Interaction of calcium with bovine plasma protein C

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Abstract
The binding of 45Ca2+ to bovine plasma protein C (PC) and to activated bovine plasma protein C (APC) was examined by equilibrium ultrafiltration at pH 7.4 and 25.degree. C. Under these conditions, PC possesses 16.0 .+-. 2.0 equivalent Ca2+ binding sites of average KD (8.7 .+-. 1.5) .times. 10-4 M, and APC contains 9.0 .+-. 1.0 equivalent Ca2+ binding sites, with an average KD of (4.3 .+-. 1.1) .times. 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. 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.