PLASMIN DEGRADATION OF INSULIN-LIKE-GROWTH-FACTOR BINDING PROTEIN-5 (IGFBP-5) - REGULATION BY IGFBP-5-(201-218)

Using the major bone insulin-like growth factor-binding protein (IGFBP ) IGFBP-5, we took a mechanistic approach in evaluating the role of th e heparin-binding domain of IGFBP-5 in regulating plasmin (Pm) proteol ysis of IGFBP-5. Using synthetic IGFBP-5 peptide fragments, we determi ned that the heparin-binding domain, IGFBP-5-(208-218), inhibits Pm pr oteolysis of intact IGFBP-5. The mechanism of action of IGFBP-5-(201-2 18) was by inhibiting Pm binding to substrate IGFBP-5. IGFBP-5-(201-21 8) action was independent of site of proteolysis, fluid, or solid phas e interaction. In addition, IGFBP-5-(201-218) was found to inhibit pla sminogen (Pg) activation to Pm. IGFBP-5-(201-218) did not directly inh ibit the activity of Pm, urokinase Pg activator (PA), or tissue-type P A but acted as a competitive inhibitor of Pg activation by PA, which i s in contrast to the stimulating effect of heparin on Pg activation. T hese data indicate that the heparin-binding domain contains the serine protease (Pg-to-Pm) binding site region of IGFBP-5, and that this reg ion, which is presumed to represent a Pm-induced proteolytic product o f IGFBP-5, is capable of regulating Pm action.