These experiments measured the effect of γ radiation on the nuclear envelope using doxyl-fatty acid spin-label probes. Nuclei were isolated from cultured MOLT-4 cells, a radiation-sensitive human T-cell lymphocyte. Membrane fluidity was measured from the electron paramagnetic resonance spectra of the probes. MOLT-4 cells were grown under standard conditions, and suspensions were exposed to60 Co γ radiation at room temperature. The spectra of 5-doxylstearic acid in the nuclei were those of a strongly immobilized label. A difference in the membrane fluidity was detected in a series of experiments comparing labeled irradiated and nonirradiated nuclei. The change in fluidity was measured by comparing the changes in the order parameter, S, of the spin label in irradiated nuclei with those in control nuclei. The change in the S ratio is dependent on radiation dose, increasing with doses up to 15 Gy. The maximum change of the order parameter with time after irradiation occurs 16-20 h after radiation exposure. These observations are correlated with changes in cell viabilities.

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