The effects of singlet oxygen produced by photodynamic action on the mitochondrial permeability transition differ in accordance with the localization of the sensitizer

We have examined whether the effects of singlet oxygen (O-1(2)) produced by photodynamic action on the mitochondrial permeability transition (PT) can be modulated by the localization of photosensitizers in irradiated mitochondria. We have previously shown that oxidation due to O-1(2) photogenerated in hematoporphyrin (HP)-loaded mitochondria can prevent opening of the PT pores, likely after degradation of some critical histidines (Salet et al., 1997, J. Biol. Chem. 272, 21938-21943), Equally, in the present study we have irradiated mitochondria in the presence of a structurally different photosensitizer producing O-1(2), namely 4,5',8-trimethylpsoralen (TMP), Fluorescence studies show that TRIP binds to protein sites which differ from those of HP, In sharp contrast with HP, TRIP-driven photodynamic action triggers per se pore opening. Interestingly, this inducing effect is inhibited when TRIP-treated mitochondria are irradiated after addition of mersalyl, a specific reagent protecting thiol groups of the inner mitochondrial membrane that are oriented toward the external hydrophilic phase. This fact suggests that O-1(2)-mediated thiol oxidation is responsible for TRIP-photoinduced pore opening. Taken together, these findings suggest that O-1(2) can activate or inactivate a cellular function such as mitochondrial PT depending on the site where it is produced in the mitochondrial membrane. (C) 2001 Academic Press.