HGH Fragment 176-191: The Native C-Terminal Segment and Its Analytical Pitfalls

The native C-terminal 16 residues of human growth hormone, made synthetically and studied for the fat-mobilising activity ascribed to that part of the parent hormone — that is HGH Fragment 176-191. It reproduces the sequence precisely as it occurs in hGH, which distinguishes it from AOD-9604, where the same region has been altered by design. Our research-grade material comes as lyophilized powder in sealed vials, for laboratory use only.

The name notwithstanding, this is not growth hormone and does not behave like it. Somatropin itself, the intact 191-residue biologic, is prescription-only and appears separately in our prescription biologics reference section.

Defining the fragment

Built as a four-helix bundle of 191 residues, human growth hormone signals by bringing two copies of its receptor together. Papers published across the 1980s and 1990s reported that a short C-terminal stretch could reproduce the hormone's effects on fat tissue, and that doing so did not call for the receptor-binding surfaces the whole hormone uses. Residues 176 to 191 synthesised alone are that stretch: 16 residues carrying the same intramolecular disulfide loop found in the parent protein.

It is worth being precise about fragment versus analogue. An analogue is a deliberately modified version of a molecule, changed to improve its properties. A fragment is an authentic sub-sequence of a larger protein, and nothing guarantees it behaves like the whole: it may keep one activity, lose another and even show behaviour the parent never had, because cutting it out removes the structural context that used to constrain it.

Origin and structure

• Sequence. Residues 176-191 of human growth hormone exactly as they occur — nothing added, no acylation, no protective substitutions.
• Disulfide bridge. A Cys-Cys bond within the molecule closes the loop that the reported activity requires; the formula's two sulfur atoms correspond to it.
• Identifiers. Formula C78H125N23O23S2, CAS 66004-57-7, molecular weight 1817.12 Da.
• Absence of stabilising modifications. With nothing built in to resist proteolysis or slow clearance, the peptide is a short-lived species in any biological setting.

Observe how close its mass is to AOD-9604's: 1817.12 Da against 1815.08 Da, some two daltons apart. That proximity is analytically consequential, because a low-resolution mass measurement cannot separate them with any confidence.

A second naming issue complicates literature searches. The fragment is numbered 176-191 by some sources and 177-191 by others, and papers, catalogues and patents apply the two conventions inconsistently. Whenever a study reports activity for a C-terminal growth hormone fragment, check the exact residue span and whether a residue was appended before treating the paper as evidence about your material. That is not fussiness: the gap between the two conventions is exactly what divides the native fragment from AOD-9604.

The proposed mechanism

What is proposed here matches the account given for AOD-9604, uncertainty included. Preclinical studies report that the fragment drives lipolysis and suppresses lipogenesis in fat tissue, as seen in isolated adipocyte preparations and rodent models. Several papers implicate beta-3 adrenergic receptor signalling, though as a permissive or indirect participant rather than by direct agonism, and knockout work has made the picture less clear.

What it reportedly fails to do matters just as much. No report has it increasing IGF-1, driving linear growth or producing the glucose effects that accompany intact growth hormone. That dissociation is the whole reason the fragment is useful as a research tool: it lets a laboratory test whether a lipolytic signal attributed to growth hormone can be obtained without touching the growth hormone receptor or the GH-IGF axis.

What has been studied

• Cell and adipocyte work. Release of glycerol and free fatty acids from isolated adipocytes, activity of lipogenic enzymes, and readouts of lipolytic signalling — the core of the evidence base.
• Rodent work. Body composition and adiposity in obese mouse and rat models, usually with IGF-1 measured as a negative control to verify dissociation from growth hormone signalling.
• Comparative studies. Head-to-head work on the native fragment and the tyrosine-extended AOD-9604, asking whether the modification changes activity or only handling.
• Human evidence. No substantial published human trial programme exists for the unmodified fragment. Clinical work in this field used AOD-9604 and did not reproduce the preclinical weight results. Claims of human efficacy for the native fragment find no support in published trials.

How the two versions came to diverge is traced in our article on the growth hormone lipolytic fragment.

Deciding between the native fragment and its tyrosine-extended relative depends on the question being asked. Where the interest lies in the native sequence as it exists inside growth hormone — whether, say, the region is intrinsically active once freed from the parent protein — the unmodified fragment is the right reagent, since an appended residue adds a variable the parent protein never had. Where the point is comparability with the AOD-9604 literature, or where detection at 280 nm is needed analytically, the modified version fits better. Comparative studies often run both arms, and that is the only way to determine whether the tyrosine is merely an analytical convenience or pharmacologically significant.

Presentations available

The HGH Fragment 176-191 product page lists sizes and prices, inside the fat-loss and metabolic peptides collection.

Reconstitution and storage at the bench

The usual arithmetic: 2 mL of bacteriostatic water added to a 10 mg vial gives 5 mg/mL, that is 5,000 mcg/mL, so 500 mcg of peptide sits in every 0.1 mL. Reconstituting a small, unmodified, water-soluble peptide asks little, but the disulfide bridge is the weak point: reducing agents, thiol traces in a badly chosen diluent, or extended alkaline exposure can open the loop, and an opened loop makes a different molecule even while the purity certificate still looks clean.

Hold sealed lyophilized powder at minus 20 degrees Celsius, away from light and moisture, and reconstituted solution at 2 to 8 degrees Celsius for use within the study window. Aliquot at reconstitution so the material avoids repeated cycles of freezing and thawing. Our reconstitution guide has the detail.

Purity, the COA and how to read it

A certificate should present an HPLC trace with main-peak area percentage, a mass spectrometry identity result against the theoretical 1817.12 Da, net peptide content, the lot number and the analysis date. Two analytical traps are specific to this molecule. One is the roughly two-dalton separation from AOD-9604, beyond the reach of a low-resolution instrument — so a purity certificate on its own cannot establish which of the pair your vial contains. The other is the disulfide state, where correctly folded and reduced forms likewise differ by two mass units. Where either distinction bears on your work, ask about the resolution of the analysis instead of taking the headline figure at face value. Mass spectrometry for identity and how to read a COA cover this ground.

Clearance is the practical limit on any design carried out in vivo with this peptide. Carrying no acylation and no protective substitutions, and weighing under 2 kDa, it is cleared renally and broken down quickly, so a single administration leaves only a brief window of systemic exposure. That is why published rodent studies typically administer frequently. A design that administers once and then measures a distant endpoint will probably detect nothing, and a negative result from such a design describes exposure rather than activity — a distinction worth stating outright whenever short unmodified peptides are reported or reviewed.

Regulatory position

This fragment is approved as a medicine nowhere and has no clinical development programme behind it; it is a research chemical. Note too that its name is a recurring source of confusion with somatropin, a prescription-only biologic falling under a wholly different regulatory regime — the fragment neither substitutes for it nor offers a lawful path to its effects.

Related peptides and comparisons

AOD-9604 vs HGH Fragment 176-191 is the comparison that matters. The intact hormone appears as a reference listing under HGH (somatropin).