Species characteristics of hepatic biotransformation enzymes in two tropical freshwater teleosts, Tilapia (Oreochromis niloticus) and mudfish (Clarias anguillaris)

Tilapia (Oreochromis niloticus) and mudfish (Clarias anguillaris) were given a single intraperitoneal injection of beta-naphthoflavone (BNF, 50 mg/kg) and Clophen A50 (Clo A50, 20 mg/kg). Hepatic levels of CYP1A protein, 7-ethoxyresorufin O-deethylase (EROD), glutathione S transferase (GST) and UDP-glucuronasyl transferase (UDP-GT) activities were determined at days 3 and 10 after treatment. Highest levels of EROD activity (5-6-fold) and CYP1A protein (3-5-fold) were observed in both species at day 3 on treatment with BNF. The same treatment also increased GST and UDP-GT activities in mudfish at day 3, but only UDP-GT activity was induced in tilapia at day 10. In tilapia treated with Clo A50, EROD activity was highest at day 3, whereas CYP1A protein level, GST and UDP-GT activities were highest at day 10. In mudfish, Clo A50 appeared to inhibit EROD activity, but CYP1A protein level was increased at day 10, whereas GST and UDP-GT activities reached highest levels at day 3. The control EROD levels did not differ significantly between the species, hut control GST levels were three to five times higher in tilapia, whereas those of UDP-GT were six times higher in mudfish. Although there are many common features in the hepatic xenobiotic biotransformation systems of the two freshwater teleosts, a number of species characteristics exist; most importantly, the differences in response to polychlorinated biphenyls and the different temporal pattern of induction response. These factors should be taken into account when using these species in studies of xenobiotic biotransformation and biomarker responses in tropical freshwater fish.