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 the sequence of reactions leading to the formation of a hemostatic plug. This reaction is catalyzed by the prothrombinase complex, composed of factors Xa and Va, which is assembled on a phospholipid surface through Ca-mediated interactions with the lipid polar headgroups. In this paper we describe experiments indicative for a major role of the stereochemical configuration of phosphatidylserine in the binding of the prothrombinase complex to a phospholipid surface. Using two stereoisomers of phosphatidylserine, i.e., L-alpha-glycerophosphoryl-L-serine (PLS) and L-alpha-glycerophosphoryl-D-serine (PDS), we demonstrate that membranes containing PLS are appreciably more favorable than membranes containing PDS in promoting assembly of the prothrombinase complex and catalysis of prothrombin conversion. Ellipsometric analysis of the binding of factor Va and factor Xa to a surface composed of phosphatidylcholine 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. When PLS is replaced by phosphatidyl-beta-lactate (PLac), a phospholipid resembling PS but lacking the amino group, a similar decrease in prothrombinase activity is found as observed with PDS, implicating the importance of both the amino group and the stereoconfiguration of the serine moiety for the assembly of the prothrombinase complex. The much higher efficiency in supporting prothrombinase of membranes containing PLS as compared to those containing PDS or PLac is further illustrated by their higher capacity to retain their catalytic properties upon changes in surface charge. Unlike PLS, the incorporation of positively charged stearylamine leads to a considerable reduction in prothrombinase activity when PDS or PLac are used as anionic phospholipid. Taken together, these findings demonstrate the important and unique character of phosphatidyl-L-serine as procoagulant phospholipid.