Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor-related protein

The interaction between the endocytic receptor low density lipoprotein rece ptor-related protein (LRP) and either coagulation factor IX or its active d erivative factor IXa was studied. Purified factor IX was unable to associat e with LRP when analyzed by surface plasmon resonance. By contrast, factor XIa-mediated conversion of factor IX into factor IXa resulted in reversible dose and calcium-dependent binding to LRP. Active-site blocking of factor IXa did not affect binding to LRP, whereas LRP binding was efficiently inhi bited in the presence of heparin or antibodies against factor IX or LRP. Th e factor IXa-LRP interaction could he described by a a-site binding model w ith equilibrium dissociation constants of 27 nmol/L and 69 nmol/L. Consiste nt with this model, it was observed that factor IXa binds to 2 different re combinant receptor fragments of LRP (denoted cluster II and cluster IV) wit h equilibrium dissociation constants of 227 nmol/L and 53 nmol/L, respectiv ely. The amount of factor IXa degraded by LRP-deficient cells was 35% lower than by LRP-expressing cells, demonstrating that LRP contributes to the tr ansport of factor IXa to the intracellular degradation pathway. Be cause li gand binding to LRP is often pre ceded by binding to proteoglycans, the con tribution of proteoglycans to the catabolism of factor IXa was addressed by employing proteoglycan-deficient cells. Degradation of factor IXa by prote oglycan-deficient cells proceeded at a 83% lower rate than wildtype cells. In conclusion, the data presented here indicate that both LRP and proteogly cans have the potential to contribute to the catabolism of factor IXa. (C) 2000 by The American Society of Hematology.