Synthetic peptide KKRR corresponding to the human ACTH fragment 15–18 is an antagonist of the ACTH receptor

Abstract

Tritium-labeled synthetic fragments of human adrenocorticotropic hormone (ACTH) [³H]ACTH (11–24) and [³H]ACTH (15–18) with a specific activity of 22 and 26 Ci/mmol, respectively, were obtained. It was found that [³H]ACTH-(11–24) binds to membranes of the rat adrenal cortex with high affinity and high specificity (K _d 1.8 ± 0.1 nM). Twenty nine fragments of ACTH (11–24) were synthesized, and their ability to inhibit the specific binding of [³H]ACTH (11–24) to adrenocortical membranes was investigated. The shortest active peptide was found to be an ACTH fragment (15–18) (KKRR) (K _i 2.3 ± 0.2 nM), whose [³H] labeled derivative binds to rat adrenocortical membranes (K _d 2.1 ± 0.1 nM) with a high affinity. The specific binding of [³H]ACTH-(15–18) was inhibited by 100% by unlabeled ACTH (11–24) (K _i 2.0 ± 0.1 nM). ACTH (15–18) in the concentration range of 1–1000 nM did not affect the adenylate cyclase activity of adrenocortical membranes and, therefore, is an antagonist of the ACTH receptor.