EFFECT OF SURFACE-IMMOBILIZED HEPARIN ON THE ACTIVATION OF ADSORBED FACTOR-XII

Two different heparin surfaces, structurally closely related and of si milar negative charge characteristics, were compared with regard to ad sorption and activation of coagulation Factor XII (FXII). One surface was prepared by immobilization of unfractionated heparin, which yielde d a surface containing both heparin molecules with high and with low a ffinity for antithrombin (unfractionated [UF] heparin surface). The ot her surface consisted of a fraction of heparin molecules with low affi nity for antithrombin (LA heparin surface) and essentially devoid of a ntithrombin-binding as well as anticoagulant activity. Both surfaces a dsorbed FXII from plasma to a similar extent, and essentially the same quantities of bound factor could be recovered from the surfaces. The two heparin surfaces, however, differed markedly with regard to activa tion of the adsorbed FXII. On the LA heparin surface, a major portion of the surface-bound FXII was recovered in its enzymatically active fo rm (FXIIa), but only trace amounts of the FXII taken up by the UF hepa rin surface had undergone activation. When FXII-deficient plasma was u sed instead of normal plasma, no surface-associated enzyme activity co uld be recovered on either surface. The presence of free standard hepa rin or low molecular weight heparin in the plasma exposed to the LA he parin surface did not prevent conversion of FXII to FXIIa.