Ca²⁺ binding to skeletal muscle troponin C in skeletal and cardiac myofibrils

Ca²⁺ binding to skeletal muscle troponin C in skeletal or cardiac myofibrils was measured by the centrifugation method using ⁴⁵Ca. The specific Ca²⁺ binding to troponin C was obtained by subtracting the amount of Ca²⁺ bound to the CDTA-treated myofibrils (troponin C-depleted myofibrils) from that to the myofibrils reconstituted with troponin C. Results of Ca²⁺ binding measurement at various Ca²⁺ concentrations showed that skeletal troponin C had two classes of binding sites with different affinity for Ca²⁺. The Ca²⁺ binding of low-affinity sites in cardiac myofibrils was about eight times lower than that in skeletal myofibrils, while the high-affinity sites of troponin C in skeletal or cardiac myofibrils showed almost the same affinity for Ca²⁺. The Ca²⁺ sensitivity of the ATPase activity of skeletal troponin C-reconstituted cardiac myofibrils was also about eight times lower than that of skeletal myofibrils reconstituted with troponin C. These findings indicated that the difference in the sensitivity to Ca²⁺ of the ATPase activity between skeletal and cardiac CDTA-treated myofibrils reconstituted with skeletal troponin C was mostly due to the change in the affinity for Ca²⁺ of the low-affinity sites on the C troponin C molecule.