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Effect of the membrane potential on the binding of Merocyanine 540 to Human erythrocytes Johan W.M. Lagerberg, John VanSteveninck and Tom M.A.R. Dubbelman Sylvius Laboratories, Department of Molecular Cell Biology, Leiden University, Leiden, the Netherlands ## ABSTRACT Illumination of erythrocytes in the presence of MC540 resulted in changed binding characteristics of MC540, i.e. a red shift of the emission maximum of bound dye with an increase in the relative fluorescence quantum yield. AlPcS4-mediated photodynamic treatment, before addition of MC540, resulted in a comparable change of the MC540 binding characteristics with, in addition, an increase in the concentration of MC540 in the membrane. Both photodynamic treatments induce depolarization of the red cell membrane, with a dose-dependency comparable with that of changed MC540 binding. Also depolarization, induced by incubation of the cells with A23187 in the presence of Ca2+ in high [K+]-buffer, resulted in similar changes of the MC540 binding characteristics. Hyperpolarization induced by incubation with A23187 in low [K+]-buffer resulted in decreased binding of MC540. In accordance, the MC540-mediated photodamage to the red cells was decreased upon hyperpolarization of the cells. The results indicate that the binding of MC540 to erythrocytes is strongly dependent on the membrane potential, and that hyperpolarization of the membrane could be a possibility to protect erythrocytes against MC540-mediated photodynamic damage.