IGF-1 LR3 Research Peptide

IGF-1 LR3 is an 83-residue analogue of human insulin-like growth factor 1: the complete 70-residue IGF-1 chain with a 13-residue N-terminal leader and arginine in place of glutamic acid at position 3. Crown Peptides supplies it in 1mg vials of white freeze-dried powder. Australian researchers first described it as Long [Arg3]-IGF-I, which is why it is also listed as Long R3 IGF-1, LR3-IGF-1, Long Arg3 IGF-1 or IGF1 LR3.

L is for Long, referring to the 13-residue extension at the N-terminus, and R3 uses the single-letter code for arginine (R) at position 3 of the IGF-1 sequence, where the native hormone carries glutamic acid. The name mirrors its sister compounds from the same project, Long IGF-I and Long [Gly3]-IGF-I.

Surprisingly, from growth hormone. The leader reads MFPAMPLSSLFVN: methionine and the following ten residues of porcine growth hormone’s N-terminus, then valine and asparagine. It was introduced as part of an efficient bacterial expression system, and its hydrophobic character also helped the protein fold correctly. Even with the authentic IGF-I sequence behind it, the leader alone made Long IGF-I more potent than IGF-I in cells that secrete binding proteins.

They behave differently wherever binding proteins are present. Native IGF-1’s activity in a culture well or a tissue depends on how much IGFBP surrounds it, so results can drift with cell density, medium changes or species. LR3 largely sidesteps that variable and gives a cleaner read-out of receptor signalling. Where no binding proteins are present, the advantage disappears and native IGF-1 can be the more potent of the two.

Slightly less tightly, as it happens. The original Adelaide group put its affinity for the type 1 IGF receptor at about a third of native IGF-1’s. Its higher potency in most systems comes from being far less captured by binding proteins, which leaves more of it free to reach the receptor.

They are different molecules. Mecasermin is recombinant human IGF-1 with the native 70-residue sequence, and it has been approved in the US and the EU under the brand name Increlex for severe primary IGF-1 deficiency in children. IGF-1 LR3, with its leader and arginine swap, has never been approved as a medicine anywhere. In sport, IGF-1 and its analogues are banned under the World Anti-Doping Agency’s list of prohibited substances.

Because nothing holds on to it. In catheterised rats given radiolabelled tracers, native IGF-1 travelled mainly in the 150 kDa complex and cleared slowly, at about 0.9 ml/min per kg in non-pregnant animals, while LR3 circulated mostly as free peptide and cleared roughly ten times faster, at about 9.8 ml/min per kg. Free peptide moves into tissues and is degraded more readily, so the idea of a “longer-lasting” IGF-1 applies to culture media rather than to the bloodstream.

Mainly as a tool for asking how growth factors reach a wound. Across a model endothelial barrier, binding proteins slowed the passage of IGF-1 but left LR3 unaffected. In rats fitted with subcutaneous wound chambers, more intact LR3 than IGF-1 was recovered from wound fluid after an intravenous tracer, although only a tiny fraction of either arrived, and the authors concluded that circulating IGF-1 plays only a minor role in surface wound repair.

At the far end of the same axis. GHRH analogues such as Tesamorelin and CJC-1295 with DAC act on the pituitary to raise growth hormone, which then prompts the liver and other tissues to make native IGF-1, and that IGF-1 is promptly taken up by binding proteins. LR3 skips the pituitary and liver entirely and acts at the IGF-1 receptor itself, so comparing the two levels helps separate the effects of GH from those of IGF-1 signalling.

They are the central quality question for any IGF-1 protein. Its six cysteines can pair correctly (Cys6–Cys48, Cys18–Cys61 and Cys47–Cys52 in IGF-1 numbering) or in a mismatched arrangement, and the mispaired form has exactly the same mass. Mass spectrometry confirms the 9.11 kDa molecule, and high-resolution instruments can resolve the 6 Da by which the folded protein is lighter than its fully reduced chain, but reversed-phase HPLC is what separates correctly folded material from isomers.

Here the LR3 sequence is much better behaved than IGF-1: in a refolding study it gave 85% native structure and 10% of the alternative form, against 45% and 24% for IGF-1. The nine sulphur atoms in its formula are the six cysteines plus three methionines, two of which sit in the leader.

Dissolving IGF-1 proteins in dilute acid is common practice; the culture-grade liquid form of LR3 is made up at 1 mg/mL in 100 mM acetic acid, and its maker recommends low-protein-binding tubes for reconstituted stock so that material is not lost to plastic surfaces. Our IGF-1 LR3 is shipped as a white lyophilised powder, sealed in glass; keep unopened vials frozen at -20°C and shielded from warmth and light, as our storage guide explains.

Growth-factor work runs at very small concentrations: culture-grade LR3 is typically used at 10 to 100 micrograms per litre of medium, so a single milligram goes a long way and one vial is usually ample for method development. Boxes of five or ten suit labs feeding several cell lines over months or running a bioprocess series, where staying on one lot number avoids re-qualifying every new batch, and they lower the cost per vial.