97 Fish Physiology and Biochemistry 18: 97-106, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. Hepatic P450 monooxygenase response in rainbow trout (Oncorhynchus mykiss (Walbaum)) administered aquaculture antibiotics K.A. Moutou 1 *, M.D. Burke 2 and D.F. Houlihan 1 1 Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 3TZ, Scotland; 2 Department of Pharmaceutical Sciences, De Montfort University, The Gateway, Leicester LE7 9BH, U.K. Accepted: May 3, 1997 Keywords: rainbow trout, liver, cytochrome P450 monooxygenases, oxolinic acid, flumequine Abstract The effects of the 4-quinolones, oxolinic acid and flumequine on hepatic microsomal cytochrome P450 monooxygenases in rainbow trout were examined during this study. Following antibiotic administration in the diet for 10 days at a representative commercial medicated feed concentration, fish were killed for assay at various periods up to 12 days following a return to normal diet. Ethoxyresorufin O-deethylase (EROD) and benzyloxyresorufin O-dealkylase (BROD) activities were significantly elevated as a result of antibiotic treatment. The effects of oxolinic acid were delayed and longer-lasting compared with flumequine. The effects of flumequine were detectable 4 days earlier than those of oxolinic acid and lasted for less than ten days after treatment cessation. In contrast, oxolinic acid effects were apparent even on day 12 of the recovery period. The results of O-dealkylation, isoform-selective inhibition, and immunoblotting showed that the effects of both oxolinic acid and flumequine were related to P4501A subfamily. P450-binding spectra and experiments in vitro suggested that both antibiotics are weak dose- independent inhibitors of P450 activity, with flumequine exhibiting slightly higher binding affinity and inhibitory activity. *Correspondence to: Centre of Marine Sciences, University of Algarve, 8000 Faro, Portugal. Introduction Treatment of bacterial diseases via medicated di- ets comprises an intergral part of the fish farming practices. Oxolinic acid and flumequine are mem- bers of the 4-quinolones, a family of systemic synthetic antibiotics which were introduced in aquaculture chemotherapy in the early 1970s, and have been routinely and successfully used for the control of furunculosis and red mouth disease in trout (Alderman 1988). Chemotherapeutic agents are either activated or inactivated by the cytochrome P450 at the first step of their biotransformation; the insertion of one atom of molecular oxygen to lipophylic com- pounds by the cytochrome P450 is the first of a series of metabolic reactions which lead either to activation of the parent compound or to increased polarity and water solubility, drug elimination and/or excretion from the body (Ioannides and Parke 1990; Black 1993; Gibson and Skett 1994). Fish, like mammals, have multiple forms of cytochrome P450 as confirmed by the isolation of purified P450s from scup (Stenotomus chrysops), cod (Gadus morhua) and rainbow trout (Onchor- hynchus mykiss) (Williams and Buhler 1982, 1984; Klotz et al. 1983, 1986; Goksoyr 1985). Rainbow trout liver, in particular, has been shown to contain at least five different constitutive iso- enzymes (designated LMC1 to LMC5) as well as a β-naphthoflavone-inducible member of the P4501A subfamily (Williams and Buhler 1983, 1984; Heilmann et al. 1988; Miranda et al. 1989, 1990).