 isoforms, environmental toxicology, human CYP enzymes, liver microsomes, monooxygenases, toxicology