The effect of paraquat on the mutagenicity of benzo( a )pyrene

Elevated activities of superoxide dismutase (SOD) were detected in histidine‐requiring strains of Salmonella typhimurium after the bacteria were preincubated for 1 h at 37°C with S‐9 mix and paraquat (methylviologen, PQ 2+ ) at 10 −4 M. A fivefold increase in SOD level was found for strains TA 98 and TA 100. These elevated levels of SOD activity were correlated with a significant reduction of the mutagenicity of metabolically activated benzo( a )pyrene (B( a )P) in these tester bacteria when evaluated in a preincubation assay system. A 69.0–92.5% and 23.5–66.9% reduction was noticed when 0.5–4.0 μg per plate of B( a )P was used in TA 98 and TA 100, respectively. However, exogenous superoxide dismutase at 10–100 μg ml −1 added to top agar had no significant effect on the number of revertants produced by activated B( a )P. These data indicate a major role of intracellular superoxide anion in promoting mutagenicity of B( a )P.