N-ethylmaleimide sensitization of X-irradiated hypoxic Chinese hamster cells.

Chinese hamster cells were X irradiated either aerobically or hypoxically, after flushing with nitrogen plus carbon dioxide. In agreement with earlier data, for asynchronous cells, the oxygen enhancement ratio (OER) was approximately three. If the sulfhydryl-binding 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 as well as aerobic cells and the OER for NEM-treated cells was also about three. 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 three times as large was necessary to reduce survival to the same level for hypoxic cells as for aerobic cells, whether or not th...