摘要：

Liposome permeability to net proton-hydroxide flux was compared under two conditions that have produced extreme values in previous studies. In the first [Nichols, J. W. & Deamer, D. W. (1980) Proc Natd Acadi Sci USA 77, 2038-2042] small pH gradients were permitted to decay near pH 7 and permeability coefficients in the, range of 10-4 cm/sec were obtained. In the second.[Nozaki, Y. & Tanford, C. (1981}Proc. Nat Acad& Sci USA 78, 4324-43281 the decay rate of large pH gradients (=3 pH units) was measured and apparent permeability coefficients in the range of 10-9 cm/sec were reported. We have repeated the conditions of the latter study and have found that the decay of large pH gradients produces a diffusion potential that limits net proton-hydroxide flux. The permeability coefficients calculated from the flux are therefore underestimated and do not reflect intrinsic proton permeability. With the same liposome preparations under conditions in which small gradients -decay so that diffusion potentials are negligible, the measured permeability coefficient-is near 10' cm/sec, as reported earlier. There is a large variance in reported'values ofproton-hydroxide, permeability coefficients in liposomes, which range between 10-4 and 10-9 cm/sec. We have compared two liposome systems that produced extreme values, those of Nichols and.Deamer (1) and Nozaki and Tanford (2). The former workers prepared internally buffered liposomes (pH 7) in potassium sulfate solutions by ether vaporization and used acid-base pulses to produce small pH gradients across the liposome membranes. The latter workers prepared aspartate-buffered liposomes (pH 4) in sodium nitrate and chloride solutions by octyl glucoside dialysis and produced large pH gradients by adjusting the external pH to near 7. Both pairs of workers estimated protonhydroxide permeability by measuring the decay kinetics of the pH gradients. In order to clarify this issue, we have used the octyl glucoside liposome system and directly compared the monitoring methods ofthe two studies. Certain assumptions made by Nozaki and Tanford were addressed. These are (i) that significant proton diffusion potentials do not develop and (ii) that most of the apparent proton flux occurs as neutral species of HC1 or HNO3. The first assumption was tested by addition of valinomycin in the presence of potassium while monitoring proton flux. We tested the second assumption by using sulfate as the major anion in the medium, because Gutknecht and Walter (3) showed that sulfate cannot carry protons across lipid bilayer membranes. We were able to obtain results similar to those ofNozaki and Tanford in regard to decay rates of the buffered pH gradient. However, when counterion current was permitted by valinomycin addition, the pH gradient rapidly decayed, suggesting that proton flux is limited by diffusion potentials under their conditions. When we used the same liposome system and monitored the, decay.of small pH gradients near pH 7 (conditions in which counterion-current is not limiting) we obtained proton-hydroxide permeabilities in the-range of 10' cm/sec.

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