Observations on the photosensitized breakage of DNA by 2-thiouracil and 334-nm ultraviolet radiation

Breaks induced in purified DNA by 334-nm ultraviolet (UV) radiation are enhanced 30 times when 2-thiouracil (s2Ura) is present during aerobic irradiation. This enhancement by s2Ura is maximally effective at a concentration of about 1 mM. Anoxic irradiation reduces the s2Ura-enhanced breakage by 90%, indicating a Type II photosensitization. Benzoate, glycerol, diazabicyclo[2.2.2.]octane (DABCO) and histidine all inhibit formation of s2Ura photosensitized breaks, unlike diethylenetriaminepenta-acetic acid (DETAPAC) and catalase, which do not. The relationships between the concentration of DABCO, benzoate and histidine and their protection against induction of single strand breaks (SSBs) were similar, with little inhibition below 10 mM and maximal inhibition near 0.1 M for all compounds. Irradiation of the DNA-s2Ura mixture dissolved in D2O instead of H2O enhanced the rate of induction of SSBs in DNA by 334-nm light almost five times. Addition of superoxide dismutase (40, 80 and 200 μg/ml) decreased the rate of induction of breaks in DNA by 334-nm radiation plus s2Ura (in H2O) by about 40%. Boiled superoxide dismutase had no effect. (author)