Lethal Effects of Iodine-125 Decay by Electron Capture in Escherichia Coli and in Bacteriophage T1

Summary

Iodine-125 decays by electron capture, with resultant vacancy cascades and multiple ionization (Auger effect). This decay mode is known to cause extensive destruction of small organic molecules. ¹²⁵I has been incorporated into the DNA of four strains of E. coli with differing repair capabilities as well as into coliphage T1, in the form of 5-iododeoxyuridine (IUDR), a thymidine analogue. Lethal effects of the decay of incorporated ¹²⁵I have been studied in these micro-organisms stored at −196°c. The lethal efficiency per decay under these conditions is 0·49 for T1 phage, 0·45 for E. coli B_s−1, and 0·38 for E. coli K-12 AB2463, both repair-deficient strains; but it is 0·22 for E. coli B/r and 0·15 for E. coli 15THU, which are capable of normal DNA repair. These lethal efficiencies are roughly an order of magnitude greater than those observed for ³²P decay in micro-organisms. According to approximate dose calculations, 5 per cent of this killing in bacteria and 33 per cent in T1 phage may be accounted for by the external ionizing radiation emitted during ¹²⁵I decay. There is no evidence of substantial radiosensitization of these micro-organisms by the low degree of substitution of IUDR for thymidine in DNA obtained in these experiments. ¹²⁵I killing of T1 phage shows little or no host-cell reactivation and is nearly independent of storage temperature. These results are consistent with the hypothesis that the severe lethal effects of ¹²⁵I decay in the DNA of micro-organisms are largely due to direct physical disruption of the DNA molecule by the Auger effect. However, a significant role for localized ionizing radiation from decaying ¹²⁵I atoms cannot be ruled out, particularly in T1 phage.