EFFECTS OF UV AND VISIBLE RADIATION ON DNA - FINAL BASE DAMAGE

Several mechanisms are likely to be involved in the solar radiation-me diated modifications of cellular DNA. Direct excitation of DNA bases b y the UVB component (290-320 nm) of solar light gives rise, mostly thr ough oxygen independent reactions, to the formation of dimeric pyrimid ine lesions including cyclobutadipyrimidines, pyrimidine (6-4) pyrimid one photoproducts and related valence Dewar isomers, In addition, phot oexcitation of cytosine and guanine may lead to the formation in relat ively minor yields of 6-hydroxy-5,6-dihydrocytosine and 8-oxo-7,8-dihy droguanine, respectively, A second mechanism that requires the partici pation of endogenous photosensitizers together with oxygen is at the o rigin of most of the DNA damage generated by the UVA (320-400 nm) and visible light, Singlet oxygen, which arises from a type II mechanism, is likely to be mostly involved in the formation of 8-oxo-7,8-dihydrog uanine that was observed within both isolated and cellular DNA. Howeve r, it may be expected that the latter oxidized purine lesion together with DNA strand breaks and pyrimidine base oxidation products are also generated with a lower efficiency through Fenton type reactions. A mo re definitive assessment of these mechanisms would require further stu dies aimed at the identification and quantification of the different D NA photolesions including both dimeric pyrimidine photoproducts and ph otooxidized lesions.