Modulation by small hydrophobic molecules of valinomycin-mediated potassium transport across phospholipid vesicle membranes.

The effects of small hydrophobic molecules on valinomycin-mediated K+ transport in small unilamellar soybean phospholipid vesicles have been studied by using a vesicle-entrapped pH-sensitive hydrophilic fluorescence probe to monitor counterion-limited, passive H+ diffusion into vesicles after an abrupt decrease in external pH [Clement, N. R., & Gould, J. M. (1981) Biochemistry (preceding paper in this issue)]. Under conditions where, even in the absence of valinomycin, transmembrane KL+ movement represented the primary and limiting counterion flux, less than 1 valinomycin molecule/vesicle was sufficient to accelerate the rate of H+ entry into all of the vesicles. Incorporation of the bulkily substituted molecules butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and p-di-tert-butylbenzene into soybean lipid bilayers had no effect upon K+ diffusion in the absence of valinomycin. However, the presence of these hydrophobic molecules increased the apparent efficacy for K+ transport of a given valinomycin concentration by as much as 4-6 fold. The less bulky membrane perturbants tert-butyl alcohol, phenol, and heptane showed very much less dramatic effects. While the rate of valinomycin-mediated K+ transport (in the presence or absence of BHT) was very sensitive to temperature-induced changes in membrane fluidity, the degree of synergistic interaction between valinomycin and BHT was independent of temperature. Furthermore, BHT, BHA, and p-di-tert-butylbenzene, at levels which alter valinomycin-mediated K+ transport, did not by themselves induce changes in membrane fluidity. It is postulated that changes in phospholipid head-group packing and/or surface charge density brought about by the presence of bulky perturber molecules leads to changes in partitioning of valinomycin or the valinomycin-K+ complex between the aqueous and membrane phases.