EXAMINATION OF THE ADDITIVITY ASSUMPTION USING THE SPIRAL AND STANDARD SALMONELLA ASSAYS TO EVALUATE BINARY COMBINATIONS OF MUTAGENS

Binary combinations of pure chemicals and complex mixtures were evalua ted for their ability to produce additive mutagenicity responses in th e spiral and standard Salmonella mutagenicity assays. Single chemicals were selected that were representative of the primary chemical class responsible for much of the mutagenic activity of each complex: mixtur e. The following agents were evaluated in the absence of S9: 1-nitropy rene, diesel exhaust extract, and the chlorinated drinking water mutag en -chloro-4-dichloromethyl-5-hydroxy-2-[5H]-furanone (MX). In the pre sence of S9, the following agents were evaluated: 4-aminobiphenyl, ben zo[a]pyrene, and an organic extract from the particulate emissions res ulting from the combustion of polyethylene in a rotary kiln incinerato r. Binary combinations of the agents within each S9 group were tested. The results were analyzed for additivity by determining whether the d ifference between the expected response of the binary mixture was sign ificantly different from the observed response. In general, the combin ations of mutagens produced additive responses at low doses in both as says. However, at high doses, the results generally departed from addi tivity, especially for combinations of indirect-acting mutagens. A req uirement for exogenous metabolic activation imposes conditions, such a s different S9 optima for each mutagen, that complicate the applicatio n of the additivity assumption to binary combinations of indirect-acti ng mutagens. Although the two bioassays generally gave similar results for each binary combination of mutagens, the spiral assay permitted a gradient of mutagen and/or S9 concentrations to be evaluated, and it allowed the mutagens to be evaluated in the absence of solvents, elimi nating any potential solvent-mutagen interactions. These features of t he spiral assay may be advantageous when studying chemical interaction s.