Circulating binding proteins for the insulinlike growth factors.

Six distinct insulinlike growth factor-binding proteins (IGFBP-1 to IGFBP-6) of core molecular mass 20-30 II have been identified, of which one, IGFBP-3, carries most o f the circulating IGF-I and IGF-II in ternary complexes that also contain an acid-labile glycoprotein subunit. Although the role of circulating IGFBP-3 in IGF stabilization and transport is becoming increasingly well understood, the functions of the other IGFBPs in the circulation are less clear, and some redundancy of function is possible. IGFBP-l, which is metabolically regulated by insulin and carbohydrates, may act as a counterregulator in blood glucose regulation and could also be important in targeting IGF delivery from the circulation to the tissues, while recent studies of IGFBP-2 physiology suggest that this protein may function as an additional IGF carrier when IGFBP-3 levels are inadequate. Research into the roles of circulating IGFBP-4, -5, and -6 is, as yet, poorly developed and will depend on the ready availability of analytical methods for these proteins.