title: Ion transport mediated by the valinomycin analogue cyclo(L-Lac-L-Val-D-Pro-D-Val)3 in lipid bilayer membranes.
abstract: Cyclo(L-Lac-L-Val-D-Pro-D-Val)3 (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 selectively sequence for PV-Lac is identical to that of valinomycin; i.e., Rb+ greater than K+ greater than Cs+ greater than or equal to NH+4 greater than Na+ greater than Li+. The steady-state zero-voltage conductance, G(0), is a saturating function of KCl 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 membrane-water 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 x 10(6) M-1 s-1; dissociation rate constant, 4 x 10(5) s-1; translocation rate constant for complex, 5 x 10(4) 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.
mesh_terms: D001692:Biological Transport; D002412:Cations / Q000378:metabolism; D002462:Cell Membrane / Q000032:analysis; D002463:Cell Membrane Permeability / Q000187:drug effects*; D008051:Lipid Bilayers / Q000378:metabolism*; D008433:Mathematics; D008564:Membrane Potentials; D014634:Valinomycin / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000494:pharmacology
chemical_list: D002412:Cations; D008051:Lipid Bilayers; D014634:Valinomycin; C030271:cyclo(L-lactoyl-L-valyl-D-prolyl-D-valyl)3