Negatively charged phospholipids stimulate factor XI activation by thrombin

Introduction: Coagulation factor XI (FXI) is the zymogen of the serine-protease FXIa, which contributes to thrombin formation by activating factor IX. FXI can be activated by factor XIIa or thrombin, but thrombin-catalysed FXI activation is highly inefficient, unless stimulated by polyanions such as dextran sulphate (DXS) or polyphosphate (PolyP). Aim: To investigate whether negatively charged phospholipids can also enhance thrombin-catalysed FXI activation and to determine the dependence of this reaction on phospholipid concentration and composition, thrombin exosites and ionic strength. Methods: FXI was incubated with thrombin in the absence and presence of DXS, PolyP or phospholipid vesicles, and FXIa generation was followed using a chromogenic substrate. The role of thrombin exosites was probed using specific aptamers. The ionic strength was varied using either sodium chloride (NaCl) or tetrapropylammonium chloride. Results: Similar to DXS and PolyP, phospholipid vesicles with a high percentage of phosphatidic acid or phosphatidylserine enhanced thrombin-catalysed FXI activation by 1–2 orders of magnitude. Moreover, thrombin and FXI bound to negatively charged phospholipids in direct binding assays. Both thrombin exosites contributed to the ability of thrombin to bind to phospholipids and activate FXI, with a predominance of exosite II. Decreasing the ionic strength greatly stimulated FXI activation by thrombin, both in the absence and presence of phospholipids. However, at equal ionic strength, the reaction was more efficient with NaCl than with tetrapropylammonium chloride, suggesting a specific stimulatory effect of Na+. Conclusions: Negatively charged phospholipids, just as DXS and PolyP, stimulate FXI activation by thrombin via a ‘template mechanism’.