The rate constants of valinomycin-mediated ion transport through thin lipid membranes.

Electrical relaxation experiments have been performed with phosphatidylinositol bilayer membranes in the presence of the ion carrier valinomycin. After a sudden change of the voltage a relaxation of the membrane current with a time constant of about 20 musec is observed. Together with previous stationary conductance data, the relaxation amplitude and the relaxation time are used to evaluate the rate constants of valinomycin-mediated potassium transport across the lipid membrane. It is found that the rate constants of translocation of the free carrier S and the carrier-ion complex MS(+) are nearly equal (2.10(4) sec(-1)) and are of the same order as the dissociation rate constant of MS(+) in the membrane-solution interface (5.10(4) sec(-1)). The equilibrium constant of the heterogeneous association reaction M(+) (solution) + S (membrane) --> MS(+) (membrane) is found to be approximately 1 M(-1), about 10(6) times smaller than the association constant in ethanolic solution.