Protective Action of Flavins on Ultraviolet Irradiated Dna

A postulated capability of riboflavin and flavin mononucleotide to protect DNA from damage by far-ultraviolet irradiation was investigated in purified reaction mixtures and in cultured mammalian cells. Purified calf thymus DNA, admixed with flavin at approximately physiological ratios, was protected from undergoing irradiation-induced changes in spectral absorbance and viscosity compatible with those accompanying pyrimidine dimerization. Irradiation-induced changes in unprotected DNA were reversed by a subsequent treatment of the DNA with flavin in the presence of visible light. There was no apparent interaction between flavin and non-irradiated DNA. These results suggest that photoexcited flavin interacts directly with the ultraviolet photoproduct in DNA and does not function simply as an energy sink to dissipate photons absorbed by DNA. Cultured embryonic hamster cells were protected from ultraviolet-induced inhibition of growth by a preliminary exposure to relatively large amounts of flavin. This effect was not due to a vitamin- like promotion of cell growth by the flavin. The observation that flavin concentrated transiently in the cytoplasm of exponentially growing hamster cells during a 6 hr incubation period suggested that a shielding action may cooperate with the protective photochemical mechanism in vivo. It was concluded that flavins may function physiologically to protect the genetic material of tissues exposed to actinic radiations, and that when flavin levels in these tissues are inadequate, either because of nutritional deficiency or excessive intensity of irradiation, the damage which results may provide one pathogenetic avenue for the characteristic integumentary lesions of riboflavin deficiency disease