Cardiolipin enhances protein C pathway anticoagulant activity

The anticoagulant activity of activated protein C (APC) was studied using f actor Xa-1-stage assays of both the procoagulant and anticoagulant activiti es of phospholipid vesicles containing phosphatidylserine or cardiolipin as active phospholipids. In the absence of APC, phosphatidylserine vesicles s howed higher procoagulant activity than cardiolipin vesicles whereas cardio lipin vesicles supported APC-dependent anticoagulant activity better than p hosphatidylserine vesicles. Enhancement of APC anticoagulant activity in pl asma by cardiolipin was markedly stimulated by the APC cofactor protein S. In purified reaction mixtures, cardiolipin in phospholipid vesicles dose-de pendently enhanced APC anticoagulant activity. This effect of cardiolipin w as partially dependent on protein S, and immunoblotting studies showed that cardiolipin enhanced the APC-mediated cleavage of the factor Va heavy chai n at Arg506 and Arg306. In solid-phase binding assays, increasing amounts o f cardiolipin in multicomponent phospholipid vesicles increased the affinit y for protein S and to a lesser extent APC. These data are consistent with the hypothesis that cardiolipin stimulates the anticoagulant protein C path way by increasing the affinity of phospholipid surfaces for protein S:APC a nd by enhancing inactivation of factor Va by APC due to cleavages at Arg506 and Arg306 in factor Va. Based on this, it is further hypothesized that an ti-cardiolipin or anti-oxidized cardiolipin antibodies may be thrombogenic because they inhibit phospholipid-dependent expression of the anticoagulant protein C pathway. (C) 2000 Academic Press.