Abstract

The affinity of corticosterone to organic cation transporters (OCTs) is subtype- and species-dependent. For example, the IC₅₀ values for corticosterone inhibition of cation uptake by transporters rOCT1 and rOCT2 are ∼150 and ∼4 μM, respectively. By introducing domains and amino acids from rOCT2 into rOCT1, we found that the exchange of three amino acids in the presumed 10th transmembrane α helix is sufficient to increase the affinity of rOCT1 for corticosterone to that of rOCT2. Replacement of these amino acids in rOCT2 decreased the affinity for corticosterone. These amino acids (Ala443, Leu447, and Gln448 in rOCT1 and Ile443, Tyr447, and Glu448 in rOCT2) are probably located within the substrate binding region because in rOCT1 mutants, the K_m values for uptake of tetraethylammonium (TEA) and 1-methyl-4-phenylpyridinium (MPP) were decreased in parallel with a decrease of the IC₅₀ values for the inhibition of cation uptake by corticosterone. In mutant rOCT1(L447Y/Q448E), the IC₅₀ value for the inhibition of [³H]MPP (0.1 μM) uptake by corticosterone (24 ± 4 μM) was significantly higher compared with the IC₅₀ value for inhibition of [¹⁴C]TEA (10 μM) uptake (5.3 ± 1.7 μM). This finding suggests an allosteric interaction between transported cation and corticosterone. Because this substrate-specific effect cannot be explained by differential replacement of corticosterone by MPP versus TEA and was observed after point mutations within the presumed substrate region, the data suggest that MPP or TEA bind to the substrate binding region simultaneously with corticosterone and cause a short-range allosteric effect on the corticosterone binding site.