DETECTION OF PYRIMIDINE DIMERS AND MONOADDUCTS INDUCED BY 7-METHYLPYRIDO(3,4-C) PSORALEN AND UVA IN CHINESE-HAMSTER V79 CELLS BY ENZYMATIC CLEAVAGE AND HIGH-PRESSURE LIQUID-CHROMATOGRAPHY

The photochemotherapeutically active psoralen derivative 7-methylpyrid o(3,4-c) psoralen (MePyPs) has been recently shown to be able to photo induce monoadducts of the C-4-cycloaddition type as well as pyrimidine dimers in DNA in vitro. In the present study, we report on the induct ion of these two types of photolesions in mammalian cells in culture. The MePyPs photocycloadducts were quantified in V79 Chinese hamster ce lls after treatment with MePyPs plus UVA following enzymatic hydrolysi s of the DNA by DNase I, S1 nuclease and acidic phosphatase treatments . Concomitantly induced pyrimidine dimers were determined by two metho ds, high-pressure liquid chromatography and alkaline gel electrophores is after dimer-specific endonucleolytic cleavage. The results show tha t, in Chinese hamster cells treated with MePyPs plus UVA, the yield of pyrimidine dimers is approximately 5-10% that of MePyPs-DNA photocycl oadducts. Because psoralen monoadditions to DNA alone are generally no t considered as being very phototoxic, a synergistic interaction of mo noadditions with pyrimidine dimers may be expected to occur in order t o explain the high photobiological effectiveness of this psoralen deri vative.