Internalization by cells of radiolabeled protein ligands bound to the cell surface is frequently
analyzed by extraction of the cells with low pH buffers. This treatment supposedly strips the
ligands from the cell surface, and remaining molecules are considered to be internalized.
However, we show herein that:(1) low molecular weight catabolic products that are trapped
within lysosomes (residualizing radiolabels) are efficiently extracted by low pH buffers,
under the same conditions used to remove cell surface-bound material, and (2) low pH …