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 [14C-Phe][Gln5]β -corticotropin 1-20 amide but failed to bind significant amounts of [14C-Phe][Gln5]β -corticotropin 1-10. The former peptide exhibits a high degree of in vivo adrenocorticotropic activity in the rat; the latter is inactive. [14C-Phe][Gln5]β -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 β -corticotropin 1-20 amide to displace [14C-Phe][Gln5]β -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.