WAVELENGTH DEPENDENCE OF OXIDATIVE DNA-DAMAGE INDUCED BY UV AND VISIBLE-LIGHT

DNA damage induced by UV radiation and visible light (290-500 nm) in A S52 Chinese hamster cells was analysed by an alkaline elution assay wi th specific repair endonucleases, Cells were exposed to extensively fi ltered monochrome or broad-band radiation, Between 290 and 315 nm, the ratio of base modifications sensitive to Fpg protein (i.e. 8-hydroxyg uanine and formamidopyrimidines) and T4 endonuclease V (i.e. cyclobuta ne pyrimidine dimers) was constant (similar to 1:200), indicating that the direct excitation of DNA is responsible for both types of damage in this range of the spectrum, While the yield of pyrimidine dimers pe r unit dose continued to decrease exponentially beyond 315 nm, the yie ld of Fpg-sensitive modifications increased to a second maximum betwee n 400 and 450 nm, The damage spectrum in this wavelength range consist ed of only a few other modifications (strand breaks, abasic sites and pyrimidine modifications sensitive to endonuclease LII) and is attribu ted to endogenous photosensitizers that give rise to oxidative DNA dam age via singlet oxygen and/or type I reactions. The generation of Fpg- sensitive modifications by visible light was not linear with dose but followed a saturation curve, It is calculated that the exposure of the cells to low doses of solar radiation results in the formation of cyc lobutane pyrimidine dimers and Fpg-sensitive modifications in a ratio of 10:1.