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The study was conducted to investigate the responses of the hepatic microsomal cytochrome P450 monooxygenase system in the rock bream Oplegnathus fasciatus after chronic exposure to 0, 1, 2, 4, and 8 μg/L tributyltin (TBT) concentrations for 4 weeks. Hepatic cytochrome 450 content and ethoxyresorufin O -deethylation (EROD) activity were found to significantly increase in fish treated with the higher concentration of TBT (≥4 μg/L); however, no significant changes were observed in penthoxyresorufin O -deethylation (PROD) activity in all treated groups compared to the control group. These findings suggest that exposure to a low TBT concentration (≥4 μg/L) has the potential to induce cytochrome 450 content and EROD enzyme activity in hepatic tissue in the rock bream. Key words: Oplegnathus fasciatus , Tributyltin, Cytochrome 450, EROD, PROD Introduction Many aquatic ecosystems are faced with spatially or tem-porally alarmingly high levels of xenobiotic chemicals (Brack et al., 2002; Diez et al., 2002). Tributyltin (TBT) is a typi-cal synthetic pollutant, which was selected for investigation in this study. TBT is an organotin compound used primarily as a biocide in antifouling paints for ships, boats, and fishing nets. The use of TBT as an antifouling agent in paint has been banned for small boats and fishing nets in most countries as it has been shown to be toxic to aquatic life and is an endo-crine disrupting chemical that generates severe reproductive effects in aquatic animals (Horiguchi et al., 1994). Reproduc-tive impairment in gastropods has been reported at low TBT concentrations (Gibbs et al., 1988). TBT has been shown to induce imposex and intersex in brosobranchs (Oehlmann et al., 1998), and impaired reproductive function in fish has also been described (Mc Allister and Kime, 2003).Some studies have reported toxic effects of TBT such as morphological and functional alterations of teleost gills in aquatic media (Byrne et al., 1989; Schwaiger et al., 1992; Tsu -da et al., 1992; Wang and Huang, 1998). TBT has also been shown to markedly inhibit fish hepatic CYP and in particu-lar the isozyme cytochrome P4501A1 (CYP1A), both in vivo and vitro (Fent and Bucheli, 1994; Morcillo and Porte, 1997). The CYP1A isozyme plays a key role in the biotransformation of xenobiotic compounds, leading to either their detoxifica-tion or bioactivation, while other CYP forms are important

Reponses of the Hepatic Microsomal Cytochrome P450 Monooxygenase System in Rock Bream Oplegnathus fasciatus Exposed to Tributyltin (TBT)