Shape of the potential energy barrier of the iodine-mediated halide transport.

Voltage-clamp experiments were performed on lipid bilayer membranes to study the voltage dependence of the iodine-mediated halide transport. Under all experimental conditions only one exponential current relaxation, apart from the capacitive spike, could be resolved up to a clamp voltage of 200 mV. The current relaxation could be described by an initial conductance, G0, the relaxation time constant, tau, and the relaxation amplitude, alpha, that is the difference between the initial current, I0, and the steady state current, I chi, divided by the steady state current. The occurrence of one single exponential relaxation suggested that one of the different transport steps involved in the carrier-mediated ion transport according to the Lüger-model is always in equilibrium. This is most probably the transport of the free carriers across the membrane. The voltage dependence of G0, tau, and of alpha were used to determine the voltage dependence of the translocation rate constants of the complexed carriers, kAS. In the case of the iodine-mediated iodide transport, G0, tau and alpha were only mediate voltage-dependent, which means the voltage dependent translocation of the complex encounters a trapezoidal barrier shape. For the iodine-mediated bromide translocation G0, tau and alpha exhibited no dependence on the applied clamp-voltage, which suggested that a square Nernst-Planck barrier limits the transport of the corresponding complex.