title: Plasma lipoproteins support prothrombinase and other procoagulant enzymatic complexes.
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 micromol/L prothrombin, physiological levels of very low density lipoprotein (VLDL) (0.45 to 0.9 mmol/L triglyceride, or 100 to 200 micromol/L phospholipid) yielded rates of 2 to 8 nmol Flla x L(-1) x s(-1) in a donor-dependent manner. Low density lipoprotein (LDL) and high density lipoprotein (HDL) also supported prothrombinase but at much lower rates (< or =1.0 nmol FIIa x L(-1) x s[-1]). For comparison, VLDL at 2 mmol/L triglyceride yielded approximately 50% the activity of 2X10(8) thrombin-activated platelets per milliliter. Although the FIIa production rate was slower on VLDL than on synthetic phosphatidylcholine/phosphatidylsenne vesicles (approximately 50 nmol FIIa x L(-1) x s[-1]), the prothrombin Km values were similar, 0.8 and 0.5 micromol/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 x L(-1) x 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.
authors.affiliation: Department of Biochemistry, University of Vermont, Burlington 05446, USA.
authors.forename: M P
authors.initials: MP
authors.lastname: Moyer
authors.forename: R P
authors.initials: RP
authors.lastname: Tracy
authors.forename: P B
authors.initials: PB
authors.lastname: Tracy
authors.forename: C
authors.initials: C
authors.lastname: van't Veer
authors.forename: C E
authors.initials: CE
authors.lastname: Sparks
authors.forename: K G
authors.initials: KG
authors.lastname: Mann
mesh_terms: D001777:Blood Coagulation / Q000502:physiology*; D001782:Blood Donors; D001792:Blood Platelets / Q000201:enzymology; D004798:Enzymes / Q000502:physiology*; D015942:Factor VIIa / Q000502:physiology; D015943:Factor Va / Q000037:antagonists & inhibitors; D065427:Factor Xa Inhibitors; D006801:Humans; D007700:Kinetics; D008074:Lipoproteins / Q000097:blood*; D008077:Lipoproteins, LDL / Q000378:metabolism; D008079:Lipoproteins, VLDL / Q000378:metabolism / Q000494:pharmacology; D013925:Thromboplastin / Q000378:metabolism* / Q000502:physiology
chemical_list: D004798:Enzymes; D065427:Factor Xa Inhibitors; D008074:Lipoproteins; D008077:Lipoproteins, LDL; D008079:Lipoproteins, VLDL; D015943:Factor Va; D013925:Thromboplastin; D015942:Factor VIIa