Plasma lipoproteins support prothrombinase and other procoagulant enzymatic complexes

The prothrombinase complex (factor [F]Xa, FVa, calcium ions, and lipid membrane) converts production to thrombin (FIIa). To determine whether plasma lipoproteins could provide a physiologically relevant surface, we determined the rates of FIIa production by using purified human coagulation factors, and isolated fasting plasma lipoproteins from healthy donors. In the presence of 5 mmol/L FVa, 5 mmol/L FXa, and 1.4 mu mol/L prothrombin physiological levels of very low density lipoprotein (VLDL) (0.45 to 0.9 mmol/L triglyceride, or 100 to 200 mu mol/L phospholipid) yielded rates of 2 to 8 mmol FIIa.L(-1).s(-1) in a donor-dependent manner. Low density lipoprotein (LDL) and high density lipoprotein (HDL) also supported prothrombinase but at much lower rates (less than or equal to 1.0 nmol FIIa.L(-1).s(-1)). For comparison, VLDL at 2 nmol/L triglyceride yielded approximate to 50% the activity of 2 x 10(8) thrombin-activated platelets per mililiter. Although the FIIa production rate was slower on VLDL than on synthetic phosphatidylcholine/phosphatidylserine vesicles (approximate to 50 nmol FIIa.L(-1).s(-1)), the prothrombin K(m) values were similar, 0.8 nad 0.5 mu mol/L, respectively. Extracted VLDL lipids supported rates approaching those of phosphatidylcholine/phosphatidylserine vesicles, indicating the importance of the intact VLDL conformation. However, the presence of VLDL-associated, factor-specific inhibitors was ruled out by titration experiments, suggesting a key role for lipid organization. VLDL also supported FIIa generation in an assay system comprising 0.1 nmol/L FVIIa; 0.55 nmol/L tissue factor, physiological levels of FV, FVIII, FIX, and FX; and prothrombin (3 nmol/L FIIa.L(-1).s(-1)). These results indicate that isolated human VLDL can support all the components of the extrinsic coagulation pathway, yielding physiologically relevant rates of thrombin generation in a donor-dependent manner. This support is dependent on the intact lipoprotein structure and does not appear to be regulated by specific VLDL-associated inhibitors. Further studies are needed to determine the extent of this activity in vivo.