RT Journal Article
SR Electronic
T1 Different Affinities of Inhibitors to the Outwardly and Inwardly Directed Substrate Binding Site of Organic Cation Transporter 2
JF Molecular Pharmacology
JO Mol Pharmacol
FD American Society for Pharmacology and Experimental Therapeutics
SP 1037
OP 1047
DO 10.1124/mol.64.5.1037
VO 64
IS 5
A1 Christopher Volk
A1 Valentin Gorboulev
A1 Thomas Budiman
A1 Georg Nagel
A1 Hermann Koepsell
YR 2003
UL http://molpharm.aspetjournals.org/content/64/5/1037.abstract
AB The rat organic cation transporter 2 (rOCT2) was expressed in Xenopus laevis oocytes and cation-induced outward and inward currents were measured in whole cells and giant patches using voltage clamp techniques. Tetrabutylammonium (TBuA) and corticosterone were identified as nontransported inhibitors that bind to the substrate binding site of rOCT2. They inhibited cation-induced currents from both membrane sides. Increased substrate concentrations could partially overcome the inhibition. At 0 mV, the affinity of TBuA from the extracellular side compared with the intracellular side of the membrane was 4-fold higher, whereas the affinity of corticosterone was 20-fold lower. The data suggest that the substrate binding site of rOCT2 is like a pocket containing overlapping binding domains for ligands. These binding domains may undergo separate structural changes. From the extracellular surface, the affinity for uncharged corticosterone was increased by making membrane potential more negative. This implies potential-dependent structural changes in the extracellular binding pocket and existence of a voltage sensor. Interestingly, at 0 mV, an 18-fold higher affinity was determined for trans-inhibition of choline efflux by corticosterone compared with cis-inhibition of choline uptake. This suggests an additional high affinity-conformation of the empty outwardly oriented substrate binding pocket. A model is proposed that describes how substrates and inhibitors might interact with rOCT2. The data provide a theoretical basis to understand drug-drug interactions at polyspecific transporters for organic cations.