Effects of hemin on porphyrin-induced photodamage of membrane-bound Mg2+-ATPase and tryptophan oxidation

The effects of photoactivated porphyrins on membrane-bound Mg 2 + -ATPase activity and oxidation of free tryptophan were studied. The efficiency of the porphyrins to induce photoinhibition of Mg 2 + -ATPase activity decreases in the order: protoporphyrin > hematoporphyrin derivative > hematoporphyrin > uroporphyrin. When the photooxidation of tryptophan in solution was followed, the photodamage decreases in a reversed order: uroporphyrin > hematoporphyrin > hematoporphyrin derivative > protoporphyrin. These effects were dose-dependent and intensity increased with time of illumination. Thus, tryptophan, which is water-soluble, is more easily destroyed by a hydrophylic porphyrin, whereas membrane Mg 2 + -ATPase, which is lipophilic, is more severely damaged by hydrophobic porphyrin. Hemin reduced the photodamage induced by protoporphyrin on mem- branal Mg 2 + -ATPase, but was without effect on photooxidation of tryptophan by porphyrins. Electron microscopic examination of membrane vesicles treated with hemin showed association between hemin and the membrane bilayer. The results suggest competition between hemin and photosensitive porphyrin for membranal sites. In order to observe the photodamage, the porphyrin should associate with the effective site of the target or at least be very close to it. Thus, the association between the sensitizer and the effective site of the target is the determining factor in photosensitization.