Genotoxicity testing with fish hepatic S9 for evaluation of complex mixtures in the aquatic environment: the use of channel catfish as a model

A Salmonella/fish S9 mutagenicity assay, a modification of the traditional rodent Salmonella/microsome mutagenicity test of Ames, was evaluated and a working test protocol was established with channel catfish (Ictalurus punctatus) as a model system for monitoring genotoxins in complex mixtures extracted with organic solvents from freshwater sediments. Post-mitochondrial supernatant fractions (S9) from catfish mediated the biotransformation of the arylamines 2-aminoanthracene (2-AA) and 2-aminofluorene (2-AF) as well as polyaromatic hydrocarbon benzo(a)pyrene (BaP) models to Salmonella (strain TA98) mutagens, confirming other freshwater fish studies with bream, carp; pike, and rainbow trout. The enzymatic activity of catfish S9 was cytochrome P-448-450 like: inducible, heat liable, cofactor dependent, and inhibitor sensitive. Bacterial mutagenesis in the Salmonella/catfish S9 test was dose dependent and sensitive to both S9 concentrations and preincubation activation temperature. A liquid-preincubation modification of the standard plate-incorporation method significantly improved the catfish poikilothermic activation system. The hepatic S9 activity of catfish and the traditional Arochlor-1254-induced rat were qualitatively and quantitatively similar in the detection of model arylamines and benzo(a)pyrene genotoxins. The relative sensitivity of the tests, that is the lowest detectable concentration of 2-AA, 2-AF, and BaP, was <1.0 mu g/plate with both activation systems. The sensitivity of the Salmonella/fish S9 assay as a screening tool to detect environmental genotoxins was clearly influenced by three test factors: preinduction of the fish, preincubation of the sample, and optimal preincubation temperature of the fish S9. The successful use of a fish activation system to mediate the biotransformation of progenotoxins establishes this Salmonella/fish S9 mutagenicity test as an ecologically relevant assay to monitor the potential aquatic hazards of environmental genotoxins influencing the aquatic resource of concern - the freshwater fisheries.