MEMBRANE-TRANSPORT OF TETRAPHENYLPHOSPHONIUM AND ITS HOMOLOGS 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(max) values were calculated to be 4.1 x 10(-8)-242 x 1 0(-8) S/cm2 and 13-489 muM, 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 1 0 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.