title: Interaction of calcium with bovine plasma protein C.
abstract: The binding of 45Ca2+ to bovine plasma protein C (PC) and to activated bovine plasma protein C (APC) has been examined by equilibrium ultrafiltration at pH 7.4 and 25 degrees C. Under these conditions, PC possesses 16.0 plus or minus 2.0 equivalent Ca2+ binding sites, of average KD (8.7 plus or minus 1.5) x 10(-4) M, and APC contains 9.0 plus or minus 1.0 equivalent Ca2+ binding sites, with an average KD of (4.3 plus or minus 1.1) x 10(-4) M. Both Mn2+ and Sr2+ were capable of ready displacement of Ca2+ from a Ca2+-PC complex, while Mg2+ was less effective in this regard. The alpha-thrombin-catalyzed activation of PC was inhibited by the presence of Ca2+. A kinetic analysis of this effect demonstrated that it was, in large part, due to an increase in the Km of the reaction. Addition of other divalent cations, e.g. Mn2+, Sr2+, and Mg2+, in place of Ca2+ also resulted in inhibition of the alpha-thrombin-catalyzed activation of PC in a manner which paralleled their ability to displace Ca2+ from a Ca2+-PC complex. On the other hand, the activation of PC by the coagulant protein from Russell's Viper venom was augmented by the presence of Ca2+. Other divalent metal ions, such as Sr2+ and Mn2+, in the absence of Ca2+, also weakly stimulated this reaction. Mg2+ was without notable effect.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D001779:Blood Coagulation Factors / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002413:Cations, Divalent; D002417:Cattle; D002942:Circular Dichroism; D004789:Enzyme Activation; D006023:Glycoproteins / Q000378:metabolism*; D007700:Kinetics; D011485:Protein Binding; D011486:Protein C; D011487:Protein Conformation; D013917:Thrombin / Q000378:metabolism
chemical_list: D001779:Blood Coagulation Factors; D002413:Cations, Divalent; D006023:Glycoproteins; D011486:Protein C; D013917:Thrombin; D002118:Calcium