Factors affecting the response of N,N-dimethylaminoazobenzene in the Ames microbial mutation assay

The mutagenicity of N,N-dimethylaminoazobenzene (DAB) is difficult to demonstrate in Ames' test. Usually there are specific requirements for activation by post-mitochondrial supernatant fluid (S-9) from Aroclor-treated rat livers and the pre-incubation modification of the test. Results from this laboratory suggest, however, that pre-incubation is not essential; also, that, contrary to published reports, concentrations of S-9 greater than 10% in S-9 mix do not reduce the mutagenic response. Induction of enzyme activity well above normal levels, on the other hand, is necessary, but this requirement can be substituted by the addition of norharman. If a competent S-9 mix is used, pre-incubation with or without shaking does not alter the response and supplementation with ATP or NADH similarly has no effect. It is concluded that interlaboratory differences in the ability to demonstrate DAB mutagenicity reflect differences in the level of induction of liver enzymes and, possibly, the concentration of endogenous co-factors.