Structural damage to lymphocyte nuclei by H₂O₂ or gamma irradiation is dependent on the mechanism of OH^. radical production

Normal human lymphocytes were exposed to OH^. radicals produced indirectly by exposure to H₂O₂ or directly by gamma irradiation. Using a flow cytometry technique to measure changes in nucleoid size, it was found that generation of OH^. in each system produced a characteristic relaxation in nuclear supercoiling. Exposure of cells to H₂O₂ produced a metal-dependent step-wise relaxation in extracted nucleoids, while gamma irradiation induced a gradual dose-dependent increase in nucleoid size. The site-specific metal-dependent changes produced in lymphocytes incubated in H₂O₂ should also occur in gamma irradiation cells, but the characteristic effects on nuclear supercoiling would not be detected within the background of random DNA damage. The importance of metals in maintaining the supercoiled loop configuration of DNA within the protein matrix suggests that free radical damage at metal locations may be particularly toxic for the cell.