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CONVEX CURVATURES OF ALKALINE ELUTION PROFILES OF DNA FROM HUMAN CELLS IRRADIATED WITH 405 nm UVA: EVIDENCE FOR INDUCTION OF SLOWLY DEVELOPING ALKALI‐LABILE SITES

The alkaline (pH 12.1) elution profiles of DNA from human P3 cells exposed to monochromatic 405 nm UVA radiation deviate from exponential: on a logarithmic plot of eluted fraction of DNA vs time of elution, the rate of elution accelerates for the first 6 h. Following this period, the profiles become exponential. In contrast, the elution profiles of DNA after 520 nm green light or ionizing radiation exposures (x‐ and gamma rays, and fission spectrum neutrons) are always strictly exponential, evidence that the convex profiles were not due to an artifact caused by elution technique. Holding the DNA at pH 12.1 for 6 h after 405‐nm exposures before initiating elution resulted in profiles that were close to exponential, with slopes similar to the final slopes observed following the 6‐h elution period in the original experiments. This is evidence that some DNA breaks develop slowly during the first 6 h of elution, as a result of exposure to alkali. Therefore, the DNA lesions induced by 405‐nm light as measured by the alkaline elution technique are apparently heterogeneous and include a major class of alkali‐labile sites that develop slowly during incubation at pH 12.1. Convex profiles also occur following exposure of the cells td visible light at 434 and 512 nm.