Effect of membrane surface potential on the uptake of anionic compounds by liposomes

The effect of membrane surface potential on the uptake of several anionic compounds by liposomes (large unilamellar vesicles), which contain various amounts of dipalmitoylphosphatidylserine (DPPS), was investigated. The uptake amount of four tested anionic compounds (cefixime, benzyloxyindoleacetic acid (BOIAA), ceftibuten and S-1006) decreased with an increase in the DPPS content of liposomes, and was correlated with the membrane surface potential monitored using a fluorescent dye, 8-anilino-1-naphthalene sulfonate (ANS). Moreover, for all of the tested anionic compounds, a good correlation was observed between the ratio of the uptake value (5 min) by each of the liposomes comprising various amounts of DPPS to the uptake value by liposomes containing 10% DPPS and a relative membrane surface potential monitored by ANS. On the other hand, the uptake of zwitterionic compounds (enoxacin, cephradine and benzyloxytryptophan (BOTP)) was independent of DPPS content. These results suggest that the uptake of tested anionic compounds by large unilamellar lipid vesicles is dependent on the membrane surface potential which originates in the surface negative charge.