*2.5. Cell-Surface Biotinylation*

Cell surface hOAT3 expression was assayed using the procedures introduced by our group [39]. The hOAT3 cells were labeled with sulfo-NHS-SS-biotin solution (0.5 mg/mL in PBS/CM) on ice, with slow shaking for two continuous 20 min. After discarding the biotin solution, the cells were washed once with glycine solution (100 mM in PBS/CM) and incubated with glycine solution for 20 min to completely quench the unbound sulfo-NHS-SS-biotin. The cells were then lysed in lysis buffer consisted of 10 mM Tris-HCl, pH 7.5, 150 mM NaCl, 1 mM EDTA, 0.1% SDS, 1% Triton X-100, and 1% proteinase inhibitor cocktail. The cell lysates were centrifuged at 16,000× *g* at 4 ◦C, and the supernatant was then mixed with streptavidin agarose resin (40 μL) to separate the cell surface proteins. The hOAT3 at the cell surface was detected by immunoblotting using the anti-Myc antibody.
