STUDIES OF ADSORPTION, ACTIVATION, AND INHIBITION OF FACTOR-XII ON IMMOBILIZED HEPARIN

The aim of the present investigation was to clarify whether immobilize d heparin does, as previously suggested, prevent triggering of the pla sma contact activation system. Purified FXII in the absence or presenc e of antithrombin and/or C1 esterase inhibitor as well as plasma was e xposed for 1 to 600 seconds to a surface modified by end-point immobil ization of heparin, With purified reagents, a process including surfac e adsorption and activation of FXII occurred within 1 second. In the p resence of antithrombin, the resulting surface-bound alpha-FXIIa was i nhibited within that time. Likewise, the adsorption of native FXII fro m plasma was a rapid process. However, the inhibition of surface-bound alpha-FXIIa was slightly slower than with purified components, Nevert heless, neither beta-FXIIa nor FXIa were found in the plasma phase. Ex posure of a surface prepared from heparin molecules, lacking antithrom bin binding properties, to plasma resulted in surface-bound alpha-FXII a within 1 second. In the liquid phase, beta-FXIIa was detected after 2.5 seconds and, 12 seconds later, FXIIa and FXIa in complex with the C1 esterase inhibitor appeared. Addition of heparin to plasma prior to surface exposure did not prevent activation of surface-bound FXII, no r did it increase the beta-FXIIa inhibition rate and prevent FXI activ ation in plasma, although beta-FXIIa and FXIa-AT complex formation occ urred. It is concluded that surface-immobilized heparin, unlike hepari n in solution, effectively inhibits the initial contact activation enz ymes by an antithrombin-mediated mechanism, thereby suppressing the tr iggering of the intrinsic plasma coagulation pathway. (C) 1998 Elsevie r Science Ltd.