Selective photosensitization of mitochondria by the lipophilic cationic porphyrin POR10

The lipophilic cationic porphyrin derivative 5.10.15.20-tetrakis(1-decylpyridinium-4-yl)-21H.23H-porphin tetrabromide (POR10) binds to the mitochondria of living HeLa cells and can be used for the selective photosensitization of mitochondria. Cytotoxic and phototoxic effects were studied by means of electron microscopy and respiratory activity and compared with the effects of the hydrophilic cationic 5.10.15.20tetrakis(1-methylpyridinium-4-yl)-21H.23H-porphin (POR1) and the anionic 5.10.15.20-tetrakis(1-sulphophen-4-yl)-21H.23H-porphin (TPPS). Lipophilic and hydrophilic molecules behave quite differently. The mitochondria are much more attacked by the lipophilic POR10 than by the hydrophilic POR1 and TPPS. At low POR10 concentrations and short incubation periods (C-D<0.1 mu M; t(1)<1 h), the mitochondrial ultrastructure scarcely alters, but, on increasing concentrations and prolonged incubation periods, in particular the cristae were affected. Irradiation (lambda>425 nm) of incubated HeLa cells also strongly attacks the cristae. For example, after incubation under mild conditions (0.1 mu M; 1 h) and subsequent irradiation (12 min at 65 mW cm(-2)) the cristae were partially destroyed. After incubation under intensified conditions (1 mu M; 1 h) and subsequent irradiation (15 min at 25 mW cm(-2)) the cristae disappeared completely and the mitochondria were transformed into globular vesicles surrounded by a double membrane. Surprisingly, respiratory activity increased (115%) after POR10 treatment under mild conditions (0.1 mu M; 30 min). However, subsequent irradiation for only 2 min (65 mW cm(-1)) decreased the respiratory activity to 18% relative to the untreated control (100%). Much smaller effects were observed with POR1-treated cells. We assume that the lipophilic POR10 cations, like other lipophilic dye cations, bind to the very lipophilic enzymes of the respiratory chain, which are constituents of the cristae. In contrast with POR10 the hydrophilic porphyrin derivatives are spread over several cellular compartments and are less phototoxic.