Sermorelin: GRF(1-29) Chemistry, Receptor Data and Peptide Stability

Sermorelin is the C-terminally amidated 29-residue N-terminal fragment of human growth hormone-releasing hormone, catalogued as GRF(1-29)NH2, hGHRH(1-29)NH2 and Groliberin. Every residue is a standard L-amino acid in the native order, so the molecule is a truncation of the endogenous 44-residue hormone rather than an engineered analogue, and its formula is C149H246N44O42S at 3357.9 g/mol.

The residue numbering matters more here than in most peptides, because four specific positions carry the whole reactivity profile. Ala2 defines the dipeptidyl peptidase IV cleavage site. Asp3 is the residue that isomerises in aqueous solution. Asn8 is the deamidation site. Met27 is the only sulfur atom in the molecule and therefore the only oxidation-sensitive side chain. Lys12 and Lys21 are the two free primary amines that conjugation chemistry targets. Each of those assignments comes from published work summarised in the sections below.

Because the sequence is native, sermorelin has no D-amino acids, no non-proteinogenic residues and no appended chemistry. That makes it the reference point against which the modified members of this peptide family are described, and it is why the chemical literature frequently reports sermorelin numbers alongside those for longer or substituted relatives such as GRF(1-44)-NH2 and GRF(1-20)-NH2.

The unambiguous identifiers are PubChem CID 16132413, InChIKey WGWPRVFKDLAUQJ-MITYVQBRSA-N and CAS 86168-78-7 for the free peptide. Records that describe the acetate carry the composite formula C149H246N44O42S · C2H4O2 at 3417.97 g/mol, roughly 60 g/mol heavier. Documentation should state which of the two a given container holds, since peptide content per milligram differs between them.

Sermorelin retains the complete recognition determinant of full-length GHRH, so it engages the GHRH receptor through the same interface as the endogenous hormone. Gaylinn and colleagues cloned that receptor from a human pituitary library in 1993: a 423-amino-acid seven-transmembrane protein of the secretin family, sharing 47 percent identity with the vasoactive intestinal peptide receptor.

Class B1 peptide recognition proceeds in two stages: the C-terminal half of the ligand docks against the receptor extracellular domain, then the N-terminal residues descend into the transmembrane bundle and trigger activation. The 29-residue fragment carries both halves of that pharmacophore. Sequence identity with the remaining members of the family falls off from there, at 42 percent for the secretin receptor, 35 percent for calcitonin and 28 percent for parathyroid hormone. Zhou and colleagues resolved the arrangement in 2020 by cryo-electron microscopy of the human receptor with GHRH and heterotrimeric Gs at 2.6 angstroms, showing the helical peptide contacting the extracellular domain, every extracellular loop, and all transmembrane helices except helix 4.

A 2026 letter in Protein and Cell added the states that the 2020 work did not cover. Wang and colleagues determined a ligand-free receptor complex at 3.04 angstroms and a complex with the small-molecule agonist PCO371 at 2.88 angstroms, and set both against the peptide-bound form and against splice variant 1 of the receptor. Those two structures define the conformational baseline and an allosteric entry point distinct from the orthosteric peptide site the 29-residue fragment occupies.

Downstream coupling is to Gs and adenylate cyclase, and cyclic AMP accumulation is the standard functional readout in receptor-expressing cell lines. No published structure shows sermorelin itself bound to the receptor. The structural inferences above are drawn from complexes with the full-length hormone whose first 29 residues sermorelin reproduces exactly, which is a strong basis but still an inference rather than a direct measurement.

Sermorelin carries an alanine at position 2, and that single residue is the reason the peptide disappears from plasma within minutes. Dipeptidyl peptidase IV removes the N-terminal Tyr-Ala dipeptide, converting the molecule into sermorelin(3-29)-NH2, a fragment that has lost the N-terminal residues the receptor transmembrane bundle recognises.

Bongers and colleagues quantified the reaction in 1992 using purified human placental enzyme with HPLC readout. Working at 37 degrees Celsius and pH 7.8 from a starting substrate concentration of 0.15 mM, they recorded initial cleavage rates near 5 micromoles per minute per milligram of enzyme, and found the same figure for GRF(1-44)-NH2, GRF(1-29)-NH2 and GRF(1-20)-NH2. Truncating the chain from 44 residues to 20 therefore does nothing to protect the scissile bond.

The same study mapped what the enzyme will and will not accept. Analogues bearing a D-configuration residue at P2, P1 or P1', along with desamino-Tyr1 and N-methyl-Tyr1 variants, resisted cleavage entirely. Across a substituted series, catalytic efficiency peaked for a hydrophobic side chain at position 2 measuring roughly 0.25 nanometres, matching alpha-aminobutyric acid. Related work on bovine sequences put relative cleavage rates at 100 percent for Ala2, 4 percent for Ser2, 2.5 percent for Thr2, 0.53 percent for Val2 and zero for Ile2, which is the quantitative basis for every position-2 substitution in this peptide family.

Cleavage products of this family are analytically important even where they are pharmacologically inert. Memdouh and colleagues built sermorelin(3-29)-NH2 into their reference panel precisely because the metabolite persists in a matrix where the parent may already be gone, and later urine methods carry it as a named target alongside the intact peptide.

Sermorelin degrades in aqueous solution by three routes independent of any enzyme: isomerisation at Asp3, deamidation at Asn8, and oxidation at Met27. Bongers and colleagues isolated the products of the first two from GRF(1-29)-NH2 in 1992 and reported that the peptide is most stable between pH 4 and pH 5.

The pH dependence has a clean chemical explanation, and it held across solutions from 15 to 200 micromolar. Below pH 4 the Asp3 side chain is protonated and the dominant route is chain cleavage at the Asp3-Ala4 bond, yielding GRF(4-29)-NH2. Above pH 4 the ionised carboxylate instead attacks the backbone, and isomerisation to beta-Asp3 predominates. Both processes follow simple first-order kinetics whose rates track pH and are independent of buffer concentration.

Asn8 deamidation is the faster of the two and proceeds through a cyclic imide intermediate at pH 5 and above, generating beta-Asp8 and Asp8 products in roughly a 4:1 ratio under general base catalysis. Below pH 2 a separate direct hydrolysis of the protonated amide gives the alpha-aspartyl product exclusively. Notably, deamidation at pH 8.0 proved relatively insensitive to temperature below 37 degrees Celsius, which the authors attributed to conformational constraint. Asp25 and Asn35 showed no comparable degradation under the same conditions.

The potency cost of deamidation was measured on a closely related analogue. Friedman and colleagues reported in 1991 that the beta-Asp8 product of a GRF(1-32)NH2 analogue was 400 to 500 times weaker than the parent in cultured bovine anterior pituitary cells, and the alpha-Asp8 product 25 times weaker. In phosphate buffer at pH 7.4 and 37 degrees Celsius the parent peptide showed a disappearance half-life of 202 hours, while replacing Asn8 with serine extended it to 1550 hours. Oxidation of Met27 to the sulfoxide is the third documented route and is the reason substituted analogues in this family commonly carry leucine at that position.

Sermorelin has published human clearance figures from constant-infusion work. Soule and colleagues infused GHRH-(1-29)-NH2 into ten normal men at 25 nanograms per kilogram per minute and reported a metabolic clearance rate of 39.7 plus or minus 3.9 millilitres per kilogram per minute and a half-life of 4.3 plus or minus 1.4 minutes.

The comparison built into that study is what makes the numbers interpretable. A D-Ala2 analogue run under identical conditions cleared at 21 plus or minus 1.2 millilitres per kilogram per minute, significantly slower at P below 0.001, with a half-life of 6.7 plus or minus 0.5 minutes. Inverting one stereocentre at the dipeptidyl peptidase IV site therefore cut clearance roughly in half, which links the plasma figure directly back to the enzymology.

Other reported values sit in the same region. A 2003 review of GRF conjugation chemistry by Esposito and colleagues described the plasma half-life of GRF(1-29) in humans as approximately 10 to 20 minutes, a range consistent with the infusion data once differences in sampling and assay are allowed for. That review also identified Lys12 and Lys21 as the positions where mono-PEGylated conjugates carrying a 5000-dalton polymer retained in vitro activity comparable to the unmodified peptide.

On the pharmacodynamic side, Wilton and colleagues studied thirty healthy men aged 19 to 43 and found that 0.25 micrograms per kilogram was already sufficient to produce a measurable rise in circulating growth hormone, with peaks near 90 milliunits per litre at 1 to 2 micrograms per kilogram. The same study measured intranasal bioavailability at only 3 to 5 percent, an unsurprising figure for a 3.4 kDa peptide with a computed XLogP of minus 12.1 and 52 hydrogen-bond donors.

Sermorelin is confirmed by liquid chromatography coupled to tandem or high-resolution mass spectrometry, verified against a monoisotopic mass of 3355.8187 Da and an average molecular weight of 3357.9 g/mol. Anti-doping laboratories have driven most method development for this compound, and that literature is where the quantitative sensitivity figures live.

Four method papers bracket the current position. Memdouh and colleagues incubated four of the larger GHRH-family analogues in fortified urine in 2021, characterised nineteen major in vitro metabolites, synthesised them as reference materials, and reached detection limits generally at or below 1 nanogram per millilitre. Cristea and colleagues published a weak cation-exchange extraction with triple-quadrupole instrumentation in 2023, reporting a detection limit of 0.2 and a quantification limit of 0.6 nanograms per millilitre with linearity from 0.1 to 1.2. Thomas and colleagues consolidated the entire 2 to 10 kilodalton analyte window into one extraction in 2024, controlled by five internal standards. Ucakturk and Nemutlu combined ultrafiltration with solid-phase extraction ahead of nano-flow chromatography and Orbitrap detection in 2026, reaching detection limits of 0.5 nanograms per millilitre or better and identification limits between 0.5 and 0.75.

Fragment behaviour under proteolysis has been characterised separately, and one result is directly useful in the laboratory. Gonzalez-Lopez and colleagues synthesised a panel of doping-relevant peptides by solid-phase chemistry in 2023 and challenged them with human blood, trypsin and chymotrypsin under HPLC-DAD monitoring. The C-terminal octapeptide sermorelin(22-29) survived all three challenges intact, which qualified it as an in-house internal standard for quantification work in biological matrices.

Two constraints recur across all of these methods. Urinary concentrations of this peptide family are small, which is why preconcentration and nano-flow separations keep appearing. And the analyte is fragile in matrix, so extraction chemistry and cold handling carry as much weight in the reported sensitivity as the mass spectrometer does.

Sermorelin has an unusually well documented degradation profile, and the practical handling guidance follows from that chemistry rather than from supplier catalogue text. The peer-reviewed maximum-stability window for the 29-residue amide in aqueous solution is pH 4 to 5, with distinct failure modes on either side: backbone cleavage at Asp3-Ala4 in acid, and Asp3 isomerisation plus Asn8 deamidation as pH rises.

Buffer choice therefore carries real consequence. At pH 5 and above the Asn8 route is general base-catalysed, meaning stronger buffer bases accelerate it, and it produces beta-Asp8 and Asp8 species in roughly a 4:1 ratio that are far weaker than the parent in pituitary cell assays. Cooling helps less than expected on this particular route: deamidation at pH 8.0 was reported to be relatively insensitive to temperature below 37 degrees Celsius.

Three degradation markers are worth watching by mass spectrometry. Loss of 130 daltons from the N-terminus indicates dipeptidyl peptidase IV activity and the sermorelin(3-29)-NH2 metabolite. A gain of 1 dalton indicates Asn8 deamidation, though the isomeric beta-Asp and alpha-Asp products require chromatographic separation to distinguish. A gain of 16 daltons indicates oxidation of Met27 to the sulfoxide, the only sulfur-dependent route in the molecule and the reason elemental sulfur content is a useful identity check against substituted analogues that lack it.

Supplier specifications describe the material as a white to off-white lyophilized solid at 98 percent purity or higher by HPLC. Those are catalogue figures rather than published measurements, and they describe the neat solid, not solution behaviour. Salt form should be recorded explicitly, since the free peptide at 3357.9 g/mol and the acetate at 3417.97 g/mol differ by roughly 1.8 percent in mass per unit of peptide content. The computed profile, an XLogP of minus 12.1 with 52 hydrogen-bond donors and 1470 square angstroms of polar surface, describes a strongly hydrophilic molecule that dissolves in aqueous buffer without organic co-solvent and has no expectation of passive membrane permeability.