Fuchs F, Black B
Biochim Biophys Acta. 1980 Mar 26;622(1):52-62. doi: 10.1016/0005-2795(80)90157-9.
The effect of Mg2+ on Ca2+ bound to glycerinated rabbit psoas muscle was studied by means of a double-isotope technique. The troponin-C content of the fibers was analyzed by quantitative sodium dodecyl sulfate polyacrylamide gel electrophoresis and found to be 0.5 mumol/g fiber protein. In the absence of Mg2+ the fibers bound a maximum of 3.5 mumol Ca2+/g protein. This value could be readily accounted for in terms of the four Ca2+-binding sites of troponin-C and the two divalent cation-binding sites of myosin. In the presence of 1 mM Mg2+ the entire Ca2+ titration curve was shifted downward with a maximum bound Ca2+ of slightly more than 2 mumol/g, or 4 mol Ca2+/mol troponin-C. Further increase in Mg2+ concentration to 10 mM had little effect on Ca2+ binding when the free Ca2+ concentration was in the upper part of the physiological range (5 . 10(-7)-5 . 10(-6) M) but caused a marked reduction when the Ca2+ concentration was low. The results are consistent with biochemical data showing two groups of (Mg2+/Ca2+)-binding sites. One group, located in myosin, has a high affinity for Mg2+ while the other group, located on troponin-C, has a low affinity for Mg2+. If the free Mg2+ concentration in muscle is in the range of 2--5 mM, as suggested by recent data, it can be inferred that the binding sites on myosin will never be occupied by Ca2+ under physiological conditions.
采用双同位素技术研究了Mg2+对结合在甘油处理的兔腰大肌上Ca2+的影响。通过定量十二烷基硫酸钠聚丙烯酰胺凝胶电泳分析纤维中的肌钙蛋白C含量，发现其为0.5 μmol/g纤维蛋白。在没有Mg2+的情况下，纤维结合的最大Ca2+量为3.5 μmol Ca2+/g蛋白。就肌钙蛋白C的四个Ca2+结合位点和肌球蛋白的两个二价阳离子结合位点而言，这个值很容易得到解释。在存在1 mM Mg2+的情况下，整个Ca2+滴定曲线向下移动，最大结合Ca2+量略高于2 μmol/g，即4 mol Ca2+/mol肌钙蛋白C。当游离Ca2+浓度处于生理范围的上限（5×10−7 - 5×10−6 M）时，将Mg2+浓度进一步提高到10 mM对Ca2+结合几乎没有影响，但当Ca2+浓度较低时会导致显著降低。这些结果与显示两组（Mg2+/Ca2+）结合位点的生化数据一致。一组位于肌球蛋白中，对Mg2+具有高亲和力，而另一组位于肌钙蛋白C上，对Mg2+具有低亲和力。如果如最近的数据所表明的，肌肉中的游离Mg2+浓度在2 - 5 mM范围内，那么可以推断在生理条件下肌球蛋白上的结合位点永远不会被Ca2+占据。
