The effect of Mg2+ on the Ca2+ binding to troponin C in rabbit fast skeletal myofibrils.
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 x 10(6) and 1.94 x 10(2) M-1, respectively, for the low affinity sites, and 1.58 x 10(8) and 1.33 x 10(3) 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 x 10(6) and 276 x 10(2) 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.
Morimoto
S
S
Department of Pharmacology, Faculty of Medicine, Kyusyu University, Fukuoka, Japan.
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002132:Calcium Radioisotopes; D004492:Edetic Acid / Q000494:pharmacology / Q000031:analogs & derivatives; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology*; D009210:Myofibrils / Q000187:drug effects / Q000378:metabolism*; D011817:Rabbits; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
D002132:Calcium Radioisotopes; D014336:Troponin; D019209:Troponin C; C005390:CDTA; D004492:Edetic Acid; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002118:Calcium