Split-dose Irradiation of Escherichia coli in the Absence and Presence of Mercaptoethylamine

Abstract

SUMMARYA single dose of y-radiation to Escherichia coli B gave a semi-logarithmic dose-response curve. A second irradiation 3, 6, or 18 hr after a priming dose of 8 h a d showed that the cells had become more radio-sensitive during the incubation between irradiations.When mercaptoethylamine was present during irradiation, protection against the lethal effects of the y-radiation occurred. After a priming dose to the bacteria in the presence of the radio-protective chemical, and incubations for 3,6, or I 8 hrinits absence, asecond irradiation in the absence of the chemical showed them to be much more radio-resistant than the normal unprotected Escherichia coli B. This indicates that the presence of the protector during irradiation alters the type of damage induced or the development and repair of that damage.

A Test of the ‘Mixed-disulphide’ Hypothesis of Cysteamine Radioprotection

“…6 (figure 3) . This result agrees with an earlier report by Prew, Gresham and Hodkinson (1969) . The addition of 50 mM diamide before irradiation of cysteamine-treated bacteria caused the biphasic survival curve to become linear Figure 3 .…”

“…Some workers believe that radical scavenging and electrondonation reactions account for cysteamine's protective efplcacy ; others that the amino-thiol forms a mixed disulphide with protein-bound sulphydryl groups and that this confers protection (Eldjarn and Pihl 1960) . Modig and his coworkers proposed a variation on the latter theme, hypothesizing that formation of the mixed disulphide releases glutathione from binding sites on cellular proteins and that this glutathione is the protector (Revesz and Modig 1965, Modig 1968 .…”

Effect of Fractionation Treatment With Gamma-Rays on the Mutation Frequency in Maize

“…In experiments with E. coli, the increase of mutational response to the second irradiation was also observed by double irradiations with X-ray and/or UV, probably due to some modifications in the cells exposed to the first irradiation and subsequently incubated in minimal medium (Kada et al 1966) . Other experiments with bacteria also demonstrated a cellular sensitization by a priming dose of fractionation treatment (Prew et al 1969) . On the other hand, significantly lower mutation frequencies were described in mice when a total dose of 400 R was given in eight fractions each spaced 75 minutes apart (Russell 1968) .…”