, Volume 43, Issue 4, pp 367-394

Alkali ion transport through lipid bilayer membranes mediated by enniatin A and B and beauvericin

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Stationary conductance measurements with lipid bilayer membranes in the presence of enniatin A and B and beauvericin were performed. For comparison, some valinomycin systems were investigated. It was found that the conductance in the case of enniatin A and B is caused by a carrier ion complex with a 1∶1 stoichiometry, whereas for beauvericin, a 3∶1 carrier ion complex has to be assumed to explain the dependence of the conductance on carrier and ion concentration in the aqueous phase. The current-voltage curves measured with dioleoyl phosphatidylcholine membranes show a superlinear behavior for the three carriers in the presence of potassium. On the other hand, supralinear current-voltage curves were observed with membranes from different monoglycerides, except for beauvericin. The results obtained with enniatin A and B are in a satisfactory agreement with an earlier proposed carrier model assuming a complexation between carrier and ion at the membrane water interface.

The discrimination between potassium and sodium ions is much smaller for the enniatins than for valinomycin. This smaller selectivity as well as the fact that potassium ions cause the highest conductance with lipid bilayer membranes may be due to the smaller size of the cyclic enniatin molecules, which contain 6 residues in the ringvs. 12 in the case of valinomycin. Charge-pulse relaxation studies were performed with enniatin A and B, beauvericin, and valinomycin. For monoolein membranes only in the case of valinomycin, all three relaxations predicted by the model could be resolved. In the case of the probably more fluid membranes from monolinolein (Δ9, 12-C18: 2) and monolinolenin (Δ9, 12, 15-C18: 3) for all carrier systems except for beauvericin, three relaxations were observed.

The association rate constantk R , the dissociation rate constantk D , and the two translocation rate constantsk MS andk s for complexed and free carrier, respectively, could be calculated from the relaxation data. The carrier concentration in the aqueous phase had no influence on the rate constants in all cases, whereas a strong saturation of the association rate constantk R with increasing ion concentration was found for the enniatins. Because of the saturation,k R did not exceed a value of 4×105 m −1 sec−1 with 1m salt irrespective of carrier, ion, or membrane-forming lipid.

A similar but less pronounced saturation behavior was also observed for the translocation rate constantk S of the free carrier. The other two rate constants were independent of the ion concentration in the aqueous phase. In the case of the enniatins, the translocation rate constantk MS was not independent from the kind of the transported ion. In the series K+, Rb+ and Cs+,k MS increases about threefold. The turnover numbers for the carriers as calculated from the rate constants range between 104 sec−1 and 105 sec−1 and do not show a strong difference between the individual carriers. The conductance difference in the systems investigated here is therefore mainly caused by the partition coefficients, which are smaller for the enniatins than for valinomycin.