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 troponin C molecule.
Morimoto
S
S
Department of Pharmacology, Faculty of Medicine, Kyushu University, Fukuoka.
Ohtsuki
I
I
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D009124:Muscle Proteins / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D009210:Myofibrils / Q000201:enzymology / Q000378:metabolism*; D011817:Rabbits; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
D009124:Muscle Proteins; D014336:Troponin; D019209:Troponin C; D000251:Adenosine Triphosphatases; D002118:Calcium