Cho, TM; Rose, RL; Hodgson, E
The polycyclic aromatic hydrocarbon naphthalene is an environmental pollutant, a component of jet fuel and since 2000 reclassified as a potential human carcinogen. Few studies of in vitro human metabolism of naphthalene are available, and these studies are mostly focused on lung metabolism. Therefore, the current studies were performed to characterize the metabolism of naphthalene mediated by human cytochromes P450 (CYP) using pooled human liver microsomes (pHLM) and CYP isoforms. The major metabolites from naphthalene mediated by pHLM were trans- 1, 2-dihydro-1, 2-naphthalenediol (dihydrodiol), 1-naphthol (1-ol), and 2-naphthol (2-ol). Their kinetic parameters showed Km values of 23, 40, and 115 ?M, and Vmax values of 2860, 268, and 22 pmol/mg protein/min., respectively. Through human CYP isoform screening of naphthalene metabolism, CYP1A2 was identified as the most efficient isoform among those tested for producing dihydrodiol and 1-ol while CYP3A4 was the most effective for 2-ol production. Only CYP1A2 generated 1, 4-naphthoquinone from naphthalene. Metabolism studies of primary metabolites of naphthalene were also performed in order to identify secondary metabolites. While 2-ol was readily metabolized by pHLM to produce 2, 6- and 1, 7-dihydroxynaphthalene, dihydrodiol and 1-ol were inefficient substrates for pHLM. To further explore the metabolism of dihydrodiol and 1-ol, a series of human CYP isoforms was applied. 1, 4-naphthoquinone and four unknown metabolites from 1-ol 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 unidentified metabolites with CYP2A6 having the greatest ability toward this substrate among those tested. The metabolism of dihydrodiol by CYP isoforms was lower than that of 1-ol. These studies identify both primary and secondary metabolites of naphthalene mediated by pHLM and CYP isoforms. The dihydrodiol appears to be a relatively stable biomarker of human exposure to naphthalene.