Biosensor measurement of the interaction kinetics between insulin-like growth factors and their binding proteins

The binding kinetics of human insulin-like growth factor binding protein (IGFBP) 1-6 for recombinant human insulin-like growth factor (IGF) I and II were measured and compared in the present study using surface plasmon resonance biosensor technique. Different concentrations of IGFBPs (5-100 nM) were allowed to interact with the immobilized IGF-I or IGF-II on sensor chip surface. Both des(1-3)IGF-I and insulin are known to bind weakly to the IGFBPs and therefore are used as negative controls for the binding experiments. The resultant sensorgrams were analyzed by using simple 1:1 binding model to derive both the association rate (k(a)) and dissociation rate (k(d)) constants for IGFBP-IGF interactions. The k(a) values of IGFBPs are in the range of 1x10(4) to 9x10(5) M(-1) s(-1) for IGF-I and 7x10(3) to 1.7x10(6) M(-1) s(-1) for IGF-II, respectively. The orders of k(a) for both IGF-I and IGF-II are IGFBP-3>IGFBP-5>IGFBP-6>IGFBP-4>IGFBP-2>++ +IGFBP-1. The k(d) values of IGFBPs are in the range of 1.5x10(-5) to 2x10(-4) s(-1) for IGF-I and 3.6x10(-5) to 3.7x10(-4) s(-1) for IGF-II, respectively. The order of k(d) for IGF-I is IGFBP-6>IGFBP-5>IGFBP-4>IGFBP-3>IGFBP-2>++ +IGFBP-1 and that for IGF-II is IGFBP-5>IGFBP-6>IGFBP-2>IGFBP-4>IGFBP-3>++ +IGFBP-1, respectively. The equilibrium affinity constants (K(A)) were calculated based on the ratio of k(a)/k(d) and were more precise than the published literature values based on competitive radioligand binding assays. The systematic study enables a direct comparison on the IGF-binding properties among the various IGFBPs, and the kinetic data provide additional information to delineate the physiological role of different IGFBPs in vivo.