CONDUCTANCE CHANGE IN PHOSPHOLIPID-BILAYER MEMBRANE BY AN ELECTRONEUTRAL IONOPHORE, MONENSIN

Monensin is a polyether antibiotic ionophore and is considered an elec troneutral Na/H antiporter. Its addition, however, increased the condu ctance of phospholipid bilayer membrane, and this increase was observe d only when the medium contained Na+. Analysis of the current-voltage curve suggested that the increase was due to the formation and the tra nslocation of an univalently charged species. The conductance at zero external voltage was proportional to the second power of monensin conc entration and increased with the decrease in pH of the medium. Modifie d monensin whose terminal carboxyl was esterified showed much larger i ncrease (ca. 100 times) in conductance than intact monensin. We conclu ded that the complex between the dimer of protonated monensin and Nacontributed to the electrogenic transport of monensin. This complex be ars a + 1 charge, which is consistent with the analysis of current-vol tage curves. Contrary to the conductance, the Na+ transfer rate of lip osomal membrane measured with Na-23-NMR was proportional to the monens in concentration, meaning that the electrogenic component contributes little to the total monensin-mediated Na+ transport in the present sys tem. It should be noted that this electrogenic component may change th e membrane potential.