Abstract
Factor VIIa participates in blood clotting by activating factor X and/or factor IX by limited proteolysis. The proteolytic activity of factor VIIa is absolutely dependent on a lipoprotein cofactor designated tissue factor. We have examined the ability of purified preparations of human plasma high density, low density and very low density lipoproteins, as well as apolipoproteins A-I and A-II, to inhibit the factor VIIa-tissue factor mediated activation of either factor X or factor IX before and after treatment of the lipoprotein preparation with polyclonal antibody directed against partially- purified human plasma extrinsic pathway inhibitor (EPI). In the absence of anti-EPI IgG, HDL, LDL, VLDL, and apolipoprotein A-II noncompetitively inhibited factor X activation by factor VIIa-tissue factor with apparent Ki values of 3.39 mumol/L, 124 nmol/L, 33 nmol/L, and 10.5 mumol/L, respectively. Apolipoprotein A-I had no effect on this reaction. The inhibitory activity of HDL, LDL, VLDL, and apolipoprotein A-II in this reaction was unaffected by the presence of high levels of anti-EPI IgG. In the absence of exogenous factor Xa, none of the lipoproteins studied inhibited the activation of factor IX using the tritiated peptide release assay. In the presence of added factor Xa (1 nmol/L), LDL and VLDL, but not HDL and apolipoprotein A- II, inhibited the activation of factor IX by factor VIIa-tissue factor. This inhibition was completely blocked by prior incubation of the lipoprotein with anti-EPI IgG indicating association of EPI with these particles. Taken collectively, our data indicate that HDL, LDL, and VLDL, at or below their plasma concentration, each selectively inhibits the factor VIIa-tissue factor mediated activation of factor X by a mechanism that appears to be distinct from extrinsic pathway inhibitor. These lipoproteins may not only play a role in the regulation of extrinsic blood coagulation, but may also selectively promote the activation of factor IX by factor VIIa-tissue factor in vivo at low tissue factor concentrations.
