Photosensitized DNA damage in human cells is localized in chromatin sensitive to DNAse I digestion

Human NHIK 3025 cells were sensitized to light by incubation with Photofrin II (PII) and aluminum phthalocyanine tetrasulphonate (AlPCS4). Light exposure of sensitized cells at 1 degree C induced DNA strand breaks causing unwinding of DNA in alkali. Subsequent to light exposure, cells were treated with deoxyribonuclease I (DNAse I) which produces DNA strand breaks in active chromatin. The combination of photo-damage, killing > 95% of the cells, and treatment with DNAse I caused the same degree of DNA unwinding as DNAse I treatment alone. This was not the case for X-irradiation. Thus, the photosensitized DNA strand breaks were selectively induced in or close to DNAse I-sensitive sites.