HGH Fragment 176-191 and AOD-9604 are often treated as the same compound in popular discussions, but they are structurally distinct peptides with differences that matter in research contexts. Understanding what separates them requires looking at the original HGH molecule, how the fragment was identified, and what the tyrosine modification in AOD-9604 actually does.
The C-Terminus of HGH: Where the Fragment Comes From
Human Growth Hormone is a 191-amino acid peptide. Research investigating the biological properties of different regions of the HGH molecule found that the C-terminal end — specifically the region around amino acids 176 to 191 — exhibited distinct biological activity related to lipid metabolism, independent of the N-terminal region that drives growth-promoting effects.
This finding emerged from systematic research at Monash University in Australia during the 1990s, where researchers methodically tested peptide fragments of HGH to map which regions were responsible for which biological activities. The C-terminal fragment was identified as the region with the most pronounced effects on fat cell biology — specifically, stimulating lipolysis and inhibiting lipogenesis in adipocyte preparations.
HGH Fragment 176-191: The Native Fragment
| Item | Detail |
| Compound name | HGH Fragment 176-191 |
| CAS Number | 66004-57-7 |
| Sequence | Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (15 amino acids) |
| Molecular weight | ~1817 Da |
| Origin | Direct fragment of native HGH sequence positions 176-191 |
| N-terminus | Free leucine (native sequence start) |
HGH Fragment 176-191 is the native peptide sequence — positions 176 through 191 of the full HGH molecule, extracted as a standalone compound. In this form, it retains the disulfide bond between the two cysteine residues (positions 182 and 189 in the fragment), which contributes to its structural conformation.
AOD-9604: The Modified Version
| Item | Detail |
| Compound name | AOD-9604 (Tyr-hGH Fragment 177-191) |
| CAS Number | 221231-10-3 |
| Sequence | Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (16 amino acids) |
| Molecular weight | ~1815 Da |
| Key difference | Tyrosine (Tyr) added at N-terminus; sequence starts at position 177 (not 176) of HGH |
| Origin | Synthetic, developed at Monash University as a stabilised research version |
AOD-9604 differs from HGH Fragment 176-191 in two ways: it begins at position 177 rather than 176, and a tyrosine residue is added at the N-terminus. This creates a 16-amino acid peptide rather than the 15-amino acid native fragment. The ‘9604’ designation is a research identifier from the Monash University development programme.
Why the Tyrosine Addition Matters
Adding a tyrosine at the N-terminus of a peptide is a well-established modification in peptide chemistry research. Tyrosine provides a defined N-terminal structure that can improve peptide stability by protecting the N-terminus from aminopeptidase degradation. It also provides a UV-absorbing aromatic ring useful for peptide quantification by absorbance spectroscopy — a practical benefit in research settings.
The tyrosine addition in AOD-9604 shifts the biological activity profile slightly compared to the native fragment, as the N-terminus contributes to receptor interactions and conformation. Research comparing the two compounds in adipocyte assays has reported similar overall biological activity profiles, with AOD-9604’s modifications providing improved stability characteristics.
Research Comparison: What’s Different in Practice?
| Item | Detail |
| Structural basis | Native HGH C-terminal sequence | Modified with Tyr addition |
| Length | 15 amino acids | 16 amino acids |
| Stability | Moderate | Improved (Tyr N-terminal protection) |
| GH receptor binding | Minimal | Minimal (both avoid GH receptor) |
| IGF-1 stimulation | Negligible | Negligible (both) |
| Lipolytic activity in research | Documented | Documented; similar profile |
| Primary use in research | Mechanistic studies | Extended research protocols requiring stability |
For most research applications examining fat metabolism mechanisms, the two compounds produce comparable results. The practical choice depends on the experimental protocol: AOD-9604’s improved stability makes it preferable for longer-duration studies or storage-sensitive protocols, while HGH Fragment 176-191 provides access to the native sequence for mechanistic studies where the precise N-terminal structure matters.
AOD-9604 and HGH Fragment 176-191 are related but distinct research compounds. The tyrosine addition is not cosmetic — it affects stability and potentially receptor interaction characteristics. Treating them as identical in research literature reviews can lead to errors in interpreting comparative study results.