Amphlett G W, Kisiel W, Castellino F J
Biochemistry. 1981 Apr 14;20(8):2156-61. doi: 10.1021/bi00511a013.
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.
已通过在pH 7.4和25℃下的平衡超滤法研究了45Ca2+与牛血浆蛋白C（PC）以及活化牛血浆蛋白C（APC）的结合情况。在这些条件下，PC拥有16.0±2.0个等效的Ca2+结合位点，平均解离常数KD为(8.7±1.5)×10(-4)M，而APC含有9.0±1.0个等效的Ca2+结合位点，平均KD为(4.3±1.1)×10(-4)M。Mn2+和Sr2+都能够轻易地从Ca2+-PC复合物中置换出Ca2+，而Mg2+在这方面的效果较差。Ca2+的存在会抑制α-凝血酶催化的PC活化。对这种效应的动力学分析表明，这在很大程度上是由于反应的米氏常数增加所致。加入其他二价阳离子，如Mn2+、Sr2+和Mg2+来替代Ca2+，也会以与它们从Ca2+-PC复合物中置换Ca2+的能力平行的方式抑制α-凝血酶催化的PC活化。另一方面，罗素蝰蛇毒中的凝血蛋白对PC的活化作用会因Ca2+的存在而增强。在没有Ca2+的情况下，其他二价金属离子，如Sr2+和Mn2+，也会对该反应产生微弱的刺激作用。Mg2+则没有显著影响。
