We measured the binding of IGF-I and IGF-II to recombinant human N-terminal [residues 1-97; recombinant human IGF-binding protein-31-97 (rhIGFBP-31-97)] and C-terminal (residues 98-264; rhIGFBP-398-264) IGFBP-3 fragments and compared it with IGF binding to intact IGFBP-3 using biosensor analysis. Experiments were carried out in different configurations, either with binding protein or fragment immobilized or with IGF immobilized. These experiments showed that IGF-I and IGF-II bind to IGFBP-3 with affinities of 4-5 × 109 M-1 and similar binding kinetics. The affinities of both rhIGFBP-31-97 and rhIGFBP-398-264 for IGF proteins were approximately 3 orders of magnitude less than that of full-length IGFBP-3. These results further support the concept that high affinity binding of IGF to IGF-binding proteins results from a two-site interaction of IGF with both the N- and C-terminal regions of the binding protein. Binding of insulin to IGFBP-3 and its N- and C-terminal fragments and of IGF-I and IGF-II to the structurally related proteins mac25 and connective tissue growth factor was also investigated. Weak insulin binding to full-length IGFBP-3 could be demonstrated in a few experiments, but we found that binding of IGF-I, IGF-II, and insulin to mac25 or connective tissue growth factor was below the detection limit of the biosensor instrument.
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