Plasma Lipoproteins Support Prothrombinase and Other Procoagulant Enzymatic Complexes

From the Departments of Biochemistry (M.P.M., R.P.T., P.B.T., C.v.V., K.G.M.) and Pathology (R.P.T.), University of Vermont, Burlington; and the Department of Pathology (C.E.S.), University of Rochester, Rochester, NY.

Correspondence to Russell P. Tracy, PhD, Laboratory for Clinical Biochemistry Research, University of Vermont, 55A South Park Dr, Colchester, VT 05446. E-mail rtracy{at}salus.uvm.edu

Abstract—The prothrombinase complex (factor [F]Xa, FVa, calcium ions, and lipid membrane) converts prothrombin 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 nmol/L FVa, 5 nmol/L FXa, and 1.4 µmol/L prothrombin, physiological levels of very low density lipoprotein (VLDL) (0.45 to 0.9 mmol/L triglyceride, or 100 to 200 µmol/L phospholipid) yielded rates of 2 to 8 nmol 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 (1.0 nmol FIIa · L^-1 · s^-1). For comparison, VLDL at 2 mmol/L triglyceride yielded 50% the activity of 2x10⁸ thrombin-activated platelets per milliliter. Although the FIIa production rate was slower on VLDL than on synthetic phosphatidylcholine/phosphatidylserine vesicles (50 nmol FIIa · L^-1 · s^-1), the prothrombin K_m values were similar, 0.8 and 0.5 µmol/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.

Key Words: lipoproteins • blood coagulation • prothrombinase • thrombin

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