Formation of 150-kDa binary complexes on insulin-like growth factor binding protein-3 and the acid-labile subunit in vitro and in vivo

Adult rat serum contains two types of 150-kDa IGFBP complexes: ternary complexes containing bound IGF-I, intact IGFBP-3 and the acid-labile subunit (ALS), and binary complexes that contain ALS and proteolytically-nicked IGFBP-3 but which lack bound IGF. We present evidence that the binary complexes containing proteolytically-nicked IGFBP-3 can be formed in two ways: by direct association of IGFBP-3 with ALS in the absence of IGF, and by proteolysis of IGFBP-3 within 150-kDa ternary complexes, resulting in increased dissociation of IGF-I. The relative contributions of the two mechanisms is unknown. Preliminary results indicate that binary complexes also can form in vivo. Proteolysis of IGFBP-3 in the 150-kDa ternary complex provides a regulatable mechanism by which IGF-I may be mobilized from the circulating reservoir of 150-kDa complexes to the tissues.