Fluoride inhibits photohemolysis induced by chloroaluminum phthalocyanine tetrasulfonate <tex-math>$({\rm AlPcS}_{4})$</tex-math> when it is added to dye-loaded human erythrocytes prior to light exposure (E. Ben-Hur, A. Freud, A. Canfi, and A. Livne, Int. J. Radiat. Biol. 59, 797-806, 1991). This is due to formation of a complex of F- with Al3+, leading to selective release and/or modified dye binding with some proteins so that the effective photochemical reaction is prevented. In this work we used F- as a probe to evaluate the involvement of the plasma membrane functions of Chinese hamster ovary cells in photocytotoxicity induced by chloroaluminum phthalocyanine (AlPc). Fluoride was found to protect against killing of cells photosensitized by AlPc but not <tex-math>${\rm AlPcS}_{4}$</tex-math>. Plasma membrane damage induced by AlPc photosensitization was manifested by K+ leakage, membrane depolarization, inhibition of glucose and amino acid uptake, and Na+/ K+- ATPase inactivation. The latter enzyme system was found to be the one most sensitive to inhibition by the combination of AlPc and PDT among the membrane functions studied, and was completely protected by F- in the dose range at which up to 95% of the cells are killed. Of the other membrane functions only glucose transport was slightly protected by F-. It is concluded that damage to the plasma membrane is involved in cell killing induced by AlPc photosensitization and that the plasma membrane enzyme Na+/ K+- ATPase is a probable candidate as a critical target.

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