Altered ligand specificity of proteolysed insulin-like growth factor binding protein-3.

IGF binding protein-3 (IGFBP-3) undergoes limited proteolysis in human pregnancy serum, altering its electrophoretic mobility and its binding of radioiodinated IGF tracers. IGF-I tracer, monoiodinated on Tyr31, discriminated less than other tracers between intact and proteolysed IGFBP-3. In competitive binding curves using this tracer, intact IGFBP-3 showed comparable reactivity towards IGF-I and analogs with substitutions at Tyr24, Tyr31 or Tyr60. In contrast, proteolysed IGFBP-3 reacted equally with IGF-I and [Ala31]IGF-I (approximately 10-fold lower potency than with intact IGFBP-3), but showed a marked selectivity against [Ser24]IGF-I and [Leu60]IGF-I (approximately 100-fold lower potency than with intact IGFBP-3). The affinity of IGF-I binding to proteolysed, but not intact, IGFBP-3 was increased by the addition of the acid-labile subunit of the IGFBP-3 complex. This study defines more fully the lesion in IGFBP-3 caused by serum proteolysis during pregnancy and demonstrates that tyrosine-substituted IGF-I derivatives are valuable tools in studying IGFBP-3 proteolysis.