THE EFFECTS OF PHOTOOXIDATION BY PROFLAVINE ON HeLa CELLS—1. THE MOLECULAR MECHANISMS*

Abstract— DNA and RNA syntheses were inhibited immediately after proflavine treated HeLa cells were irradiated with visible light (400–500 nm). The molecular mechanism for this photooxidation may be either a free radical-mediated (Type I) or singlet oxygen-mediated (Type II) reaction. Non-toxic free radical and singlet oxygen quenchers were added to cells and sensitizer before irradiation to quench the appropriate excited state intermediate. Photooxidative damage (the inhibition of incorporation of ¹⁴C]-thymidine) in this system was greatly reduced in the presence of free radical quenchers (glutathione, penicillamine) and not significantly affected by the presence of singlet oxygen quenchers (α-tocopherol, β-carotene, DABCO). This suggests that at least part of the photodynamic damage in HeLa cells is via a Type I mechanism.