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

Abstract

A Salmonella/fish S9 mutagenicity assay, a modification of the traditional rodent Salmonella/musome 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 μg/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.