Insulin Receptors in the Liver: Specific Binding of 125 IInsulin to the Plasma Membrane and Its Relation to Insulin Bioactivity

With 125 Iinsulin at 7 × 10 -10 M, 25% of the radioactivity was bound to plasma membranes purified from rat liver. 20% of the 125 Iinsulin binding was inhibited by unlabeled insulin at 10 -9 M (6 ng/ml), equivalent to insulin concentrations in hepatic portal blood; inhibition of 125 Iinsulin binding was 80% at 10 -7 M and 90% at 10 -5 M. Eight insulins and derivatives with biological potencies that differed over a 100-fold range inhibited the binding of 125 Iinsulin to liver membranes in direct proportion to their ability to stimulate glucose oxidation in isolated fat cells. Inactive insulin chains, as well as glucagon, ACTH, and human growth harmone were without effect. The binding of 125 Iinsulin increased 55-fold as plasma membrane was purified from crude homogenate. Binding was time- and temperature-dependent, and addition of excess insulin produced rapid dissociation of 125 Iinsulin. This study demonstrates directly the binding of insulin to its biologically important receptors.