HGH 191aa Peptide: The 191 Amino Acid Form of Growth Hormone

By What Peptides Editorial Team · Updated 2026-09-14 · Part of Peptide Research, Industry Updates & Practical Guides

HGH 191aa is shorthand used in supplier listings for human growth hormone specified by residue count. The number refers to the mature 191 amino acid chain, which is the form that remains after the signal peptide is removed during secretion. Growth hormone is a protein rather than a short peptide, and its size places it at the large end of what peptide chemistry handles routinely: about 22 kDa, a four helix bundle, and two disulfide bonds. This page is descriptive biochemistry. It describes what the molecule is, which forms circulate, and what the production nomenclature means. It makes no performance or body composition claim and states no dose.

The residue count is worth taking seriously, because it is one of the few facts on a listing that a reader can check independently. At an average residue mass of about 110 Da, a 191 residue chain falls near 21 kDa before end corrections and close to 22 kDa after them, which matches the mass quoted in sequence databases. The sequence, the disulfide pattern and the annotated variants are recorded in UniProt, so a listing printing a different count describes something other than the native chain. For chain vocabulary see peptide structure and classification, and for size thresholds elsewhere see labelling categories in sports supplements.

The 191 residue chain and the two major circulating forms

The GH1 gene encodes a prehormone of 217 residues. The first 26 residues are a signal peptide: an N-terminal stretch with a positively charged n-region, a hydrophobic h-region and a polar c-region carrying a cleavage site, recognised by the signal recognition particle during translation and removed by signal peptidase. What remains is the mature 191 residue chain. That cleavage is why the count matters. A listing reading 191 refers to the mature circulating form, a listing reading 217 refers to the prehormone, and the two are not the same molecule even though they share a name.

Circulating growth hormone is not one species. The predominant form is the 191 residue chain of roughly 22 kDa, produced from the full length messenger RNA. A second major form of about 20 kDa arises from alternative splicing that skips one exon and deletes a block of internal residues. A third group of species includes deamidated and other modified variants, N-terminally truncated chains, disulfide-linked dimers and oligomers, and fragments generated after secretion. Because these appear after translation as well as during it, an immunoassay and a mass spectrometry method can return different pictures of one sample. That is an analytical point, not a claim about effect.

Recombinant production and the nomenclature around it

The word recombinant describes a production method, not a different molecule in the ideal case. A host cell carries an expression construct, translates the chain, and either secretes it or accumulates it; purification follows, and the product is characterised for identity, purity, potency and impurity profile. Commonly used hosts include Escherichia coli, which accumulates the chain intracellularly and requires refolding, and mammalian cells such as Chinese hamster ovary cells, which secrete a folded chain. Each route leaves a different impurity signature, and host cell protein and residual DNA are specified limits rather than incidental details.

Naming conventions follow from that history. Somatropin is the international nonproprietary name for growth hormone of recombinant DNA origin with the 191 residue sequence. Somatrem is a related name for a 192 residue variant carrying an additional N-terminal methionine, which was an early recombinant product. A listing printing 191aa is asserting sequence identity with the native mature chain, and a report supporting that claim would show an identity method such as LC-MS or peptide mapping alongside a purity figure from reversed phase HPLC with a stated gradient. Background on purity reporting sits in how third-party peptide purity reports are read.

• 191aa is a residue count assertion about the mature chain, not a grade and not a purity claim.
• Somatropin is the INN for the recombinant 191 residue form; somatrem carries an extra methionine.
• Host choice sets the impurity profile: bacterial hosts need refolding, mammalian hosts secrete folded chain.
• Identity is established by mass measurement or peptide mapping; purity is a separate figure from a separate method.
• Any specification should name the method used, not only the result obtained.

Where the 22 kDa figure comes from and how to check it

The 22 kDa figure is arithmetic a reader can reproduce. Take an average residue mass of about 110 Da, a convention that already accounts for the 18.01056 Da of water lost per peptide bond formed, and multiply by 191 residues: the chain lands near 21 kDa before end corrections. Add the retained terminal water of 18.02 Da, subtract about 2.016 Da for each of the two disulfide bonds formed by oxidising four cysteine thiols, and the total approaches 22.1 kDa. That is the figure quoted for the protein in sequence databases, and every term in it is checkable.

Checking a listing is then a matter of looking for the methods behind the number. Identity should be supported by a mass measurement or a peptide map, purity by a chromatographic method with its gradient and detection wavelength stated, and residual moisture by Karl Fischer titration where a lyophilised powder is supplied. Reference texts place lyophilised powder at around minus 20 degrees C and reconstituted material at 2 to 8 degrees C, with repeated freeze-thaw cycles as the commonly cited failure mode. Those are laboratory handling conventions, not instructions. See how subunit assembly is described, and more reporting in this cluster sits under peptide research and industry updates.

This page is part of the Peptide Research, Industry Updates & Practical Guides guide.