Ion transport mediated by the valinomycin analogue cyclo(L-Lac-L-Val-DPro-D-Val)3 in lipid bilayer membranes

Abstract

Cydo(L-Lac-L-Val-n-Pro-D-Val)a (PV-Lac) a structural analogue of the ion-carrier valinomycin, increases the cation permeability of lipid bilayer membranes by forming a 1:1 ion-carrier complex. The selectivity sequence for PV-Lac is identical to that of valinomycin, i.e., Rb+ > K+ > Cs+ ≥ NH+4 > Na+ > Li+. The steady-state zero-voltage conductance, G(0), is a saturating function of KCI concentration. A similar behavior was found for Rb+, Cs+, and NH+4. However, the ion concentration at which G(0) reaches a plateau strongly depends on membrane composition. The current-voltage curves present saturating characteristics, except at low ion concentrations of Rb+ K+, or Cs+. The ion concentration at which the saturating characteristics appear depends on membrane composition. These and other results presented in this paper agree with a model that assumes complexation between carrier and ion at the membranewater interface. Current relaxation after voltage-jump studies were also performed for PV-Lac. Both the time constant and the amplitude of the current after a voltage jump strongly depend on ion concentration and membrane composition. These results, together with the stationary conductance data, were used to evaluate the rate constants of the PV-Lac-mediated K+ transport. In glycerolmonooleate they are: association rate constant, 2 × 106 M-1 s-1; dissociation rate constant, 4 × 105 s-1; translocation rate constant for complex, 5 × l04 s-1; and the rate of translocation of the free carrier (ks), 55 s-1. ks is much smaller for PV-Lac than for valinomycin and thus limits the efficiency with which the carrier is able to translocate cations across the membrane. © 1981, Rockefeller University Press., All rights reserved.