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

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Abstract
The effects of small hydrophobic molecules on valinomycin-mediated K+ transport in small unilamellar soybean phospholipid vesicles were studied 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. Under conditions where, even in the absence of valinomycin, transmembrane K+ movement represented the primary and limiting counterion flux, < 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. 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. BHT, BHA and p-di-tert-butylbenzene, at levels which alter valinomycin-mediated K+ transport, did not alone induce changes in membrane fluidity. Changes in phosholipid 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.