IGF-Binding Protein (IGFBP)
A family of six proteins that bind to and regulate the activity of IGF-1 and IGF-2 in the bloodstream. IGFBPs extend the half-life of IGFs, control their tissue distribution, and modulate their interaction with receptors. IGFBP-3 is the most abundant and clinically measured binding protein.
Technical Context
Six IGFBPs (IGFBP-1 through IGFBP-6) regulate IGF bioavailability. IGFBP-3 is the most abundant, carrying approximately 75-90% of circulating IGF-1 in a ternary complex with ALS. This complex extends IGF-1's half-life from minutes to approximately 12-16 hours and regulates its delivery to tissues. IGFBP levels change with nutritional status, GH activity, and various diseases. IGFBP-3 is sometimes measured alongside IGF-1 in diagnostic evaluation of GH disorders. Proteolysis of IGFBPs by specific proteases (including pregnancy-associated plasma protein-A, PAPP-A) releases free IGF-1 for receptor binding, adding another layer of regulation to the IGF system.