Influence of surface potentials on the mitochondrial H+ pump and on lipid-phase transitions

It has been shown that the surface potential of lipid membranes, as well as of mitochondria, can be shifted more positive by absorption of alkylbiguanides. Both phospholipid vesicles and natural membranes respond in an analogous way to this shift. Ion activities at the immediate membrane surface are influenced by sign and magnitude of the surface charge. Corresponding effects on ion transport and on fluorescence-probe binding can be observed. The mitochondrial H+ pump is inhibited when the surface charge is shifted more positive. In contrast,the absolute charge density determines the temperature of the ordered-fluid transition. The latter is increased by biguanides, suggesting that the membrane is rendered more rigid. The experiments make obvious that physical relations derived from model systems apply equally well to lipid-containing natural membranes.