Role of Peroxide in the Radioprotective Action of Thiols in E. Coli

Summary

The radioprotective action of cysteamine (MEA) and cysteine in E. coli is due partly to autoxidatively generated hydrogen peroxide (H₂O₂). This effect, which predominates at low concentrations of the thiols (1–2 mM in neutral solution), is regularly correlated with a metabolic block, measured as inhibition of RNA synthesis. In experiments with E. coli 15 (autotroph) under exponential growth in complete medium, the role of H₂O₂ was demonstrated by (a) a decreased radioprotective action if catalase was present in the medium; (b) a radioprotective action of H₂O₂ added to the medium; (c) a decreased protective action in the absence of catalytically active copper; and (d) oxygen being required for the radioprotective action to develop. At higher concentrations of the thiols, their radioprotective action, and the accompanying metabolic block, are less dependent on H₂O₂ generation and presumably due to a different mechanism. The radioprotective action of H₂O₂ is possibly related to the radioprotective action in mammals of catalase inhibitors.