What they do
A family of binding proteins carries IGF-1 in circulation, extending its half-life and controlling how much is available to receptors at any moment. The great majority of circulating IGF-1 is bound rather than free.
Why analogues alter affinity for them
An analogue with reduced binding-protein affinity has a larger free fraction. IGF-1 LR3 is the common example: its N-terminal extension and position-three substitution reduce binding-protein affinity substantially compared with native IGF-1.
The consequence is a different availability profile, not a different receptor.
The experimental implication
Cell culture usually lacks binding proteins, so an analogue designed to evade them shows less advantage there than in a system where they are present. A result from one setting does not predict the other, and this is a frequent source of confusion when reading the literature.
Research use only. This page is explanatory and does not recommend any use, quantity or procedure. REVIVE LAB supplies reference materials for laboratory research. These are not medicines and are not for human or veterinary consumption. Anyone considering the use of any compound in humans should do so only under qualified medical supervision.