The biotransformation of the dietary carcinogen aflatoxin B1 (AFB1) was examined in hepatic microsomal and cytosolic fractions from channel catfish, an aquatic species shown to be refractory to AFB1 toxicity and reported to be resistant to AFB1 hepatocarcinogenesis. Catfish liver microsomes catalyzed the in vitro oxidation of AFB1 to the reactive AFB1-8,9-epoxide (AFBO) at high substrate concentrations (128 microM AFB1) but not at low substrate concentrations (16 microM AFB1) which were more representative of environmental exposure. A similar trend was observed for the production of the hydroxylated metabolite aflatoxin M1 (AFM1). In contrast, hepatic microsomes prepared from rainbow trout, a species sensitive to AFB1 toxicity and hepatocarcinogenesis, activated AFB1 at 16 and 128 microM AFB1, with the rate of AFB1 epoxidation by trout microsomes exceeding that of catfish by more than sixfold. Treatment of channel catfish with 5,6-benzoflavone (beta NF, 20 mg/kg) resulted in a threefold increase in AFM1 formation but did not affect AFBO formation. AFB1 was rapidly reduced to aflatoxicol (AFL), a putative detoxification product of AFB1, at both low and high substrate concentrations. The rate of AFL production by channel catfish hepatic cytosol was 40- and 65-fold greater than observed for rainbow trout at 16 and 128 microM AFB1, respectively. Western blotting of catfish cytosols revealed the presence of a catfish cytosolic protein of approximately 25 kDa that displayed immunological cross-reactivity to rat GST Ya, but not to other rat or mouse alpha class GSTs with high AFBO-conjugating activity. Furthermore, catfish cytosolic GSTs did not catalyze the conjugation of AFBO with GSH. The results of these studies indicate that AFB1 is poorly oxidized by channel catfish microsomes, and suggest that the lack of microsomal AFB1 activation together with the rapid conversion of AFB1 to AFL contributes to the apparent resistance of channel catfish to AFB1 toxicity and hepatocarcinogenesis.