What Does Long R3 Mean in IGF-1 LR3? The Structure Behind the Name

LynxLab · 24 October 2026 · 8 min read

Why IGF-1 LR3 has 83 amino acids rather than 70, what the Arg3 swap and 13-residue extension do to binding protein affinity, and who designed it in Adelaide.

Photo: Tara Winstead, Pexels

The name IGF-1 LR3 is a compressed description of two deliberate changes to a human growth factor. "Long" refers to 13 extra amino acids bolted onto the front of the molecule. "R3" records that the third amino acid of the natural sequence, a glutamate, was replaced with an arginine. Both changes were made in Adelaide in the early 1990s for one reason: to stop the peptide sticking to the binding proteins that normally hold IGF-1 in check.

IGF-1 itself is a 70-amino-acid peptide

Insulin-like growth factor 1 is made mainly in the liver in response to growth hormone and carries much of growth hormone's effect on tissue. The mature hormone is a single chain of 70 amino acids, cut from a longer precursor; UniProt entry P05019 records the mature chain as residues 49 to 118 of a 195-residue precursor. Its three-dimensional fold resembles insulin, which is where the name comes from.

In blood and in tissue, almost none of that IGF-1 floats free. It is carried by a family of six IGF binding proteins, IGFBP-1 to IGFBP-6, which extend its life in circulation but also block it from reaching its receptor until released. Any experiment with native IGF-1 in a system that contains binding proteins is really an experiment on the IGF-1 that escapes them.

That is the problem the Adelaide group set out to solve, and it is why igf-1 lr3 exists as a separate reagent rather than as a brand of IGF-1.

Why glutamate 3 was swapped for arginine

The earlier clue came from a truncated form, des(1-3)IGF-1, which lacks the first three amino acids and is far more potent than IGF-1 in cell assays. Losing residue 3, the glutamate, seemed to be what mattered.

R. King, Geoffrey Francis, John Wallace and colleagues tested that directly. In a 1992 paper in the Journal of Molecular Endocrinology they expressed human IGF-1 in Escherichia coli as a fusion protein and produced two variants with glutamate 3 replaced by either glycine or arginine. Both [Gly3]-IGF-1 and [Arg3]-IGF-1 were more potent than IGF-1 at stimulating protein and DNA synthesis in rat L6 myoblasts.

The binding data explained why. Both analogues "bound very poorly to bovine IGF-binding protein-2" while binding "slightly less well than IGF-I to the type-1 receptor". The authors' conclusion was that "reduced binding to IGF-binding proteins rather than increased receptor binding is the likely explanation for the greater biological potency". The arginine version became the R3 in igf1 lr3.

Where the 13-amino-acid tail came from

The "Long" part began as a manufacturing trick. Expressing small peptides like IGF-1 in bacteria is inefficient, so the Adelaide team built fusion proteins: an IGF-1 sequence preceded by a fragment of porcine growth hormone. In the second 1992 paper, Francis and colleagues describe a version made of the first 11 amino acids of methionyl porcine growth hormone, then the dipeptide Val-Asn, then IGF-1. That is 13 residues in total, and it was left on the finished molecule.

The Francis paper names the three constructs Long IGF-1, Long [Gly3]-IGF-1 and Long [Arg3]-IGF-1, the last of which is the molecule now sold as IGF-1 LR3. The hydrophobic extension "appears to facilitate the correct folding of the IGF-I analogues", which raised yields, and it also reduced binding-protein affinity in its own right.

The combined molecule is 83 amino acids, 70 from IGF-1 plus the 13-residue extension, and Sigma-Aldrich's datasheet for its LONG R3 IGF-1 product gives a molecular mass of about 9,110 daltons. It describes the peptide as "the complete human IGF-I amino acid sequence with substitution of Arg for Glu 3 and a 13 amino acid N-terminal extension peptide", expressed in E. coli, recovered from inclusion bodies and refolded before chromatography.

How much the binding changed

Francis's team ranked the constructs by potency in cell lines that secrete binding proteins into their medium. The order was Long [Arg3]-IGF-1 roughly equal to des(1-3)IGF-1, then Long [Gly3]-IGF-1, then Long IGF-1, then native IGF-1 last. The two changes together, extension plus arginine, outperformed either alone.

The control experiment is the telling one. In chicken embryo fibroblasts, a cell line that secretes no detectable binding proteins, Long [Arg3]-IGF-1 was actually less potent than IGF-1. Strip out the binding proteins and the modified molecule loses its advantage, because at the receptor itself it is a slightly weaker ligand. The potency gain is entirely about avoiding IGFBPs.

That distinction matters when choosing between native IGF-1 and long r3 igf-1 for research. In a defined medium with no binding proteins the two behave similarly or the native form wins. In serum, in tissue, or in any system where IGFBPs are present, the LR3 form is the one that reaches the receptor.

The animal data from the same group

Frank Tomas, Francis, John Ballard and colleagues took the comparison into whole animals. In a 1993 study in the Journal of Endocrinology, growing female rats received either IGF-1 or LR3IGF-1 for 14 days through implanted osmotic pumps. The LR3 peptide was supplied by GroPep Pty Ltd of Adelaide.

Native IGF-1 at the highest dose, 278 micrograms a day, produced 18 to 26% increases in body weight gain, nitrogen retention and food conversion efficiency. LR3IGF-1 at 44 micrograms a day produced similar effects, a potency difference of roughly six-fold. The authors noted that human growth hormone at 213 micrograms a day did not stimulate growth in the same model.

Those are preclinical figures from a specific rat model, reported here as the study reported them. They are the origin of the claim, repeated on many igf-1 lr3 uk listings without a citation, that the LR3 form is "more potent" than IGF-1. The potency is real but it is a binding-protein effect, not a stronger receptor signal.

From an Adelaide lab to a commercial reagent

The authors on the 1992 and 1993 papers were based at CSIRO and the Cooperative Research Centre for Tissue Growth and Repair in Adelaide, and the molecule was commercialised by GroPep, the company that supplied the peptide for the Tomas study. GroPep's business became almost entirely the sale of Long R3 IGF-1 as a cell culture supplement.

In August 2006 Chemical and Engineering News reported that the Danish enzyme company Novozymes had agreed to acquire GroPep for almost 65 million US dollars. GroPep's sales the previous year were 12.7 million dollars, "almost all from Long R3 IGF-1", which the magazine described as a growth factor supplement for making vaccines, monoclonal antibodies and other recombinant proteins in serum-free mammalian cell culture.

A companion post explains why cell culture labs use IGF-1 LR3 in serum-free media, where the binding-protein story becomes a manufacturing one. Researchers who buy igf-1 lr3 today are buying a reagent whose design has not changed since those 1992 papers.

Reading the structure on a certificate of analysis

The structural facts above are what a lab report should confirm. Mass spectrometry should show a parent mass close to 9,110 daltons, well above that of native IGF-1. HPLC purity should be reported against that single peak. The N-terminal sequence, where a supplier reports it, should begin with the porcine growth hormone fragment rather than the Gly-Pro-Glu start of native IGF-1.

LynxLab stocks igf-1 lr3 1mg for research use in 1mg vials at £49.99, tested to 99% purity with the lab report on the product page. The batch lab reports are published for each vial so the mass can be checked against the figures in this post.

One regulatory note belongs here. The World Anti-Doping Agency's 2026 Prohibited List prohibits "insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues" at all times under section S2.3, and IGF-1 LR3 is an analogue by definition. It is not a licensed medicine in the UK and is sold for laboratory research only.

Frequently asked questions

Is IGF-1 LR3 the same as IGF-1?

No. IGF-1 LR3 contains the full 70-amino-acid IGF-1 sequence with one substitution, arginine for glutamate at position 3, plus a 13-amino-acid extension at the N-terminus, making 83 residues in total. The changes cut its affinity for IGF binding proteins while leaving receptor binding slightly weaker than native IGF-1.

What does the R3 stand for in IGF-1 LR3?

R is the single-letter code for arginine and 3 is the position in the IGF-1 sequence. King and colleagues showed in 1992 that replacing glutamate 3 with arginine made the peptide bind "very poorly" to IGF binding protein 2, which is why it is more potent in cells that secrete binding proteins.

Why is IGF-1 LR3 more potent than IGF-1 in cell culture?

Because most cultured cells secrete IGF binding proteins that soak up native IGF-1. Francis and colleagues found in 1992 that Long [Arg3]-IGF-1 was the most potent construct in binding-protein-secreting lines but less potent than IGF-1 in chicken embryo fibroblasts, which secrete none. The advantage comes from escaping the binding proteins.

Who made IGF-1 LR3 first?

Researchers at CSIRO and the Cooperative Research Centre for Tissue Growth and Repair in Adelaide, including Geoffrey Francis, John Ballard, John Wallace and Frank Tomas, published the design in 1992 and 1993. The Adelaide company GroPep manufactured it, and Novozymes bought GroPep in 2006 for almost 65 million US dollars.

Sources

Research use only

IGF-1 LR3 from LynxLab

1mg vials from £49.99. ≥99% purity, lab report on the page. Same-day dispatch, UK stock.

Buy IGF-1 LR3