HIGH-MOLECULAR-WEIGHT KININOGEN REGULATES PREKALLIKREIN ASSEMBLY AND ACTIVATION ON ENDOTHELIAL-CELLS - A NOVEL MECHANISM FOR CONTACT ACTIVATION

The consequences of assembling the contact system of proteins on the s urface of vascular cells has received little study. We asked whether a ssembly of these proteins on the surface of cultured human endothelial cells (HUVEC5) results in the activation of prekallikrein (PK) and it s dependent pathways. Biotinylated PK binds specifically and reversibl y to HUVECs in the presence of high molecular weight kininogen (HK) (a pparent K-d of 23 +/- 11 nmol/L, B-max of 1.7 +/- 0.5 x 10(7) sites pe r cell [mean +/- SD, n = 5 experiments]). Cell-associated PK is rapidl y converted to kallikrein. Surprisingly, the activation of cell-associ ated HK.PK complexes is entirely independent of exogenous factor XII ( K-m = 30 nmol/L, V-max = 12 +/- 3 pmol/L/min in the absence v K-m = 20 nmol/L, V-max = 9.2 +/- 2.1 pmol/L/min in the presence of factor XII) . Rather, kallikrein formation is mediated by an endothelial cell-asso ciated, thiol protease. Cell-associated HK is proteolyzed during the c ourse of prekallikrein activation, releasing kallikrein from the surfa ce. Furthermore, activation of PK bound to HK on HUVECs promotes kalli krein-dependent activation of pro-urokinase, resulting in the formatio n of plasmin. These results indicate the existence of a previously und escribed, factor XII-independent pathway for contact factor activation on HUVECs that regulates the production of bradykinin and may contrib ute to cell-associated plasminogen activation in vivo. (C) 1998 by The American Society of Hematology.