PHOTOTOXICITY OF HALOGENOFLUORESCEIN DERIVATIVES IN DICTYOSTELIUM-DISCOIDEUM AMEBAS - COMPARISON OF ',4',5',7'-TETRABROMOFLUORESCEIN-DIIODOFLUORESCEIN AND 4',5'-DIIODOFLUORESCEIN DEXTRAN

The halogenated fluorescein derivatives: 2',4',5',7'-tetrabromofluores cein isothiocyanate dextran (Br4FD) and 4',5'-diiodofluorescein isothi ocyanate dextran (1,FD), were found to be efficient photosensitizers f or the production of singlet oxygen. The singlet oxygen quantum yields were determined by reaction with acceptors in aqueous solution. Their comparison showed that the singlet oxygen quantum yield of Br4FD was threefold higher than that of I2FD. Br4FD was more resistant to photob leaching than I2FD. Both derivatives were internalized by fluid-phase pinocytosis in amoebae of the cellular slime mould Dictyostelium disco ideum. Subsequently, illumination of cells led to a dose-dependent los s of viability consistent with a role for singlet oxygen generated ins ide the endosomal compartments in the mechanism of photoinjury. Br4FD showed a three-fold higher efficiency than I2FD for photoinduced cytot oxicity.