Proton/hydroxide conductance through lipid bilayer membranes.

A simple method of measuring proton/hydroxide conductance (GH/OH) through planar lipid bilayer membranes is described. First the total conductance (Gm) is measured electrically. Then the H+/OH- transference number (TH/OH) is estimated from the diffusion potential (Vm) produced by a transmembrane pH gradient. The pH gradient is produced by a pair of buffered solutions which have identical concentrations of all ions except H+ and OH-. Thus, Vm is due entirely to H+/OH- diffusion and GH/OH can be calculated from the relations, Vm = TH/OHEH/OH and GH/OH = TH/OHGm, where EH/OH is the equilibrium potential for H+ and OH-. In bilayers made from bacterial phosphatidylethanolamine (PE) in n-decane, GH/OH is nearly independent of pH, ranging from about 10(-9) S cm-2 at pH 1.6 to 10(-8) S cm-2 at pH 10.5. Because GH/OH is nearly independent of pH, the calculated permeability coefficients to H+ and/or OH- are extremely pH dependent, which partly explains the wide range of values reported for phospholipid vesicles and biological membranes. GH/OH appears to be independent of the membrane surface charge, because titrating either the phosphate or the amino group of PE has little effect on GH/OH. GH/OH is reduced about 10-fold when the water activity is reduced 33% by replacement with glycerol. Although the mechanism of H+/OH- conductance is not known, the relation between GH/OH and water activity suggests that several water molecules are involved in the H+/OH- transport process.