An electrical and structural characterization of H+/OH- currents in phospholipid vesicles

Paramagnetic amphiphiles have been utilized to measure and characterize electrogenic H+/OH- ion transport in a series of model membrane systems. Membrane conductivity to H+/OH- ions varies with the method of vesicle preparation and with the level of saturation of the membrane phospholipid. Small sonicated vesicles have the lowest conductivities by approximately an order of magnitude compared to reverse-phase or ether-injection vesicle systems. This conductivity is particularly sensitive to the presence of polyunsaturated lipids in the vesicle membrane. The current-delta pH dependence of the H+/OH- conductivity shows a nonideal behavior and renders the phenomenological membrane permeability dependent upon the experimental value of delta pH that is chosen. These factors can account for much, if not all, of the variability in the published values for the H+/OH- permeability of model membranes. A procedure has been developed to establish and estimate changes in the dipole potential of vesicle bilayers. Using this method, we demonstrate that H+/OH- currents are insensitive to alterations in the membrane dipole field, a result that suggests that these currents are not rate limited by diffusion over simple electrostatic barriers in the membrane interior. In addition, conduction in D2O has been examined, and we find that there is little difference in the magnitudes of D+/OD- currents compared to H+/OH- currents in vesicle systems.