INDUCTION OF DNA STRAND BREAKS IN NORMAL HUMAN FIBROBLASTS EXPOSED TO MONOCHROMATIC ULTRAVIOLET AND VISIBLE WAVELENGTHS IN THE 240–546 nm RANGE

The induction of DNA single‐strand breaks in normal human fibroblasts exposed to monochromatic wavelengths from 240–546 nm was measured by the alkaline elution assay. The cells were irradiated at 1°C to prevent both repair of induced breaks and formation of enzymatically induced breaks through excision repair. The cultures were also washed with and irradiated while suspended in phosphate buffered saline to prevent the formation of DNA damaging photoproducts from medium components. The action spectrum for DNA strand breakage was found to exhibit one peak at 265 nm, consistent with DNA absorption, and a second peak at 450 nm. The normalized action spectrum in the visible is similar to the normalized absorption spectrum for riboflavin, a known photosensitizing agent, implicating this molecule as the absorbing chromophore.