Relationship between fluidity and ionic permeability of bilayers from natural mixtures of phospholipids

Summary Proton and calcium permeability coefficients of large unilamellar vesicles made from natural complex mixtures of phospholipids were measured in various conditions and related to membrane fluidity. Permeability coefficients at neutral pH and 25°C were in the range of 10⁴ cm sec¹ and 2.5×10¹¹ cm sec¹ for protons and calcium, respectively. With the exception of two cases. (H⁺)>10⁴m and (Ca²⁺)>10³m, fluidity increases correspond to permeability increases. Theoretical analysis shows that, for both ions, the measured values of permeability coefficients imply that the permeation process is controlled by the productD₁D₂ of the diffusion coefficient from the medium into the membrane (D₁) by the diffusion coefficient in the membrane (D₂). Further analysis ofD₁ values deduced from combined use of permeability and fluidity data shows that the solubilization should occur in a medium of dielectric constant of about 12, suggesting the involvement of the hydration water of membranes. High proton concentrations, although having virtually no effect on fluidity, trigger the appearance of (i) lateral heterogeneity in membranes, as seen by³¹P NMR, and (ii) large permeability increases. It is proposed that the main effect of fluidity and/or lateral heterogeneity on permeability may bevia the membrane hydration control. We conclude that the current assumption that permeability is controlled by fluidity should be regarded with caution, at least in the case of ions and natural mixtures of phospholipids.