[Synthetic peptide KKRR corresponding to the human ACTH fragment 15-18 is an antagonist of the ACTH receptor].

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 (Kd 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) (Ki 2.3 +/- 0.2 nM), whose [3H] labeled derivative binds to rat adrenocortical membranes (Kd 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) (Ki 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.