The effect of Mg2+ on the Ca2+ binding to rabbit fast skeletal troponin C and the Ca2+ dependence of myofibrillar ATPase activity was studied in the physiological state where troponin C was incorporated into myofibrils. The Ca2+ binding to troponin C in myofibrils was measured directly by 45Ca using the CDTA-treated myofibrils as previously reported (Morimoto, S. and Ohtsuki, I. (1989) J. Biochem. 105, 435-439). It was found that the Ca2+ binding to the low and high affinity sites of troponin C in myofibrils was affected by Mg2+ competitively and the Ca2+- and Mg2+-binding constants were 6.20 · 106 and 1.94 · 102 M-1, respectively, for the low affinity sites, and 1.58 · 108 and 1.33 · 103 M-1, respectively, for the high affinity sites. The Ca2+ dependence of myofibrillar ATPase was also affected by Mg2+, with the apparent Ca2+- and Mg2+-binding constants of 1.46 · 106 and 2.76 · 102 M-1, respectively, suggesting that the myofibrillar ATPase was modulated through a competitive action of Mg2+ on Ca2+ binding to the low affinity sites, though the Ca2+ binding to the low affinity sites was not simply related to the myofibrillar ATPase.
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