Assay Conditions Influence Affinities of Rat Organic Cation Transporter 1: Analysis of Mutagenesis in the Modeled Outward-Facing Cleft by Measuring Effects of Substrates and Inhibitors on Initial Uptake.

ABSTRACT

The effects of mutations in the modeled outward-open cleft of rat organic cation transporter 1 (rOCT1) on affinities of substrates and inhibitors were investigated. Human embryonic kidney 293 cells were stably transfected with rOCT1 or rOCT1 mutants, and uptake of the substrates 1-methyl-4-phenylpyridinium+ (MPP+) and tetraethylammonium+ (TEA+) or inhibition of MPP+ uptake by the nontransported inhibitors tetrabutylammonium+ (TBuA+), tetrapentylammonium+ (TPeA+), and corticosterone was measured. Uptake measurements were performed on confluent cell layers using a 2-minute incubation or in dissociated cells using incubation times of 1, 5, or 10 seconds. With both methods, different apparent Michaelis-Menten constant (Km) values, different IC50 values, and varying effects of mutations were determined. In addition, varying IC50 values for the inhibition of MPP+ uptake and varying effects of mutations were obtained when different MPP+ concentrations far below the apparent Km value were used for uptake measurements. Eleven mutations were investigated by measuring initial uptake in dissociated cells and employing 0.1 µM MPP+ for uptake during inhibition experiments. Altered affinities for substrates and/or inhibitors were observed when Phe160, Trp218, Arg440, Leu447, and Asp475 were mutated. The mutations resulted in changes of apparent Km values for TEA+ and/or MPP+ Mutation of Trp218 and Asp475 led to altered IC50 values for TBuA+, TPeA+, and corticosterone, whereas the mutation of Phe160 and Leu447 changed the IC50 values for two inhibitors. Thereby amino acids in the outward-facing conformation of rOCT1 could be identified that interact with structurally different inhibitors and probably also with different substrates.