THE MECHANISM BY WHICH HEPARIN PROMOTES THE INHIBITION OF COAGULATION-FACTOR XIA BY PROTEASE NEXIN-2

Previous kinetic studies have shown that protease nexin-2 is a potent, reversible, and competitive inhibitor of factor XIa. Here we show tha t high molecular weight heparin potentiates the ability of protease ne xin-2 to inhibit factor XIa with a parabolic concentration dependence, predominantly because of an increase of the association rate constant with little perturbation of the dissociation rate constant. No effect on factor XIa inhibition by protease nexin-a was observed with hepari n preparations of 6-22 saccharide units (0.1 nM-10 mu M), whereas hepa rin preparations with 32-64 saccharide units potentiated factor XIa in hibition by protease nexin-2 in a size-and concentration-dependent man ner. We propose a model wherein heparin exerts this effect by providin g a template for the assembly of factor XIa-protease nexin-a complexes , and only heparin polymers consisting of greater than 32 saccharide u nits (M-r similar to 10,000) are sufficiently long to provide a templa te to which factor XIa and protease nexin-a molecules can bind simulta neously. Heparin-mediated enhancement of factor XIa inhibition by prot ease nexin-2 was partially abrogated by high molecular weight kininoge n, suggesting that high molecular weight kininogen may play a role in regulating factor XIa activity.