Interstrand DNA crosslinking by 4,5'8-trimethylpsoralen plus monochromatic ultraviolet light. Studies by alkaline elution in mouse L1210 leukemia cells

DNA crosslinking by 4,5',8-trimethylpsoralen plus monochromatic ultraviolet LIGHT of wavelength 365 nm was studied in mouse L1210 leukemia cells. DNA breaks and crosslinking were evaluated by alkaline elution of DNA from poly(vinyl chloride) filters. Trimethylpsoralen plus 365 nm LIGHT produced DNA crosslinks but not breaks. The kinetics of crosslinking were linear with respect to concentration and second-order with respect to LIGHT exposure time. The latter finding supports the proposed two photon mechanism for the formation of diadducts. In contrast to DNA crosslinking agents such as nitrogen mustard, nitrosoureas and platinums, trimethylpsoralen crosslinks were resistant to proteolytic digestion. Thus, trimethylpsoralen plus 365 nm LIGHT produced interstrand crosslinks, as proposed for a bifunctional agent binding to bases on opposite DNA strands.