Open AccessInhibition of human vitamin‐K‐dependent protein‐S‐cofactor activity by a monoclonal antibody specific for a Ca 2+ ‐dependent epitope
Author(s) -
MALM Johan,
PERSSON Ulla,
DAHLBÄCK Björn
Publication year - 1987
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1987.tb11191.x
Subject(s) - epitope , chemistry , thrombin , protein s , biochemistry , monoclonal antibody , protein a/g , calcium , protein g , matrix gla protein , cofactor , binding protein , protein c , chymotrypsin , microbiology and biotechnology , antibody , biology , enzyme , trypsin , platelet , fusion protein , recombinant dna , hyperphosphatemia , organic chemistry , phosphate , gene , immunology
Protein S is an anticoagulant vitamin‐K‐dependent plasma protein functioning as a cofactor to activated protein C in the degradation of factors V a and VIII a . A murine monoclonal antibody, HPS 7, specific for a calcium‐stabilized epitope in human protein S, is described. The epitope was available in intact protein S, both in its free form and when protein S was bound to C4b‐binding protein. It disappeared upon reduction of disulfide bridges and also after thrombin of chymotrypsin cleavage of protein S. Thrombin cleaves protein S close to the calcium‐binding region containing γ‐carboxyglutamic acid (Gla). The cleaved protein still contains the Gla region, linked by a disulfide bridge, but it has a lower affinity for calcium and no protein C cofactor activity. The thrombin‐mediated cleavage of protein S could be inhibited by HPS 7. The K a for the interaction between protein S and the monoclonal was estimated to be approximately 0.7 × 10 8 M −1 . Half‐maximal binding between HPS 7 and protein S was observed at a calcium concentration of 0.50 mM, indicating that saturation of the Gla region with calcium was required for the interaction. The recently reported Gla‐independent high‐affinity calcium binding did not induce the epitope. The calcium‐dependent binding of protein S to phospholipid vesicles as well as the protein C cofactor activity was inhibited by HPS 7. The data suggests that the epitope for HPS 7 is located in the Gla region of protein S or in the closely positioned thrombin‐sensitive region.