Hepatic metabolism, phase I and II biotransformation enzymes in Atlantic salmon (Salmo Salar, L) during a 12 week feeding period with graded levels of the synthetic antioxidant, ethoxyquin.

The synthetic antioxidant ethoxyquin (EQ) is a widely used additive in animal feeds, including farmed fish feed. The use of EQ as food additive is prohibited and it is also undesirable in farmed meat and fish products. The possible negative aspects of EQ in fish feeds, such as modulation of hepatic detoxifying enzymes and possible effects through "carry-over" to edible parts of fish are not known. In addition, the subsequent consequences for human consumers have not been previously studied. In the present work, the alteration in gene and protein expression patterns, and catalytic activities of phase I and II hepatic biotransformation enzymes due to prolonged exposure to graded levels of dietary EQ in the range of 11-1800 mg EQ/kg feed were studied. The kinetics of parent EQ and its major metabolite, ethoxyquin dimer (EQDM) was also studied. In general two weeks seem to be the critical point in the entire toxicological response of salmon to dietary consumed EQ. Biotransformation of EQ to EQDM is shown to be a rapid process. However, the decrease in biotransformation rate results in the accumulation of EQ metabolites, high concentration of which was postulated to alter translation and post-translational modification of CYP3A, GST and UDPGT at feeding day 14 and 42, with subsequent decreases in the biotransformation of consumed EQ. Decrease in the biotransformation of consumed EQ produced the retention of un-metabolized EQ rather than metabolites in salmon liver. This may be considered as undesirable effect, since it could lead to the transport and accumulation in other organs and edible tissues. It may also cause a new wave of biotransformation with formation of metabolites inhibiting detoxifying enzymes. In general, these processes may prolong the excretion of dietary EQ from the fish body and produce EQ-derived residues in the ready-to-consume salmon or fish products. These EQ residues may have higher toxicological effects for human consumers than the parent compound and therefore need to be studied in more detail.