Oxidation of specific SH protein of mitochondria by photodynamic action of hematoporphyrin. Relevance to uncoupling of oxidative phosphorylation.

Photoexcited hematoporphyrin (Hp) induces the uncoupling of oxidative phosphorylation of mitochondria. The uncoupling was inhibited by pre-incubation of mitochondria with a fluorescent SH reagent, eosin-5-maleimide, which has been shown to react specifically with an essential SH group of the Pi/H+ symporter [Houstek and Pedersen, J Biol Chem 260: 6288-6295, 1985]. Eosin-5-maleimide labeled 33, 34.5 and 36 kDa proteins in untreated rat liver mitochondria. When eosin-5-maleimide was added after the treatment with Hp plus light, the proteins were not labeled. Singlet oxygen detection by the ESR spin trapping method during photoradiation of Hp was inhibited by amino acids. Cysteine inhibited it more efficiently than histidine, methionine, tryptophan, tyrosine or alanine under the conditions used. HPLC demonstrated that Hp plus light oxidizes cysteine to cystine together with a smaller amount of cysteinesulfinic acid. These results suggest that Hp plus light oxidizes the SH group of mitochondrial protein, probably the Pi/H+ symporter, with singlet oxygen as a mediator. The possibility of the uncoupling of oxidative phosphorylation through such a modification of the Pi/H+ symporter is discussed.