| Abstract: | Apolipoprotein A-II (apoA-II) has been shown to inhibit tissue factor participation in the activation of coagulation factor X by factor VIIa. The magnitude of inhibition was dependent on the concentration of the enzyme (factor VIIa) and substrate (factor X) present in the reaction. With factor VIIa at 0.86 nM, 0.41 microM apoA-II inhibited factor X activation as much as 50% at 200 nM factor X, with inhibition decreasing to 39% at 3 nM factor X. When factor X was held constant at 100 nM, 0.41 microM apoA-II inhibited its activation by 80% when factor VIIa was present at 26.7 pM, but the inhibition decreased to 47% when factor VIIa was increased to 1.75 nM. Kinetically, increasing apoA-II decreased the reaction Vmax. ApoA-II produced little effect on the apparent Km, but the apparent K1/2 for factor VIIa in the reaction increased as apoA-II concentration increased. In the presence of 0.75 pM bovine tissue factor, reconstituted with 4.31 microM phosphatidylserine-phosphatidylcholine (30:70, w/w) vesicles, and in the absence of apoA-II, the apparent Km was near 7 nM factor X when factor VIIa was present at 0.86 nM. Under the same conditions with factor X at 100 nM, the apparent K1/2 was near 56 pM factor VIIa. As apoA-II was added to 0.41 microM, the apparent K1/2 increased to about 200 pM factor VIIa. The aggregate results support a model in which apoA-II inhibits tissue factor potentiation of factor VIIa activity. Because the apparent K1/2 increases when apoA-II is added, the factor VIIa can apparently protect tissue factor from the effects of apoA-II. Thus, apoA-II appears to inhibit factor X activation by preventing the appropriate association of tissue factor with factor VIIa. |
