Conductance routes for protons across membrane barriers

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Abstract
Simple phospholipid bilayers show a high level of permeability to protons; in spite of this fact, large proton gradients existing across such bilayers may decay very slowly. In sealed systems, the free movement of protons across a membrane barrier is severely restricted by the coincident development of a proton diffusion potential. Using the fluorescent weak acid N-[5-(dimethylamino)naphth-1-ylsulfonyl]glycine (dansylglycine) to monitor the collapse of pH gradients across barrier membranes, it is revealed that in strongly buffered systems movement of the small number of protons giving rise to this electrical potential is insufficient to perturb the proton concentration gradient; significant flux of protons (and hence significant collapse of the concentration gradient) can only occur (a) if protons traverse the membrane as part of an electroneutral complex or (b) if there is a balancing flow of appropriate counterions. In both instances, proton flux is obligatorily coupled to the translocation of species other than protons. In weakly buffered systems, the small initial uncoupled electrogenic flux of protons may significantly alter the concentration gradient. This initial rapid gradient collapse caused by uncoupled electrogenic proton movements is then electrogenic proton flux shows a temperature dependence very similar to that demonstrated for water permeation across simple lipid bilayers; upon cooling, there is a sharp decrease in flux at the temperature coinciding with the main gel-liquid-crystalline phase transition of the lipid. The coupled proton flux shows a markedly different temperature dependence with no dramatic change in rate at the phase transition temperature and strong similarity to the behavior previously seen with solutes known to be permeating as electrically neutral compounds. This temperature dependence profile for proton permeation supports the suggestion that this process has much more in common with water diffusion than with the translocation of other monovalent cations, such as Na+, across membrane barriers.