Cyclobutane-type pyrimidine photodimer formation and excision in human skin fibroblasts after irradiation with 313-nm ultraviolet light.

The formation and excision of 313-nm light-induced cyclobutane-type pyrimidine photodimers were determined in confluent cultures of human fibroblasts. A new method was developed for the resolution and determination of cytosine-thymine (CT) and thymine-thymine dimers (TT) by using sodium borohydride reduction and high-pressure liquid chromatography. This assay can detect as little as 1.8 TT or 5.6 CT per 10(8) daltons, levels induced in monolayers of human skin fibroblasts by doses of 1 and 2 kJ m-2 of 313-nm light, respectively. CT formation was 20% more efficient than TT formation in the physiological dose range of 2.25-15 k m-2 at 37 degrees C. Normal fibroblasts removed 61% TT within the first 8 h of incubation following a dose of 5.5 kJ m-2. CT was removed approximately twice as efficiently as TT during the same time period following exposure to 10 kJ m-2. The lack of removal of CT as well as TT observed in xeroderma pigmentosum fibroblasts indicates that the repair deficiency in these cells affects the repair of both classes of dimers.