INCREASED PROTHROMBIN ACTIVATION IN PROTEIN S-DEFICIENT PLASMA UNDER FLOW CONDITIONS ON ENDOTHELIAL-CELL MATRIX - AN INDEPENDENT ANTICOAGULANT FUNCTION OF PROTEIN-S IN PLASMA

Protein S is a vitamin K-dependent nonenzymatic coagulation factor inv olved in the regulation of activated protein C (aPC). In this study, w e report an aPC-independent anticoagulant function of protein S in pla sma under flow conditions. Plasma, anticoagulated with low-molecular-w eight heparin allowing tissue factor-dependent prothrombin activation, was perfused at a wall shear rate of 100 s(-1) over tissue factor con taining matrices of stimulated endothelial cells placed in a perfusion chamber. Fractions were collected in time at the outlet and prothromb in activation was determined by measuring the activation fragment F-12 of prothrombin. In normal plasma, a time-dependent prothrombin activ ation was detected by the generation of fragment(1+2). Prothrombin act ivation had ceased after 12 minutes perfusion, independent of the amou nt of tissue factor present in the matrix. Depletion of protein S from plasma or inhibition of protein S in plasma by monoclonal antibodies induced a 5- to 25-fold increase of prothrombin activation on the proc oagulant endothelial cell matrix. A prolonged prothrombin activation w as detected in protein S-depleted plasma up to 20 minutes after onset of the thrombin generation. The increased prothrombin activation in pr otein S-depleted plasma could not be explained by the absence of the c ofactor function of protein S for aPC because depletion of protein C f rom plasma did not result in increased prothrombin activation. These d ata provide further evidence for a strong anticoagulant function of pr otein S in plasma independent from activated protein C. (C) 1995 by Th e American Society of Hematology.