A nonradioactive iodide uptake assay for sodium iodide symporter function

The standard assay for sodium iodide symporter (NIS) function is based on the measurement of radioiodide uptake (125I) in NIS-expressing cells. However, cost and safety issues have limited the method from being used widely. Here we describe a simple spectrophotometric assay for the determination of iodide accumulation in rat thyroid-derived cells (FRTL5) based on the catalytic effect of iodide on the reduction of yellow cerium(IV) to colorless cerium(III) in the presence of arsenious acid (Sandell–Kolthoff reaction). The assay is fast and highly reproducible with a Z′ factor of 0.70. This procedure allows the screening of more than 800 samples per day and can easily be adapted to robotic systems for high-throughput screening of NIS function modulators. Using this method, the potency of several known inhibitors of NIS function was evaluated in a single day with high accuracy and reliability. Measured IC50 values were essentially identical to those determined using Na125I.