Changes of intrinsic membrane potentials induced by flip-flop of long-chain fatty acids

The passive transbilayer movement-flip-flop-was investigated on planar bila yer Lipid membranes (BLMs), containing myristic, stearic, or linoleic long- chain fatty acids (FA). In response to a transbilayer pH gradient, a differ ence in the surface charges between inner and outer leaflets appeared. Beca use the BLM was formed from FA and neutral lipid, a surface potential diffe rence was originated solely by a concentration difference of the initially equally distributed ionized FA. As revealed by S-potential measurements, th e corresponding surface potential difference Delta Phi(s) was at least twic e the value expected from a titration of the FA alone. The additional surfa ce charge was attributed to FA flip-flop induced by the transbilayer pH gra dient. Delta Phi(s) was derived from capacitive current measurements: carri ed out with a direct current (dc) bias and was corrected for changes of mem brane dipole potential Phi(d). Dual-wavelength ratiometric fluorescence mea surements have shown that Phi(d) values of the pure DPhPC bilayers and BLMs containing 40 mol % FA differ by less than 6%. It is concluded that fast F A Rip-flop is not restricted to membranes with high curvature. The role of pH gradient as an effective driving force for the regulation of FA uptake i s discussed.