Summary
Protein S (PS) is a vitamin K-dependent anticoagulant that acts as a cofactor to activated
protein C (APC). To date PS has not been shown to possess anticoagulant activity in
the absence of APC.
In this study, we have developed monoclonal antibody to protein S and used to purify
the protein to homogeneity from plasma. Affinity purified protein S (PSM), although
identical to the conventionally purified protein as judged by SDS-PAGE, had significant
anticoagulant activity in the absence of APC when measured in a factor Xa recalcification time. Using SDS-PAGE we have demonstrated that prothrombin cleavage
by factor X awas inhibited in the presence of PSM. Kinetic analysis of the reaction revealed that
PSM competitively inhibited factor X amediated cleavage of prothrombin. PS preincubated with the monoclonal antibody, acquired
similar anticoagulant properties. These results suggest that the interaction of the
monoclonal antibody with PS results in an alteration in the protein exposing sites
that mediate the observed anticoagulant effect. Support that the protein was altered
was derived from the observation that PSM was eight fold more sensitive to cleavage
by thrombin and human neutrophil elastase than conventionally purified protein S.
These observations suggest that PS can be modified in vitro to a protein with APC-independent
anticoagulant activity and raise the possibility that a similar alteration could occur
in vivo through the binding protein S to a cellular or plasma protein.
Keywords
Protein S - Thrombosis - Coagulation
