Radiolysis of Chromatin Extracted from Cultured Mammalian Cells: Formation of DNA–protein Cross Links

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Abstract
Chromatin extracted from Chinese hamster [V79-753] lung fibroblasts was examined for the formation of radiation-induced DNA-protein cross links using a membrane filter assay. The relative efficiencies of the aqueous radical intermediates OH.cntdot. (hydroxyl radical), .**GRAPHIC**. (aqueous electron) and O2- (superoxide radical), were investigated. Cross-links were found in .gamma.-irradiated isolated chromatin and in chromatin irradiated in the cell before isolation. When isolated chromatin was irradiated under conditions in which the chromosomal proteins were dissociated from the DNA, no cross-links were detectable. The most efficient radical for the production of cross links in irradiated, isolated chromatin was the hydroxyl radical, whereas the superoxide radical was essentially ineffective. For chromatin irradiated in the cell before isolation, the greatest effect was seen for cells irradiated in an atmosphere of N2O, suggesting that the hydroxyl radical may be involved in the formation of cross-links in intact cells also. The formation of cross-links in chromatin irradiated in cells before isolation was considerably less efficient than in irradiated, isolated chromatin.