Wallac 1282 CompuGamma γ counter), with the remaining cell lysate centrifuged (21,000 × g for 10 min at 4 °C) and the supernatant used to determine protein concentration following radioactive decay using a Pierce BCA assay. To quantify radiotracer uptake in cells, three standard solutions of the radioactivity-containing medium were counted on the gamma counter. Data were expressed as a percentage of total radioactivity administered to cells per mg of protein. Molar activity studies were conducted by adding [ 19 F]2 to achieve a range of molar activities.