Induction of cyclobutane pyrimidine dimers, pyrimidine(6-4)pyrimidone photoproducts, and Dewar valence isomers by natural sunlight in normal human mononuclear cells.

Immunocytochemistry was used for the direct measurement of cyclobutane pyrimidine dimers, (6-4) photoproducts, and Dewar isomers in normal human mononuclear cells following irradiation by natural sunlight or by a FS20 broad spectrum UVB sunlamp. The induction of each type of photoproduct was detected following 30-60 min sunlight exposure or with FS20 fluences as low as 50-100 Jm-2. With increasing FS20 fluences, there was a dose-dependent increase in the binding of pyrimidine dimer, (6-4) photoproduct, and Dewar isomer-specific monoclonal antibodies. The relative ratio of Dewar isomer to (6-4) photoproduct antibody binding sites was much higher following exposure to natural sunlight than to broad spectrum UVB. With the (6-4) monoclonal antibody, a small increase in binding sites was evident after a 1-h exposure to natural sunlight. This remained relatively constant with further exposure. These results are consistent with the hypothesis that, following irradiation with natural sunlight, the majority of (6-4) photoproducts are converted into Dewar valence isomers.