Role of Leaflet Asymmetry in the Permeability of Model Biological Membranes to Protons, Solutes, and Gases

Bilayer asymmetry in the apical membrane may be important to the barrier function exhibited by epithelia in the stomach, kidney, and bladder. Previously, we showed that reduced fluidity of a single bilayer leaflet reduced water permeability of the bilayer, and in this study we examine the effect of bilayer asymmetry on permeation of nonelectrolytes, gases, and protons. Bilayer asymmetry was induced in dipalmitoylphosphatidylcholine liposomes by rigidifying the outer leaflet with the rare earth metal, praseodymium (Pr³⁺). Rigidification was demonstrated by fluorescence anisotropy over a range of temperatures from 24 to 50°C. Pr³⁺-treatment reduced membrane fluidity at temperatures above 40°C (the phase-transition temperature). Increased fluidity exhibited by dipalmitoylphosphatidylcholine liposomes at 40°C occurred at temperatures 1–3°C higher in Pr³⁺-treated liposomes, and for both control and Pr³⁺-treated liposomes permeability coefficients were approximately two orders of magnitude higher at 48° than at 24°C. Reduced fluidity of one leaflet correlated with significantly reduced permeabilities to urea, glycerol, formamide, acetamide, and NH₃. Proton permeability of dipalmitoylphosphatidylcholine liposomes was only fourfold higher at 48° than at 24°C, indicating a weak dependence on membrane fluidity, and this increase was abolished by Pr³⁺. CO₂ permeability was unaffected by temperature. We conclude: (a) that decreasing membrane fluidity in a single leaflet is sufficient to reduce overall membrane permeability to solutes and NH₃, suggesting that leaflets in a bilayer offer independent resistances to permeation, (b) bilayer asymmetry is a mechanism by which barrier epithelia can reduce permeability, and (c) CO₂ permeation through membranes occurs by a mechanism that is not dependent on fluidity.