Tissue factor pathway inhibitor in complex with low density lipoprotein isolated from human plasma does not possess anticoagulant function in tissue factor-induced coagulation in vitro

Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of the extrinsic coagulation system. In human plasma 70-85% is associated with apoB-containing lipoproteins whereas 10-20% exists in a carrier free form. The purpose of the present study was to assess the anticoagulant function of TFPI in complex with low density lipoproteins (LDL) on tissue factor (TF)-induced coagulation in vitro. LDL-TFPI complexes were isolated by preparative density gradient ultracentrifugation, LDL-free TFPI by preparative gel filtration and the anticoagulant properties were assessed by a diluted prothrombin time assay (dPT). LDL-free TFPI (0-0.46 U/ml) added to the dPT mixture, caused a prominent dose-dependent prolongation of dPT (0-42.2 sec.) which could be abolished by the addition of blocking anti-TFPI IgG. Contrary, increasing amounts of LDL-bound TFPI (0-4.0 U/ml) shortened dPT by 11.4 sec at the highest concentration. LDL-bound TFPI was not immunodetected by anti-TFPI IgG directed against the distal portion of the C-terminus, and appeared on Western blotting with a major band at 67 kDa and a weak band at 34 kDa which suggest that LDL-bound TFPI lack anticoagulant function due to carboxy terminal truncation. Our data provide evidence for the hypothesis that the anticoagulant function of TFPI is restricted to its carrier free form in human plasma.