ROLE MAGNESIUM IN RELAXATION OF MYOFIBRILS

Mg was necessary for relaxation of intact myofibrils even under conditions where it did not displace bound calcium. After most of the bound calcium had been removed by treatment with ethylene glycol bis(2′-aminoethyl ether)-N,-N′-tetraacetic acid and magnesium so that the myofibrils contained only 0.5 μmole of calcium/g of protein (a residual amount of bound calcium which could not be removed under any circumstances), myofibrils in the presence of magnesium remained fully relaxed until sufficient calcium had been added to raise the amount of bound calcium above 1.4 μmole of calcium/g of protein. If, however, magnesium was removed from the medium, syneresis was 85% complete with only 0.6 μmole of bound calcium/g of protein and remained constant on increasing the amount of bound calcium. It appeared that syneresis was independent of bound calcium when the magnesium concentration (contaminating magnesium was present) was very low. Syneresis was incomplete because of the low magnesium. With 0.6 μmole of bound calcium/g of protein the rate of adenosine triphosphate hydrolysis in the absence of added magnesium was twice that in its presence. Adenosine triphosphatase activity, however, increased with increasing bound calcium but this activity probably was not related to contraction but represented calcium-activated hydrolvsis without energy transfer. Relaxation as well as syneresis and adenosine triphosphatase activity required magnesium as magnesium adenosine triphosphate. The activation of the adenosine triphosphatase activity with low magnesium adenosine triphosphate was determined only by the concentration of magnesium adenosine triphosphate and was not influenced by the concentration of free magnesium ion, but the inhibition of adenosine triphosphatase activity at higher concentrations of total magnesium depended upon both the concentrations of magnesium adenosine triphosphate and the free magnesium ion. While most of the bound calcium could be removed and 40% of the residual calcium was exchangeable, the myofibrils contained 4 μmole/g of protein unexchangeable magnesium. Since this amount of unexchangeable magnesium equaled the content of monomeric actin in myofibrils, it appeared likely that myofibrillar actin contains magnesium as the tightly bound divalent cation rather than calcium. © 1969, American Chemical Society. All rights reserved.