Electrostatic potentials arising due to adsorption of Na+, K+-ATPase-containing membrane fragments on bilayer lipid membrane

The adsorption of Na+,K+-ATPase-containing membrane fragments (MF) on the surface of planar bilayer membranes (BLM) was studied by the method of capacitive current second harmonic compensation extended to laterally heterogenic systems. The suspension of MF added to water solution adjacent to BLM creates the electrostatic potential measured by the method mentioned above and changes the BLM capacity and its conductivity induced by nonactine or pentachlorophenole. It is shown that the electrostatic potential can be caused by two different mechanisms. The first is the adsorption on BLM of charged molecules which can be either phospholipids dissolved in water, or detergent, sodium dodecyl sulfate, present in the sample as an impurity. The second mechanism is the adsorption of charged MF leading to the creation of an electric field inside the BLM in the contact region with the MF, if the distance between the membranes is comparable to the thickness of the diffuse electric double layer. To distinguish between both possibilities, experiments were carried out with ex-change of the aqueous solution adjacent to the BLM by perfusion of the cell. The perfusion leads to a potential drop due to washing of single molecules adsorbed on the membrane surface. The remaining potential (about 10 mV) is probably caused by irreversible adsorption of ME The surface charge of MF was about -10 mC/m(2), determined from their electrophoretic mobility in suspension. Most likely, this charge is caused by charged lipids present in ME It was confirmed by the experiments with liposomes made with the lipids extracted from ME The addition of these liposomes into the cell created the potential changes in BLM of the same sign as in the case of ME The electric field inside MF contacting with BLM was estimated using different values of unknown parameters of the system, namely, the thikness of water gap between BLM and MF, and the surface coverage of BLM by fragments. This field may be very large and has to be taken into account in studies of Na+, K+-ATPase functioning in such experimental systems.