Formation of the main UV-induced thymine dimeric lesions within isolated and cellular DNA as measured by high performance liquid chromatography-tandem mass spectrometry

UVB radiation-induced formation of dimeric photoproducts at bipyrimidine si tes within DNA has been unambiguously associated with the lethal and mutage nic properties of sunlight. The main lesions include the cyclobutane pyrimi dine dimers and the pyrimidine (6-4) pyrimidone adducts. The latter compoun ds have been shown in model systems to be converted into their Dewar valenc e isomers upon exposure to UVB Light. A new direct assay, based on the use of liquid chromatography coupled to tandem mass spectrometry, is now availa ble to simultaneously detect each of the thymine photoproducts. It was appl ied to the determination of the yields of formation of the thymine lesions within both isolated and cellular DNA exposed to either UVC or UVB radiatio n. The cis-syn cyclobutane thymine dimer was found to be the major photopro duct within cellular DNA, whereas the related (6-4) adduct was produced in an approximately 8-fold lower yield. Interestingly, the corresponding Dewar valence isomer could not be detected upon exposure of human cells to biolo gically relevant doses of UVB radiation.