Endogenous thiol levels in heterogeneous murine tumor cells as a function of the physiological state and the response to X-irradiation.

The endogenous thiols (PSH, protein sulfhydryls; NPSH, nonprotein sulfhydryls; and GSH, glutathione) were measured in the 66 and 67 murine carcinoma cells growing under different physiological conditions in vitro (e.g., proliferation, P; nutrient-deprived quiescence QI; and QI cells stimulated by refeeding the monolayer in situ and assayed 4 (St4) and 14 (St14) h later). The aerobic radiation response was also studied as a function of the physiological state and thiol concentration. The changes in PSH levels suggest that the proportion of thiol-containing proteins changed whenever the cells were in transition between different physiological states (e.g., when QI cells were stimulated by refeeding, the proportion of PSH was elevated dramatically over either QI or P cells). The NPSH and GSH levels were both down significantly in the QI vs. P cells as was the total thiol level (PSH plus NPSH). Fourteen h but not 4 h after stimulation, the NPSH and GSH levels had returned to or exceeded the P-cell levels. Also, the proportion of GSH in the NPSH fraction varied as a function of the physiological state. The 66 and 67 QI cells were both more radiosensitive than the respective P cells. Also, the 66 cell radiation-induced cytotoxicity had returned to the P response by about 4 h after refeeding but the stimulated 67 cells had not. However, no overall correlation was apparent between the various aerobic radiation responses and the pool sizes of either the total thiols or of the various subsets of thiols. The depressed total thiol level and the increased radiosensitivity of the QI cells could represent a cause-and-effect relationship or these parameters could be independent phenomena only related indirectly through the reduced metabolic activity of the quiescent cells.