Binding of insulin to isolated nuclei.

Specific binding sites for 125I-labeled insulin were detected in purified nuclei isolated from rat liver. Binding was rapid, reversible and directly proportional to the number of nuclei employed. Unlabeled native insulin, at concentrations as low as 1ng/ml, significantly inhibited the binding of labeled hormone, whereas unlabeled proinsulin and desoctapeptide insulin were less potent. In contrast, glucagon, thyrotropin, growth hormone (somatotropin), and prolactin were without effect. Under identical incubation conditions, 125I-labeled glucagon bound to liver plasma membranes 5- to 10-fold more strongly than did insulin; in contrast glucoagon did not bind to liver nuclei. These studies demonstrate the presence of specific binding sites for insulin in purified nuclei isolated from rat liver. In addition, they suggest that the nucleus may be an intracellular site of insulin action.