[Effects of Ca2+ and Mg2+ on the enzymatic properties of cardiac muscle myosin].

Objective. To study the influence of Ca2+ and Mg2+ on the enzymatic properties of cardiac muscle myosin. Method. A convenient method for the purification myosin from the left ventricle of rabbit heart was described. The Km and Vmax of Ca(2+)-activated and Mg(2+)-activated ATPase and the effects on the enzymatic properties of myosin ATPase in different ionic concentration and different pH range were determined from the rate of Pi release in enzymatic reaction. Result. The Km values of Ca2+, Mg(2+)-activated myosin ATPase at high ionic [correction of ironic] strength were 5.27 +/- 2.10 mmol, 7.04 +/- 2.06 mmol and the Vmax values were 1.10 +/- 0.13 micromoles mg-1 min-1, 0.617 +/- 0.09 micromoles mg-1 min-1 respectively. The Km of Ca(2+)-activated ATPase was higher than that of Mg(2+)-activated ATPase. But the ATPase activity of Ca2+ was influenced by the concentrations of MgCl2. The effect of Ca(2+)-activated ATPase increase was found at lower MgCl2 concentrations. As the MgCl2 concentration increased above 6 mmol/L, Ca2+ sensitivity was decreased. The pH-activity profiles showed that Mg(2+)-activated myosin ATPase activity was more stable than that of Ca(2+)-activated. Conclusion. The mechanism of Ca2+ and Mg2+ effect on myosin ATPase were different. Mg2+ is essential to maintain the conformation of enzymatic activity of myosin in cardiac muscle contraction. Ca2+ is likely acted as a role conducting signals and regulating function.