Summary
Lupus anticoagulant (LA) IgGs have been reported to inhibit more effectively and consistently the Xa/Va/phospholipid complex-catalyzed activation of human prothrombin than the Xa/Va/phospholipid complex-catalyzed activation of bovine prothrombin. This led us to carry out studies to determine whether the ability to inhibit the activation of prothrombin of LA IgGs, separated from the plasma of 15 patients by protein A affinity chromatography, could be related to the ability of the LA IgGs to bind to prothrombin under various experimental conditions. Of 14 LA IgG preparations tested all prolonged to a variable but substantial extent the dilute Russell’s viper venom time (dRVVT) of human plasma but only minimally prolonged the dRVVT of bovine plasma. In a purified prothrombin activation system with a rate limiting concentration of phospholipid, all 15 LA IgG preparations inhibited the activation of human prothrombin with the majority showing >50% of inhibition. In contrast, only one LA IgG markedly inhibited (>50%) the activation of bovine prothrombin and five others moderately inhibited (25-40%) the activation of bovine prothrombin. Nevertheless, the majority of LA IgG preparations bound to immobilized bovine prothrombin on a Western blot and also to immobilized bovine prothrombin on a microtiter well. In an ELISA in which phosphatidylserine (PS) was immobilized on microtiter wells, bovine prothrombin supported the binding of 10 of 15 LA IgG preparations to PS. However, the extent of binding was lower than that observed with human prothrombin. In experiments with 125I-human prothrombin or 125I-bovine prothrombin in a solution containing Ca2+, the addition of PS/PC vesicles enhanced the binding of both human and bovine prothrombin to some LA IgG preparations. The enhanced binding was particularly evident for bovine prothrombin. Although seemingly related for some preparations, the ability of a LA IgG to bind to bovine prothrombin, either in the presence or absence of PS, and the ability of that LA IgG to inhibit the activation of bovine prothrombin was not consistently related for all preparations.
