A radioimmunoassay of the human
insulin receptor was developed employing a potent rabbit polyclonal antibody to the human
insulin receptor and a highly purified human placental
insulin receptor preparation. The receptor, obtained by sequential affinity chromatography with
insulin receptor monoclonal antibody-
agarose and
wheat germ agglutinin-
agarose, was radiolabeled with 125I-Bolton-Hunter
reagent at specific activities of 2,100-3,300 Ci/mmol. Over 75% of this
ligand was immunoprecipitable with the polyclonal antireceptor antibody and remained immunoprecipitable for greater than 45 days. The assay was sensitive to unlabeled receptor concentrations as low as 0.2 ng/0.5 ml; unlabeled
insulin did not cross-react and unlabeled
insulin-like growth factor (
IGF)-I receptor cross-reacted weakly. The radioimmunoassay was applicable to the measurement of
insulin receptors in tissues and cells that were extracted by solubilization in 1%
Triton X-100; no purification of the extracted receptor was necessary. Of the three major target tissues for
insulin action studied, liver had the highest concentration of receptors (47.6 ng/mg
protein); fat and muscle had lower levels. Other studies with the radioimmunoassay indicated that
insulin receptors were decreased both in monocytes from obese hyperinsulinemic subjects and in fibroblasts from patients with
leprechaunism.