HGH 24iu Peptide

HGH is recombinant human growth hormone: the complete 191-residue mature chain of the protein encoded by GH1 on human chromosome 17, produced in bacteria rather than extracted from tissue. UniProt P01241 describes a 217-residue precursor whose first 26 residues are a secretory signal peptide. Cleaving that signal leaves the circulating hormone, running from FPTIPLSRLF at the N terminus to SVEGSCGF at position 191. The international non-proprietary name for the recombinant protein with that exact sequence is somatropin, and the trade shorthand 191AA exists to distinguish it from somatrem, the earlier 192-residue bacterial product that carried an extra N-terminal methionine.

It is worth being precise about vocabulary. At 22,124 g/mol across 191 residues, with two internal disulfide bridges and a defined tertiary fold, this is a protein, not a peptide. Almost everything else in a research peptide catalogue is a synthetic chain of five to forty residues with no fixed structure in solution. That difference drives everything downstream: how the material is made, how it is assayed, how fragile it is in a vial, and why it is the only item on the shelf whose content is stated in International Units instead of milligrams.

Growth hormone also occupies an unusual place in the history of molecular biology. Its complex with its own receptor was the first cytokine receptor structure ever solved, the alanine scan that mapped its binding surface invented a technique still used across protein engineering, and the disaster that ended pituitary extraction in 1985 is the reason recombinant production exists at all. Those stories are covered below, along with the arithmetic that turns 24iu into a mass.

Somatropin folds into an up-up-down-down four-helix bundle. The topology is unusual: in most four-helix bundles adjacent helices run antiparallel, whereas here helices 1 and 2 run parallel to each other and the connecting loops cross the full length of the bundle. Two disulfide bridges hold it together. Cys53 to Cys165 closes a large loop spanning most of the molecule, and Cys182 to Cys189 closes a small loop at the C terminus, which happens to be the loop that the separately sold HGH Fragment 176-191 consists of.

The hormone carries two entirely different receptor binding surfaces, and neither resembles the other structurally. Site 1 is the high-affinity face, built mainly from helix 4 and the loop between helices 1 and 2. Site 2 is a lower-affinity face on the opposite side, built from helices 1 and 3. Cunningham and Wells mapped site 1 by replacing every residue in three discontinuous stretches of the sequence, 62 single alanine substitutions in total, and found roughly a dozen large side chains whose loss cost more than fourfold in receptor affinity. The same scan found one residue, Glu174, whose removal improved affinity by more than fourfold, which is why engineered growth hormone variants are frequently more avid than the natural hormone.

One hormone molecule binds two receptor molecules. The 2.8 angstrom crystal structure of somatropin with the extracellular domain of its receptor, solved at Genentech and deposited as PDB 3HHR, shows a 1:2 complex in which both receptor copies present essentially the same residues to the hormone despite the two hormone surfaces having no structural similarity to each other. Binding is sequential: a receptor occupies site 1 first, and only then does a second receptor engage site 2. That order is what makes an excess of hormone self-inhibitory in vitro, since at high concentration every receptor is capped by its own hormone at site 1 and no dimer forms.

The intracellular consequence is JAK2 activation. The growth hormone receptor has no kinase domain of its own. It recruits Janus kinase 2 to a proline-rich Box1 motif just inside the membrane, and Argetsinger and colleagues showed in 1993 that growth hormone drives complex formation between JAK2 and the receptor, activates JAK2 kinase activity, and produces tyrosine phosphorylation of both proteins. Activated JAK2 phosphorylates the receptor tail, which recruits and phosphorylates STAT5B, which dimerises, enters the nucleus and transcribes the growth hormone response genes, IGF1 and IGFALS among them. Src family kinases bound to the upper cytoplasmic domain account for a smaller set of responses, and SOCS2 provides the negative feedback that terminates the signal.

Downstream, the classical somatomedin arm runs through the liver. Hepatic STAT5B activation raises IGF-1 output, and IGF-1 in turn drives the anabolic and growth-promoting effects historically attributed to growth hormone itself. The direct arm does not go through IGF-1 at all: growth hormone acts on the adipocyte to promote lipolysis and to antagonise insulin action, which is why the intact hormone is diabetogenic in a way that its C-terminal fragment is not.

This is the most-asked question about growth hormone and the one most product listings answer badly or not at all. Growth hormone is labelled in International Units because the unit predates reliable protein mass assays. When the hormone could only be extracted from pituitary tissue in impure form, the sensible way to define an amount was by biological effect against a shared reference ampoule, not by weighing a preparation whose composition nobody could pin down. The convention stuck long after the assay problem went away.

The current anchor is the Second WHO International Standard for somatropin, ampoule code 98/574, established in 2001 in place of the first standard 88/624. Its calibration study ran across 16 laboratories in nine countries, used size-exclusion HPLC as the recognised assay, and produced two numbers: an ampoule content of 1.95 mg of total somatropin plus somatropin-related proteins, and a specific activity of 3.0 IU per mg of somatropin. That second figure is the conversion factor. At 3.0 IU per mg, a 24iu presentation corresponds to 8.0 mg of protein, a 10iu presentation to 3.33 mg, and a 100iu kit to 33.3 mg.

Two details in that definition matter and are almost universally dropped. First, the 3.0 IU per mg figure is defined against somatropin monomer, not against total protein in the vial. Dimers, higher aggregates and related proteins count towards mass but not towards the assigned activity, so a vial holding 8.0 mg of total protein delivers 24iu only if essentially all of it is monomeric. Size-exclusion HPLC, not reverse-phase HPLC, is the method that answers that question. Second, the conversion is specific to the recombinant standard. The older pituitary-derived International Standard, code 80/505, was assigned 4.4 IU per ampoule against roughly 1.70 mg of hGH extract, which works out near 2.6 IU per mg. Literature from the pituitary era therefore does not convert with the modern factor, and comparing an old IU figure against a new one without accounting for the change in standard introduces a systematic error of about fifteen percent.

The practical consequence for anyone reading a certificate of analysis: a stated IU figure is a claim about biological potency that depends on monomer content, whereas a stated milligram figure is a claim about mass. They are not interchangeable statements about the same thing, and the arithmetic that turns one into the other silently assumes a pure monomeric preparation.

Between 1989 and 1992 a group at Genentech worked out how a cytokine receptor is switched on, using growth hormone as the test case. The sequence of results is worth following because each step depended on the last.

The first was alanine-scanning mutagenesis, published by Cunningham and Wells in 1989. They replaced each residue in three discontinuous segments of the hormone, 62 substitutions, and measured receptor affinity for every variant while confirming with a panel of seven conformation-sensitive monoclonal antibodies that the fold had not been disturbed. The result was a functional map of the binding surface at single-side-chain resolution. The technique outgrew the problem and became a standard tool for protein interfaces generally.

The second was the 1991 demonstration that one hormone binds two receptors. Size-exclusion chromatography, calorimetry and a fluorescence quenching assay converged on a 1:2 complex, and engineered hormone variants revealed two distinct but adjacent binding sites on the hormone matched against two overlapping sites on the receptor. Crucially, the binding was shown to be ordered: site 1 first, site 2 second.

The third was the crystal structure. De Vos, Ultsch and Kossiakoff solved the complex at 2.8 angstroms and found exactly what the solution work predicted, plus something it could not have predicted: both receptors donate essentially the same residues to the hormone even though the two hormone surfaces have no structural resemblance to each other. The structure also revealed a large contact area between the C-terminal domains of the two receptors, and the relative sizes of those interfaces supported the sequential mechanism as the route to signal transduction.

The fourth confirmed the model by breaking it. Fuh and colleagues built a hybrid receptor carrying the growth hormone receptor extracellular domain on the transmembrane and cytoplasmic domains of the murine G-CSF receptor, then showed that bivalent antibodies to the receptor were agonists while their monovalent fragments were not. Substituting Gly120, which sits at the centre of site 2, produced a hormone that binds one receptor and cannot recruit the second, blocking signalling. That antagonist design principle later became the basis for a marketed growth hormone receptor antagonist used in acromegaly.

The textbook version of the story, that the hormone brings two separate receptor monomers together, turned out to be wrong in an interesting way. Work from Michael Waters's group at the University of Queensland showed that dimerisation alone is not sufficient to activate the full-length receptor, and that the extracellular domain does not measurably change conformation on ligand binding: the liganded and unliganded crystal structures are essentially the same shape.

Their 2005 model proposed instead that receptor subunits are already associated in the membrane and that the hormone works by rotating them relative to each other, a consequence of the asymmetric placement of site 1 and site 2 on the hormone. FRET, BRET and co-immunoprecipitation all supported specific transmembrane interactions that exist without hormone present. Inserting a defined number of alanine residues into the transmembrane domain, which rotates the helix without adding any ligand, activated the receptor on its own.

The 2014 follow-up filled in the rest of the mechanism. Ligand binding converts the two transmembrane helices from a parallel pair into a left-handed crossover, which separates the lower parts of the helices and therefore separates the Box1 motifs that carry JAK2. That separation pulls each JAK2 pseudokinase domain away from the kinase domain of its partner, releasing the inhibition and pairing the two kinase domains for trans-activation. The model is now taken to generalise across class I cytokine receptors, which is a reasonable claim to make about the receptor whose structure defined the family.

The somatomedin hypothesis held that growth hormone acts on the body almost entirely by causing the liver to release IGF-1. The cleanest test of that idea came in 1999, when Yakar and colleagues used Cre/loxP recombination to delete igf1 only in mouse liver. Hepatic igf1 mRNA disappeared and circulating IGF-1 fell dramatically, confirming that the liver supplies most of what is in the blood. But body weight, body length and femur length matched wild-type littermates. Circulating IGF-1 turned out not to be required for normal postnatal growth; locally produced IGF-1 acting in an autocrine and paracrine fashion is doing that work.

Almost none of the IGF-1 in blood travels free. Roughly three quarters of it circulates in a 150 kDa ternary complex made of IGF-1, IGFBP-3 and the acid-labile subunit, a leucine-rich glycoprotein of hepatic origin encoded by IGFALS. Free IGF-1 has a plasma half-life measured in minutes; inside the ternary complex it is measured in hours, because the assembled complex is too large to cross the capillary endothelium. Growth hormone regulates all three components through STAT5B, which is why IGFBP-3 and the acid-labile subunit rise and fall alongside IGF-1 rather than independently of it.

This architecture explains a result that otherwise looks paradoxical. Because the ternary complex both extends IGF-1 half-life and restricts its exit from circulation, a large rise in total serum IGF-1 does not necessarily mean a proportionate rise in IGF-1 reaching tissue. Reading IGF-1 as a proxy for growth hormone exposure requires knowing what fraction of it is complexed, which is why IGFBP-3 and acid-labile subunit measurements accompany IGF-1 in careful work.

This catalogue carries four things that relate to growth hormone in four different ways, and they are frequently conflated. Intact somatropin is the hormone itself, engaging the receptor directly. HGH Fragment 176-191 is the last sixteen residues cut away from the rest of the molecule, containing the small Cys182 to Cys189 disulfide loop and nothing else. AOD-9604 is that same C-terminal region with a tyrosine added at the N terminus. The secretagogues, GHRP-2, GHRP-6, hexarelin, ipamorelin and the GHRH analogues, do not touch the growth hormone receptor at all.

The fragments are defined by what they lack. Neither site 1 nor site 2 of the intact hormone survives amputation to sixteen residues, so no receptor dimerisation is possible and the JAK2/STAT5 arm is not engaged. That was shown directly: in cells transfected with the growth hormone receptor, AOD-9604 neither competed with labelled hormone for binding nor drove proliferation, while the intact hormone did both. The entire premise of the fragment programme was that lipolytic activity and growth-promoting activity map to different parts of the protein and can be separated, which also means fragment data cannot be extrapolated to the intact hormone or the reverse.

The secretagogues work one level upstream, at the ghrelin receptor GHS-R1a on pituitary somatotrophs, provoking release of whatever growth hormone the gland has already made. That preserves pulsatility, preserves hypothalamic negative feedback, and caps the response at the pituitary's secretory capacity. Direct receptor agonism does none of those things: it produces a concentration profile set by absorption rather than by the hypothalamus, and it bypasses the feedback loop entirely. In comparative work this is the distinction that matters, because a secretagogue and the hormone can produce similar IGF-1 responses through completely different upstream mechanisms and with completely different ceilings.

IGF-1 LR3, also in this catalogue, sits one step further downstream still. It is an IGF-1 analogue engineered to evade the binding proteins described in the previous section, so it acts at the IGF-1 receptor without reference to growth hormone, its receptor, or the ternary complex.

Growth hormone in blood is not one molecule. The GH1 transcript can be spliced at an alternative acceptor site inside exon 3, removing the codons for residues 32 to 46 of the mature chain, the stretch EEAYIPKEQKYSFLQ. The product is a 176-residue protein of roughly 20.3 kDa that shares both disulfide bonds and most of the fold with the 22 kDa form. It typically accounts for something on the order of a tenth of pituitary growth hormone output.

The deletion falls in the loop between helices 1 and 2, which contributes to site 1, and the biology reflects that. The 20 kDa variant showed no specific binding to growth hormone receptors in human liver plasma membranes, whereas the 22 kDa form bound the same preparation with high affinity, implying that the 20 kDa form contributes little to the hepatic somatogenic pathway in humans. Its plasma transport is different too: roughly 80 percent of the protein-bound fraction is carried by a low-affinity binding protein specific to the 20 kDa form, with only a small fraction on the receptor-derived high-affinity binding protein that carries almost all of the 22 kDa form.

Two practical points follow. Recombinant somatropin is the 22 kDa form only, so a preparation of it is more homogeneous than the pituitary hormone it replaced, which is one of the reasons the recombinant material assays more cleanly. And the ratio of 22 kDa to 20 kDa forms in a blood sample is the basis of the isoform differential immunoassay used in anti-doping testing, since exogenous 22 kDa hormone suppresses endogenous secretion of both forms and skews the ratio in a way that endogenous secretion never does.

Growth hormone is species-specific: bovine or porcine hormone does not usefully activate the human receptor. Until the 1980s the only source was human cadaver pituitary glands, pooled by the tens of thousands and extracted in national programmes. In 1985 the first cases of Creutzfeldt-Jakob disease appeared in young adults who had received that material as children, with incubation periods running from a few years to several decades.

The final tally is documented. Reviewing the entire era of iatrogenic transmission, Brown and colleagues counted 226 cases of Creutzfeldt-Jakob disease attributable to contaminated cadaveric growth hormone, making it one of the two largest iatrogenic prion outbreaks on record alongside dura mater grafts. A single infected donor gland pooled into a batch could contaminate the whole batch, and no purification step in use at the time inactivated the agent.

Distribution of pituitary-derived hormone was halted in 1985, the same year recombinant somatropin became available. The switch was not an incremental improvement in manufacturing; it removed the source of the hazard. Every gram of growth hormone produced anywhere in the world since then has been recombinant, and that is the direct reason a bacterial expression system, rather than tissue extraction, is the only route by which this protein now exists.

Growth hormone was one of the first proteins ever expressed recombinantly. Goeddel and colleagues reported direct expression in Escherichia coli in Nature in 1979, using a hybrid gene assembled from chemically synthesised DNA joined to enzymatically prepared cDNA under the lac promoter. That first route had a defect: bacterial translation begins with methionine, so the product carried an extra N-terminal methionine and was 192 residues rather than 191. That molecule became somatrem, and the extra residue was associated with higher rates of anti-hormone antibody formation than the natural sequence.

The fix was to make the bacterium do the processing. Gray and colleagues showed in 1985 that placing either the natural human signal sequence or the E. coli alkaline phosphatase signal sequence ahead of the coding region caused the protein to be secreted into the periplasmic space, where signal peptidase cleaves it. N-terminal sequencing confirmed correct processing in both cases, and non-reducing gels confirmed that the periplasmic product was monomeric and carried the same two disulfide bonds as authentic hormone. The oxidising environment of the periplasm, unlike the reducing cytoplasm, allows those bonds to form.

That is what 191AA denotes: the periplasmically processed, correctly disulfide-bonded 191-residue sequence with no extra methionine, identical in primary structure to the hormone the pituitary secretes. It is a claim about identity, not about purity or about who made it. Confirming it requires intact mass measurement, since a 191-residue and a 192-residue chain differ by 131 Da, well within the resolution of routine electrospray mass spectrometry but invisible to an HPLC purity trace.

A 191-residue protein needs a different analytical package from a synthetic peptide, and the difference is not cosmetic. Reverse-phase HPLC purity, the number that dominates peptide certificates of analysis, answers a narrow question here: it separates the protein from chemically distinct impurities but is poorly suited to distinguishing monomer from dimer and higher aggregates, because those species share the same chemistry.

Size-exclusion HPLC is the method that answers the aggregation question, and it is the method the WHO standard was calibrated by for exactly that reason. Since the assigned 3.0 IU per mg refers to monomer, size-exclusion data is what converts a mass figure into a potency figure. A preparation can show high reverse-phase purity and still be substantially dimeric.

Intact mass by electrospray or MALDI mass spectrometry confirms the 22,124 Da chain and rules out the 192-residue somatrem variant, the reduced form (4 Da heavier than the oxidised form), and the common degradation products. Peptide mapping after tryptic digestion confirms the sequence and, run under non-reducing conditions, confirms that Cys53 pairs with Cys165 and Cys182 with Cys189 rather than scrambling into the alternative pairings. Circular dichroism reports on helical content and is the fastest way to detect a preparation that has lost its fold, since an unfolded four-helix bundle is analytically distinct while being chemically identical.

Degradation chemistry worth watching on a stability trace: deamidation, which UniProt records at Gln163 and Asn178 of the precursor numbering as a deterioration product, methionine oxidation, and disulfide scrambling. Each shifts mass or charge in a characteristic way, which is why ion-exchange chromatography is a useful orthogonal method alongside the size-exclusion and reverse-phase traces.

Almost every handling rule that applies loosely to synthetic peptides applies strictly here, because a folded protein has a state to lose that a short linear peptide does not. Lyophilised somatropin is comparatively robust, which is why the WHO ampoules are shipped at ambient temperature despite being stored at -20 degrees Celsius: the freeze-dried solid has little conformational freedom to speak of. In solution the picture changes completely.

The dominant failure mode in solution is aggregation, not hydrolysis. Growth hormone is surface active and unfolds at air-water interfaces, so agitation, foaming and vigorous mixing all drive irreversible aggregate formation. Freeze-thaw cycling does the same thing by a different route, concentrating solutes at the ice front and denaturing protein at the growing crystal boundary. The consequence is measurable in the assay that matters: aggregates count towards mass but not towards assigned activity, so an agitated preparation loses potency without losing protein.

Zinc is worth knowing about here. Cunningham and colleagues showed that two Zn2+ ions bind cooperatively per hormone dimer, coordinated by His18, His21 and Glu174, and that the zinc-stabilised dimer resists guanidine denaturation better than the monomer does. This is thought to be how the hormone is packed into pituitary secretory granules, and it is why zinc-containing buffers behave differently from zinc-free ones in stability studies.

The WHO standard formulation is itself informative about what stabilises this protein: each ampoule of 98/574 contains 1.95 mg of somatropin against 20 mg of glycine, 2 mg of mannitol, 2 mg of lactose and 2.5 mg of sodium bicarbonate. The excipient mass exceeds the protein mass by more than a factor of thirteen, which is a fair indication of how much scaffolding a lyophilised protein needs to survive the drying step intact.

The regulatory position needs stating precisely because it is unlike anything else in a research catalogue. Recombinant somatropin is a licensed biologic medicine, dispensed on prescription, in the United States, the European Union, Canada, the United Kingdom, Japan and elsewhere, with marketing authorisations covering growth hormone deficiency in children and adults and several other endocrine indications. Multiple manufacturers hold those authorisations, and the products are made under pharmaceutical good manufacturing practice with lot-level regulatory oversight.

Material supplied for laboratory research is a separate category of thing. It carries no marketing authorisation, is not manufactured or released against a pharmacopoeial monograph, and is not supplied for administration to humans or animals. The reference material this article cites carries the same restriction: the WHO instructions for use of 98/574 state plainly that the preparation is not for administration to humans or to animals in the human food chain. Growth hormone additionally sits in a distinct legal category in the United States, where distribution for purposes other than those authorised is separately restricted under 21 U.S.C. 333(e), which is not true of the synthetic peptides elsewhere in this catalogue.

In sport, growth hormone is prohibited at all times under section S2 of the World Anti-Doping Agency prohibited list, as are its fragments and the secretagogues that release it. Two independent detection approaches are in routine use: the isoform differential test described above, which exploits the suppression of endogenous 20 kDa hormone by exogenous 22 kDa hormone, and the biomarker test, which reads IGF-1 and the collagen fragment P-III-NP together against population reference ranges. The isoform test has a short detection window measured in hours to a day or two; the biomarker test extends it to a week or more.