Summary
Protein S is a vitamin-K dependent glycoprotein involved in the regulation of the
anticoagulant activity of activated protein C (APC). Recent data showed a direct anticoagulant
role of protein S independent of APC, as demonstrated by the inhibition of prothrombinase
and tenase activity both in plasma and in purified systems. This anticoagulant effect
of protein S can be explained either by a direct interaction of protein S with one
of the components of the complexes and/or by the interference 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 discrepant APC cofactor and direct
anticoagulant activity. In order to elucidate these differences and to study the mechanism
of the APC-inde-pendent activity of protein S, we compared the protein S preparations
in phospholipid-binding properties and anticoagulant activity. The dissociation constant
for the binding of protein S to immobilized phospholipids ranged from 7 to 74 nM for
the different protein S preparations. APC-independent inhibition of both prothrombinase
and tenase activity performed on phospholipid vesicles and in plasma showed a strong
correlation 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 protein 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 APC-cofactor
activity suggesting that the affinity of protein S for phospholipids is of less importance
in the expression of APC-cofactor activity of protein S.
We conclude that high affinity interactions of protein S with the membrane surface
are essential for the direct anticoagulant activity of protein S and we suggest that
inhibition of the prothrombinase and the tenase complex by protein S is a consequence
of the occupation of the phospholipid surface by protein S molecules.
