ACRIDINE ORANGE-MEDIATED PHOTODAMAGE TO CULTURED-CELLS

Photosensitization mediated by the lysosomotropic, weakly basic dye ac ridine orange (AO) was studied on cultured J-774 cells. The phototoxic ity was found to be potentiated by elevated oxygen tension and reduced at low oxygen tension. Moreover, cell cultures pre-exposed to the sin glet oxygen scavenger sodium azide showed pronounced protection agains t the loss of viability induced by AO and blue light. AO-mediated phot osensitization was neither increased by pre-exposure of cell cultures to ferric chloride or the catalase-inhibitor aminotriazole nor decreas ed by exposure to deferoxamine. These observations suggest that type I I (singlet oxygen-mediated) reactions predominate over type I reaction s (radical-mediated). A rapid and pronounced decrease in lysosomal cat hepsin L activity (up to 60%) was observed after an initial 10 min irr adiation, indicating the lysosomal compartment to be an early target. This irradiation time did not, however, result in any substantial loss of viability. Levels of cytosolic lactate dehydrogenase were unaffect ed even after 30 min irradiation, indicating that neither cytosol nor plasma membrane is a primary target of the AO-mediated photodamage. Gl utathione depletion by pre-exposure to buthionine-S,R-sulfoximine (BSO ) much enhanced the sensitivity of J-774 cells to AO-mediated photosen sitization, indicating a protective role for thiol-containing compound s against AO-mediated photodamage.