Pheophorbide a-induced photo-oxidation of cytochrome c: implication for photodynamic therapy.
Pheophorbide a-induced photo-oxidation, in vitro, of cytochrome c oxidase and cytochrome c results in irreversible modifications to both protein components. Photo-oxidation of cytochrome c, as exhibited by change in its heme oxidation state, displays exponential kinetics and is detected with a lag period. Both the photo-induced inactivation of the enzyme, and destruction of the substrate ability of cytochrome c occur as complex multi-process events. Under similar experimental conditions, the loss of the substrate capability of cytochrome c develops approximately three times faster than inactivation of the enzyme. The slight lag in the photo-oxidation of cytochrome c is due to pheophorbide a-induced superoxide production. However, the relative amount of photo-oxidant produced is considerably more effective than the cytochrome c reducing capacity of the superoxide. Neither hydroxyl radical nor hydrogen peroxide are involved in the photo-oxidation of the heme function. The possibilities of heme oxidation by a singlet oxygen mediated pathway or direct electron abstraction involving the heme or apoprotein are not excluded. It is proposed that a multi-site oxidation of numerous reduced energy cofactors within cells may augment collateral enzyme inactivation in maximizing photosensitizer-induced cytotoxicity. Accordingly, amphipathic photosensitizers, capable of accessing both lipid and aqueous compartments containing reduced cofactors, may be more effective agents for photodynamic therapy than those which exhibit a high specificity of subcellular localization.
Chernomorsky
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Department of Molecular Biology and Biochemistry, Rutgers University, Piscataway, NJ 08855-1059.
Wong
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Poretz
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D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism / Q000528:radiation effects*; D066298:In Vitro Techniques; D010084:Oxidation-Reduction; D010777:Photochemistry; D010778:Photochemotherapy; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology; D013481:Superoxides / Q000378:metabolism
D003574:Cytochrome c Group; D011838:Radiation-Sensitizing Agents; D013481:Superoxides; D002734:Chlorophyll; C032623:pheophorbide a