A Role for Hydrogen Peroxide in the Pro-apoptotic Effects of Photodynamic Therapy

Although the first reactive oxygen species (ROS) formed during irradiation of photosensitized cells is almost invariably singlet molecular oxygen (O-1(2)), other ROS have been implicated in the phototoxic effects of photodynamic therapy (PDT). Among these are superoxide anion radical (O-center dot(2)-), hydrogen peroxide (H2O2) and hydroxyl radical ((OH)-O-center dot). In this study, we investigated the role of H2O2 in the pro-apoptotic response to PDT in murine leukemia P388 cells. A primary route for detoxification of cellular H2O2 involves the peroxisomal enzyme catalase. Inhibition of catalase activity by 3-amino-1,2,4-triazole led to an increased apoptotic response. PDT-induced apoptosis was impaired by addition of an exogenous recombinant catalase analog (CAT-SKL) that was specifically designed to enter cells and more efficiently localize in peroxisomes. A similar effect was observed upon addition of 2,2'-bipyridine, a reagent that can chelate Fe+2, a co-factor in the Fenton reaction that results in the conversion of H2O2 to (OH)-O-center dot. These results provide evidence that formation of H2O2 during irradiation of photosensitized cells contributes to PDT efficacy.