DIFFERENCES IN THE INTERACTIONS OF LUPUS ANTICOAGULANT IGG WITH HUMANPROTHROMBIN AND BOVINE PROTHROMBIN

Lupus anticoagulant (LA) IgGs have been reported to inhibit more effec tively and consistently the Xa/Va/phospholipid complex-catalyzed activ ation of human prothrombin than the Xa/Va/phospholipid complex-catalyz ed activation of bovine prothrombin. This led us to carry out studies to determine whether the ability to inhibit the activation of prothrom bin of LA IgGs, separated from the plasma of 15 patients by protein A affinity chromatography, could be related to the ability of the LA IgG s 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 p hospholipid, all 15 LA IgG preparations inhibited the activation of hu man prothrombin with the majority showing >50% of inhibition. In contr ast, only one LA IgG markedly inhibited (>50%) the activation of bovin e prothrombin and five others moderately inhibited (25-40%) the activa tion of bovine prothrombin. Nevertheless, the majority of LA IgG prepa rations bound to immobilized bovine prothrombin on a Western blot and also to immobilized bovine prothrombin on a microtiter well. In an ELI SA in which phosphatidylserine (PS) was immobilized on microtiter well s, bovine prothrombin supported the binding of 10 of 15 LA IgG prepara tions to PS. However, the extent of binding was lower than that observ ed with human prothrombin. In experiments with I-125-human prothrombin or I-125-bovine prothrombin in a solution containing Ca2+, the additi on of PS/PC vesicles enhanced the binding of both human and bovine pro thrombin to some LA IgG preparations. The enhanced binding was particu larly evident for bovine prothrombin. Although seemingly related for s ome preparations, the ability of a LA IgG to bind to bovine prothrombi n, either in the presence or absence of PS, and the ability of that LA IgG to inhibit the activation of bovine prothrombin was not consisten tly related for all preparations.