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ToxSci Advance Access published online on June 21, 2006
Toxicological Sciences, doi:10.1093/toxsci/kfl044
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© The Author 2006. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received April 24, 2006
Accepted June 12, 2006

Biotransformation and Toxicokinetics

Hepatic Biotransformation and Metabolite Profile During a Two-Week Depuration Period in Atlantic salmon Fed Graded Levels of the Synthetic Antioxidant, Ethoxyquin

Victoria J. Berdikova Bohne 1, Kristin Hamre 1, and Augustine Arukwe 2 *

1 National Institute for Nutrition and Seafood Research (NIFES), P.O. box 2029, Nordnes, N-5817 Bergen, Norway
2 Department of Biology, Norwegian University of Science and Technology (NTNU), Høgskoleringen 5, N-7491 Trondheim, Norway

* To whom correspondence should be addressed.
Augustine Arukwe, E-mail: arukwe{at}bio.ntnu.no


  Abstract

The synthetic antioxidant ethoxyquin (EQ) is increasingly used in animal feeds and has been candidate for carcinogenicity testing. EQ has the potential for toxicological and adverse health effects for both fish and fish consumers through "carry-over" processes. The toxicological aspects of EQ have not been systematically investigated. The present study was performed to investigate the hepatic metabolism, metabolite characterization and toxicological aspects of EQ in salmon during a 2-weeks depuration after a 12-week feeding period with 18 mg (low), 107 mg (medium) and 1800 mg/kg feed (high). The alteration in gene expressions and catalytic activities of hepatic biotransformation enzymes were studied using real-time PCR with specific primer pairs and by kinetics of two identified hepatic metabolites. Analysis of EQ metabolism was performed using HPLC method and showed the detection of 4 compounds of which two were quantified; parent EQ and EQ dimer (EQDM). Two metabolites were identified as de-ethylated EQ (DEQ) and quinone imine (QI), but these were not quantified. The concentration of the quantified EQ related compounds in the liver at day 0 showed a positive linear relationship with measured dietary EQ (R2=0.86 and 0.92 for parent EQ and EQDM, respectively). While the low EQ feeding group showed a time-specific induction of AhR mRNA expression, the medium dose group showed decreased AhR mRNA at depuration day 7. Expression of CYP1A1 was decreased during the depuration period. Consumption of dietary EQ produced the expression of CYP3A, GST and UDPGT mRNA during the depuration period. A similar pattern of effect was observed for both CYP3A and phase-II genes and supports our previous postulation of common regulation of these enzymes by the same inducer, namely EQ metabolites. The induction of CYP3A, UDPGT and GST gene expressions at day 7 was in accordance with the low concentration of DEQ. The low concentration of putative DEQ may induce the CYP3A with subsequent increase in the biotransformation of EQ into DEQ. The increase in UDPGT may seem to be a synchronising mechanism required for the excretion of DEQ. The biotransformation of dietary EQ is proven by simultaneous induction of both phase-I and II detoxification system in the liver of Atlantic salmon. Therefore, the apparent low concentration of putative DEQ may account for the induced phase-I and -II detoxifying enzymes at least during depuration. This speculated hypothesis is currently a subject for systematic investigation in our laboratory using in vitro and genomic approaches.

Keywords: Synthetic antioxidant; Ethoxyquin; Salmon; Biotransformation; Metabolites.
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