PLASMIN CAN REDUCE THE FUNCTION OF HUMAN BETA(2) GLYCOPROTEIN-I BY CLEAVING DOMAIN-V INTO A NICKED FORM

beta(2)-Glycoprotein I (beta(2)GPI) is a highly glycosylated plasma pr otein with the ability to bind negatively charged substances such as D NA, heparin, dextran sulfate, and negatively charged phospholipids. Th e most relevant physiological role of beta(2)GPI is supposed to be the regulation of the function of anionic phospholipids like cardiolipin (CL). beta(2)GPI consists of a single polypeptide chain (326 amino aci d residues) with a molecular mass of about 50 kD and with five tandem repeated domains (I, II, III, IV, and V). In the previous study, we fo und that factor Xa can produce the nicked form by cleaving Lys 317-Thr 318, using recombinant human domain V (r-Domain V). However, the reac tion was extremely slow. In the present paper, we found that plasmin c an produce the nicked form of domain V, using recombinant domain V (r- Domain V) and beta(2)GPI from human plasma. On sodium dodecyl sulfate- polyacrylamide gel electrophoresis, r-Domain V was rapidly cleaved int o a nicked form by plasmin, very slowly by factor Xa, but not by throm bin, tissue-type plasminogen activator, urokinase, and tissue factor/f actor VIIa. The cleavage site of r-Domain V and beta(2)GPI by plasmin was proved to be Lys 317-Thr 318 by amino acid sequence analysis of th e digest and of the C-terminal peptide isolated by high-performance li quid chromatography. The cleavage was completely inhibited by plasmin inhibitor (alpha(2)PI). The nicked form was demonstrated to show reduc ed affinity for CL with a dissociation constant of one order of magnit ude larger than that of the intact beta(2)GPI, To determine whether th e specific cleavage of beta(2)GPI by plasmin can occur also in plasma, human plasma was first acid-treated to inactivate alpha(2)PI and then incubated with urokinase. About 12% of beta(2)GPI in plasma was nicke d when alpha(2)PI activity decreased to 80%. The nicked form was not g enerated in plasminogen-depleted plasma. These results suggest that pl asmin can produce the nicked form of beta(2)GPI with the reduced abili ty to bind phospholipids in vivo. (C) 1998 by The American Society of Hematology.