Kinetics of the valinomycin-induced potassium ion leak from liposomes with potassium thiocyanate enclosed

Abstract

The kinetics of the valinomycin-induced K+ leak from egg lecithin liposomes with KCNS enclosed were studied by measuring the initial increase of the leak following the addition of the ionophore. Variation of liposome and valinomycin concentrations enabled a discrimination between the affinity of the ionophore for the lipid bilayer and the turnover rate of the transport process. The results indicate that valinomycin is distributed immediately between the outer liposomal bilayer and the aqueous phase. A partition constant of 0.0222 cm at 30°C was obtained for egg lecithin liposomes. By varying the temperature it was shown that the partition constant has a negative temperature coefficient. The Arrhenius activation energy of the transport process itself is 15.7 ± 1.1 kcals/mole. Concerning the mechanism of the valinomycin-mediated transport it was found that the complex formation can be described kinetically by the formation of a ternary complex in which K+ and a thiocyanate ion are bound by the ionophore.

By varying the fatty acid composition of the membrane constituents it was found that the turnover rate of the transport increased with increasing unsaturation of the phospholipids present in the membrane. This result suggests that the translocation of the complex is the rate-limiting step of the transport cycle. At 30 °C a turnover constant of 328 ± 44 potassium ions/molecule valinomycin per second was obtained for egg lecithin lipsomes.