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 degrees C with S-9 mix and paraquat (methylviologen, PQ2+) 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 micrograms per plate of B(a)P was used in TA 98 and TA 100, respectively. However, exogenous superoxide dismutase at 10-100 micrograms 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.