Summary
The objective of this study was to determine whether (β2-glycoprotein I (β2GPI) has procoagulant activity by inhibiting the anticoagulant activity of activated
protein C (APC). β2GPI inhibited significantly the APC-catalyzed inactivation of factor Va in an assay
using factor V-deficient plasma and physiological levels of protein S and factor Va.
This inhibitory effect was diminished by the addition of increasing concentrations
of phospholipids, suggesting that β2GPI competitively inhibits the binding of APC to the phospholipid surface. β2GPI inhibited weakly factor Va- and phospholipid-dependent prothrombinase activity
at concentrations similar to those to inhibit APC activity. The depletion of β2GPI from plasma led to only a slight shortening of the diluted Russell’s viper venom-dependent
clotting time, but to a strong and significant potentiation of the anticoagulant activity
of APC. These results suggest that under certain physiological conditions β2GPI has procoagulant property by inhibiting the phospholipid-dependent APC anticoagulant
activity.