The endocytotic rate constant. A cellular parameter for quantitating receptor-mediated endocytosis.

of endocytosis have been mainly restricted to the qualitative level (6-9).We have recently presented a set of steady state equations that accurately describe the cellular binding, internalization, and degradation of polypeptide ligands under physiological conditions (5).A central concept in our steady state model is the endocytotic rate constant ( k , ) which we defined as the probability of an occupied receptor being internalized in 1 min at 37 "C.The ligand-receptor k, is important because its value ' The abbreviations used are: CHAPS, 3[(3-~holamidopropyl)-dimethylammoniol-I-propane sulfonate; DV medium, Dulbecco's modified Eagle's medium; EGF, epidermal growth factor; HF cells, human neonatal foreskin fibroblasts; In/Sur, the ratio between internalized ligand and cell surface-bound ligand PMSF, phenylmethylsulfonyl fluoride; BSA, bovine serum albumin.