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 whethe r plasma lipoproteins could provide a physiologically relevant surface , we determined the rates of FIIa production by using purified human c oagulation factors, and isolated fasting plasma lipoproteins from heal thy 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 phosp holipid) yielded rates of 2 to 8 mmol FIIa.L-1.s(-1) in a donor-depend ent manner. Low density lipoprotein (LDL) and high density lipoprotein (HDL) also supported prothrombinase but at much lower rates (less tha n 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) t hrombin-activated platelets per mililiter. Although the FIIa productio n rate was slower on VLDL than on synthetic phosphatidylcholine/phosph atidylserine vesicles (approximate to 50 nmol FIIa.L-1.s(-1)), the pro thrombin K-m values were similar, 0.8 nad 0.5 mu mol/L, respectively. Extracted VLDL lipids supported rates approaching those of phosphatidy lcholine/phosphatidylserine vesicles, indicating the importance of the intact VLDL conformation. However, the presence of VLDL-associated, f actor-specific inhibitors was ruled out by titration experiments, sugg esting a key role for lipid organization. VLDL also supported FIIa gen eration in an assay system comprising 0.1 nmol/L FVIIa; 0.55 nmol/L ti ssue factor, physiological levels of FV, FVIII, FIX, and FX; and proth rombin (3 nmol/L FIIa.L-1.s(-1)). These results indicate that isolated human VLDL can support all the components of the extrinsic coagulatio n pathway, yielding physiologically relevant rates of thrombin generat ion in a donor-dependent manner. This support is dependent on the inta ct lipoprotein structure and does not appear to be regulated by specif ic VLDL-associated inhibitors. Further studies are needed to determine the extent of this activity in vivo.