GHRP-6 is a synthetic hexapeptide of the growth hormone secretagogue class, and an agonist at the growth hormone secretagogue receptor type 1a (GHS-R1a), the receptor for ghrelin. It was one of the first compounds in the series that led to the identification of that receptor, and it is used in receptor binding, calcium mobilisation and inositol phosphate assays and in pituitary cell culture. NuVion supplies GHRP-6 as a laboratory chemical for in vitro research use only.
Key facts
| Type / class | Synthetic peptide, growth hormone secretagogue, GHS-R1a agonist |
| Amino acid count | 6 residues, C-terminal amide |
| Sequence | His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 |
| Molecular formula | C46H56N12O6 |
| Molecular weight | 873.0 g/mol |
| CAS number | 87616-84-0 |
| Synonyms / other names | Growth hormone releasing peptide 6, GH-releasing hexapeptide, SKF-110679 |
| Supplied form | Lyophilised powder in a sealed vial |
Structure and chemistry
GHRP-6 is a short linear peptide with a free N-terminal histidine and an amidated C-terminal lysine. Two of the six residues are D-enantiomers, D-Trp at position 2 and D-Phe at position 5, and the C-terminus is capped as the amide. Both features are deliberate. D-residues are not substrates for the common aminopeptidases and endopeptidases, and blocking the C-terminal carboxylate removes the charge that carboxypeptidases recognise, so the molecule survives longer in serum-containing media than an all-L hexapeptide would.
The pharmacophore is the pair of aromatic side chains at positions 2 and 4, the D-Trp and the L-Trp, which occupy a hydrophobic pocket in the receptor’s transmembrane bundle. The lineage runs back to met-enkephalin analogues; the series was built by systematic substitution around that opioid-derived scaffold, and the activity that emerged has nothing to do with opioid receptors. GHRP-6 carries no acyl chain, which distinguishes it chemically from ghrelin, whose Ser3 must be octanoylated by ghrelin O-acyltransferase before the natural ligand can activate the same receptor.
Two tryptophans and one phenylalanine give strong absorbance near 280 nm, so peptide content can be estimated spectrophotometrically and the compound is easy to detect in RP-HPLC with UV detection. Tryptophan is also the residue most prone to photo-oxidation, so solutions are kept out of direct light. There is no cysteine and no disulfide bridge, and the lysine side chain leaves the molecule positively charged at neutral pH.
Mechanism of action
GHS-R1a is a class A G protein-coupled receptor expressed on pituitary somatotroph cells, in hypothalamic neurons and at lower levels in several peripheral tissues. GHRP-6 binds it and stabilises the active conformation. The receptor couples principally to Gq/11: the activated alpha subunit stimulates phospholipase C-beta, which hydrolyses phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate and diacylglycerol. IP3 opens IP3 receptors on the endoplasmic reticulum and raises cytosolic calcium, while diacylglycerol with that calcium activates protein kinase C. In somatotroph cells, the calcium transient is what couples receptor occupancy to exocytosis of GH-containing secretory granules, and GH appearing in the culture medium is the classic cellular readout.
GHS-R1a is unusual among GPCRs for its high constitutive activity: in transfected cells it signals at roughly half its maximal rate with no ligand present. That property makes the receptor a standard system for studying inverse agonism, and it means assays have to be designed against a high basal signal. Agonist occupancy is followed by GRK phosphorylation of the receptor tail and beta-arrestin recruitment, which desensitises G protein coupling and drives internalisation, so beta-arrestin recruitment assays give a second, G protein-independent measure of activity. A truncated splice variant, GHS-R1b, does not signal on its own and heterodimerises with GHS-R1a to reduce its surface expression.
GHRP-6 acts at a receptor distinct from the GHRH receptor, which is a class B GPCR coupled to Gs and cAMP. Because the two receptors converge on the same secretory cell through different second messengers, co-application of a GHS-R1a agonist and a GHRH analogue in pituitary cell culture gives a larger response than either alone, and that additivity is a standard demonstration of two independent pathways in one cell type. GHRP-6 and the related hexapeptides also bind CD36, a scavenger receptor, at higher concentrations, which is worth controlling for in cell types that express it. The widely used antagonist [D-Lys3]-GHRP-6 is a single-residue analogue of this peptide and appears as the negative control in much of the GHS-R1a literature.
Research applications
GHRP-6 sits in the NuVion Growth Hormones category. Typical study contexts include:
- Radioligand and fluorescent-ligand competition binding on GHS-R1a membrane preparations, giving Ki values against ghrelin and other secretagogues.
- Calcium mobilisation assays (FLIPR or fluorescent indicator dyes) in GHS-R1a-expressing HEK293 or CHO lines, the fastest functional readout of Gq coupling.
- Inositol phosphate accumulation assays (IP-One HTRF) to quantify PLC-beta activation, including measurement of constitutive activity and inverse agonism against it.
- Beta-arrestin recruitment and receptor internalisation assays, used to compare the balance of G protein and arrestin signalling across the secretagogue series.
- Primary pituitary cell culture with GH in the medium quantified by ELISA, alone and with a GHRH-receptor agonist, to separate the two secretory pathways.
- RP-HPLC method development and peptide stability studies, where the tryptophan absorbance at 280 nm and the D-residue content are the analytical features of interest.
Handling in the laboratory
GHRP-6 is supplied as a lyophilised powder in a sealed vial. It is reconstituted with bacteriostatic water; the reconstitution calculator gives the volume needed for a target stock concentration. Keep the sealed vial dry, away from light and refrigerated as described in the product documentation. Keep the reconstituted solution refrigerated, protect it from light because of the tryptophan residues, and use it within the period stated in the documentation. Aliquoting the stock avoids repeated freeze-thaw cycles, and low-binding tubes limit adsorption losses at working concentrations.
The peptide is characterised by RP-HPLC for purity and by mass spectrometry for identity, where the singly and doubly protonated ions are compared against the calculated monoisotopic mass of the amidated hexapeptide.
Testing and supply from NuVion
NuVion has GHRP-6 independently tested by Janoshik Analytical. Most batches are tested, and the Certificate of Analysis for a tested batch, showing RP-HPLC purity and mass spectrometry identity, is published on the product page and in the Certificates of Analysis library. Manufacture is GMP-audited, the peptide is supplied lyophilised in sealed vials, and orders are dispatched from within Australia.
Related compounds
In the same category, GHRP-2 is the closest structural comparator at the same receptor, and Ipamorelin is the selective pentapeptide agonist often run alongside both in receptor pharmacology work.
Frequently asked questions
What is GHRP-6 used for in research?
It is used as a reference agonist at GHS-R1a. Typical work includes competition binding against ghrelin, calcium and inositol phosphate assays in receptor-expressing cell lines, beta-arrestin recruitment assays, and pituitary cell culture where GH in the medium is measured by ELISA.
How does GHRP-6 differ from ghrelin?
Ghrelin is a 28-residue peptide that must carry an octanoyl group on Ser3 to activate GHS-R1a. GHRP-6 is a six-residue synthetic ligand with no acyl chain and two D-amino acids, so it activates the same receptor while being simpler to synthesise and more resistant to proteases in culture medium.
Is GHRP-6 a therapeutic good in Australia?
No. GHRP-6 is not included in the Australian Register of Therapeutic Goods and has not been assessed by the TGA. NuVion supplies it as a laboratory chemical for in vitro research, and nothing on this page is a representation about therapeutic use.
How should GHRP-6 be stored?
Keep the sealed vial of lyophilised powder dry, away from light and refrigerated as stated in the product documentation. After reconstitution, keep the solution refrigerated and protected from light, and use it within the period the documentation states. Aliquoting avoids repeated freeze-thaw cycles.
Research use only. This product is a laboratory chemical supplied for in vitro research. It is not included in the Australian Register of Therapeutic Goods and has not been assessed by the Therapeutic Goods Administration for quality, safety or efficacy. It is not for human or veterinary use, and nothing on this page is a representation about therapeutic use.
