N-Ethylmaleimide and the Cyclic Response to X-Rays of Synchronous Chinese Hamster Cells

In addition to DNA synthesis, at least one other factor appears to exert a controlling influence on the cyclic dependence of the lethal radiation damage in mammalian cells. Experiments designed to identify this factor have been conducted with the sulfhydryl binding agent, N-ethylmaleimide (NEM) in synchronous Chinese hamster cells. Low concentrations of NEM (0.75 μM) sensitize oxygenated Chinese hamster cells most in the late S period. The effect is primarily on the slope of the survival curve, rather than its shoulder, and the maximum sensitization factor for late S cells is about 1.6. Experiments with NEM and cysteamine indicate that NEM forms a stable complex in the cell, and thus exposure to NEM before irradiation sensitizes the cell equally well as when NEM is present during irradiation. The sensitizing effect is less the longer the interval between pretreatment with NEM and irradiation. N-ethylmaleimide also has a pronounced postirradiation effect: when given immediately after irradiation, the effect is the same as for pretreatment or during irradiation. The effect decreases in magnitude with time for up until 2 hours after irradiation in late S cells and for longer times in the case of G₁ cells. N-ethylmaleimide also suppresses the rise in survival normally seen late in the cell cycle of cells inhibited by hydroxyurea from synthesizing DNA and thus presumably modifies a factor which controls cell survival in the absence of DNA synthesis. Late S cells irradiated in the presence of NEM have the same capacity to recover from sublethal damage as do untreated cells. These results suggest that when oxygenated cells are exposed to a low concentration of NEM, for some time, repair of lethal damage is inhibited. The mechanism by which NEM accomplishes this inhibition is not known, since the evidence as to whether the -SH binding is involved is conflicting and other actions of NEM also take place.