Anomalous potentials on bilayer lipid membranes in the presence of usnic acid: Markin-Sokolov versus Nernst-Donnan equilibrium

[Display omitted] •Ionophoric properties of usnic acid (UA) are probed in bilayer lipid membranes (BLM)•With UA, BLM potential at 1-pH unit gradient is twice the Nernst equilibrium level.•At Mg2+ or Ca2+ gradient, BLM potential has opposite sign with respect to Nernst one.•Mg2+ (Ca2+)/H+ exchange activity of UA is confirmed with pyranine-loaded liposomes.•The BLM data agree with a concept of membrane equilibrium for coupled ion transport. To gain insight into the mechanisms of ionophoric activity of usnic acid (UA), we examined the UA-induced generation of potentials on a planar bilayer lipid membrane (BLM) in the presence of concentration gradients of hydrogen and magnesium or calcium ions under open-circuit conditions. Remarkably, the BLM potential generated by UA at the proton concentration gradient of 1 pH unit was approximately twice the Nernst equilibrium level. With a concentration gradient of magnesium or calcium ions, the BLM potential generated by UA had the opposite sign. The observed anomalies in the membrane potentials were consistent with a theory developed by Markin and Sokolov (Bioelectrochem. Bioenerg. 1990) for the case of ionophore-mediated coupled fluxes of several ions across a membrane.