Protection by mercaptoethylamine (MEA) and thiourea against the induction of DNA single-strand breaks by irradiation under aerated and anoxic conditions and in the frozen state was studied with cultured mouse leukemic L5178Y cells. The results were compared with those previously obtained for colony-forming ability. Under aerated conditions, good correlation was observed between protection against DNA strand breaks and cell lethality over a wide range of concentrations of the protective agents. Under anoxic conditions, DRF values varied from 1.0 to 1.6 for 50 mM MEA and from 1.0 to 1.8 for 100 mM thiourea over the dose range studied. Protection against DNA strand breaks was observed only for doses greater than 20 krad. Protection against DNA strand breaks after irradiation at -196°C was essentially the same as that for cell lethality. MEA interfered with the rejoining of DNA strand breaks induced by irradiation under aerobic conditions, even when it was removed before the postirradiation incubation.

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