title: Switch from inhibition to activation of the mitochondrial permeability transition during hematoporphyrin-mediated photooxidative stress. Unmasking pore-regulating external thiols.
abstract: We have studied the mitochondrial permeability transition pore (PTP) under oxidizing conditions with mitochondria-bound hematoporphyrin, which generates reactive oxygen species (mainly singlet oxygen, (1)O(2)) upon UV/visible light-irradiation and promotes the photooxidative modification of vicinal targets. We have characterized the PTP-modulating properties of two major critical sites endowed with different degrees of photosensitivity: (i) the most photovulnerable site comprises critical histidines, whose photomodification by vicinal hematoporphyrin causes a drop in reactivity of matrix-exposed (internal), PTP-regulating cysteines thus stabilizing the pore in a closed conformation; (ii) the most photoresistant site coincides with the binding domains of (external) cysteines sensitive to membrane-impermeant reagents, which are easily unmasked when oxidation of internal cysteines is prevented. Photooxidation of external cysteines promoted by vicinal hematoporphyrin reactivates the PTP after the block caused by histidine photodegradation. Thus, hematoporphyrin-mediated photooxidative stress can either inhibit or activate the mitochondrial permeability transition depending on the site of hematoporphyrin localization and on the nature of the substrate; and selective photomodification of different hematoporphyrin-containing pore domains can be achieved by fine regulation of the sensitizer/light doses. These findings shed new light on PTP modulation by oxidative stress.
authors.affiliation: C.N.R. Institute of Neurosciences at the Department of Biomedical Sciences, University of Padova, Italy.
authors.forename: Valeria
authors.initials: V
authors.lastname: Petronilli
authors.forename: Justina
authors.initials: J
authors.lastname: Sileikyte
authors.forename: Alessandra
authors.initials: A
authors.lastname: Zulian
authors.forename: Federica
authors.initials: F
authors.lastname: Dabbeni-Sala
authors.forename: Giulio
authors.initials: G
authors.lastname: Jori
authors.forename: Silvano
authors.initials: S
authors.lastname: Gobbo
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Tognon
authors.forename: Peter
authors.initials: P
authors.lastname: Nikolov
authors.forename: Paolo
authors.initials: P
authors.lastname: Bernardi
authors.forename: Fernanda
authors.initials: F
authors.lastname: Ricchelli