ACTH 11-24 (Adrenocorticotropic Hormone (11-24))

Interactions of adrenocorticotropic hormone with its adrenal receptors. Degradation of ACTH-1-24 and ACTH-11-24

Crude membranes (20,000 times g pellet) prepared from human, rat, and ovine adrenals bind 125-I-corticotropin-(1-24)-tetracosapeptide (125-I-ACTH-1-24) and degrade unbound hormone. The degradation is dependent on temperature and the concentration of membrane proteins. The degradation of 125-I-[9-tryptophan(o-nitrophenylsulfenyl)]-corticotropin-(1-24)-tetracosapeptide (125-I-NPS-ACTH-1-24) is similar to 125-I-ACTH-1-24, but that of 125-I-corticotropin-(11-24)-tetradecapeptide (125-I-ACTH-1-24 is inhibited by ACTH-1-24 and corticotropin-(1-10)-decapeptide (ACTH-1-10), but ACTH-11-24 at the same molar concentration has no effect. On the other hand, the degradation of 125-I-ACTH-11-24 is protected by ACTH-11-24 and ACTH-1-24, but not by ACTH-1-10. This suggests two systems of degradation, one will have the NH-2-terminal sequence of ACTH-1-24 as substrate, and the other the 11-24 COOH-terminal sequence. The main label product from the degradation of the 125-I-ACTH-1-24 and 125-I-ACTH-11-24 behaves as [125-I]monoiodotyrosine on Sephadex G-50 and paper chromatography. The independence of ACTH binding to its receptor and degradation is demonstrated by the following facts. (a) Calcium and pancreatic trypsin inhibitor completely inhibit the binding at concentrations when the degradation is not altered; (b) the sequences of peptides of ACTH which inhibit the binding and degradation of 125-I-ACTH-1-24 are different.

ACTH-(11-24) antagonizes ACTH-(1-24)-induced behavioral syndrome

The possible involvement of the ACTH-(11-24) fragment on the stretchings, yawnings and penile erections induced by the intracerebroventricular injection of ACTH-(1-24) in rats, was studied. The results indicate that this C-terminal fragment is devoid of any behavioral activity, but inhibits the behavioral syndrome induced by ACTH-(1-24). This suggests that the fragment-(11-24) of the ACTH molecule may contain or represent an address sequence for brain ACTH receptors and may be involved in the termination of the behavioral response to melanocortins.

[Structure-function organization of ACTH: fragment ACTH 11-24--a functionally important site of the hormone molecule]

The steroidogenic and lipolytic activities of ACTH fragments (ACTH11-24--I, ACTH11-19--II, ACTH11-16--III and ACTH 17-24--IV) were studied. Fragments I--IV exert a steroidogenic effect in isolated fasciculata rat adrenal cells at concentrations of 1--500 micrograms/ml. The inner activity (alpha) and concentration at which a half-maximum effect is achieved (EC50) for fragments I and IV are 0.64+/-0.09 and 0.5--2.0 micrograms/ml, for fragment III--0.49+/-0.07 and 0.7 microgram/ml, respectively. Fragments I--IV have no effect on the lipolysis in isolated rat fat cells. The results obtained are indicative of the functional importance of fragment ACTH11-24 in manifestation of steroidogenic action of ACTH and suggest that the second active site of ACTH is enclosed within this amino acid sequence.

[Structural and functional organization of ACTH: synthesis and properties of analogs of ACTH-(11-24)-tetradeca- and ACTH-(1-24)-tetracosapeptides containing hexa-amino acids instead of a natural sequence of ACTH 19-24 amino acids]

To assess the role of amino acid sequence ACTH 19-24 in the corticotropin structure and steroidogenic activity, the analogues of ACTH-(11-24)-tetradeca- and ACTH-(1-24)-tetracosapeptides containing hexaglycine, hexaphenylalanine, hexaglutamic acid or hexalysine instead of the natural 19-24 sequence have been synthesized by conventional methods. All these compounds in water have the CD curves characteristic of random coil, CD spectra of analogue ACTH-(1-24)-tetracosapeptide and hexalysine-containing analogue ACTH-(11-24)-tetradecapeptide in trifluoroethanol indicate the presence of alpha-helices. The latter compound manifested higher steroidogenic activity than ACTH-(11-24)-tetradecapeptide. All the other analogues were either less active than ACTH-(1-24)-tetracosapeptide or inactive over the concentration range 10(-5)-10(2) mg/ml, thereby testifying to functional importance of the 19-24 sequence for manifesting full steroidogenic activity.

Adrenocorticotropin regulates angiotensin II receptors in bovine adrenal cells in vitro

The role of ACTH-(1-24) on angiotensin II receptors has been studied in bovine adrenal glomerulosa cells in primary culture. Angiotensin II receptors were measured in cells pretreated or not by ACTH-(1-24) on day 4 of culture. ACTH-(1-24) decreased angiotensin II binding sites in a time and a dose-dependent manner. After 24 hours of treatment the minimal effective dose of ACTH-(1-24) was 10(-11)M and the maximal effect was obtained with 10(-8)M. Moreover, ACTH-(1-24) 10(-8)M decreased significantly angiotensin II receptors after 6 hours of treatment. Scatchard plot analysis showed that ACTH-(1-24) treatment did not modify the affinity of angiotensin II receptors (Ka = 0.42 and 0.44 X 10(9)M-1 in control and treated cells respectively) but reduced by about half the number of angiotensin II sites per cell. Like ACTH-(1-24), 8-Bromo-cAMP, forskolin and cholera toxin decreased angiotensin II receptors. Factors such as prolactin, somatostatin, ACTH-(11-24) and dopamine which are bound to adrenal membranes without increasing cAMP production had no effect. In conclusion, these studies in vitro demonstrate for the first time that ACTH decreases angiotensin II receptors by a direct mechanism acting on glomerulosa cells, and they also suggest that this effect could be mediated by cAMP.