HUMAN HAGEMAN-FACTOR (FACTOR-XII) AND HIGH-MOLECULAR-WEIGHT KININOGENCOMPETE FOR THE SAME BINDING-SITE ON HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS

Factor XII (FXII, plasma concentration 375 nM) is a critical member of the plasma contact activation system and is the zymogen form of FXII( a), a serine protease involved in intrinsic coagulation, complement ac tivation, activation of factor VII, and generation of the vasoactive p eptide bradykinin. As such its interaction with cells involved in infl ammatory pathways can be of physiologic and pathologic significance. W e have studied the binding of FXII to cultured human umbilical vein en dothelial cells (HUVEC). HUVEC were incubated with I-125-FXII, and cel l-bound factor FXII was measured. FXII bound to HUVEC saturably in a z inc-dependent manner. The optimal zinc concentration was 50-60 muM. Bi nding of labeled FXII was drastically reduced when a 200-fold molar ex cess of unlabeled FXII was included in the incubation mixture at time zero or when added at 60 min during a 150-min time course experiment. Quantitative binding experiments indicated a dissociation constant of 144 nM with 10-12 million binding sites/endothelial cell. Unlabeled hi gh molecular weight kininogen (HK) inhibited the binding of labeled FX II with a K(i) of 98 nM, whereas unlabeled FXII inhibited the binding of labeled HK to HUVEC with a K(i) of 152 nM. SDS-polyacrylamide gel e lectrophoresis and autoradiography of cell-bound I-125-FXII showed tha t factor XII underwent limited proteolysis and the molecular weights o f the fragments were similar in size to activated FXII. The cell-bound activated factor XII was also able to activate prekallikrein. These d ata suggest that (i) FXII binds to HUVEC specifically, saturably, and reversibly in a zinc-dependent manner, (ii) HK and FXII may compete wi th each other for the same cell-surface receptor/s, and (iii) cell-bou nd FXII is capable of undergoing activation to FXII(a).