N-Ethylmaleimide Sensitization of X-Irradiated Hypoxic Chinese Hamster Cells

Abstract

Chinese hamster cells were X irradiated aerobically or hypoxically, after flushing with N plus CO2. In agreement with earlier data, for asynchronous cells, the O2 enhancement ratio (OER) was approximately 3. If the sulfhydrylbinding agent N-ethylmaleimide (NEM) was present during or immediately after irradiation, the principal effect was a pronounced decrease in the extrapolation number of the survival curve of NEM-treated cells compared to nontreated cells. This was observed with hypoxic and aerobic cells, and the OER for NEM-treated cells was about 3. For NEM treatments which were essentially nontoxic, NEM acts synergistically with X-rays, suggestive of an inhibition by NEM of a cell''s ability to repair sublethal damage. For synchronous cells obtained by mitotic selection, a result consistent with the above was obtained; a dose 3 times as large was necessary to reduce survival to the same level for hypoxic cells as for aerobic cells, whether or not cells were treated with NEM. The OER was independent of NEM treatment throughout the cell cycle, with the possible exception of mitosis which was not studied with the methods used. The action of NEM at low concentrations (0.75 .mu.M) was largely independent of tension. O2 acted to produce more damage per unit dose in the cell while NEM sensitizes apparently by preventing repair of sublethal damage.