Formation of products of the 5,6-dihydroxydihydrothymine type by ultraviolet light in HeLa cells

The formation of products of the 5,6-dihydroxydihydrothymine-type (t/sup UV/) and cyclobutane-type pyrimidine photodimers (TT) and tritiated water (/sup 3/H/sub 2/O) by monochromatic light at 240, 265, 280, and 313 nm (6-nm half-band-width) was investigated in HeLa S-3 cells which were labeled in their DNA with (methyl-/sup 3/H)thymine. The efficiency of the formation of all three products was maximal at 280 nm and dropped towards longer and shorter wavelengths. The efficiency of TT formation dropped more strongly towards longer and shorter wavelengths than the efficiency of t/sup UV/ formation (comparison at a dose of 5 x 10/sup 3/ J m/sup -2/). Total monomeric thymine ring saturation (t/sub sat/) was estimated from the t/sup UV/ data. It was calculated that 0.06 t/sub sat/ was formed for each TT at 280 nm, but 0.73 t/sub sat/ per TT at 313 nm. It follows that monomeric thymine ring saturation represents a minor photochemical reaction relative to pyrimidine dimerization in the far-ultraviolet but a major reaction in the near-ultraviolet. The formation of /sup 3/H/sub 2/O by ultraviolet light from (methyl-/sup 3/H) thymidine-labeled HeLa cells most likely indicates the attack of hydroxyl radicals on the cellular DNA; 5-methyleneuracil radicals formed as a consequence of the reaction may be important intermediates in ultraviolet-induced DNA-DNA and DNA-protein cross-linking.