Direct demonstration of insulin receptor internalization. A quantitative electron microscopic study of covalently bound 125I-photoreactive insulin incubated with isolated hepatocytes.

When 125I-insulin is incubated with isolated rodent hepatocytes at 37 degrees C, the ligand initially binds to the plasma membrane of the cell and is subsequently internalized by adsorptive endocytosis. To confirm directly that the insulin receptor is internalized with the ligand, we covalently linked photoreactive 125I-N sigma B29 (azidobenzoyl) insulin to its specific hepatocyte receptor and followed its fate by quantitative electron microscopic autoradiography. We found that the covalently linked photoreactive insulin is internalized by the cell in fashion analogous to the internalization of ordinary 125I-insulin, indicating that, at least under these conditions, the insulin receptor is internalized with the ligand.