Influence of membrane structure on the kinetics of carrier-mediated ion transport through lipid bilayers.

Charge-pulse relaxation experiments of valinomycin-mediated Rb+ transport have been carried out in order to study the influence of membrane structure on carrier kinetics. From the experiment data the rate constants of association (kR) and dissociation (kD) of the ion-carrier complex as well as the rate constants of translocation of the complex (kMS) and of the free carrier (kS) could be obtained. The composition of the planar bilayer membrane was varied in a wide range. In a first series of experiments, membranes made from glycerolmonooleate dissolved in different n-alkanes (n-decane to n-hexadecane), as well as solvent-free membranes made from the same lipid by the Montal-Mueller technique were studied. The translocation rate constants kS and kMS were found to differ by less than a factor of two in the membranes of different solvent content. Much larger changes of the rate constants were observed if the structure of the fatty acid residue was varied. For instance, an increase in the number of double bonds in the C20 fatty acid from one to four resulted in an increase of kS by a factor of seven and in an increase of kMS by a factor of twenty-four. The stability constant K = kR/kD of the ion-carrier complex as well as the translocation rate constants kS and kMS were found to depend strongly on the polar headgroup of the lipid. The incorporation of cholesterol into glycerolmonooleate membranes reduced kR, kMS and kS up to seven-fold.