Abstract
Charge-pulse relaxation experiments with the negatively charged lipophilic ions, dipicrylamine and tetraphenylborate, (as well as with the positively charged carrier system Rb +-valinomycin) have been carried out in order to study the influence of sterols on the ion transport through the lipid bilayer membrane. The mol fraction of the sterols (cholesterol, epicholesterol, ergosterol, stigmasterol, dihydrocholsterol, epicoprostanol and cholesterololeate) as referred to total lipid was varied in a wide range (mol fractions 0–0.8). The monoolein/sterol or dioleoylphosphatidylcholine/sterol mixtures were dissolved in n-hexadecane in order to minimize effects of the sterol on the membrane thickness. Cholesterol had a strong influence on the transport of the lipophilic ions. Its incorporation into monoolein membranes increased the rate constant i of translocation up to 8-fold, but incorporation into phosphatidylcholine membranes had virtually no influence on k i . The other sterols with one hydroxy group and cholesterololeate had no influence on the rate constant or the partition coefficient β. The results are discussed on the basis of a possible change of dipole potential of the membrane caused by cholesterol and its derivatives. In the case of valinomycin-mediated Rb + transport only cholesterol had a strong influence on transport properties. The rate constants of association ( k R ) as well as the rate constants of translocation of the complex ( k MS ) and of the free carrier ( k S ) were reduced by incorporation of cholesterol up to eight-fold. The decrease of k S and k MS are possibly caused by a decrease of membrane fluidity, whereas the decrease of k R may be due to an increase of surface potential. The different action of cholesterol on the two transport systems is discussed under the assumption that the adsorption plane of the lipophilic ion is located more towards the aqueous side and that of the ion-carrier complexes more towards the hydrocarbon side of the dipole layer.
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