Extracellular superoxide provokes glutathione efflux from Escherichia coli cells

The aim of the study was to elucidate a possible relationship between transmembrane cycling of glutathione and changes in levels of external superoxide. Exposure of growing Escherichia coli to exogenous reactive oxygen species (ROS) generated by xanthine and xanthine oxidase (XO) stimulates reversible glutathione (GSH) efflux from the cells that is considerably lowered under phosphate starvation. This GSH efflux is prevented by exogenous SOD, partially inhibited by catalase, and is not dependent on the GSH exporter CydDC. The γ-glutamyl transpeptidase (GGT) deficiency completely prevents a return of GSH to the cytoplasm. In contrast to wild-type E. coli, mutants devoid of GGT and glutathione reductase (GOR) show enhanced accumulation of oxidized glutathione in the medium after exposure to xanthine and XO. Under these conditions, sodC, ggt and especially gshA mutants reveal more intensive and prolonged inhibition of growth than wild-type cells. Treatment with XO does not influence E. coli viability, but somewhat increases the number of cells with lost membrane potential. In summary, data obtained here indicate that transmembrane cycling of GSH may be involved in E. coli protection against extracellular ROS and may promote rapid growth recovery.