ACTH Antagonists

Abstract

Structural modifications within the active site of the ACTH molecule have produced analogs that inhibit the hormone sensitive adenylate cyclase system of bovine adrenal cortical plasma membranes. It is demonstrated that the tryptophan residue of the ACTH molecule is essential for stimulation of the enzyme. Substitution of tryptophan by phenylalanine or by NG-methyltryptophan as in [Gln5, provides ACTH analogs that exhibit high affinity for the ACTH receptor(s) but fail to activate the adenylate cyclase system. It is concluded that affinity for the receptors alone is not sufficient for expression of hormonal activity. The observation that adrenal cortical adenylate cyclase activated by fluoride ion is not inhibited by the antagonists indicates that hormonal and fluoride activation proceed via different mechanisms.The discovery of competitive inhibitors of polypeptide hormones is of considerable importance from a fundamental as well as from a practical point of view. Not only are compounds exhibiting such properties valuable tools for probing the mechanism of action of polypeptide hormones but they may have clinical importance in controlling specific endocrinopathies. Considerable efforts are presently directed toward discovery of such compounds. Knowledge of the essential structural features (active site) of the parent hormone may provide clues for the development of competitive antagonists.We have regarded the S-peptide-S-protein system* (1) as a model for the mode of action of peptide hormones (2). In this system two enzymically inactive components, S-peptide and S-protein, combine in a highly specific manner to form the enzymically fully active ribonuclease S. In the model, Sprotein is analogous to the receptor; S-peptide represents a peptide hormone. Structure-function studies (3) have demonstrated that [His"2] is the active site of S-peptide and that the rest of the molecule provides the vehicle to bring this residue into the correct stereochemical position on S-protein to form the active site of ribonuclease S. Since mechanistic studies (4) had shown that the pK of the imidazole ring of [His12] in Speptide is of prime importance for enzymic activity, it was a logical step to replace this histidine with an isosteric amino acid exhibiting a different pK from histidine in order to develop an S-peptide antagonist. It has now become evident that many peptide hormones combine with receptors on the cell surface and bring about stimulation of hormone sensitive adenylate cyclase(s) (6). This activation appears to provide the key to physiological function. Details of the mechanisms involved are missing but peptide hormones, like S-peptide, do indeed function as enzyme activators.The sequence -His-Phe-Arg-Trp-Gly-(positions 6-10 in Fig. 1) was postulated to be the essential structural feature (active site) of the ACTH molecule (2) and structure-function studies based on in vivo assays have largely confirmed this hypothesis (7). Based on observations with the S-peptide-S-protein system, we speculated that a...

Modulation of the response of bovine adrenocortical adenylate cyclase to corticotropin

“…When the sensitivity to hormonal stimulation of cyclic AMP production in intact adrenal cells (Mackie et al, 1972;Seelig & Sayers, 1973;Peytremann et al, 1973;Finn et al, 1976) is compared with that of the adenylate cyclase activity in membrane preparations from adrenal glands of the same species (Hoffmann et al, 1974;Londos & Rodbell, 1975;Dazord et al, 1975;Glossmann & Gips, 1975), the former system is commonly more sensitive. However, we have consistently found a KO.5 value in the range 0.3-1.0nM for all batches of bovine adrenal membranes.…”

The Chemistry of Tryptophan in Peptides and Proteins

“…It is worth noting here that HoFFMAN et al (184) recently found that a phenylalanine analog of ACTH was devoid of enzymatic activity, but exhibited affinity for the ACTH receptor. For these reasons, Cov et al (90) suggested that the essential feature oftryptophan in maintaining hormonal action was due to its electron-donating properties.…”

Approaches to the rational design of selective melanocortin receptor antagonists

“…Shorter peptides ACTH 1-24 and ACTH 1-23 were shown to have agonist properties similar to that of the full length ACTH, whereas a 13-residue peptide ACTH 11-24 was shown to have antagonist properties (Table 2) [29]. It was also shown that a peptide with an N-methyltryptophan instead of Trp at the 9 position in ACTH 1-24 had antagonist activity [30]. In a recent study by Kovalitskaya et al , it was shown that the cationic C-terminal sequence -Lys-Lys-Arg-Arg- (ACTH 15-18 ) had antagonist activity [31].…”