Lethal and mutagenic actions of N-methyl-N'-nitro-N-nitrosoguanidine potentiated by oxidized glutathione, a seemingly harmless substance in the cellular environment.

Both the lethal and the mutagenic actions of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on cells of Streptococcus pneumoniae were greatly potentiated by a component of yeast extract added to the cellular environment. This component was found to be an oxidation product of glutathione, glutathione disulfide (GSSG). At low concentrations in the medium, both GSSG and glutathione potentiated MNNG action, but at high concentrations, glutathione (and other sulfhydryl compounds) abolished the effect. Point mutations in a cellular gene conferred resistance to the potentiating effect, and they blocked uptake of either GSSG or glutathione into the cells as well. This gene apparently encodes a component of the system for glutathione transport in S. pneumoniae. The mechanism by which GSSG, an apparently innocuous substance in the environment, renders low levels of MNNG genotoxic and cytotoxic thus depends on its transport into the cell, where it is reduced by glutathione reductase and then activates intracellular MNNG. Also, it was observed that mutants of S. pneumoniae defective in DNA mismatch repair are more resistant to MNNG than are wild-type cells by a factor of 2.5.