INTRACELLULAR AND EXTRACELLULAR RADIOSENSITIZATION OF SERRATIA-MARCESCENS BY BIPYRIDINIUM COMPOUNDS

The one-electron reduced form of the bipyridinium compounds benzylviologen and methylviologen have been found to diffuse across the cytoplasmic membrane of Serratia marcescens cells. Subsequent reoxidation of the viologens to the dicationic form traps the compound inside the cells. Cells at a density of 4 X 109 ml-1 took up approximately half of the compound when incubated with an initial extracellular concentration of 200 μM of either reduced viologen. The degree of radiosensitization in anoxia afforded by the compounds parallels the rise in internal concentration and reaches a maximum enhancement ratio of 2.0 ± 0.1 for both compounds. This level in sensitization is similar to that found when the compounds are external to the cell. No additivity in sensitization is found when the viologens are both internal and external to the cells at the time of irradiation suggesting that the same target site is sensitized. This site is probably some membrane-associated structure.