Influence of substrate structure on substrate binding to the renal organic cation/H+ exchanger

The carrier-mediated exchange of H+ for organic cations ("OC/H+ exchange") is the active step in OC secretion in renal proximal tubules. Although hydrophobicity is known to be an important criterion for binding of substrates to this transporter, the degree to which steric parameters of substrate structure influence binding to the exchanger is unclear. We examined this issue by measuring the inhibition of OC/H+ exchange produced by a group of quaternary ammonium compounds which share a common structural motif: an N1-pyridinium residue. Activity of the OC/H+ exchanger was determined by measuring transport of [14C]tetraethylammonium (TEA) in brush-border membrane vesicles (BBMV) from rabbit renal cortex. Transport was measured in the presence of a pH gradient (pHin 6.0; pHout 7.5) to maximize TEA/H+ exchange. Apparent inhibitory constants (Ki values) for each test agent were measured. The test agents included 4-phenylpyridiniums and 3-phenylpyridiniums, quinoliniums and acridiniums. The planar structure of these compounds permits a direct test of whether the presence of planar hydrophobic mass in different orientations relative to the pyridinium motif exerts a systematic effect on substrate binding to the OC/H+ exchanger. The hydrophobicity of each group of compounds was systematically varied by addition of different substituents at the quaternary nitrogen. Whereas decreases in Ki proved to be proportional to hydrophobicity, the position of the phenyl-ring substituent(s) had no effect on substrate interaction with the exchanger. The results led to the development of a preliminary quantitative structure-activity relationship (QSAR) correlating substrate hydrophobicity and substrate binding to the OC/H+ exchanger. This QSAR was used to predict the binding of 1-methyl-4-phenylpyridinium (MPP+), (+) and (-)nicotine, (+) and (-)ephedrine, quinine and quinidine to the OC/H+ exchanger. Molecular graphics representation of the 3D structures of the test agents was used to develop a working model of a hydrophobic, planar receptor surface on the OC/H+ exchanger against which substrates are suggested to interact during binding. Development of the QSAR and receptor surface model open the way to quantitative tests of the specific physical and structural determinants of substrate selectivity by the renal OC/H+ exchanger.