Inhibition of phospholipid and platelet-dependent prothrombinase activity in the plasma of patients with lupus anticoagulants

Prothrombinase activity was analysed in the plasma of a series of patients with lupus anticoagulants (LAC). In the presence of purified PS-PC (20-80%) vesicles the prothrombinase activity triggered by kaolin was retarded by 2-3 min with respect with normal plasma. The maximal values of prothrombinase activity increased by increasing the amount of phospholipid vesicles. However, in the plasma of the patients they were always lower than those of normal plasma at each phospholipid concentration. Platelet-dependent prothrombinase activity was subsequently investigated. Again, both a delay in appearance and reduced peak values of prothrombinase activity were observed in the plasma of the patients. This inhibition was partially overcome by the addition of an excess of purified phospholipids. Finally, the effect of LAC IgG on platelet rich plasma-dependent prothrombinase activity was investigated. The main effect observed was a delay of the peak time of prothrombinase activity, while the maximal peaks were affected only by one IgG preparation. We conclude that LAC antibodies can react with both purified negatively-charged phospholipids and platelet procoagulant phospholipids and inhibit prothrombinase activity in a similar way in both cases.