THE INTERACTION OF PROTEIN-S WITH THE PHOSPHOLIPID SURFACE IS ESSENTIAL FOR THE ACTIVATED PROTEIN C-INDEPENDENT ACTIVITY OF PROTEIN-S

Protein S is a vitamin-K dependent glycoprotein involved in the regula tion of the anticoagulant activity of activated protein C (APC). Recen t data showed a direct anticoagulant role of protein S independent of APC,as demonstrated by the inhibition of prothrombinase and tenase act ivity both in plasma and in purified systems. This anticoagulant effec t of protein S can be explained either by a direct interaction of prot ein S with one of the components of the complexes and/or by the interf erence with the binding of these components to phospholipid surfaces. During our investigation we noted that protein S preparations purified in different ways and derived from different sources, expressed discr epant APC cofactor and direct anticoagulant activity. In order to eluc idate these differences and to study the mechanism of the APC-independ ent activity of protein S, we compared the protein S preparations in p hospholipid-binding properties and anticoagulant activity. The dissoci ation constant for the binding of protein S to immobilized phospholipi ds ranged from 7 to 74 nM for the different protein S preparations. AP C-independent inhibition of both prothrombinase and tenase activity pe rformed on phospholipid vesicles and in plasma showed a strong correla tion with the affinity for phospholipids. The APC-independent activity could be abolished by monoclonal antibodies that were either calcium- dependent and/or directed against epitopes in the Gla-region of protei n S, suggesting that the protein S-phospholipid interaction is crucial for the APC-independent anticoagulant function of protein S. Protein S preparations with a low APC-independent activity expressed a high AP C-cofactor activity suggesting that the affinity of protein S for phos pholipids is of less importance in the expression of APC-cofactor acti vity of protein S. We conclude that high affinity interactions of prot ein S with the membrane surface are essential for the direct anticoagu lant activity of protein S and we suggest that inhibition of the proth rombinase and the tenase complex by protein S is a consequence of the occupation of the phospholipid surface by protein S molecules.