Inactivation of Mammalian Cells after Disintegrations of3H or125I in Cell DNA at −196°C

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Abstract
Chinese hamster cells, strain V79–S171, or mouse leukaemic cells, strain L5178Y, were permitted to incorporate either 3H-IUdR, 3H-TdR or 125IUdR, into cell DNA so that the DNA was unifilarly labelled. The cells were then frozen and stored at −196°C to accumulate damage due to isotope disintegration. At various times, cell samples were thawed out and colony-forming ability was measured. Tritium disintegrations in the DNA of Chinese hamster cells were found to be less efficient (α0 = 1/970; Dq = 1500 disintegrations) than the same number of disintegrations in the DNA of mouse leukaemic cells (α37 = 1/306 − 1/175). This difference was similar to the difference in radiosensitivity seen when the two cell-lines were exposed in the frozen state to external radiation in the form of X- or gamma-rays. This result is contrasted with the killing efficiences of 125I disintegrations, which were almost identical in the two cell-lines (L5178Y cells α37 = 1/46; V79 cells α0 = 1/39). An analysis of the data suggests that the damage formed when 125I decays within the DNA is inefficiently repaired or not repaired and may involve events specifically associated with the Auger effect, including molecular disruption and possible effects analogous to those seen after high-LET radiation exposure.