Characterization of aldrin epoxidation in flathead mullet (Mugil cephalus) liver microsomes: Involvement of CYP3A

Toxic organochlorine pesticides, aldrin and its metabolite dieldrin, have been reported to contaminate the aquatic environment highly. In the present study, epoxidation reaction of aldrin to dieldrin was studied in mullet liver microsomes and contribution of cytochrome P450 isozyme(s) was determined by using specific cytochrome P450 inhibitors and substrate. Flathead mullet samples (Mugil cephalus) were caught from the West Black Sea Region of Turkey. Fish liver microsomes were prepared by differential centrifugation. Epoxidation of aldrin to dieldrin was determined by measuring the amount of dieldrin produced using gas chromatography and electron capture detector. Aldrin epoxidation was linear with time up to 60 min and with protein concentration up to 10 mg/mL. Maximal fish liver aldrin epoxidase activity was observed at pH 7.6. Aldrin epoxidase exhibited monophasic kinetics with apparent Km value of 140 μM for aldrin. The enzyme activity was inhibited approximately by 9% and 16% by methanol and DMSO, respectively. On the other hand, aldrin epoxidation was enhanced by about 113% with 2% ethanol in the incubation medium. Ketoconazole (CYP3A inhibitor), potentially inhibited the metabolism of aldrin, tolbutamide (CYP2C substrate), and alpha-naphthoflavone (CYP1A inhibitor) did not show inhibition. The results of this study strongly suggest that CYP3A is the cytochrome P450 isozyme involved in aldrin epoxidation in mullet liver microsomes whereas CYP2C and CYP1A are not involved