Rat liver microsomal azoreductase activity on four azo dyes derived from benzidine, 3,3'-dimethylbenzidine or 3,3'-dimethoxybenzidine.

The ability of rat liver microsomes from phenobarbitone pretreated animals to reduce the azo groups of amaranth, sunset yellow, congo red, trypan blue, chloramine sky blue FF and direct black 38 was measured spectrophotometrically in vitro. The dyes amaranth and sunset yellow acted as positive controls. Of the dyes derived from benzidine or its congeners, only direct black 38 was reduced to an appreciable extent; the rate of reduction was 10% of that for amaranth. The dyes were tested for mutagenicity in the Salmonella/microsome assay, the only active compound being direct black 38. Mutagenicity of this dye may be due in part to the mutagen 1,2,4-triaminobenzene. Mutagenic activity and azo-reduction of direct black 38 was independent of the presence of oxygen. The results presented suggest that mammalian liver may play only a minor or negligible role in the azo-reduction of dyes derived from benzidine or its congeners.