Kinetics of proton-hydroxyl transport across lecithin vesicle membranes as measured with a lipoid pH-indicator.

When unilamellar vesicles are prepared in the presence of 3-Palmitoyl-7-ocy-coumarin abbreviation 3P-UBF) this lipoid pH-Indicator is anchored by its fatty acid chain to the membrane and can be used to measure pH-changes at the outer and inner membrane surface (ranging from pH to pH 9.5). By rapidly changing the pH of the outer aqueous phase a pH-gradient is set up across the vesicle membrane. The rate of the subsequent H+ or OH- influx into the vesicles can be measured as a change of the 3P-UBF absorbance at 424 nm. This was done with a stopped-flow -spectrophotometer at temperatures between 10 degree C and 50 degree C. Suspensions of vesicles prepared from egg-lecithin or L-dipalmitoyl-lecithin were investigated in buffered salt solutions. The influence of Na+, K+, Cl-, SO2-(4) and valinomycin on the rate of absorbance changes was studied at different temperatures. It was found that the rate of the pH-equilibration between the aqueous phase outside and inside the vesicles depends on the direction of the pH-gradient. This new result together with a high H+/OH- permeability of vesicle membranes found in recent studies from other laboratories and confirmed by this investigation is interpreted to indicated a higher permeability of the vesicle membrance to OH--ions compared to H+-ions. (Calculated values are: POH = 1.4 x 10(-4) cm/s at pH9 and PH = 8.3 x 10 (-7) cm/s at pH 5 and 20 degree C.) All data described in the literature in detail agree with this suggestion but a pH-dependence of POH and PH cannot be excluded.