Membrane transport of tetraphenylphosphonium and its homologues through the planar phospholipid bilayer: Concentration dependence and mutually competitive inhibition in membrane passive transport

The concentration dependence and mutually competitive inhibition in the membrane transport of the lipophilic cations tetraphenylphosphonium (TPP⁺) and its homologues were studied by use of a planar phospholipid bilayer. The current‐voltage characteristics were analyzed by use of a model developed by Ketterer et al. (J. Membr. Biol. 1971, 5, 225–245). The conductance at 0 mV [G(0)], which represents the transport ability, increased with an increase in concentration, and the G(0) values revealed saturation at higher concentrations. The relationship between G(0) and concentration was well fitted to a Michealis‐Menten‐type equation with a saturable component {maximum conductance [G(0)_max] and concentration yielding one‐half of G(0)_max [K_m]}. The G(0)_max and K_m values were calculated to be 4.1 x 10⁻⁸−242 x 10⁻⁸ S/cm² and 13–489 μM, respectively, depending on the lipophilic cations used. The G(0) values normalized by the corresponding G(0)_max values were plotted against the concentrations normalized by the corresponding K_m values. Normalized curves for TPP⁺ homologues were all superimposed on a single curve. The mechanism for the saturation of G(0) values may involve the adsorption of TPP⁺ homologues to a planar phospholipid bilayer, because the K_m values calculated in the present study were comparable to those for adsorption to phosphatidylcholine liposomes. We further determined the mutual inhibition of membrane transport by these homologues. Tetraphenylmethylphosphonium at 1 mM, a concentration threefold higher than the K_m value for this compound, reduced the G(0) for other TPP⁺ homologues by 38–55%, whereas reduction by 10 mM tetraethylammonium was minimal. The type of inhibition was classified as mutually competitive. In experiments on membrane transport, it is often assumed that the observation of saturation and mutually competitive inhibition indicates the presence of a carrier. The present paper, however, presents facts contradicting this assumption: greater caution should be taken in interpreting transport data to indicate carrier‐mediated transport.