Nonmediated flip-flop of anionic phospholipids and long-chain amphiphiles in the erythrocyte membrane depends on membrane potential

The nonmediated inward translocation (flip) of the anionic fluorescent N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)- (NBD-)labeled phospholipid phosphatidylmethanol (PM) from the outer to the inner membrane leaflet of human erythrocytes and vice versa depends on membrane potential. Interestingly, inside-positive potentials due to chloride gradients and the native chloride conductance of the cells resulted in an increase of the flip rates. This flip enhancement could be suppressed by addition of gramicidin D, which increases cation conductance, or 4,4'-diisothiocyanatostilbene-2,2'-disufonate (DIDS), which inhibits anion conductance. Conversely, inside negative potentials established by an outward-directed K+ gradient in the presence of gramicidin on DIDS-treated cells resulted in a decrease of flip rate. Flip rate exhibited an exponential dependence on membrane potential. The opposite effects of the positive and negative potentials were obtained for the outward translocation (flop) from the inner to the outer membrane leaflet. Similar potential dependencies were found for the nonmediated flip of anionic NBD-labeled phosphatidic acid (PA) and 2-(N-decyl)aminonaphthalene-6-sulfonic acid (2,6-DENSA) following blockage of the band-3-mediated component of flip. The membrane potential also influences the stationary distribution of the anionic lipids between the inner and outer leaflets. The distribution is shifted to the inner leaflet by increasingly positive potentials and to the outer leaflet by increasingly negative potentials. It is concluded that nonmediated flip-flop of the anionic phospholipids and the long-chain sulfonate represents electrogenic translocation of the unprotonated charged lipids across the hydrophobic barrier.