The effect on the Km for radiosensitization at 0 degree C of thiol depletion by diethylmaleate pretreatment: quantitative differences found using the radiation sensitizing agent misonidazole or oxygen.
Pretreatment of V79- WNRE cells with 150 microM diethylmaleate for 1 hr at 37 degrees C caused a decrease in intracellular glutathione levels to approximately 10-15% of control levels (0.5 vs 5.0 nmol/10(6) cells). The cells could be washed free of diethylmaleate and held at 0 degree C for several hours without toxicity and with no increase in glutathione concentration, although the glutathione concentration rapidly increased to normal levels at higher temperatures. Survival curves were determined as a function of oxygen or misonidazole concentration (the latter in the absence of oxygen). A new "thin-film" technique was used to avoid changes in oxygen concentration because of radiochemical or cellular oxygen consumption. Glutathione depletion itself caused a small but consistent radiosensitization of hypoxic cells (dose enhancement ratio of 1.2). However, glutathione depletion caused a profound change in the radiosensitizing efficiency of misonidazole, with a decrease in Km of about sevenfold from 0.6 to 0.09 mM. In contrast, only a 2.5-fold decrease was found in the Km for radiosensitization by oxygen with diethylmaleate pretreatment. These results suggest a fundamental problem with the conventional theory of radiosensitivity whereby one considers a first-order competition for reaction with target radicals between radical-fixing versus radical-repairing species. It also suggests difficulties in the interpretation of glutathione as the only endogenous protective species.
D000818:Animals; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects*; D002478:Cells, Cultured / Q000187:drug effects; D002588:Cesium Radioisotopes; D006224:Cricetinae; D003412:Cricetulus; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D005720:Gamma Rays; D005978:Glutathione / Q000502:physiology*; D008298:Maleates / Q000494:pharmacology; D008920:Misonidazole / Q000494:pharmacology*; D009593:Nitroimidazoles / Q000494:pharmacology*; D010100:Oxygen / Q000494:pharmacology*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*
D002588:Cesium Radioisotopes; D008298:Maleates; D009593:Nitroimidazoles; D011838:Radiation-Sensitizing Agents; D008920:Misonidazole; C014476:diethyl maleate; D005978:Glutathione; D010100:Oxygen