Role of phosphatidylserine in the activation of blood coagulation factor VIII.

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It is well known that two consecutive reactions of blood coagulation cascade catalyzed by the tenase and prothrombinase complex are accelerated greatly in the presence of a particular membrane phospholipid, phosphatidylserine. The aim of this project is to elucidate the molecular mechanism underlying the specific activation of blood coagulation cascade by PS.We have employed a series of immunochemical approach to identify the PS-specific binding sites on the blood coagulation factors. We first established a monoclonal antibody (mAb) , PS4A7, which binds specifically to PS and determine the amino acid sequences of the heavy-and light-chain variable regions. We found that the 12 amino acid residue synthetic peptide derived from the CDR-3 region of heavy chain of PS4A7 bound specifically to PS,indicating that the CDR-3 region paly a dominant role in the interaction with PS and from the peptide motif which is responsible for the specific interaction with PS.We then raised a series of the anti-idiotypic mAbs against the combining site of the PS-specific mAb. We found that one anti-idiotypic mAb, named Id8F7, cross-reacts extensively with proteins kinase C,a enzyme which requires PS for its enzymatic activation, and blood coagulation factor VIII (FVIII) and factor V (FV) . This finding indicates that the PS-specific mAb and the blood coagulation factors share the common structure, which is responsible for the specific interaction with PS.To identify the PS-specific binding site, we mapped the Id8F7-binding site on FV using the various recombinant fragments of the protein. The anlaysis showed that the Id8F7 binding site, which is a putative PS-specific binding site of the protein, locates in the C2-region of the FV light chain.