Somatropin (HGH 191-AA) Research Guide: Structure, IU vs mg, Reconstitution and Quality

Somatropin research material is recombinant human growth hormone with the full 191-amino-acid sequence of the native 22 kDa pituitary hormone. It is a folded four-helix protein rather than a short synthetic peptide, it is quantified in international units as well as milligrams, and in the United Kingdom it is a Class C controlled drug. This guide covers its structure, the IU-to-mg convention, concentration reference figures for solution preparation, and the quality checks that matter for a 191-AA product.

Updated 10 September 2026~14 min readReviewed by the GenoPept technical team

What is somatropin (HGH 191-AA)?

Somatropin is the international non-proprietary name for recombinant human growth hormone produced with the identical 191-amino-acid sequence of the principal human pituitary isoform. The “191-AA” in product names is simply that residue count, used to distinguish full-length material from truncated variants and fragments.

DrugBank records somatropin under CAS 12629-01-5 with the molecular formula C990H1532N262O300S7 and a molecular weight of 22,129 Da, which is why the molecule is conventionally described as the 22 kDa isoform.¹ Analytical work on human serum confirms the mature protein comprises 191 residues with cysteines at positions 53 and 165 and at positions 182 and 189 joined by disulfide bridges.²

Two consequences follow from that size. First, somatropin is a protein, not a peptide in the synthetic-chemistry sense: it is produced by recombinant expression and refolding rather than by solid-phase synthesis, so its quality attributes include correct folding and aggregation state, not just sequence purity. Second, its biological activity depends on three-dimensional structure, which makes it more sensitive to handling than a short linear peptide.

Structure: the four-helix bundle and its disulfide bridges

Somatropin adopts an up-up-down-down antiparallel four-helix bundle, the fold shared across the growth hormone and cytokine family. The crystal structure of human growth hormone bound to the extracellular domain of its receptor, solved by de Vos, Ultsch and Kossiakoff in 1992, established both the fold and the way two receptor molecules engage a single hormone.³

Two disulfide bridges cross-link the chain. The larger, Cys53 to Cys165, closes a substantial loop through the core of the molecule. The smaller, Cys182 to Cys189, forms a compact loop near the C-terminus.² Together they restrain the fold, and disruption of either — by reduction, by scrambling, or by unfolding at an interface — is a genuine loss of the molecule as characterised, even when the amino acid sequence is untouched.

Why this matters analytically

For a short synthetic peptide, reverse-phase HPLC purity and an observed mass usually settle the question of identity. For a 22 kDa folded protein they do not. A correctly sequenced somatropin molecule can be misfolded, disulfide-scrambled, or present as dimer and higher-order aggregate, and none of those states necessarily separates cleanly on a standard reversed-phase gradient.

Meaningful characterisation therefore adds orthogonal methods: size-exclusion chromatography to resolve aggregate from monomer, and peptide mapping after enzymatic digestion to confirm sequence and disulfide connectivity. When evaluating a certificate of analysis for a 191-AA product, the presence or absence of an aggregate measurement is the single most informative difference from a peptide COA.

IU vs mg: where 1 mg ≈ 3 IU comes from

Growth hormone has historically been quantified by bioassay, and the international unit is a potency measure defined against a reference preparation rather than a mass. The WHO Second International Standard for somatropin, coded 98/574 and held at NIBSC, carries an assigned value of 3.0 International Units per mg somatropin.⁴ That assignment is the whole basis of the conversion used throughout the field.

Two directions follow directly:

1. Milligrams to units. IU = mg × 3. So 1 mg is 3 IU, and 10 mg is 30 IU.
2. Units to milligrams. mg = IU ÷ 3. So 10 IU is approximately 3.33 mg, and a 100 IU kit contains approximately 33.3 mg of protein.

The word “approximately” is doing real work. The international unit is a potency unit, so the mass equivalent of a given number of units depends on the specific activity of the material being measured. For a well-characterised, correctly folded 191-AA product the 3 IU/mg figure holds closely; for degraded or partially aggregated material the true potency per milligram would be lower, which is precisely why the standard exists.

Mechanism described in published research

The mechanism of growth hormone is unusual and well characterised. A single hormone molecule carries two distinct receptor-binding surfaces, conventionally called site 1 and site 2, and engages two growth hormone receptor molecules sequentially. That 1:2 stoichiometry was established by the 1992 crystal structure of the hormone bound to the receptor’s extracellular domain.³

DrugBank summarises the downstream consequence: the hormone “binds to the human growth hormone receptor (GHR), which is a dimeric receptor expressed in target cells in the liver and cartilage”, the receptor dimerises and interacts with Janus kinase 2, and this triggers tyrosine phosphorylation of JAK2 and the receptor, initiating the signal transducer and activator of transcription pathway. Binding in liver and cartilage promotes production of IGF-1, which acts through type 1 IGF receptors.¹

Framed for a research context, this makes somatropin useful as a defined agonist in cell-culture systems examining GHR signalling, JAK-STAT pathway activation, or IGF-1 transcription. What matters analytically is that the readout depends on correctly folded, monomeric material — an aggregated preparation will under-perform in a receptor assay regardless of what the mass spectrum says.

UK legal status: a Class C controlled drug

Somatropin is a controlled drug in the United Kingdom. The Misuse of Drugs Act 1971 (Modification) Order 1996 added a group of anabolic and androgenic steroids, an adrenoceptor stimulant, and polypeptide hormones to the Class C list under the Act.⁵ Somatropin was among the polypeptide hormones brought into that classification.

Its regulatory handling is set by Schedule 4 Part II of the Misuse of Drugs Regulations 2001, which lists “Chorionic Gonadotrophin (HCG) … Clenbuterol … Non-human chorionic gonadotrophin … Somatotropin … Somatrem … Somatropin”.⁶ Part II of that Schedule covers substances excepted from the prohibition on possession, and excluded from importation and exportation offences when carried out in person for administration to that person, while remaining subject to specified record-keeping and supply requirements.

Practically, the point for a research buyer is straightforward. Somatropin is not an unregulated catalogue chemical. Supply is a regulated activity, a supplier should acknowledge that status openly rather than treating the product as an ordinary line, and any transaction remains subject to the usual research-use-only conditions in addition to controlled-drug requirements. This is general information, not legal advice.

Kit sizes and total protein content

Somatropin research material is commonly supplied as a multi-vial kit labelled by total international units, typically from 60 IU up to 360 IU across a set of individual vials. Converting a kit label to total protein mass uses nothing more than the 3.0 IU/mg factor.

Vial splits vary between presentations, so always read the individual vial label rather than inferring it from the kit total. The mass per vial is what enters any concentration calculation.

Concentration reference for solution preparation

Somatropin follows the same arithmetic as any lyophilised material: concentration equals mass divided by diluent volume. The only additional step is that the label is in international units, so the solution concentration can be expressed in IU/mL as well as mg/mL. Table 3 gives both, together with the amount in a 0.1 mL withdrawal and the corresponding U-100 graduation.

These figures describe the composition of a prepared laboratory solution. They are concentration and volume values only. The reconstitution calculator will run the same arithmetic for any vial mass and diluent volume.

Quality, identity and handling a folded protein

Handling somatropin well means protecting a fold, not just a sequence. Three practices carry most of the benefit.

1. Reconstitute gently. Run the diluent down the inner wall of the vial rather than onto the cake, and swirl rather than shake. Foaming creates an air-water interface where proteins unfold and aggregate; a folded 22 kDa protein is more vulnerable to this than a short linear peptide.
2. Avoid repeated freeze-thaw. Each freezing and thawing transition stresses the tertiary structure and promotes aggregation. Where a reconstituted solution must be frozen, aliquot it first so each aliquot is thawed once.
3. Keep the dry material dry. Manning and colleagues note that all the major degradation routes — deamidation, oxidation, hydrolysis, disulfide scrambling and aggregation — run far more slowly in the lyophilised state than in solution.⁷ The single most effective stability measure is not reconstituting material until it is needed.

What to look for on a somatropin certificate of analysis

A peptide-style certificate showing only reverse-phase purity and an observed mass is incomplete for a 191-AA product. Look additionally for a size-exclusion result reporting monomer content and related proteins, and for peptide mapping or another identity method confirming the sequence and disulfide arrangement. Appearance, moisture and pH after reconstitution are also meaningful for a lyophilised protein.

Our guides on reading a certificate of analysis and purity testing set out the underlying methods in more detail.

Somatropin versus growth hormone secretagogues

A frequent point of confusion is the relationship between somatropin and the growth hormone secretagogues. They are not variants of one another and they act at different points in the axis.

Somatropin is the hormone. It is recombinant human growth hormone applied directly, engaging growth hormone receptors on target cells with no involvement of the pituitary.

Secretagogues act upstream. GHRH analogues such as sermorelin, tesamorelin and CJC-1295 act at the GHRH receptor on pituitary somatotrophs; GHRPs such as ipamorelin, GHRP-2, GHRP-6 and hexarelin act at the ghrelin receptor GHS-R1a. Both families are studied for their effect on release of endogenous hormone rather than as hormone themselves.

Structurally the contrast is stark: somatropin is a 191-residue folded protein of about 22 kDa, while ipamorelin is a pentapeptide of under 1 kDa. That difference drives everything from synthesis route to analytical method to handling requirement. The secretagogues guide covers that family in full.