IGF binding proteins (IGFBPs) are a family of six high-affinity carrier proteins, IGFBP-1 through IGFBP-6, that bind IGF-1 and IGF-2 in circulation and in tissue. Their affinity for IGF-1 equals or exceeds that of the IGF-1 receptor itself, so they act as a competitive reservoir rather than a passive transport system.
What they do
More than 95% of circulating IGF-1 travels bound, most of it in a ternary complex of IGF-1, IGFBP-3 and the acid-labile subunit. That complex is too large to cross the capillary endothelium, so it extends the circulating half-life from a few minutes to several hours while limiting how much peptide reaches receptors. Proteases that cleave IGFBP-3 release free IGF-1 locally — a regulated step, not a leak. Several IGFBPs also have reported IGF-independent activities, including effects on cell migration and apoptosis.
Why it matters
IGFBPs explain the design of research analogs. IGF-1 LR3 and IGF-1 DES(1-3) exist specifically to reduce IGFBP affinity, leaving more unbound peptide in an assay system. It also means total-IGF-1 assays and free-IGF-1 assays answer different questions.
Related terms
growth hormone secretagogue · ELISA. See the IGF-II research family and IGF and muscle peptides.