The site of action of cysteamine in irradiated bacteria: An electron spin resonance study

Electron spin resonance spectra showed that radicals produced in Escherichia coli B/r and Micrococcus radiodurans by exposure to γ-rays at −195° undergo a series of changes on warming. Cysteamine modifies these reactions in a characteristic manner, but the full effect is only seen if the bacteria are ruptured by mechanical methods prior to the addition of cysteamine. With intact M. radiodurans cysteamine has almost no effect on the ESR spectra seen after irradiation, while with E. coli B/r cysteamine already has some influence before irradiation possibly because the cells are damaged to some extent by freeze-drying. The data are interpreted as showing that cysteamine does not penetrate into the interior of non-metabolizing bacteria, yet it protects against the lethal action of ionizing radiations under these conditions. Physicochemical experiments had shown earlier that main-chain scission of DNA extracted from irradiated bacteria was not protected by cysteamine. The hypothesis is advanced that cysteamine protects a metabolic repair system of DNA which is situated at or near the cell membrane, and in this way makes the restitution of DNA lesions more efficient in irradiated bacteria.