AOD-9604: A Modified Growth Hormone Fragment (hGH 176-191)

The AOD 9604 peptide is a synthetic 16-residue fragment derived from the C-terminal end of growth hormone (GH). It corresponds to residues 177 to 191 of the 191-residue hormone, with a tyrosine added at the N-terminus, which is why the AOD9604 research peptide is often labelled as an analogue of the hGH 176-191 fragment. Its interest to researchers comes from a structural question: can a short segment cut from a large signalling protein retain any biological activity once it is separated from the receptor-binding architecture of the parent molecule?

This profile describes the parent hormone, the origin and sequence of AOD-9604, its disulfide-bridged structure, what has been reported about its receptor pharmacology in cell and animal models, and the analytical checks that matter for a cysteine-containing peptide. AOD-9604 is supplied for laboratory research use only and is not for human or veterinary use.

The Parent Protein: Growth Hormone

Pituitary growth hormone is a single-chain protein of 191 amino acids with a mass of about 22 kDa. It folds into a four-helix bundle with an unusual up-up-down-down topology, stabilised by two disulfide bonds: one between Cys53 and Cys165, linking distant parts of the chain, and a second, smaller loop between Cys182 and Cys189 near the C-terminus.

GH signals through the growth hormone receptor (GHR), a class I cytokine receptor. One hormone molecule engages two receptor chains through two distinct surfaces, termed site 1 and site 2, and this asymmetric binding repositions the intracellular domains of a receptor dimer so that the associated Janus kinase JAK2 becomes active. JAK2 then phosphorylates the receptor and downstream signalling proteins, most prominently STAT5. Both receptor-binding surfaces are formed by residues spread across the helical bundle, so no short linear segment of the hormone reproduces them.

Origin of the 176-191 Fragment

During the 1980s and 1990s, several groups examined whether isolated regions of GH carried activities separable from receptor-mediated growth signalling. Work at Monash University in Australia focused on the C-terminal region, synthesising peptides spanning residues 176 to 191 and testing them in cell and rodent systems. AOD-9604 emerged from that programme as a modified version in which residue 176 of the native sequence, a phenylalanine, is absent and a tyrosine is attached ahead of residue 177. The product name is a development code rather than a chemical descriptor. Literature naming conventions vary, and “176-191” in a title does not always mean the identical sequence.

Reported Receptor Pharmacology

The central pharmacological claim in the AOD-9604 literature is a negative one. As reported by its developers, the fragment does not bind the growth hormone receptor in the way intact GH does and does not reproduce the receptor-driven responses associated with the full hormone, such as JAK2-STAT5 activation or downstream IGF-1 production in rodent models. That is consistent with structural expectations: the site 1 and site 2 receptor contacts of GH lie mostly in the helical bundle, and residues 177 to 191 form only a small part of the molecule.

What the fragment does do at a molecular level is less clear. The published research context is metabolic signalling in cell and animal models, and those studies are preclinical. A specific receptor or binding partner for AOD-9604 has not been firmly established in the literature, and much of the early data come from a single research programme. Anyone reviewing this work should separate direct biochemical evidence from inference, note which findings come from cultured cells versus rodents, and look for independent replication. Researchers comparing GH-axis tools can find related sequences, including GHRH analogues and ghrelin receptor agonists, among the single research peptides in our catalog; our article on growth hormone secretagogues explains how those compounds differ mechanistically from a GH fragment.

Sequence and Structure of AOD-9604

The sequence of AOD-9604 is Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe. Its two cysteines correspond to Cys182 and Cys189 of GH, and in the oxidised form they are joined by an intramolecular disulfide bond that closes an eight-residue ring, mirroring the small C-terminal loop of the parent protein.

Property	AOD-9604
Length	16 residues
Relation to GH	Tyr + GH residues 177-191
Disulfide	Cys7-Cys14 (fragment numbering), equivalent to Cys182-Cys189 in GH
Average mass, oxidised	about 1815 Da
Average mass, reduced	about 1817 Da
Charged residues	Arg x2, Glu x1, plus free termini

At neutral pH the two arginines and the free N-terminal amine outweigh the glutamate and the C-terminal carboxylate, giving a net charge of about +1. A cluster of hydrophobic residues (Leu, Ile, Val, Phe and the added Tyr) gives the peptide moderate retention in reversed-phase chromatography. The added tyrosine also contributes a convenient chromophore for UV detection at 275 to 280 nm and a site suitable for radioiodination in binding studies. In isolation, a 16-residue peptide of this kind is expected to be largely flexible in solution, with the disulfide ring providing the main conformational constraint. For context on how isolated fragments relate to their parent proteins, see our comparison of peptides vs proteins.

Analytical Considerations for a Disulfide Peptide

The disulfide bond is the main analytical variable. The oxidised and reduced forms differ by about 2 Da, which is easily resolved by high-resolution mass spectrometry but can be missed at low resolution when the isotope envelope is broad. Under multiply charged electrospray conditions the difference shrinks further on the m/z scale, so deconvolution or careful isotope analysis is needed. Our guide to peptide mass spectrometry explains these considerations.

Free thiols can also form intermolecular disulfides, producing dimers and higher oligomers at about twice the monomer mass, and disulfide-linked species often elute separately in reversed-phase HPLC. Ellman’s assay for free thiols is a simple orthogonal check on oxidation state. Methionine and tryptophan are absent, which removes two common oxidation sites, but the sequence contains Gln at an internal position and Ser-Gly motifs that are worth watching for deamidation or cleavage under stress conditions. As with other lyophilised peptides, storage sealed, cold and protected from moisture, with aliquoted stock solutions, limits these changes.

What the Structure Tells Researchers

AOD-9604 is a disulfide-constrained, 16-residue peptide that reproduces the C-terminal loop of growth hormone with an added tyrosine. Structurally, it lacks the helical surfaces through which GH engages its receptor, which fits the reported absence of classical GHR signalling. Its own molecular target remains unsettled, and its literature is preclinical and concentrated in a small number of groups. Laboratories working with the AOD-9604 (Tyr-GH 177-191) fragment should confirm sequence, oxidation state and purity on each lot certificate before interpreting any cell or animal result.

References

1. UniProt Consortium. UniProtKB entry P01241, Somatotropin (GH1).
2. Brooks AJ, Dai W, O’Mara ML, et al. Mechanism of activation of protein kinase JAK2 by the growth hormone receptor. Science, 2014.
3. ICH Q6B. Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products.
4. Nelson DL, Cox MM. Lehninger Principles of Biochemistry. W.H. Freeman.