Lanthanide-ion transport across phospholipid vesicular membranes: a comparison of alamethicin 30 and A23187 using 1H-NMR spectroscopy.

The kinetics of Pr 3+ transport by the ionophores alamethicin 30 and A23187 across unilamellar phospho-lipid vesicular membranes has been compared by following the time-dependent changes in the IH-NMR spectrum of the vesicles. The measured rates of transport allow stoichiometries of the transporting species to be deduced which are consistent with channel-and carrier-mediated mechanisms respectively. The method provides a useful complement to planar bilayer conductivity studies of these systems. It has recently been emphasized that in order to investigate transmembrane transport mechanisms the development of a range of physical methods is required which can probe the membrane systems at a molecular level (l). Several groups have already begun to explore the application of NMR techniques to the problem, initially using model systems (2-.5). By employing unilamellar vesicular membranes we have demonstrated that a number of mechanisms can be distinguished for the transport of lanthanide ions by membrane-active substances including surfactants (6,7) and carrier-type ionophores (g). Recently it has been confirmed that the pore-forming polypeptide, alamethicin 30, can transport lanthanide ions across planar lipid bilayers in a manner which differs in its voltage dependence than for univalent ions (9). We have therefore examined by NMR the transport of the ion Pr 3+ by alamethicin 30 in a vesicular system and report here a comparison of the results with those using the carrier-type ionophore A23157. In contrast to previous work (10-12) we see no sign of vesicle fusion at the concentration of alamethicin used and are able to follow the kinetics of transport, which leads to a value for the stoichiometry consistent with a channel mechanism. Our initial results with alamethicin 30 also indicate that it provides a system well-suited to the investigation of the effects of a range of substances such as drugs and anaesthetics on channel-mediated transport.