Tissue factor is the membrane-associated protein which mediates activation of factors IX and X by factor VII. In a purified, reconstituted bovine system, factor X activation by the tissue factor-factor VIIa complex is inhibited by the mixed apoproteins from human high density lipoprotein (HDL) and by isolated apolipoprotein A-II (apo A-II). Other proteins found associated with plasma lipoproteins, apolipoprotein A-I (apo A-I), C-reactive protein (CRP), and β2-glycoprotein I (β2GPI), have been examined for effects on the activation of factor X by tissue factor-factor VIIa. In these experiments, bovine tissue factor, reconstituted into phosphatidylserine-phosphatidylcholine (PS/PC; 30 70) vesicles, was used at a single concentration while factor X (the substrate), factor VIIa (the enzyme), and the potentially inhibitory proteins were varied in a continuous chromogenic assay. Apo A-II and CRP clearly inhibit tissue factor-factor VIIa activation of factor X, while apo A-I and β2GPI have little or no effect. These results demonstrate that different lipid binding proteins vary in their effects on tissue factor activity.
- Tissue factor
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