ACTH, structure

As would be expected of active protein secreting cells, glandular epithelial tissue, the cytokine secreting cells of the immune system and the blood vessel endothelium, have an extensive internal structure consisting of rough endoplasmic reticulum and numerous mitochondria. Peptide hormones, growth factors and cytokines like all proteins are synthesized by DNA transcription and mRNA translation. The primary transcript of the mRNA may code for an inactive prohormone which requires careful proteolysis to produce the active hormone, as for example in the case of insulin. Adrenocorticotropic hormone (ACTH) is particularly interesting in this respect because... 

Receptors exhibit structural complementarity with their ligand in the same way that enzymes are complementary to their substrate. Often the actual binding of the hormone to its receptor involved just a small portion of both molecules. The peptide ACTH secreted by the pituitary gland contains 39 amino acids, but only about 12 of these near the N-terminal are required to engage the receptor. Furthermore, and as noted in Section 4.4.1, LH, FSH, TSH and hCG all share a common a subunit and their receptors recognize only the [3 unit. 

Structure-Behavioral Activity Relationships of Peptides Derived from ACTH... 

The influence of chain length and side-chain modifications of ACTH-derived peptides on active avoidance behaviour in rats will be discussed. H-Met(02)-Glu-His--Phe-D-Lys-Phe-OH (Org 2766) emerged from these studies as an orally active peptide with an increased potency and selectivity of action. Physico-chemical data (from the literature) on the reference peptide ACTH--(4-10) did not point to a preferred conformation in solution, whereas in the crystalline state an antiparallel 3-pleated sheet structure was found. At the receptor site we suggested an a-helical conformation in which the Phe and Met residues are close together. Additional support for this suggestion came from the behavioural activity of [des-Tyr", Met ]enkephalin and of cyclo--(-Phe-Met-cAhx-), eAhx merely serving as a spacer. 

The relationship between the structure of a series of peptides derived from the adrenocorticotropic hormone ACTH, and the behavioural activity in an active avoidance behaviour test, has been studied over the past 10-20 years. The results obtained were quite different from those in which endocrine activity relationships were studied. From the outcome of a quantitative study on the structure--behavioural activity of the ACTH-related peptides, suggestions about the spatial interactions at the receptor site were made. This receptor-bound conformation differed from those suggested by solution experiments or found in crystal structures. 

Physico-chemical properties. In the fifties and sixties, several studies on the conformation of ACTH in solution were carried out. Among the used techniques were ORD, CD, fluorescence depolarization studies and kinetics of deuterium hydrogen exchange (for a review see ref. 2). The results pointed to a highly flexible random coil in solution however, Eisinger (40) found that the distance between Tyr and Trp [in ACTH-(1-24)] as measured by excitation spectroscopy, was in better agreement with some form of loop or helical structure. In addition. Squire and Bewley noted 11-15% helical content, located in the N-terminal 1-11 part of the molecule, when measuring the ORD of ACTH at pH 8.1 (41) (a random coil was found at neutral and acidic pH values, 2). 

In the mid-seventies. Law et al. studying ACTH-(1-39) and fragments, found a transition from random coil to a helix structure when the solvent water was gradually replaced by trifluoroethanol (TFE) (43). This latter solvent had been shown to favour the establishment of ordered structures ( 3-sheet or a-helix) like a membrane-receptor environment ("membrane-receptor mimetic" properties) (47). Subsequent work of the groups of Low and Fermandjian on ACTH-(1-39) and several large fragments and... 

Small ACTH fragments related to ACTH-(4-10) have also been investigated for the presence of ordered structure. CD of ACTH--(5-10) in TFE showed a random structure (50) as was found with H-NMR for fragment 4-10 (51). The addition of anionic or cationic surfactants to an aqueous solution of ACTH-(4-11) dit not promote any a-helix or 3-form in this peptide (CD experiments S2). When ACTH-(1-14) and 1-10 were measured by CD and NMR respectively, indications for a helical or ordered structure were found (90, ). Thus it seems that the addition of the non-helix "prone" fragment 1-3 or 1-4 can promote the formation of a helical structure in the adjacent sequence. Arguments in favour of this come from the theoretical work of Argos and Palau (53) on amino acid distribution in protein secondary structures. They found that Ser and Thr frequently occur at the N-terminal helical position (cf. Ser in ACTH) to provide stability the position adjacent to the helical C-terminus is often occupied by Gly or Pro (adjacent toTrp in ACTH we have Gly ) acidic amino acid residues are frequently found at the helix N-terminus (cf. Glu in ACTH) and/or basic residues at the C-terminus (cf. Arg ). 

Crystal structure(s) of ACTH-(1-39) or 1-24 are not known. Suitable crystals for X-ray diffraction experiments could be obtained however, for the heptapeptide 4-10 (54, 55) and the smaller tetrapeptide 4-7 (54, 56). In the former case, an anti--parallel p-pleated sheet structure of the backbone was found with clustering of hydrophobic (Met, PheandTrp) and hydrophilic (Glu, His, Arg) side-chains as remarkable features. ACTH-(4-7)... 

In summary, theoretical approaches point to the potential for an a-helical structure for the fragment 4-10 and the conformation found in solutions of this and several other fragments of ACTH seem to be confirmatoryi the crystal structure reveals an antiparallel 3-pleated sheet. 

For activity of ACTH-derived peptides at the receptor for pole-jumping activity, the basic requirement seems to be the presence of a Phe and Met residue in close proximity. It is interesting to see that Phe and Met are close together in an a-helical structure in ACTH peptides (and as intra-chain neighbours in Met-enkephalin) and in the crystalline state in ACTH-(4-10) as a 3-pleated sheet and in ACTH-(4-7) in the form of a horseshoe this close proximity is in line with the results of a Free-Wilson type of analysis. 

Steady-state fluorescence polarization studies have been carried out with a number of peptides, including model peptides, ACTH, glucagon, melittin, and thyrocalcitonin. This work has been reviewed 5 and will not be discussed in the present article. More recently, interesting information on the rotational behavior and structural flexibility of various peptides has been obtained from fluorescence anisotropy decay measurements. 

The THEiTHF + 5aTHF ratio is commonly greater than 15 in these disorders, the opposite to AME syndrome. While the ratio of saturated steroids demonstrates a high 11-oxo l 1/1-hydroxy ratio, this is not the case for steroids retaining a - -4-ene structure. The F E ratio is normal or elevated (i.e., > 1 Table 5.3.9). Steroid profile analysis also typically reveals an elevated excretion of DHEA and other androgen metabolites. Clearly the excessive ACTH production resultant from an apparent cortisol deficiency is responsible for the elevated adrenal androgen production, which in turn is responsible for female virilization and other manifestations of polycystic ovary syndrome. 

Occupied receptors for adrenaline, glucagon, ACTH, and histamine activate adenylate cyclase via Gs proteins. Other Gs proteins, which contain subunits designated aolf and which exist as a number of subtypes, mediate olfactory responses. Subunit aD is another specialized polypeptide which is located primarily in neural tissues. A variety of additional G proteins have been discovered in organisms ranging from bacteria to mammals.179 183-186 All have similar structures with 39- to 45-kDa a subunits, 35- to 36-kDa (3 subunits and 5- to 8-kDa y subunits. Whereas the a subunits are unique to each G protein, (3 and y subunits may be shared among several G proteins. These proteins appear to function with many kinds of hormone receptors and... 

Konishi Y, Hayakawa K, Kuriyama M, Saito M, Fujii Y, Sudo M. Effects of ACTH on brain midline structures in infants with infantile spasms. Pediatr Neurol 1995 13(2) 134-6. 

Pederson and co-workers (see Ref. 25) have isolated a peptide of Mr 2200, from ACTH-stimulated rat adrenals, which contained 15% of basic aminoacid residues. The polar side-chain groups were thought to alter membrane structure so that transference of cholesterol towards the cyt P-450scc on the inner membrane would be favoured. 

Within the framework of ACTH synthesis a broad knowledge of the structure-activity relationships has been acquired concerning the influence of the chain length on biological activity. 

Endorphins are peptides produced by the intermediate pituitary that react with the brain s opioid receptors and presumably act as endogenous analgesics. The two best known endorphins from bovine brain are the pentapeptides, met-enkephalin and leu-enkephalin, whose structures are Tyr-Gly-Gly-Phe-Met and Tyr-Gly-Gly-Phe-Leu, respectively. There is evidence that the enkephalins, MSH, and ACTH are produced from the same precursor protein, pro-opiomelanocortin, molecular weight (MW) 31,000. This protein is present in both the anterior and intermediate pituitary glands. In the anterior pituitary area, this protein loses the element of ACTH (39 amino acids). In the intermediate pituitary, this protein... 

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