Kinetics of hydrogen ion diffusion across phospholipid vesicle membranes

The membrane-impermeant, pH-sensitive fluorescence probe 8-hydroxy-1,3,6-pyrenetrisulfonate can be entrapped within the internal aqueous compartment of unilamellar phospholipid vesicles, where it serves as a reliable indicator of internal aqueous hydrogen ion concentration [Clement, N. R., & Gould, J. M. (1981) Biochemistry 20, 1534-1539]. When the external (medium) pH of a suspension of soybean phospholipid vesicles was rapidly changed from 8.2 to 6.65, the rate of subsequent H+ influx into the vesicles, measured as the change in pyranine fluorescence, was limited (in KCl media) by the rate of charge-compensating counterion redistributions. The half-time for the pyranine fluorescence change (corresponding to an internal pH change from 8.2 to 7.43), which was several minutes in the absence of valinomycin, could be decreased to approximately 300 ms, but not further, by the K+ ionophore valinomycin. Proton ionophores such as gramicidin or bis(hexafluoroacetonyl)acetone (1799), on the other hand, decreased the time required for transmembrane H+ equilibration to less than 1 ms. These findings indicate that the intrinsic permeability of unilamellar vesicle membranes to hydrogen ions is surprisingly high and much greater than the observed permeabilities of other small ions.