Differential phase I and phase II enzyme activities in brown bullhead and channel catfish exposed to β-naphthoflavone

This research explores biochemical mechanisms underlying the differential susceptibility of two related species of Ictaluridae (channel catfish and brown bullhead) to polycyclic aromatic hydrocarbon (PAH)-mediated liver cancer. The hypothesis is that the observed difference in the relatively insensitive catfish versus more sensitive bullhead is due to differences in hepatic phase II or epoxide hydrolase (EH) activities which could enhance excretion of PAHs. Fish of each species were treated with either corn oil or β-naphthoflavone (10 mg/kg in corn oil) at both 3 and 2 days prior to first sampling. Microsomal and cytosolic liver preparations from four fish per treatment were prepared 2, 7, and 14 days after the final dosing. EROD activities were higher in catfish as previously reported. In contrast, 1-chloro-2,4-dinitrobenzene-GST activities were significantly higher in bullhead. GST was not induced in either species in dosed versus control animals. An EH assay using benzo[a]pyrene-7,8-epoxide was optimized with catfish microsomes. Activities were 5.2±0.57 nmol/mg/min and did not show time or dose-related differences. Bullhead appear to have higher constitutive EH activities compared to catfish. Current studies are ongoing with EH inhibitors to investigate this difference.