The interaction of n-tetraalkylammonium compounds with a human organic cation transporter, hOCT1

Polyspecific organic cation transporters in epithelia play an important rol e in the elimination of many endogenous bioactive amines and therapeuticall y important drugs. Recently, the first human organic cation transporter (hO CT1) was cloned from liver. The purpose of the current study was to determi ne the effect of molecular size and hydrophobicity on the transport of orga nic cations by hOCT1. We studied the interaction of a series of n-tetraalky lammonium (n-TAA) compounds (alkyl chain length, N, ranging from 1 to 6 car bons) with hOCT1 in a transiently transfected human cell line, HeLa. [C-14] tetraethylammonium (TEA) uptake was measured under different experimental c onditions. Both cis-inhibition and trans-stimulation studies were carried o ut. With the exception of tetramethylammonium, all of the n-TAAs significan tly inhibited [C-14]TEA uptake. A reversed correlation of IC,, Values (rang e, 3.0 -260 mu M) with alkyl chain lengths or partition coefficients (LogP) was observed, trans-Stimulation studies revealed that TEA, tetrapropylammo nium, tetrabutylammonium, as well as tributylmethylammonium trans-stimulate d TEA uptake mediated by hOCT1. In contrast, tetramethylammonium and tetrap entylammonium did not trans-stimulate [C-14]TEA uptake, and tetrahexylammon ium demonstrated an apparent "trans-inhibition" effect. These data indicate that with increasing alkyl chain lengths (N greater than or equal to 2), n -TAA compounds are more poorly translocated by hOCT1 although their potency of inhibition increases. Similar findings were obtained with nonaliphatic hydrocarbons. These data suggest that a balance between hydrophobic and hyd rophilic properties is necessary for binding and subsequent translocation b y hOCT1.