ACTH receptors in the adrenal: specific binding of ACTH-125I and its relation to adenyl cyclase

Insulin receptors in the liver: specific binding of ( 125 I)insulin to the plasma membrane and its relation to insulin bioactivity

With [(125)I]insulin at 7 x 10(-10) M, 25% of the radioactivity was bound to plasma membranes purified from rat liver. 20% of the [(125)I]insulin binding was inhibited by unlabeled insulin at 10(-9) M (6 ng/ml), equivalent to insulin concentrations in hepatic portal blood; inhibition of [(125)I]insulin 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)I]insulin 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)I]insulin 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)I]insulin. This study demonstrates directly the binding of insulin to its biologically important receptors.

Radioreceptor assay of adrenocorticotropic hormone: new approach to assay of polypeptide hormones in plasma

Biologically active iodine-125-labeled adrenocorticotropic hormone (ACTH) binds specifically to ACTH receptors extracted from adrenals. Unlabeled ACTH at 1 picogram per milliliter significantly displaces labeled ACTH from these receptors. This system, which appears to be applicable to all polypeptide hormones, provides a rapid and sensitive method for measurements of biologically active ACTH in dilute whole plasma.

Correlation of adrenocorticotropic activity of ACTH analogs with degree of binding to an adrenal cortical particulate preparation

This study deals with the interaction of polypeptide hormones with their receptors. Specifically it involves the binding of synthetic ACTH analogs and fragments to a particulate fraction from beef adrenal cortical tissue. This fraction was found to bind synthetic [(14)C-Phe] [Gln(5)]beta-corticotropin(1-20) amide but failed to bind significant amounts of [(14)C-Phe] [Gln(5)]beta-corticotropin(1-10). The former peptide exhibits a high degree in vivo adrenocorticotropic activity in the rat; the latter is inactive. [(14)C-Phe] [Gln(5)]beta-corticotropin(1-20) amide exhibited little affinity for similarly prepared particulates from beef kidney, liver, or adrenal medulla. Using a series of synthetic homogeneous nonradioactive analogs or fragments of beta-corticotropin(1-20) amide to displace [(14)C-Phe] [Gln(5)]beta-corticotropin(1-20) amide from the particulate fraction a significant correlation between binding and in vivo adrenocorticotropic activity was established. Major "active" and "binding" sites were shown to reside in different sections of the ACTH molecule. Charged groups play a major role in the attachment of ACTH to the particulate fraction and the sequence Lys-Lys-Arg-Arg proved to be particularly significant. The high degree of binding specificity for ACTH peptides which is exhibited by the particulate suggests that it contains ACTH receptor(s). Moreover, the striking similarity between observations with the S-peptide-S-protein model system and the system described would seem to indicate that the ACTH receptor is a protein.