Glossary
IGF Binding Protein
IGFBPs hold more than 95% of circulating IGF-1, governing its half-life and tissue access. The six family members and why analogs are built to evade them.
The IGF binding proteins (IGFBPs) form a family of six carrier proteins with high affinity — IGFBP-1 to IGFBP-6 — that capture IGF-1 and IGF-2 both in the bloodstream and in tissues. They bind IGF-1 at least as tightly as the IGF-1 receptor does, so they function as a competing reservoir, not as a mere transport vehicle.
Their functions
Over 95% of the IGF-1 in circulation is bound, mostly within a three-part complex of IGF-1, IGFBP-3 and the acid-labile subunit. This complex is too big to pass through capillary endothelium, so it prolongs the circulating half-life from minutes to several hours while restricting the amount of peptide that gets to receptors. Proteases acting on IGFBP-3 set free IGF-1 locally — a controlled process rather than leakage. Some IGFBPs have also been reported to act independently of IGF, for example on cell migration and apoptosis.
Relevance for analog design and assays
These proteins explain how research analogs are designed. IGF-1 LR3 and IGF-1 DES(1-3) were created precisely to lower IGFBP affinity so that more free peptide is available in an assay system. It also follows that assays for total IGF-1 and for free IGF-1 address different questions.
Related terms
growth hormone secretagogue · ELISA. See the IGF-II research family and IGF and muscle peptides.