IGF-1 LR3 is an 83-residue recombinant analogue of human insulin-like growth factor 1, carrying two deliberate changes: an arginine substituted for glutamate at position 3, and a 13-residue extension added to the N-terminus. Both modifications exist for one reason — to strip the molecule of its affinity for the IGF binding proteins that normally sequester native IGF-1 — while leaving affinity for the IGF-1 receptor essentially intact. The result is a growth factor that stays free in solution and in serum-containing culture, which is why it became a standard reagent in cell-culture and receptor-signalling work long before it appeared on research-chemical catalogues.
MyPeptide supplies IGF-1 LR3 as a lyophilized powder in 0.1 mg and 1 mg vials within the IGF and muscle peptides range. Research use only; not for human or veterinary use.
What is IGF-1 LR3?
The name encodes the chemistry. "LR3" is Long R3: Long for the N-terminal extension, R3 for the arginine at residue 3. The parent molecule, IGF-1, is a 70-amino-acid single-chain protein structurally related to proinsulin, produced largely in the liver under growth hormone control and acting on the IGF-1 receptor, a receptor tyrosine kinase. Native IGF-1 circulates almost entirely bound to a family of six IGF binding proteins, which control its availability and shorten its free half-life to a few minutes.
IGF-1 LR3 is produced by recombinant expression rather than solid-phase synthesis — at 83 residues and 9,117.60 g/mol it is a small protein, not a peptide that could be practically assembled stepwise. It carries CAS number 946870-92-4 and the molecular formula C400H625N111O115S9, with the three disulfide bridges of the native IGF-1 fold.
Origin and structure
Both modifications target binding-protein contact rather than receptor contact. Glutamate 3 sits in the IGF-1 N-terminal region that makes a key electrostatic contact with the binding proteins; replacing it with arginine reverses the local charge and has been reported to reduce IGFBP affinity by roughly two orders of magnitude. The 13-residue extension (MFPAMPLSSLFVN) adds further steric interference at the same interface and also raises the molecule's isoelectric point, which changes its handling characteristics in solution.
What the modifications deliberately leave alone is the receptor-binding surface. The residues that contact IGF-1R sit elsewhere on the fold, so IGF-1 LR3 remains a full agonist at the receptor. The consequence, reported consistently in culture systems, is a substantially greater apparent potency than native IGF-1 — not because the analogue binds the receptor more tightly, but because far more of it is free to do so.
How IGF-1 LR3 is thought to work
IGF-1 LR3 binds the IGF-1 receptor, a disulfide-linked α2β2 tyrosine kinase closely related to the insulin receptor. Ligand binding triggers autophosphorylation of the β-subunit kinase domain and recruitment of insulin receptor substrate proteins, which branch into two principal pathways: PI3K/Akt/mTOR, associated in the literature with protein synthesis and cell survival readouts, and Ras/Raf/MEK/ERK, associated with proliferation readouts. In myoblast systems these are the pathways investigators sample when they measure the response.
Two mechanistic caveats matter for experimental design. First, IGF-1R and the insulin receptor form hybrid receptors, and IGF-1 analogues retain measurable affinity for the insulin receptor's isoform A; selectivity is relative, not absolute. Second, because the analogue evades binding-protein control, it removes a regulatory layer that is part of native IGF-1 biology. A study using IGF-1 LR3 is deliberately measuring unbuffered receptor activation, which is useful for signalling questions and less representative of physiological IGF-1 exposure.
What the research has examined
Cell culture and bioprocessing
The largest and best-established use is as a serum-free medium supplement. Long R3 IGF-1 is widely reported in the biotechnology literature as a replacement for insulin in CHO and hybridoma cultures, where its resistance to binding proteins secreted by the cells themselves gives it a longer functional life in the medium than native IGF-1. This is in-vitro, industrial-scale work with well-characterised results.
Myoblast and skeletal muscle signalling
C2C12 and primary myoblast studies have used IGF-1 LR3 to examine proliferation and differentiation, Akt phosphorylation, and hypertrophic signalling in cultured myotubes. These are cell-level readouts; extrapolation from them to whole-organism outcomes is not supported by the published record.
Rodent growth and metabolic models
Animal work with Long R3 IGF-1 dates largely from the 1990s, including studies in rats reporting greater growth-promoting potency than equimolar native IGF-1, consistent with the binding-protein rationale. Metabolic studies have also reported hypoglycaemic responses, reflecting the residual insulin-receptor cross-reactivity noted above.
Comparative analogue work
IGF-1 LR3 is frequently studied alongside IGF-1 DES (1-3), the truncated analogue that solves the same binding-protein problem by deletion rather than substitution. The two behave differently in tissues where IGFBP proteolysis is active, and the choice between them is a genuine experimental decision rather than a preference — set out in our IGF-1 LR3 vs IGF-1 DES comparison and in the background piece on why these modifications exist.
Forms and sizes we supply
| Attribute | Specification |
|---|---|
| Product | IGF-1 LR3 (Long R3 IGF-1) |
| Form | Lyophilized powder, sealed glass vial |
| Available sizes | 0.1 mg, 1 mg |
| Purity | ≥99% by HPLC, lot-matched COA available |
| CAS number | 946870-92-4 |
| Molecular formula | C400H625N111O115S9 |
| Molecular weight | 9,117.60 g/mol |
| Chain length | 83 residues (70-residue IGF-1 core plus 13-residue N-terminal extension) |
| Key modifications | Arg substituted for Glu at position 3; N-terminal extension |
| Production | Recombinant expression |
| Primary target | IGF-1 receptor (IGF-1R) |
| Intended use | Laboratory research only |
The 0.1 mg vial suits single-plate culture work and receptor assays where the working concentration is in the nanogram-per-millilitre range; the 1 mg vial is the unit for extended series that need one lot throughout. Sourcing checks are covered in our IGF-1 LR3 buying guide.
Reconstitution and storage in a lab context
Recombinant growth factors are handled differently from small synthetic peptides. IGF-1 LR3 is poorly soluble at neutral pH, and published product literature reconstitutes it in a dilute acid — commonly 10 mM hydrochloric acid or 0.1% acetic acid — before diluting into buffer or medium. Reconstituting directly into a neutral buffer risks incomplete dissolution and aggregation, which shows up as inconsistent activity rather than as visible precipitate.
The arithmetic is straightforward: a 1 mg vial made up to 1 mL yields 1 mg/mL, or 1,000 mcg/mL; 0.1 mL contains 100 mcg. Working stocks for culture are usually several further dilutions down from that. Dilute solutions of any protein lose material to plastic and glass surfaces, so a carrier protein such as 0.1% bovine serum albumin is standard practice for working stocks below about 10 mcg/mL. General method is in our reconstitution guide.
Lyophilized vials are held at −20 °C or below, sealed and protected from light and moisture. Reconstituted material is aliquoted immediately and frozen; repeated freezing and thawing is the most common cause of lost activity in a 9 kDa protein with three disulfide bridges. Wider guidance is in how to store peptides.
Purity, COA and how to read it
Certificates for a recombinant protein carry different weight from those for a synthetic peptide. HPLC purity remains the headline figure, but for an 83-residue product the more informative entries are the mass spectrometry result, which should match 9,117.60 g/mol and so distinguishes the LR3 analogue from native IGF-1 at 7,649 g/mol, and any SDS-PAGE image, which reveals aggregation and truncation products that a reversed-phase trace can hide. Endotoxin content is also relevant for cell-culture use, since bacterial expression systems are the usual source. Our guide to reading a peptide certificate of analysis covers how these sections fit together.
Regulatory status
IGF-1 LR3 has no marketing authorisation as a medicine in the United States. A related but distinct molecule, recombinant human IGF-1 (mecasermin), is an approved prescription product for a rare growth disorder; IGF-1 LR3 is not that molecule and is not interchangeable with it. Material supplied here is a research-use-only reagent for laboratory investigation by qualified personnel — not a supplement, not a medicine, and not for human or veterinary administration. IGF-1 and its analogues are prohibited in sport under anti-doping rules.
Related molecules and further reading
The nearest neighbours are IGF-1 DES (1-3), the truncation-based analogue, and MGF, the E-domain peptide from the IGF-1 Ec splice variant, which is not a receptor agonist at all and answers a different question. Broader orientation is in our peptides for muscle growth research overview.