THE MECHANISM OF MONENSIN-MEDIATED CATION-EXCHANGE BASED ON REAL-TIMEMEASUREMENTS

Monensin is an ionophore that supports an electroneutral ion exchange across the lipid bilayer. Because of this, under steady-state conditio ns, no electric signals accompany its reactions. Using the Laser Induc ed Proton Pulse as a synchronizing event we selectively acidify one fa ce of a black lipid membrane impregnated by monensin. The short pertur bation temporarily upsets the acid-base equilibrium on one face of the membrane, causing a transient cycle of ion exchange. Under such condi tions the molecular events could be discerned as a transient electric polarization of the membrane lasting approx. 200 mu s. The proton-driv en chemical reactions that lead to the electric signals had been recon structed by numeric integration of differential rate equations which c onstitute a maximalistic description of the multi equilibria nature of the system (Gutman, M. and Nachliel, E. (1989) Electrochim. Acta 34, 1801-1806). The analysis of the reactions reveals that the ionic selec tivity of the monensin (H+ > Na+ > K+) is due to more than one term. B esides the well established different affinity for the various cations , the selectivity is also derived from a large difference in the rates of cross membranal diffusivities (Moll > MoNa > MoK), which have neve r been detected before. (v) Quantitative analysis of the membrane's cr ossing rates of the three neutral complexes reveals a major role of th e membranal dipolar field in regulating ion transport. The diffusion o f Moll, which has no dipole moment, is hindered only by the viscose dr ag. On the other hand, the dipolar complexes (MoNa and MoK) are delaye d by dipole-dipole interaction with the membrane. (vi) Comparison of t he calculated dipoles with those estimated for the crystalline conform ation of the [MoNa(H2O)(2)] and [MoK(H2O)(2)] complexes reveals that t he MoNa may exist in the membrane at its crystal configuration, while the MoK definitely attains a structure having a dipole moment larger t han in the crystal.