Insulin-like growth factor (IGF)-I and IGF-II binding to an IGF binding protein. An investigation using chemical modification of tyrosine residues as a structural probe for the sites of interaction.

We have investigated insulin-like growth factor (IGF)-I and IGF-II binding to bovine insulin-like growth factor binding protein-2 (bIGFBP-2) using chemical modification to locate sites on the IGF involved in the binding interaction. bIGFBP-2 was incubated with either recombinant human (hIGF-I) or purified ovine (oIGF-II) to form a mixture of bound and free IGF. Sites of interaction between the binding protein and IGF were then probed by iodination of the available tyrosine residues. Subsequently, the mixture of free IGF and IGF.bIGFBP-2 complex was resolved by neutral chromatography, and the IGF component of the complex with bIGFBP-2 was recovered by reverse-phase high performance liquid chromatography at pH 2.1. The tyrosine labeling patterns of the two populations of IGF, one iodinated while free and the other iodinated while associated with binding protein, were determined following endoproteinase Glu-C peptide mapping. Binding of hIGF-I or oIGF-II to bIGFBP-2 resulted in reduced iodination of the tyrosines in both hIGF-I and oIGF-II that are near the carboxyl-terminal, Tyr-60 and Tyr-59, respectively. The reduction in labeling of these tyrosine residues was 2-fold and 6-fold for hIGF-I and oIGF-II, respectively. On the other hand, labeling of the other 2 tyrosines in hIGF-I and oIGF-II was not different between the free and complexed growth factors. From these results we conclude that Tyr-60 and Tyr-59 in the carboxyl-terminal regions of hIGF-I and oIGF-II, respectively, are either directly involved in the binding reaction or lie in a region of the IGF molecule encompassed by the association with bIGFBP-2. Conversely, the labeling pattern of the other tyrosines, Tyr-24 and Tyr-31 in hIGF-I and Tyr-2 and Tyr-27 in oIGF-II, implies that they are not involved in binding to bIGFBP-2. To examine the role of IGF tyrosine residues in the association with bIGFBP-2, we prepared nonradioactive 127I-labeled oIGF-II. In bIGFBP-2 competition binding assays, 127I-labeled oIGF-II was 2.5-fold and 5-fold less potent than native oIGF-II when competing for binding of 125I-labeled IGF-I or IGF-II tracers, respectively. We interpret these results as indicating that at least 1 tyrosine in oIGF-II is involved in binding to bIGFBP-2.