ASSEMBLY OF THE PROTHROMBINASE COMPLEX ON LIPID VESICLES DEPENDS ON THE STEREOCHEMICAL CONFIGURATION OF THE POLAR HEADGROUP OF PHOSPHATIDYLSERINE

The conversion of prothrombin into thrombin is an imperative step in t he sequence of reactions leading to the formation of a hemostatic plug . This reaction is catalyzed by the prothrombinase complex, composed o f factors Xa and Va, which is assembled on a phospholipid surface thro ugh Ca-mediated interactions with the lipid polar headgroups. In this paper we describe experiments indicative for a major role of the stere ochemical configuration of phosphatidylserine in the binding of the pr othrombinase complex to a phospholipid surface. Using two stereoisomer s of phosphatidylserine, i.e., L-alpha-glycerophosphoryl-L-serine (PLS ) and L-alpha-glycerophosphoryl-D-serine (PDS), we demonstrate that me mbranes containing PLS are appreciably more favorable than membranes c ontaining PDS in promoting assembly of the prothrombinase complex and catalysis of prothrombin conversion. Ellipsometric analysis of the bin ding of factor Va and factor Xa to a surface composed of phosphatidylc holine and 10 mol % of either PLS or PDS reveals that the apparent K-d factor Va increases about 25-fold when substituting PDS for PLS. For factor Xa a 5-fold increase in K-d werved on replacing PDS for PLS. Wh en PLS is replaced by phosphatidyl-beta-lactate (PLac), a phospholipid resembling PS but lacking the amino group, a similar decrease in prot hrombinase activity is found as observed with PDS, implicating the imp ortance of both the amino group and the stereoconfiguration of the ser ine moiety for the assembly of the prothrombinase complex. The much hi gher efficiency in supporting prothrombinase of membranes containing P LS as compared to those containing PDS or PLac is further illustrated by their higher capacity to retain their catalytic properties upon cha nges in surface charge. Unlike PLS, the incorporation of positively ch arged stearylamine leads to a considerable reduction in prothrombinase activity when PDS or PLac are used as anionic phospholipid. Taken tog ether, these findings demonstrate the important and unique character o f phosphatidyl-L-serine as procoagulant phospholipid.