Coupled transport of protons and anions through lipid bilayer membranes containing a long-chain secondary amine.
Transport of protons and halide ions through planar lipid bilayers made from egg lecithin and a long-chain secondary amine (n-lauryl [trialkylmethyl] amine) in n-decane was studied. Net proton fluxes were measured with a pH electrode, and halide fluxes were measured with 82Br- and 36Cl-. In membranes containing the secondary amine, a large net proton flux was produced either by a Br- gradient with symmetrical pH or by a pH gradient with symmetrical Br-, but not by a pH gradient in Br--free solutions. This H+ flux was electrically silent (nonconductive), and the H+ permeability coefficient was greater than 10(-3) cm sec-1 in 0.1 M NaBr. In Br--free solutions, H+ selectivity was observed electrically by measuring conductances and zero-current potentials generated by H+ activity gradients. The permeability coefficient for this ionic (conductive) H+ flux was about 10(-5) cm sec-1, several orders of magnitude smaller than the H+ permeability of the electroneutral pathway. Large electroneutral Br- exchange fluxes occurred under symmetrical conditions, and the permeability coefficient for Br- exchange was about 10(-3) cm sec-1 at pH 5. The one-way Br- flux was inhibited by substituting SO4= for Br- on the "trans" side of the membrane. These results support a "titratable carrier" model in which the secondary amine exists in three forms (C, CH+ and CHBr). Protons can cross the membrane either as CHBr (nonconductive) or as CH+ (conductive), whereas Br- crosses the membrane primarily as CHBr (nonconductive). In addition to these three types of transport, there is also a pH-dependent conductive flux of Br- which has a permeability coefficient of about 10(-7) cm sec-1 at pH 5. Experiments with lipid monolayers suggest that the pH dependence of this conductive flux is caused by a change in surface potential of about +100 mV between pH 9.5 and 5.0.
D000588:Amines; D001693:Biological Transport, Active; D001965:Bromides / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D004553:Electric Conductivity; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials; D008567:Membranes, Artificial; D010539:Permeability
D000588:Amines; D001965:Bromides; D002712:Chlorides; D008567:Membranes, Artificial; D006859:Hydrogen