IGF-1 LR3: The Long-R3 Analog in the Research Literature

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

This article explains what IGF-1 LR3 is at the sequence level, why an Adelaide research group built it in the early 1990s, what the binding-protein and NMR studies showed, how it became a standard cell-culture media supplement, and what a certificate of analysis should show for a recombinant protein of this size rather than a short synthetic peptide.

IGF-1 LR3 is an 83-residue recombinant analog of human IGF-1 with two deliberate changes

IGF-1 LR3, written in the primary literature as Long-[Arg³]-IGF-I or LR3IGF-I, is human insulin-like growth factor 1 with a glutamate-to-arginine substitution at position 3 and a 13-residue extension on the N-terminus. Native human IGF-1 is a 70-residue single-chain protein of about 7,649 daltons with three disulfide bonds. Adding the extension gives 83 residues and a mass near 9,111 daltons, which places IGF-1 LR3 at the boundary between "peptide" and "small protein" and changes how it is made, handled and tested.

The extension is Met-Phe-Pro-Ala-Met-Pro-Leu-Ser-Ser-Leu-Phe-Val-Asn. As Milner and colleagues described in the Biochemical Journal in 1995, it derives from the first eleven residues of methionyl porcine growth hormone followed by Val-Asn, a remnant of the expression construct. The name "Long" refers to this extension; "R3" refers to Arg at position 3. The molecule is produced by recombinant expression in Escherichia coli followed by oxidative refolding, not by solid-phase synthesis. PubChem does not assign a small-molecule CID to a protein of this size; catalog listings commonly cite CAS 946870-92-4.

It was designed in 1992 to escape the IGF-binding proteins

The point of both modifications is to reduce binding to the six IGF-binding proteins (IGFBPs) while preserving binding to the type 1 IGF receptor. Francis and colleagues at the Cooperative Research Centre for Tissue Growth and Repair in Adelaide described the design in the Journal of Molecular Endocrinology in 1992. They built an E. coli expression system for three fusion analogs, Long IGF-1, Long-[Gly³]-IGF-1 and Long-[Arg³]-IGF-1, and found that the hydrophobic N-terminal extension improved the yield of correctly folded protein compared with earlier attempts to fold normal-length IGFs.

The biological comparison in that paper is the clearest demonstration of the design logic. In rat L6 myoblasts, which secrete IGFBPs into the medium, all the extended analogs stimulated protein and DNA synthesis more potently than authentic IGF-1, with Long-[Arg³]-IGF-1 and des(1-3)IGF-1 the most potent. In chicken embryo fibroblasts, which do not secrete detectable IGFBPs, Long-[Arg³]-IGF-1 was actually less potent than IGF-1. In other words, the analog's apparent extra potency in culture is a binding-protein effect, not a receptor effect: it is simply not sequestered by the IGFBPs the cells release.

The 1995 Biochemical Journal folding study added a manufacturing detail. Oxidative folding of IGF-1 yields two major disulfide isomers, and the Glu³ substitutions increased the yield of the native isomer, which was highest for Long-[Arg³]-IGF-1. The misfolded isomers bound poorly to IGFBPs and bound the type 1 receptor with only 0.5 to 25 percent of the affinity of the native fold, which is why disulfide isomer content is a legitimate quality question for this product.

What the binding-protein and clearance studies measured in animals

Animal studies from the same Adelaide group measured how an IGF-1 variant that avoids binding proteins behaves in plasma, and the answer was: it circulates mostly free and is cleared much faster. Bastian and colleagues reported in the Journal of Endocrinology in 1993 that radiolabelled LR3IGF-I was cleared from the plasma of virgin and pregnant rats at about 9 to 10 mL/min per kg, compared with roughly 1 mL/min per kg for IGF-1 in virgin rats, and that most of the LR3IGF-I tracer was detected as free peptide rather than in the 150 kDa IGFBP-3 complex.

Lord and colleagues, also in the Journal of Endocrinology in 1994, showed that the "poor binding" of LR3IGF-I is species-dependent. By size-exclusion chromatography of labelled IGFs in plasma, the variant that bound almost nothing in rat plasma bound measurably in sheep, human, pig and chicken plasma, in the order sheep > human > pig = chicken > rat. Ligand blots were consistent across species: IGF-1 bound much more than des(1-3)IGF-1, which bound more than LR3IGF-I. A laboratory working with serum from a non-rat species should not assume the rat numbers transfer.

NMR showed an IGF-1 domain that is nearly unchanged and an extension that is largely disordered

Two NMR studies from the Australian National University resolved what the modifications do to the fold. In FEBS Letters in 1997, Laajoki and colleagues assigned the backbone of ¹⁵N-labelled Long-[Arg³]-IGF-1 and found that the IGF-1 domain had an almost identical secondary structure to IGF-1. The first seven residues of the extension showed very few medium- or long-range NOEs, while the next five formed a turn-like structure close to the start of helix 1.

The full solution structure followed in the Journal of Biological Chemistry in 2000. The three alpha-helices were defined to a backbone precision of 0.82 angstroms, the IGF-1 domain matched earlier IGF-1 structures apart from minor changes remote from the N-terminus, and the main difference was a substantial reorientation of the first three residues of the IGF-1 domain, which the authors linked to the lower IGFBP affinity. Relaxation data showed considerable flexibility even within the helices, negative heteronuclear NOEs for the extension and the C-terminus, and slow amide exchange for some extension residues, suggesting a transient "molten helix."

For a certificate reader, the practical implication is that IGF-1 LR3 is a folded protein whose activity depends on three correct disulfides and an intact helical core. Mass alone does not confirm the fold. See LC-MS Identity Confirmation: What the Mass Spectrum Proves About a Peptide for what the identity line can and cannot establish.

Why IGF-1 LR3 became a standard cell-culture media supplement

IGF-1 LR3 is used in cell culture precisely because it is not neutralized by the binding proteins that cultured cells secrete, which makes its effective concentration predictable in serum-free medium. The analog has been sold for decades as a defined media component under the LONG R3 IGF-I trademark, originally from the Adelaide spin-out that developed it, and it is a common replacement for insulin in serum-free and chemically defined formulations used to grow recombinant-protein-producing cell lines.

Academic groups use the same property as an experimental control. Xi and colleagues reported in the Journal of Cellular Physiology in 2004 that recombinant porcine IGFBP-3 suppressed both IGF-1- and Long-R3-IGF-1-stimulated proliferation of L6 myogenic cells, which showed that the binding protein was acting through an IGF-independent route, since IGFBP-3 cannot sequester the analog. Pampusch and colleagues used the same logic with recombinant porcine IGFBP-5 in porcine embryonic myogenic cells in the Journal of Endocrinology in 2005: suppression of proliferation persisted even with a large molar excess of Long-R3-IGF-1, demonstrating IGF-independent activity of the binding protein.

Regulatory status and a cautionary identity case from anti-doping chemistry

IGF-1 LR3 is not an approved drug in any jurisdiction. The approved product in this family is mecasermin, which is recombinant native human IGF-1 (70 residues, Glu³, no extension); IGF-1 LR3 is a different molecule and a research vial is not mecasermin. IGF-1 and its analogs are listed in class S2 of the WADA Prohibited List, which is why anti-doping laboratories have published detection methods for the analog. See The WADA Prohibited List and Peptides: Which Classes Are Named.

One of those publications is also the best available lesson on identity testing for this product. Kohler and colleagues at the German Sport University reported in Growth Hormone & IGF Research in 2010 that a black-market vial, analyzed by immunoaffinity purification, nano-UPLC and high-resolution mass spectrometry of the intact and trypsin-digested protein, contained Long-R³-IGF-I carrying a C-terminal His₆ tag attached through a Leu-Glu linker. Polyhistidine tags are added during recombinant production to simplify nickel-column purification and are normally cleaved off; the authors concluded the material was probably a by-product of biochemical production rather than a finished product.

The point for a certificate reader is that an intact-mass measurement would have caught the tag immediately: a His₆-Leu-Glu extension adds roughly 1,060 daltons. A product page that reports only HPLC purity, with no observed intact mass, cannot rule out that kind of variant.

What a certificate should show for a recombinant protein rather than a synthetic peptide

For a 9 kDa recombinant protein the useful certificate lines are the observed intact mass, a purity method that can resolve folding variants and aggregates, and contaminant tests that reflect bacterial origin. Intact mass by electrospray with charge-state deconvolution should match the theoretical mass of the 83-residue sequence with three disulfides (each disulfide removes two hydrogens) within instrument tolerance, and should show whether the N-terminal methionine of the extension is retained. Reverse-phase HPLC reports purity, but size-exclusion chromatography or non-reducing SDS-PAGE is needed to see dimers and aggregates, which a reverse-phase gradient can hide.

Because the molecule is expressed in E. coli, endotoxin is a more prominent question than for a synthetic peptide, and the method and result should be stated; see Endotoxin Testing Explained: LAL, Recombinant Factor C, and Reading EU per mg. Net protein content by UV absorbance at 280 nm depends on an extinction coefficient calculated from the Tyr, Trp and cystine content, and the number used should be stated. Most research certificates do not include a cell-based activity assay, so purity and content should not be read as potency; see Net Peptide Content vs Purity.

US Prime Peptides posts a lot-matched certificate for IGF-1 LR3 1 mg before purchase, covering identity by mass spectrometry, HPLC purity, content, endotoxin, sterility and heavy metals, with an independent third-party report. The 1 mg label mass refers to the lyophilized fill and should be read against the content line, not assumed to equal active protein.

Frequently Asked Questions

What does LR3 mean in IGF-1 LR3?

"L" stands for Long, the 13-residue N-terminal extension, and "R3" means arginine at position 3 in place of the native glutamate. Both changes were introduced by Francis and colleagues in 1992 to reduce binding to IGF-binding proteins while keeping binding to the type 1 IGF receptor.

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

No. Native human IGF-1 has 70 residues; IGF-1 LR3 has 83 and a different residue at position 3. NMR studies show the IGF-1 domain keeps nearly the same fold, but the analog binds IGF-binding proteins far more weakly and is cleared from rat plasma about ten times faster. The approved drug mecasermin is native IGF-1, not LR3.

Why is IGF-1 LR3 used in cell culture media?

Cultured cells secrete IGF-binding proteins that sequester ordinary IGF-1, so its free concentration in medium drifts. IGF-1 LR3 escapes that sequestration, which makes it a predictable, defined growth-factor component in serum-free formulations and a useful control for separating IGF-dependent from IGF-independent binding-protein effects.

How should a lab check the identity of an IGF-1 LR3 vial?

Look for an observed intact mass near 9,111 daltons on the certificate, ideally with a tryptic peptide map. A 2010 anti-doping paper found a black-market product that carried a residual His₆ purification tag; an intact-mass measurement reveals that kind of variant, whereas HPLC purity alone does not.

Citations

1. Francis GL, Ross M, Ballard FJ, et al. Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency. J Mol Endocrinol. 1992;8:213-223. PMID 1378742
2. Bastian SE, Walton PE, Wallace JC, Ballard FJ. Plasma clearance and tissue distribution of labelled insulin-like growth factor-I (IGF-I) and an analogue LR3IGF-I in pregnant rats. J Endocrinol. 1993;138:327-336. PMID 7693845
3. Lord AP, Bastian SE, Read LC, Walton PE, Ballard FJ. Differences in the association of insulin-like growth factor-I (IGF-I) and IGF-I variants with rat, sheep, pig, human and chicken plasma-binding proteins. J Endocrinol. 1994;140:475-482. PMID 7514204
4. Milner SJ, Francis GL, Wallace JC, Magee BA, Ballard FJ. Mutations in the B-domain of insulin-like growth factor-I influence the oxidative folding to yield products with modified biological properties. Biochem J. 1995;308:865-871. PMID 8948444
5. Laajoki LG, Le Breton E, Shooter GK, et al. Secondary structure determination of 15N-labelled human Long-[Arg-3]-insulin-like growth factor 1 by multidimensional NMR spectroscopy. FEBS Lett. 1997;420:97-102. PMID 9450557
6. Laajoki LG, Francis GL, Wallace JC, Carver JA, Keniry MA. Solution structure and backbone dynamics of long-[Arg(3)]insulin-like growth factor-I. J Biol Chem. 2000;275:10009-10015. PMID 10744677
7. Xi G, Kamanga-Sollo E, Pampusch MS, White ME, Hathaway MR, Dayton WR. Effect of recombinant porcine IGFBP-3 on IGF-I and long-R3-IGF-I-stimulated proliferation and differentiation of L6 myogenic cells. J Cell Physiol. 2004;200:387-394. PMID 15254966
8. Pampusch MS, Xi G, Kamanga-Sollo E, et al. Production of recombinant porcine IGF-binding protein-5 and its effect on proliferation of porcine embryonic myoblast cultures in the presence and absence of IGF-I and Long-R3-IGF-I. J Endocrinol. 2005;185:197-206. PMID 15817840
9. Kohler M, Thomas A, Walpurgis K, Terlouw K, Schänzer W, Thevis M. Detection of His-tagged Long-R3-IGF-I in a black market product. Growth Horm IGF Res. 2010;20:386-390. PMID 20675162

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