DNA single-strand breaks and sister chromatid exchanges induced by treatment with hematoporphyrin and light or by x-rays in human NHIK 3025 cells.

Alkali-labile sites resulting in single-stranded breaks in DNA and sister chromatid exchanges are produced when human cells (NHIK 3025) in vitro are exposed to sublethal doses of light in the presence of hematoporphyrin. The irradiation doses required to reduce the survival from 1 to 0.1 for 220-kV X-rays and treatment with 10(-4) M hematoporphyrin in phosphate-buffered saline and 380 nm light were 6.2 grays and 230 J/sqm, respectively. X-rays induce about 5 times more sister chromatid exchanges and about 80% more DNA single-strand breaks than exposure to hematoporphyrin plus light when the two modalities of treatment are compared on the same level of survival. In both cases, the single-strand breaks are practically completely repaired within 15 min.